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FM-Kerzel
2. Research Plan: “Selection of perceptual dimensions for oculomotor control”
2.1 Introduction
It is well-known that perceptual dimensions such as motion, form or color are
processed in different cortical modules. There seems little doubt that motion is processed
in area MT/V5 of extrastriate cortex (e.g., Newsome, Britten, & Movshon, 1989) and
complex forms, such as faces, in inferotemporal cortex (IT, e.g., Kanwisher, McDermott,
& Chun, 1997). Also, there is some, albeit contended, evidence that color is processed in
area V4 (e.g., Bartels & Zeki, 2000). The modular processing of perceptual dimensions is
evident in selective perceptual deficits after lesions of the respective module. Color
processing is perturbed after lesions of V4, motion processing after lesions of MT/V5 and
the identification of complex forms, such as faces, after lesions of IT. It is also known
that selective attention may increase the sensibility to a particular perceptual dimension
(e.g., Treue, 2004; Treue & Maunsell, 1996). Behaviorally, this is evident in faster
responses to stimuli that vary on the selected perceptual dimension (e.g., Krummenacher,
Muller, & Heller, 2002; Muller, Reimann, & Krummenacher, 2003).
In the proposed experiments, we will investigate how dimensional attention
selects perceptual input for saccadic and smooth pursuit eye movements. We will focus
on two perceptual dimensions: color and luminance contrast. This comparison is
interesting because luminance contrast is known to drive subcortical neurons and also
provides the main input to cortical motion processing. In contrast, color is assumed to
result from distributed cortical processing and is only weakly represented in cortical
motion centers. The question we ask is whether and how dimensional attention may
select chromatic signals to drive saccadic or smooth pursuit eye movements when
achromatic signals are present at the same time. The common logic of the experiments is
to present two signals defined along different perceptual dimensions and to measure the
efficiency of selection of one dimension for oculomotor control. In one series of
experiments, this rationale will be applied to two well-known saccadic phenomena, the
distractor and the gap effect. Both effects are believed to have a subcortical origin.
Showing effects of dimensional attention to chromatic signals would demonstrate how
these presumed subcortical effects are modulated by high-level, cortical selection. In a
second set of experiments, simultaneous chromatic and achromatic motion signals will be
present and the ability to change the gain of chromatic and achromatic motion signals for
smooth pursuit will be measured. The interesting question is to what degree the (weak)
chromatic motion signals can be favored over achromatic signals.
2.1.1 “Color” channels
The human retina contains three cone types that are maximally sensitive to short
(S), medium (M) and long (L) wavelengths. The output of the three cone types is
combined to yield three “color” channels (see Figure 1). One channel sums up the
activation of L and M cones (L+M) and signals luminance (black/white). It therefore
responds best to gratings composed of dark and bright bars. The other two channels are
truly color-opponent channels. In the red/green channel, activation of L and M cones is
subtracted (L-M) and the channel is selectively activated by a grating composed of red
and green bars. In the blue/yellow channel, the L and M cone activation is summed and
subtracted from the blue cone activity: S – (L+M). This channel will respond best if a
blue-yellow grating is presented. Each color channel originates in morphologically
different types of retinal ganglion cells (Merigan & Maunsell, 1993). The magnocellular
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pathway (M-pathway) receives the additive luminance (L+M) signal. The parvocellular
pathway (P-pathway) receives color opponent information from M and L receptors (LM). Finally, the more recently discovered koniocellular pathway receives color opponent
information from (M+L) and S cones (for review see Dobkins, 2000). Each retinal
ganglion cell is associated with different layers in the corpus geniculatum laterale (CGL),
thalamus, and primary visual cortex. The emerging view on the cortical processing of
chromatic signals is that the information is distributed across a wide range of cortical
areas (e.g., Dougherty, Press, & Wandell, 1999; Gegenfurtner & Hawken, 1996).
Figure 1. Red/green, blue/yellow and light/dark color channels (copied from Dobkins,
2000). The color channels correspond to the P-, K-, and M-pathways.
For all experiments proposed below, it would be desirable to vary the color axis to
evaluate differences between the P- and K- pathways. The question is, however, whether
this will be technically possible. The phosphors of CRT-monitors are not aligned with the
physiological color channels and the maximally possible color contrast along the P- and
K-pathways is relatively low. Therefore, many researchers have used the axis defined by
the CRT-phosphors in their research: Red, green and their combination, yellow. While
this maximizes physical color contrast, it does not correspond to neurophysiological
channels, but will activate more than one channel at a time. When the experiments are set
up, pilot studies will be conducted to evaluate whether a manipulation of the color axis is
feasible. In some experiments, it is already clear that this is feasible and experiments
evaluating the contribution of color axis are proposed.
2.1.2 Neurophysiological substrate of saccades and smooth pursuit
For our present purposes, projections of the three color pathways to centers
involved in the control of smooth pursuit and saccadic eye movements are important.
While the classical view was that saccadic and smooth pursuit eye movements rely on
different neural substrates, there is a growing body of evidence suggesting that the same
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cortical (e.g., frontal eye fields, lateral intraparietal sulcus) and subcortical areas (e.g.,
superior colliculus) are involved (for review see Krauzlis, 2005). Besides their
commonalities, it is also clear that saccades and smooth pursuit use the same
neurophysiological substrate differently. MT and MST are the major source of motion
information during smooth pursuit eye movements (e.g., Dursteler & Wurtz, 1988). In
contrast, the superior colliculus (SC) may play a larger role in saccadic control, as it
contains a motor map that controls saccade direction and amplitude (for review see
Sparks, 2002).
2.1.3 Projections of color channels to MT and SC
Projections of the three “color” pathways to MT are clearly dominated by the Mpathway. That is, neurons in MT are highly sensitive to the motion of a luminancedefined grating, and far less sensitive to moving red/green or blue/yellow gratings.
Psychophysically, isoluminant stimuli that mostly excite the P- or K-channel are
perceived as moving at a slower speed (Dougherty et al., 1999; Gegenfurtner & Hawken,
1996). Nonetheless, there are (albeit few) neurons in MT that are responsive to stimuli
that preferentially excite the K- and P-channels (Gegenfurtner & Kiper, 1992; Wandell et
al., 1999). It may be the case, however, that this sensitivity of MT to isoluminant,
chromatic stimuli is partially mediated by nonlinearities in the M-channel that allow for
the detection of borders between isoluminant surfaces without signaling the color of the
surfaces themselves (Thiele, Dobkins, & Albright, 1999). Whatever the exact origin of
chromatic signals in MT may be, it is now agreed upon that color perception does not
reside in a specific neural module (V4 was a strong candidate, e.g., Zeki, 1980), but that
it is distributed across extrastriate visual cortex. Therefore, color is also represented in
MT, as it is in many other areas of the brain (Dougherty et al., 1999; Gegenfurtner &
Hawken, 1996).
Single cell recordings have shown that there are no projections from coloropponent cells to the superior colliculus and no projections from S-cones at all (de
Monasterio, 1978; Marrocco & Li, 1977; Schiller & Malpeli, 1977). Therefore, neurons
in SC do not respond to isoluminant borders (Marrocco & Li, 1977) such that chromatic
signals may be masked by luminance noise (Birch, Barbur, & Harlow, 1992). Responses
to colored stimuli therefore have to be mediated through cortical areas. That is, areas
sensitive to chromatic signals feed back into SC.
2.2. Project A: The distractor and gap effects
When observers are asked to make a saccade to a suddenly appearing target,
saccadic latencies increase if a second, irrelevant object (distractor) appears at the same
time (remote distractor effect, e.g. Walker, Kentridge, & Findlay, 1995). The increase is
largest if the distractor appears around the point of fixation prior to the saccade and
decreases with increasing eccentricity (Walker, Deubel, Schneider, & Findlay, 1997). It is
believed that the superior colliculus (SC) is the neural substrate of the remote distractor
effect (Walker et al., 1997). This structure is retinotopically organized, and cells at the
rostral end (a region extending approximately 2 deg foveally) are known to play a
significant role in visual fixation (Munoz & Wurtz, 1993a, 1993b). Therefore, this region
is referred to as fixation zone. Neurons in the fixation zone increase firing rates during
active fixation, and pause during saccades. A pause of the fixation neurons may also be
produced by the removal of a visual fixation stimulus. This pause causes a latency
decrease of the subsequent saccade (Dorris & Munoz, 1995). The latency decrease
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produced by the offset of central fixation 100-200 ms before target onset is referred to as
gap effect (e.g., Fischer & Boch, 1983). Conversely, the latency increase produced by
irrelevant visual onsets that stimulate the fixation neurons at target onset is referred to as
distractor effect (see Figure 2).
Figure 2. The gap (panel A) and distractor (panel B) paradigm. Subjects make a saccade
to an achromatic target to the left of fixation. In the gap paradigm, a blank interval is
inserted between fixation offset and target onset. In the distractor paradigm, a distractor is
shown at central fixation at the time of target onset.
Consistent with the idea that the distractor effect originates in the superior
colliculus, it has been demonstrated that isoluminant blue stimuli do not produce a
distractor effect (Sumner, Adamjee, & Mollon, 2002). As mentioned above, the SC does
not receive color opponent input from the retina. To ensure that isoluminant chromatic
stimuli were invisible to the SC, Sumner and al. presented luminance noise at the two
possible target positions (see Figure 3). The target was luminance-defined (black dot),
while the distractor was chromatically defined. Because the luminance noise made the
luminance-defined target less salient, it may have been the case that subjects had to
actively search for the target. That is, there was no sudden onset and rapid, stimuluselicited saccades were not possible. Consistent with this interpretation, saccadic latencies
in the absence of a distractor were about 100 ms longer than in previous studies on the
distractor effect in which a single luminance-defined target was presented in the absence
of noise (~270 in Sumner et al. vs. ~170 ms in Walker et al., 1997). This difference in
reaction time implies that saccades were not driven in a purely bottom-up manner and
casts doubt on the assumption of SC-involvement.
Rather, high-level explanations of the effects reported by Sumner et al. (2002)
seem plausible. For instance, it may be possible that observers searched for a luminance
target and were able to tune their attention to this dimension. As a result, effects of the
chromatic distractor on saccadic latencies were reduced. In line with this idea, Müller,
Reimann, and Krummenacher (2003) found that the detection of color or orientation
singletons was expedited if observers directed their attention to the respective perceptual
dimension. In their experiments, observers had to detect a bar that differed by its
orientation or color from the surrounding distractors (a so-called singleton). Before each
trial, a cue was given as to the more likely singleton (color or orientation) and detection
times decreased when the cue correctly indicated the upcoming target, while a cost was
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incurred when the cue did not correctly indicate the upcoming target. Similarly, a study
by Folk, Remington, and Wright (1994)showed that irrelevant color or onset-defined
distractors only perturbed subsequent search if the target was defined by the same
attribute as the irrelevant stimulus. That is, search for a color singleton was slowed down
if it was preceded by an irrelevant color stimulus, but not if it was preceded by an
irrelevant onset-defined stimulus.
Figure 3. Stimulus sequence in the experiment of Sumner et al. (2002). Observers were
asked to make a saccade to a black dot while the luminance of the two possible target
locations changed every 50 ms (luminance noise). In the chromatic distractor condition
(shown), the color of the opposite target location changed at the time of target
appearance.
Therefore, we believe that the distractor effect in Sumner et al. (2002) was
mediated by high-level, cortical information flowing back to the SC. The attentional
tuning to achromatic targets explains why Sumner et al. did not find a distractor effect
with a chromatic distractor: The sensitivity to colored distractors was reduced because
attention was focused along the luminance-dimension. Remember that Sumner et al’s
interpretation was that isoluminant stimuli did not produce a distractor effect because the
direct retino-collicular pathway is color blind. This is not the case for cortical feedback to
the SC via the frontal eye fields (FEF) to the SC. This should not be taken to mean that
these cortical areas are color-sensitive themselves, in fact, the FEF is not (e.g., Schall,
1991). However, because color sensitive neurons seem to be distributed over a large
number of cortical areas, chromatic signals may influence SC responses indirectly. One
goal of the present series of experiments is to show that recurrent color feedback may
modulate SC activity. Behaviorally, this should be evident in distractor and gap effects to
achromatic distractors and fixation points, respectively.
2.2.1 Own contributions
Previous experiments in our lab showed that the SC may process rather complex
visual information. We asked observers to saccade to a sine-wave grating that appeared to
the left or right of central fixation (White, Gegenfurtner, & Kerzel, 2005). The target
appeared either on a neutral gray background, or was accompanied by a distractor. Two
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types of distractors were used: a small structured patch of about 2x2 deg, and a large
structured background that covered 45x36 deg (see Figure 4). The two distractors
produced the same foveal, but different peripheral stimulation. We observed that
compared to a neutral gray background, saccadic latencies were longer when a patch
appeared in the fovea, but not when a background covering fovea and periphery was
shown. If the difference between the gray background and the foveal patch (distractor
effect) was due to the reengagement of fixation neurons in the SC, it needs to be
accounted for how they were differentially stimulated by a small isolated patch and the
same patch as part of a large background. The most likely explanation is that recurrent
feedback from cortical areas that are involved in figure-ground segregation feeds into the
SC. The first stage where figure-ground segregation is achieved is V1 (e.g., Lamme,
1995) (which should not be taken to mean that V1 feeds into SC). Thus, it seems likely
that cortical processing modulates the distractor effect and this finding motivates the
hypothesis that chromatic signals may also modulate the distractor effect.
Figure 4. Stimuli used in White, Gegenfurtner, and Kerzel (2005). Observers looked at
the center of the screen and were instructed to saccade to a target at 10deg. Simultaneous
to the target onset, (A) a small patch in the fovea (B) a large background filling the entire
screen was presented.
2.2.2 Research Plan
The proposed experiments will answer the question whether chromatic signals can
produce a distractor and gap effect and whether dimensional attention may modulate
these effects. The proposed experiments partly follow up on the study by Sumner et al.
(2002) and our own study (White et al., 2005). In Sumner et al.’s study, no effects of a
colored distractor on saccades to a luminance defined target were found. We believe that
dimensional attention and methodological issues may account for the absence of an
effect. We propose the following experiments to show that effects believed to originate in
the SC may be mediated by chromatic and therefore cortical signals.
2.2.2.1 Experiment 1 & 2, and pretest: Gabors on a gray background
The relation between the perceptual dimension (color, luminance) of the distractor
and the saccade target will be varied. A chromatic or an achromatic target will be
combined with a distractor that shares the target’s perceptual dimension (e.g., colored
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target and colored distractor) or the target will be paired with a distractor of another
perceptual dimension (e.g., colored target and luminance-defined distractor). Based on
the results of studies on dimensional attention, we expect larger distractor effects in
conditions where distractor and target share a perceptual dimension: When observers are
able to focus on a perceptual dimension, objects that vary on this dimension will be
treated preferentially. Therefore, achromatic distractors should interfere more with
achromatic than with chromatic targets. Conversely, chromatic distractors should
interfere more with chromatic than with achromatic targets.
Chromatic Target (Exp 1)
Achromatic Target (Exp 2)
Chromatic Distractor
Same Dimension
Different Dimension
Achromatic Distractor
Different Dimension
Same Dimension
Targets and distractors are chromatic or achromatic Gaussian patches of about
2deg width. The smooth changes in a Gaussian are necessary to compensate for
chromatic lens aberration. In both experiments, a baseline condition without distractor
presentation will be included. Each trial starts with the presentation of a fixation mark at
the screen center. Then, the fixation mark is extinguished and both the distractor and the
target are shown. While the target appears at 8deg of eccentricity randomly to the left or
right, the distractor is presented at central fixation. About 100 trials per condition
(baseline, same dimension distractor, different dimension distractor) will be presented. In
the same-dimension distractor trials, a distractor is presented that is the same or different
from the target. For instance, if a blue target is presented, the distractor may be identical
to the target (e.g., blue distractor at fixation) or different (e.g., green distractor at central
fixation). Similarly, the luminance target may be of the same polarity as the distractor
(e.g., black Gaussians on gray background), or the distractor may have the opposite
polarity (e.g., white Gaussian target and black Gaussian distractor).
Same Dimension Distractor
Different Dimension Distractor
Target
Same Feature
Different Feature
Different Features
White
White
Black
Blue
Green
Blue
Blue
Green
White
Black
In the analysis, it is possible to calculate effects of dimensional overlap (same
dimension distractor vs. different dimension distractor) and of feature overlap (identical
distractor vs. different distractor along same dimension). Feature overlap refers to
conditions in which distractor and target that share a perceptual dimension and are
identical. For instance, a blue distractor may not be identical to a green target, yet it
shares the same perceptual dimension. Stronger distractor effects are expected when
target and distractor show dimensional overlap and when they are identical.
Prior to the Experiment, the contrast of the chromatic and achromatic Gaussian
has to be adjusted such that saccadic latencies to each target type are equal. To this end, a
certain color contrast is fixed and saccadic latency to various achromatic contrasts is
measured. For instance, the maximal color contrast available is shown, and the
achromatic contrast is increased from 2% (close to threshold) to 15% (clearly above
threshold). A psychometric function is then fit to the latencies to the achromatic target,
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and the contrast where saccadic latencies to the chromatic and achromatic targets are
similar is extrapolated.
Design: Distractor type (chromatic, achromatic, no distractor) x target type
(chromatic: Exp. 1, achromatic: Exp. 2)
Hypothesis: Distractors that share a perceptual dimension with the target or are
identical to the target produce larger distractor effects because attention is focused on the
target dimension.
2.2.2.2 Experiment 3 & 4, and pretest: Gabors with luminance noise
Neurophysiological evidence suggests that the SC does not receive color
opponent signals (Marrocco & Li, 1977). However, it cannot be ruled out that chromatic
signals are transmitted to SC due to non-linearities retinal ganglion cells (Dobkins, 2000),
that is, cells in SC may be color-blind, but still receive signals about the presence of a
chromatic border. To solve this problem, Somner et al. (2002) presented luminance noise
in addition to the a/chromatic signals. A problem with this approach is that stimuluselicited saccades are no longer possible because the noise is composed of a random series
of luminance changes that may well qualify as stimulus “onsets” that would typically
elicit a saccade. To clarify whether the presence of luminance noise changes the results,
Experiments 1 and 2 are rerun with luminance noise. It seems preferable to present a
large oscillating background in order to avoid selective inhibition of the two possible
target positions. Note that the noise at two target positions (see Figure 3) may
continuously stimulate saccade generating mechanisms, and may therefore require
continuous inhibition.
The luminance noise consists of small rectangular elements of about 0.5deg that
form a structured background covering the entire screen. Each element changes its
luminance randomly between 25-30 cd/m2 at 20 Hz (Birch et al., 1992). The same
conditions are run as in Experiments 1-2, and the same procedure is used to equate
chromatic and achromatic targets; this time on the luminance background (pre-test).
Design: Distractor type (chromatic, achromatic, no distractor) x target type
(chromatic: Exp. 1, achromatic: Exp. 2)
Hypothesis: Distractors that share a perceptual dimension with the target or are
identical to the target produce larger distractor effects. These effects are stable when
chromatic signals are masked by luminance noise. In fact, larger effects may be expected
because rapid, stimulus-driven saccades are not possible due to the noise, which increases
the weight of cortical control over SC.
2.2.2.3 Experiment 5 & 6: Small vs. large distractor
The effects of distractor size are examined in the paradigm used in Experiments
1-4. White et al. (2005) have demonstrated that the same foveal stimulation may have
different effects depending on the size of the distractor. Only small, locally defined
objects in the fovea produce a distractor effect. The interesting question is whether this
effect will also hold when target and distractor are defined along different dimensions.
One may expect that a small distractor in the fovea may well be ignored when it is
defined along another dimension and that it will produce the same response time pattern
as a large textured background.
Another question is whether chromatic noise that extends beyond the fovea (large
distractor condition) will also be treated as an irrelevant background. If we find
differences between the two colored distractor conditions, this would provide strong
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evidence that color-signals reach SC (maybe via cortical pathways): As the foveal
stimulation is the same in the small and large chromatic distractor condition, differences
between the two conditions would indicate that color-defined targets may drive fixation
cells in SC.
Further, it will be interesting to see whether the large chromatic background
affects latencies when saccades to an achromatic target are required. In White et al.
(2005) it was found that the large background could facilitate responses in some
conditions compared to a neutral gray background. Warning effects may underlie this
advantage. Along these lines, one may ask whether a chromatic background would still
cue an achromatic target. It may be possible that dimensional attention prevents such
cuing effects (compared to within-dimensional cueing).
Observers are asked to saccade to either a chromatic (Exp. 5) or an achromatic
(Exp. 6) target. In the baseline condition, the target appears on a neutral gray background.
In two distractor conditions, the foveal retina is stimulated by noise elements. In the small
patch condition, the patch is confined to the fovea. In the large patch condition, the patch
extends well beyond the fovea into the far periphery. As in Experiment 1-4, the noise
elements vary either along the same dimension, or a different dimension. According to
the results of White et al. (2005), we expect shorter latencies with a large background
than with a small patch. The reason is that only object-like stimuli in the fovea drive the
fixation neurons in SC.
Design: Distractor type (small patch, large patch, neutral gray) x distractor type
(chromatic, achromatic, no distractor) x target type (chromatic: Exp. 5, achromatic: Exp.
6)
Hypothesis: The difference between the large and small distractor condition is
modulated by the congruence between target and distractor. The difference only persists
if target and distractor share the same perceptual dimension.
2.2.2.4 Experiment 7-10: Gap effect with isoluminant fixation points
Another effect that is believed to have its roots in collicular fixation cells is the
gap effect. Saccades are faster when the fixation point is extinguished 100-200 ms before
target onset (e.g., Fischer & Boch, 1983). When there is no stimulus in the fovea, fixation
neurons in the rostral SC stop firing, and the saccade-related neurons in caudal SC exhibit
preparatory firing (Dorris & Munoz, 1995). If the SC was color-blind, no gap effect was
expected with isoluminant fixation points, because chromatic targets should be
insufficient to drive fixation neurons. However, if recurrent color signals modulate SCactivity, a gap effect was expected.
The fixation point will disappear 200 or 100 ms before target presentation or at
the same time as target presentation. We present either and achromatic or a chromatic
saccade target to the left or right of fixation. The relation between the perceptual
dimension of the fixation point and the saccade target are systematically varied. Target
and fixation point dimensions may either overlap or not (details see above). In
Experiment 7 and 9, a luminance target will be presented. In Experiment 8 and 10, a
chromatic target is shown. Because the target does not change, attentional settings can be
tuned to chromatic or achromatic signals. It may be that attentional settings also influence
the activity in fixation neurons of SC: if attention is devoted to a particular dimension,
stimuli varying on this dimension may become more potent fixation marks that are less
easy to quit. In other words, same dimension fixation marks may produce a larger gap
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effect. The conditions with isoluminant fixation marks will further clarify whether
luminance input via the retino-collicular pathway is necessary to produce a gap effect. If
a gap effect with isoluminant targets obtains, this would indicate that cortical analysis of
target features modulates SC activity.
As in the previous experiments, the contrast of the achromatic target is chosen
such that saccadic latencies are equal for chromatic and achromatic targets. Exp. 7-8 will
examine the gap effect with Gabor patches on a gray background and Exp. 9-10 will
examine the gap effect with Gabor patches overlaid on luminance noise (rationale for
luminance noise see above).
Design: three stimulus-onset-asynchronies (200, 100, 0) between fixation mark
offset and target onset x two fixation mark types (chromatic, achromatic) x target type
(achromatic: Exp.7,9, chromatic: Exp. 8,10) x background type (neutral gray: Exp.7,8,
noise: Exp.9,10).
Gray Background
Noise Background
Achromatic Target
Exp. 7, a/chromatic fixation
Exp. 9, a/chromatic fixation
Chromatic Target
Exp. 8, a/chromatic fixation
Exp. 10, a/chromatic fixation
Hypothesis: The gap effect is also observed with isoluminant targets, showing that
the activity of fixation neurons in SC is modulated by cortical processing. The gap effect
is larger if the target and fixation mark share a perceptual dimension because attention is
tuned to this dimension which increases the gain of this dimension.
2.3 Project B: Selection of motion signals for smooth pursuit
In the second part of the research proposal, experiments are proposed that deal
with dimensional attention to signals for smooth pursuit eye movements. As laid out in
the introduction, smooth pursuit eye movements depend on motion processing in area
MT. Both chromatic and achromatic signals arrive in MT and may drive motion
perception. Here, we ask whether dimensional attention may select the motion signals for
smooth pursuit on the basis of the distinction between luminance and color. To this end,
both luminance-defined and color-defined motion signals are present and smooth pursuit
to one or the other are instructed. Because a large number of studies have been run on
motion perception with sinusoidal gratings, some experiments will investigate smooth
pursuit to such gratings. This allows for a better comparison between the present and
previous studies. In another set of experiments, smooth pursuit to local object motion is
examined.
2.3.1 Smooth pursuit to large moving backgrounds
When looking at a large moving background covering a substantial portion of the
visual field, two modes of viewing are possible. Either the observer is “staring” passively
at the background, or the observer is voluntarily tracking single elements contained in the
background. In both cases, the eye typically follows the background slowly for some time
until a saccade brings the eye back to its original position. These eye movements are
referred to as optokinetic nystgmus (OKN). Passive, involuntary OKN is associated with
subcortical centers such as the nucleus of the optic tract, while active, voluntary OKN is
believed to be very similar to smooth pursuit of local object motion and relies more
heavily on cortical areas (overview in, Ilg, 1997). However, predictability of target
motion affects both modes of viewing (Wyatt & Pola, 1988), suggesting higher-level
control for involuntary and voluntary OKN.
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Here, I will exclusively deal with voluntary eye movements to a large textured
stimulus, and I will not investigate the cyclic pattern between slow and fast eye
movements that is typical of studies on OKN. Therefore, I will refer to “smooth pursuit”
even if the term “active OKN” may also be appropriate. In the studies proposed here,
smooth pursuit to a stimulus containing different motion vectors in the same foveal
region will be investigated. For instance, two fields of luminous dots may be
superimposed and one field moves to the left while the other moves to the right
(Niemann, Ilg, & Hoffmann, 1994) or one field moves and the other remains stationary
(Kowler, van der Steen, Tamminga, & Collewijn, 1984). Observers are instructed to
either follow a particular direction of motion when both move, or to follow the field that
is moving when the other is stationary. It has been demonstrated that the distracting dotfield has only a negligible effect on the gain (Kowler et al., 1984; Niemann et al., 1994)
or latency of the pursuit eye movements (Niemann & Hoffmann, 1997). Accurate pursuit
was possible even when the target field was less luminuous than the distracting dot-field.
Kowler et al. (1984) suggest that “that the effectiveness of voluntary selection in
eliminating the influence of background stimuli on smooth eye movements can be
virtually complete.” (p. 1789).
The proposed experiments will investigate whether such accurate selection of
target stimuli for smooth pursuit may also be based on its chromatic properties. In
previous experiments, luminance targets were selected on the basis of the direction of
motion (Kowler et al., 1984; Niemann et al., 1994) or a color cue (Ferrera & Lisberger,
1997). In any case, the selected motion was clearly mediated by the M-pathway because
it was always luminance defined. Selection of isoluminant targets in the presence of
competing luminance-defined motion requires suppression of the dominant motion input
in favor of the weaker motion signal. In most of the experiments proposed here, large
sine-wave gratings will be used as stimuli. Observers are asked to select an element in the
grating as pursuit target.
2.3.2 Attention-defined motion perception
Motion of an isoluminant grating may be perceived even if it is overlaid with a
luminance-defined grating moving in the opposite direction. For instance, the chromatic
component of the grating moves to the left, while the achromatic component moves to the
right (see Figure 5). Variation of the contrast of one of the two components will change
its salience: The higher the contrast, the more readily the motion of the respective
component is perceived. At a certain ratio of luminance and chromatic contrast, no
consistent overall motion of the grating is perceived. Nonetheless, observers are able to
attentionally track elements of the chromatic grating if instructed to do so (Cavanagh,
1992). To this end, a clearly visible line cues the bar of the chromatic grating that is to be
tracked while the eyes remain motionless. The apparent conflict between the absence of
global pattern motion, and the ability to attend to the motion of individual elements was
explained by the existence of two different motion mechanisms: A passive motion
mechanism driven by low-level motion detectors and a high-level attention-based
mechanism that tracks local elements in the grating. Thus, not global motion may be
perceived when the passive mechanism is unable to detect coherent motion. Nonetheless,
the active mechanism may independently track single elements.
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Figure 5. Space-time plot of a composite motion stimulus (copied from Thiele et al.,
1999). A heterochromatic (red-green) grating is added to an achromatic (black-white)
grating. The two gratings move into opposite directions.
Here, we ask whether observers may follow one of two overlaid gratings by
selectively focusing on either the color or luminance contrast. As outlined above,
dimensional attention may prioritize one dimension over another. It is known that
luminance-defined targets of various contrast (Spering, Kerzel, Braun, Hawken, &
Gegenfurtner, 2005) and isoluminant color gratings (Guo & Benson, 1999) drive smooth
pursuit eye movements. However, no existing study has examined whether dimensional
attention may isolate P- or K-cell motion signals for smooth pursuit eye movements in
the presence of strong M-cell firing. As outlined in the introduction, the M-pathway
accounts for the bulk of projections into MT. Nonetheless, P-cell and K-cell input is
present. The question is whether dimensional attention may favor these inputs for motor
control.
2.3.3 Own contributions
In a series of studies, we have examined the effects of target contrast on smooth
pursuit eye movements (Spering et al., 2005). It is well-known that objects appear to
move more slowly when their contrast is low (e.g., Thompson, 1982). We have found
similar effects of contrast on the quality of smooth pursuit eye movements. At low
contrast, smooth pursuit initiation is delayed and the gain of smooth pursuit decreases
(i.e., the eye moves more slowly). In a different set of experiments, we demonstrated that
visual short-term memory capacity decreases during smooth pursuit eye movements
(Kerzel & Ziegler, 2005). However, performance dropped only when the position of
elements had to be remembered. Memory for color was not perturbed by smooth pursuit.
The selective performance drop supports the notion of dimensional attention because
capacity limitations for location are more probable during smooth pursuit. Attention has
to be focused on target position to drive smooth pursuit (at least when a luminancedefined target is employed). Finally, we demonstrated that distracting elements presented
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during ongoing pursuit produce a deviation away from the distractor (Spering,
Gegenfurtner, & Kerzel, in press), suggesting that irrelevant object motion during
ongoing pursuit is inhibited.
2.3.4 Research Plan
2.3.4.1 Experiment 1: comparison between motion perception and smooth pursuit
performance
The work of Cavanagh (1992) has demonstrated that the perceived speed of a
chromatic grating viewed with stationary eyes depends on the attentional settings of the
observer. Observers were asked to judge the velocity of a chromatic grating. The
chromatic grating varied also slightly in achromatic contrast around the isoluminant
point. Observers were asked to compare the velocity of the combined grating to an
achromatic standard of a fixed contrast. Two viewing instructions were administered:
Observers were either asked to focus on single elements (bars) in the grating, or to judge
the global motion of the grating. If the observer tracked single elements in the gratings,
the perceived velocity of the chromatic grating changed little as a function of luminance
contrast. If, however, the global motion of the pattern was judged, perceived velocity
dropped at isoluminance (see Figure 6).
Figure 6. Apparent speed of the chromatic grating as a function of its luminance contrast
(copied from Cavanagh, 1992). The speed of the chromatic grating was compared to an
achromatic grating of fixed contrast. The perceived speed of the chromatic gratings drops
more dramatically when a global match is made.
In the first experiment, we will investigate whether the velocity of smooth pursuit eye
movements follows the perceived velocity of the global pattern or the perceived velocity
of single elements. As in Cavanagh’s (1992) study, a chromatic grating is used that also
varies in achromatic contrast (e.g., +/-0, 5, 10, 15% achromatic contrast). With
motionless eyes, the velocity of the chromatic grating is compared to an achromatic
grating at a fixed contrast (e.g., 15%). For perceptual estimates of velocity, a fixation
point is presented in the center of the grating to suppress smooth pursuit eye movements.
After the perceived velocity is determined under the two attentional instructions (attend
to global vs. local motion), an oculomotor condition is run. In the oculomotor condition,
the fixation point is removed and observers are asked to follow the grating with their
eyes. The question is whether the function relating achromatic contrast and quality of
pursuit (gain, latency) follows the perceived global velocity of the grating or the
perceived local velocity. A correlational analysis between perceived velocity and gain
across different luminance contrasts will determine whether the gain varies with the
pattern of local or global motion perception.
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The two attentional instructions (attentional tracking, global match) and the motor
condition (smooth pursuit) are blocked and require a single experimental session each. A
first prediction would be that smooth pursuit and attentional tracking produce similar
velocity profiles because both require concentration on a single element. Other research,
however, suggests that the global direction of motion may also influence smooth pursuit:
If two drifting, achromatic sine-wave gratings are superimposed, the perceived direction
of motion is the average of the two gratings. For instance, if one grating moves up, and
the other to the right, the perceived motion is diagonally up/right. Early components of
smooth pursuit eye movements follow the perceived direction of grating motion (e.g.,
right), while late components of smooth pursuit follows the combined motion signals
(Masson & Castet, 2002).
Design: +/- 0, 5, 10, 15% achromatic contrast x 3 instructions (perceptual:
attentional tracking, global match; motor: smooth pursuit); each instruction is run in a
separate session
Hypothesis: The velocity match obtained by (local) attentional tracking matches
the gain of smooth pursuit. Both capacities are based on the same underlying mechanism.
2.3.4.2 Experiment 2-4: simultaneous and opposing motion signals
In this experiment we set out to explore whether smooth pursuit eye movements
to a motion stimulus defined along one dimension may be maintained while distracting
motion information along another dimension is presented. Following the concept of
dimensional attention, it may be expected that attention is highly tuned to one dimension
and irrelevant dimensions are filtered out. Physiologically, the gain of motion
mechanisms serving chromatic or achromatic motion processing may be increased or
decreased to accomplish this (e.g., Treue & Maunsell, 1996).
In one condition, a chromatic (e.g., red-green, and cardinal color axis as defined
above) and an achromatic sine-wave grating are overlaid. At the beginning of each trial,
the observer is instructed to look at the center of the blank screen. Then, both gratings
appear and start to move at the same time. They either move in the same direction, or in
opposite directions (see Figure 5). Before each trial, observers receive instructions as to
whether to follow either the chromatic or the achromatic grating with their eyes. The
response-relevant dimension could be changed blockwise, or could be indicated by means
of a cue preceding each trial. In the analysis, the latency of smooth initiation, the
direction of smooth pursuit and the steady-state gain (1-2 seconds) will be evaluated. As
a baseline condition, the gratings will be presented in isolation. Thus, the distracting
effect of a simultaneously present motion signal can be calculated. In addition, the
benefits of summing chromatic and achromatic motion are evident in the comparison
between the single grating and combined-same direction grating.
Following the logic of Experiment 1, we will also run a purely perceptual
condition in which observers fixate a stationary object while judging the global and local
speed of the chromatic or achromatic grating. The perceived speed will then be compared
with the smooth pursuit performance.
In Experiment 2, the chromatic contrast is fixed and the luminance contrast varies
between 5 and 15%. In Experiment 3, the luminance contrast is fixed at 10% and the
color contrast is varied. In experiment 4, the color-opponent channel will be varied. There
are differences between red-green (P) and blue-yellow (K) channels. Dougherty et al.
(1999) showed that blue-yellow gratings are perceived as moving more slowly than red-
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green gratings when their cone-contrast is equal. To see whether the judged local motion
(see Cavanagh, 1992) of such a stimulus follows the judged global motion, the results of
the perceptual conditions may be compared. Possibly, the local motion is not different
between the two color channels while the global motion differs (as in Dougherty et al.,
1999). Again, it will be interesting to see whether the smooth pursuit performance
follows local or global motion perception.
Design: +/- 0, 5, 10, 15% achromatic (Exp. 2/4) or chromatic contrast (Exp. 3)
contrast x 3 instructions (perceptual: attentional tracking, global match; motor: smooth
pursuit); each instruction is run in a separate session
Hypothesis: Observers are able to select motion signals on the basis of chromatic
properties. This selection becomes increasingly difficult as the chromatic or achromatic
contrast of the irrelevant signal is increased. Smooth pursuit performance follows the
velocity profile of the attentional tracking condition.
2.3.4.3 Experiment 5 & 6, and pretest
Experiments 2-4 aimed at determining the efficiency of dimensional attention
with synchronized motion of target and distractor grating. In contrast, Experiments 5 and
6 aim at determining the efficiency of dimensional attention with asynchronous motion
onset of target and distractor grating. To this end, the onset of the motion of the second
grating is delayed by various time intervals. Previous studies have reported that brief
motion of a background affects smooth pursuit eye movements (Schwarz & Ilg, 1999;
Suehiro et al., 1999): If, for instance, the background was stationary and suddenly starts
to move in the same direction as the smooth pursuit target, pursuit gain changes. Effects
on pursuit gain are most pronounced if there is a change in the direction of retinal
background motion. That is, if the background was stationary prior to the perturbation,
and starts to move opposite to the target motion, this has no effect on smooth pursuit
because the direction of retinal motion of the background is the same in both cases
(opposite to pursuit). In contrast, if the background was stationary and starts to move in
the same direction as the pursuit target, this has a strong effect on pursuit gain because
the direction of retinal motion changes from opposite to the direction of smooth pursuit to
no motion.
Here, we will investigate to what degree steady-state pursuit to a chromatic or
achromatic grating is perturbed by motion of a previously stationary background. Two
versions of the experiment will be run. In Experiment 5, chromatic and achromatic
gratings are overlaid and differing motion vectors will be present in the fovea. The
question is whether motion defined along a different dimension can nonetheless perturb
smooth pursuit and whether the perturbation is symmetrical. A plausible hypothesis
would be that the perturbation is stronger for a luminance-defined distractor when the
chromatic grating is pursuit because M-cells have a stronger input into MT (i.e.,
asymmetrical perturbation).
In Experiment 6, the target is a locally-defined, moving object consisting of a
window on a sine wave grating. Thus, the nature of the stimulus is preserved to some
degree, but it is local and not a global object. To create the object, the sine wave grating
is multiplied with a Gaussian of a certain standard deviation resulting in a structured
patch of about 2deg diameter (Gabor patch). The Gabor moves horizontally along a gray,
3-4 deg “alley” that is surrounded by a sine wave grating. Along this “alley” the target
may be pursued along the horizontal dimension. The surrounding sine wave extends
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beyond the borders of the alley into the periphery and varies either along the same, or a
different perceptual dimension. As in Experiment 5, the irrelevant grating briefly moves
during target motion and the effect on the steady-state gain is measured. Based on
dimensional attention, we expect surrounding gratings that vary along the same
dimension as the target to have a larger effect on pursuit gain: Dimensional attention to
color or luminance should increase sensitivity to this dimension and irrelevant motion
signals along this dimension will more strongly affect pursuit.
As in previous experiments, the chromatic and achromatic stimuli have to be
matched in terms of the resulting smooth pursuit performance. That is, both distractor and
target elicit smooth pursuit of the same quality in terms of steady-state gain and pursuit
onset. This has to be done in a pre-test.
Design: Target (achromatic, chromatic) x Distractor (achromatic, chromatic);
Distractor motion onset varies during target motion
Hypothesis: Motion signals from the same perceptual dimension as the target
produce larger interference than motion signals from another dimension
2.3.4.4 Experiment 7 & 8: chromatic motion on luminance noise
In the proposed experiments on the distractor effect, luminance noise was used to
mask chromatic signals to the SC. Here, a similar logic may be used but with respect to
moving stimuli. We use a cloud of dots of varying density (see Figure 7). Each dot is
shown for only 50 ms and moves in a particular direction at a particular speed (e.g.,
Newsome et al., 1989). The luminance of each dot is determined randomly and varies
randomly between certain extremes (e.g., 25-30 cd/m2). This results in a scintillating
texture that masks M-cell contribution to chromatic motion perception. The coherence of
the dots’ direction of motion is varied. They either move randomly (0% coherence), or a
certain proportion of the dots is moving in a certain direction (depending on the condition
between 2% and 20%). While each dot’s luminance varies randomly, the chromatic
properties of neighboring dots will be manipulated such that a chromatically defined
object is superimposed on the dot field. The observer is instructed to pursue the
chromatic object. The direction of motion of the chromatic object is either the same as the
dot motion, or different. At a given color contrast, the coherence of dot motion is
manipulated. We expect that smooth pursuit of the chromatic target will become
increasingly difficult as the coherency of dot motion is increased. The continuous
variation of the conflicting motion signal will enable us to quantify the ability to filter out
achromatic motion signals in favor of chromatic motion.
Figure 7. A chromatic target moving across a random dot field. The motion of the dots is
limited to 50 ms. With 0% coherence, the direction of motion of the dots varies
randomly. With 100% coherence, all dots move in the same direction. The direction of
the chromatic target may be opposite (as shown) or the same as that of the dot field.
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The present experiment is related to studies investigating smooth pursuit to
feature-based motion. Feature-based motion is frequently referred to as second-order
motion (in contrast to first-order motion that involves luminance changes in a certain
direction). Previous research has established that human observers are able to smoothly
pursue second-order motion (Butzer, Ilg, & Zanker, 1997; Hawken & Gegenfurtner,
2001). For instance, observers were able to pursue a window moving to the right that
contained dots moving to the left (Butzer et al., 1997). The window was defined by the
direction of motion of the dots: While the dots on the background moved randomly, the
dots forming the object moved coherently. Even if smooth pursuit in this situation was
possible, it was impaired: Pursuit gain was much lower during the initial phase of steadystate pursuit.
Design: coherence of dots (0%, 4%, 8%, 12%) x target and chromatic contrast (5,
10, 15%) x direction of motion (same, different for local and global motion) x target type
(chromatic target: Exp. 7, achromatic target: Exp. 8)
Hypothesis: Dimensional attention permits pursuit of the chromatic object despite
variation of global dot motion. Because attentional filtering is extremely
2.4. Note the timetable and milestones of the project duration
2.4.1 Justification for acquisition of eyetracker
The experiments are to be run on a Cambridge Research System’s (CRS) Visage
graphics system that allows for high-resolution color displays (14 bits per color channel).
Such a system is available in Dirk Kerzel’s lab at the University of Geneva. Conventional
displays are limited in their color resolution (8 bits per color channel). A problem is that
at present, there is no eye tracker connected to the Visage-system. In a previous SNFgrant (10011-107768 / 1), we received funding for an EyeLink II eyetracker by SRResearch, which has been bought and is up and running. Unfortunately, the EyeLink II
system is not compatible with the Visage system. Conversely, the (conventional) graphics
adaptor attached to the EyeLink II does not have the necessary color resolution for the
present experiments. Therefore, we need another eyetracker that is compatible with the
Visage system to carry out the experiments. Cambridge Research System offers a videobased eyetracker that is suitable for the present experiments (high spatial and temporal
resolution, low noise).
2.4.2 Justification for two doctoral positions
The two projects (A and B) are extremely diverse both on a theoretical (saccades
vs. smooth pursuit) and on a methodological level (analysis of pursuit gain vs. saccadic
latencies). Therefore, it seems improbable that one CanDoc will be able to carry out both
projects. Rather, the two projects require two doctoral students. The common basic idea
(selection of perceptual dimensions for motor control) will nonetheless enable them to
collaborate on a theoretical level. Similarly, some of the programming techniques will be
similar because the same apparatus will be used.
2.4.2 Justification for a research assistant (Hilfskräfte)
As with any physiological measurements, it is extremely time-consuming to
measure eye movements. Because the requested eye tracker by Cmbridge Research
Systems does not compensate for head movements, a bite-board has to be used to
stabilize observer’s head. Bite boards have to be prepared for each participant in a
separate session (dental impression). During experimental sessions, it is necessary to
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recalibrate the experimental apparatus (every 10-15 minutes). These extremely timeconsuming activities are to be accomplished by research assistants. Thus, the CanDocs
can focus on the programming and data analysis.
2.4.4 Timetable
Programming the visual displays and some necessary techniques to determine
isoluminance (flicker fusion and minimal motion technique) will take at least 6 months.
Programming the to-be-bought eyetracker from Cambridge Research Systems will take
another 6 months. In sum, programming the equipment and testing it will take at least a
year. This will include pilot experiments and the development of analysis routines for the
detection of smooth pursuit and saccadic onsets, smooth pursuit gain and acceleration
(which heavily depend on the equipment).
In the second year, data collection and analysis will be accomplished. In the third
year, publications will be drafted and eventual control experiments will be run. During
the course of each year, the CanDoc will present (preliminary) data at on international
conference. Two conferences are important in the domain: the European Conference on
Visual Perception (ECVP) and the meeting of the Vision Sciences Society (VSS) in the
USA. The transatlantic conference will be reserved for the third year.
st
1 year
2nd year
3rd year
Candoc 1: Saccades
Candoc 2: Smooth pursuit
programming of equipment, implementation of paradigms, development of
routines for data analysis, pilot testing
data collection, full data analysis, presentation of preliminary results
compilation of scientific publications, control experiments, presentation of
results
2.5. Explain the significance of the planned research to the scientific community and
to eventual potential users.
The proposed experiments will enhance our understanding of basic processes underlying
attention and eye movements. No applications are envisaged.
3.1 References
Bartels, A., & Zeki, S. (2000). The architecture of the colour centre in the human visual
brain: new results and a review. Eur J Neurosci, 12(1), 172-193.
Birch, J., Barbur, J. L., & Harlow, A. J. (1992). New method based on random luminance
masking for measuring isochromatic zones using high resolution colour displays.
Ophthal. Physiol Opt., 12, 133-136.
Butzer, F., Ilg, U. J., & Zanker, J. M. (1997). Smooth-pursuit eye movements elicited by
first-order and second-order motion. Exp Brain Res, 115(1), 61-70.
Cavanagh, P. (1992). Attention-based motion perception. Science, 257(5076), 1563-1565.
de Monasterio, F. M. (1978). Properties of ganglion cells with atypical receptive-field
organization in retina of macaques. J Neurophysiol, 41(6), 1435-1449.
Dobkins, K. R. (2000). Moving colors in the lime light. Neuron, 25(15-18).
Dorris, M. C., & Munoz, D. P. (1995). A neural correlate for the gap effect on saccadic
reaction times in monkey. J Neurophysiol, 73(6), 2558-2562.
Dougherty, R. F., Press, W. A., & Wandell, B. A. (1999). Perceived speed of colored
stimuli. Neuron, 24(4), 893-899.
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FM-Kerzel
Dursteler, M. R., & Wurtz, R. H. (1988). Pursuit and optokinetic deficits following
chemical lesions of cortical areas MT and MST. J Neurophysiol, 60(3), 940-965.
Ferrera, V. P., & Lisberger, S. G. (1997). The effect of a moving distractor on the
initiation of smooth-pursuit eye movements. Visual Neuroscience, 14(2), 323-338.
Fischer, B., & Boch, R. (1983). Saccadic eye movements after extremely short reaction
times in the monkey. Brain Res, 260(1), 21-26.
Folk, C. L., Remington, R. W., & Wright, J. H. (1994). The structure of attentional
control: Contingent attentional capture by apparent motion, abrupt onset, and
color. Journal of Experimental Psychology: Human Perception and Performance,
20(2), 317-329.
Gegenfurtner, K. R., & Hawken, M. J. (1996). Interaction of motion and color in the
visual pathways. Trends in Neurosciences, 19(9), 394-401.
Gegenfurtner, K. R., & Kiper, D. C. (1992). Contrast detection in luminance and
chromatic noise. J Opt Soc Am A, 9(11), 1880-1888.
Guo, K., & Benson, P. J. (1999). Grating and plaid chrominance motion influences the
suppressed ocular following response. Neuroreport, 10(2), 387-392.
Hawken, M. J., & Gegenfurtner, K. R. (2001). Pursuit eye movements to second-order
motion targets. Journal of the Optical Society of America [A], 18(9), 2282-2296.
Ilg, U. J. (1997). Slow eye movements. Progress in Neurobiology, 53(3), 293-329.
Kanwisher, N., McDermott, J., & Chun, M. M. (1997). The fusiform face area: a module
in human extrastriate cortex specialized for face perception. J Neurosci, 17(11),
4302-4311.
Kerzel, D., & Ziegler, N. (2005). Visual short-term memory during smooth pursuit eye
movements. Journal of Experimental Psychology: Human Perception and
Performance, 31(2), 354-372.
Kowler, E., van der Steen, J., Tamminga, E. P., & Collewijn, H. (1984). Voluntary
selection of the target for smooth eye movement in the presence of superimposed,
full-field stationary and moving stimuli. Vision Res, 24(12), 1789-1798.
Krauzlis, R. J. (2005). The control of voluntary eye movements: new perspectives.
Neuroscientist, 11(2), 124-137.
Krummenacher, J., Muller, H. J., & Heller, D. (2002). Visual search for dimensionally
redundant pop-out targets: parallel-coactive processing of dimensions is location
specific. J Exp Psychol Hum Percept Perform, 28(6), 1303-1322.
Lamme, V. A. (1995). The neurophysiology of figure-ground segregation in primary
visual cortex. J Neurosci, 15(2), 1605-1615.
Marrocco, R. T., & Li, R. H. (1977). Monkey superior colliculus: properties of single
cells and their afferent inputs. J Neurophysiol, 40(4), 844-860.
Masson, G. S., & Castet, E. (2002). Parallel motion processing for the initiation of shortlatency ocular following in humans. J Neurosci, 22(12), 5149-5163.
Merigan, W. H., & Maunsell, J. H. (1993). How parallel are the primate visual pathways?
Annu Rev Neurosci, 16, 369-402.
Muller, H. J., Reimann, B., & Krummenacher, J. (2003). Visual search for singleton
feature targets across dimensions: Stimulus- and expectancy-driven effects in
dimensional weighting. J Exp Psychol Hum Percept Perform, 29(5), 1021-1035.
Munoz, D. P., & Wurtz, R. H. (1993a). Fixation cells in monkey superior colliculus. I.
Characteristics of cell discharge. J Neurophysiol, 70(2), 559-575.
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Munoz, D. P., & Wurtz, R. H. (1993b). Fixation cells in monkey superior colliculus. II.
Reversible activation and deactivation. J Neurophysiol, 70(2), 576-589.
Newsome, W. T., Britten, K. H., & Movshon, J. A. (1989). Neuronal correlates of a
perceptual decision. Nature, 341(6237), 52-54.
Niemann, T., & Hoffmann, K. P. (1997). The influence of stationary and moving textured
backgrounds on smooth-pursuit initiation and steady state pursuit in humans. Exp
Brain Res, 115(3), 531-540.
Niemann, T., Ilg, U. J., & Hoffmann, K. P. (1994). Eye movements elicited by
transparent stimuli. Exp Brain Res, 98(2), 314-322.
Schall, J. D. (1991). Neuronal activity related to visually guided saccades in the frontal
eye fields of rhesus monkeys: comparison with supplementary eye fields. J
Neurophysiol, 66(2), 559-579.
Schiller, P. H., & Malpeli, J. G. (1977). Properties and tectal projections of monkey
retinal ganglion cells. J Neurophysiol, 40(2), 428-445.
Schwarz, U., & Ilg, U. J. (1999). Asymmetry in visual motion processing. Neuroreport,
10(12), 2477-2480.
Sparks, D. L. (2002). The brainstem control of saccadic eye movements. Nature Review
Neuroscience, 3(12), 952-964.
Spering, M., Gegenfurtner, K. R., & Kerzel, D. (in press). Distractor interference during
smooth pursuit eye movements. Journal of Experimental Psychology: Human
Perception and Performance.
Spering, M., Kerzel, D., Braun, D. I., Hawken, M. J., & Gegenfurtner, K. R. (2005).
Effects of contrast on smooth pursuit eye movements. Journal of Vision, 5(5),
455-465.
Suehiro, K., Miura, K., Kodaka, Y., Inoue, Y., Takemura, A., & Kawano, K. (1999).
Effects of smooth pursuit eye movement on ocular responses to sudden
background motion in humans. Neurosci Res, 35(4), 329-338.
Sumner, P., Adamjee, T., & Mollon, J. D. (2002). Signals invisible to the collicular and
magnocellular pathways can capture visual attention. Curr Biol, 12(15), 13121316.
Thiele, A., Dobkins, K. R., & Albright, T. D. (1999). The contribution of color to motion
processing in Macaque middle temporal area. J Neurosci, 19(15), 6571-6587.
Thompson, P. (1982). Perceived rate of movement depends on contrast. Vision Res,
22(3), 377-380.
Treue, S. (2004). Perceptual enhancement of contrast by attention. Trends in Cognitive
Sciences, 8(10), 435-437.
Treue, S., & Maunsell, J. H. (1996). Attentional modulation of visual motion processing
in cortical areas MT and MST. Nature, 382(6591), 539-541.
Walker, R., Deubel, H., Schneider, W. X., & Findlay, J. M. (1997). Effect of Remote
Distractors on Saccade Programming: Evidence for an Extended Fixation Zone. J
Neurophysiol, 78(2), 1108-1119.
Walker, R., Kentridge, R. W., & Findlay, J. M. (1995). Independent contributions of the
orienting of attention, fixation offset and bilateral stimulation on human saccadic
latencies. Experimental Brain Research, 103, 294-310.
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Wandell, B. A., Poirson, A. B., Newsome, W. T., Baseler, H. A., Boynton, G. M., Huk,
A., et al. (1999). Color signals in human motion-selective cortex. Neuron, 24(4),
901-909.
White, B. J., Gegenfurtner, K. R., & Kerzel, D. (2005). Effects of structured nontarget
stimuli on saccadic latency. Journal of Neurophysiology, 93, 3214-3223.
Wyatt, H. J., & Pola, J. (1988). Predictive behavior of optokinetic eye movements. Exp
Brain Res, 73(3), 615-626.
Zeki, S. (1980). The representation of colours in the cerebral cortex. Nature, 284(5755),
412-418.
3.2 Relevant Recent Publications
Kerzel, D., & Ziegler, N. (2005). Visual short-term memory during smooth pursuit eye
movements. Journal of Experimental Psychology: Human Perception and
Performance, 31(2), 354-372.
Spering, M., Gegenfurtner, K. R., & Kerzel, D. (in press). Distractor interference during
smooth pursuit eye movements. Journal of Experimental Psychology: Human
Perception and Performance.
Spering, M., Kerzel, D., Braun, D. I., Hawken, M. J., & Gegenfurtner, K. R. (2005).
Effects of contrast on smooth pursuit eye movements. Journal of Vision, 5(5),
455-465.
White, B. J., Gegenfurtner, K. R., & Kerzel, D. (2005). Effects of structured nontarget
stimuli on saccadic latency. Journal of Neurophysiology, 93, 3214-3223.
3.3 Note
(Former) graduate students that contributed to the publications by DK.
Dipl.-Psych. Miriam Spering
Brian White, MA
Dr. Nathalie Ziegler
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