Download Solution 1

23/20
1. Give three functional reasons why we have so many parallel processing pathways in the visual
system?
One function of parallel processing in retina is to condense the transfer of information to the
cortex by encoding the different qualities of an image into a set of specialized circuits, allowing
the information to pass efficiently through the blind spot. These specialized circuits also function
as a means of transmitting information about different aspects of the same region in space
simultaneously (Nassi & Callaway, 361). Later in the visual system, existence of different parallel
processing pathways allows for the information they transmit to interact if very specific ways,
providing the basis for complex computations: information from one stream can be amplified
while another is suppressed, or one might modulate the other, or they might be integrated into
a new output stream (Nassi & Callaway, 366).
3.5/5
I count only two reasons?
2. Dorsal and ventral pathways were originally thought to serve different functional purposes,
what were they? Yet, certain visual properties are processed within both visual pathways, such
as binocular disparity. Why is that?
Traditionally, the dorsal pathway is considered the “where” pathway: it is thought of as
processing information about how objects relate to the space they are in and to the other
objects in that space, carrying out motion analysis of those objects, and influencing arm and eye
motor responses to them. The ventral pathway is considered the “what” pathway and is thought
to be responsible for combining colors, patterns, and shapes in order to identify objects (Nassi &
Callaway, 367). However, it seems that they rely on some of the same types of information to
carry out their functionally different goals. One case is binocular disparity, which is the
difference in information provided by each eye, and is used to derive depth information in a
scene (Nassi & Callaway, 367). The dorsal pathway deals with coarse differences in binocular
disparity, possibly because its work involves judging greater distances of depth (exactly how far
away is that truck that’s speeding toward me?), while the ventral pathway processes fine grain
differences in disparity to judge the orientation and contours of an object in order to identify it
(“wait, is that actually a truck, or is it an SUV?”).
5/5
3. When there are two stimuli within a visual neuron’s receptive field, one preferred and one not
preferred, how is the neural response modulated by attention? What is the evidence that there
is suppression induced by attending to the non-preferred stimulus (as opposed to a relatively
smaller enhancement of response when compared to attending to the preferred stimulus)?
The paper considers a study that compared the effect of attending to a preferred and a nonpreferred stimulus (motion direction) in a neurons receptive field, and attending to a neutral
stimulus outside of its receptive field (Treue, 296). When the animal attended to the preferred
stimulus the cell became more active than when it attended to the neutral stimulus. However,
when attending to the non-preferred stimulus the cell was less active then when attending to
the neutral stimulus. First, the cell’s ability to respond differentially to stimuli within its
5/5
receptive field shows that attention modulation has a finer grained resolution than the size of
the receptive field. Furthermore, because the response to the non-preferred stimuli fell below
the response to a neutral stimulus, this shows there was active suppression.
4. In what sense does the attentional narrowing of tuning curves increase the neuron’s selectivity?
In what sense does the multiplicative increase of a neurons response increase the amount of
information communicated by the neuron?
As a tuning curve becomes narrower the regions of steepest slope, which encode the state of
the feature that the curve is tuned to respond to, cover a smaller range of values. This type of
attentional modulation restricts the range of feature values that the cell responds to, though it
also increases the magnitude of those responses. Thus the cell responds more selectively, while
also signaling smaller variations more intensely. A multiplicative modulation also causes the
regions of steepest slope to increase, but does so over the same set of feature values.
Therefore, under this type of attentional manipulation, the neuron maintains the ability to
communicate information about a wider range of stimulus states, while gaining the ability to
signal smaller variations more intensely.
5/5
5. Attentional modulation appears to increase higher up in the visual hierarchy. Why is this (we
discussed in class)?
There are two possible reasons. One is that as percepts become encoded with more semantic
meaning, attentional modulation becomes more behaviorally relevant, and therefore increases
at higher processing areas. Another possibility is that there exists an accumulative effect of
+2
attention across the visual system. When attention is “paid” at a lower level, additional
resources are recruited to process a specific set of signals. The information that those signals
carry is modulated to some degree as it is passed to higher levels. If those signals continue to be
attended to, then additional modulation will occur at each successive stage, resulting in a
compounding effect. Therefore, simply as an effect of compounding computation, attentional
modulation will appear to increase as signals proceed to higher processing areas.
6. How is featural attention different than spatial attention? Why is the discovery of featural
attention difficult for the “spotlight of attention” analogy?
Spatial attention means boosting signals that encode information about a specific region of the
visual field (i.e. something is about to appear in the left part of the scene), while featural
attention boosts signals that encode information about specific properties of the visual
environment (i.e. things that shift upward are about to appear in the scene). The “spotlight of
attention” analogy has an implied spatial component, and refers to the way we can differentially
attend to regions of the visual field. If the character of attention was entirely captured by this
analogy, then only the output of the neurons that are tuned to represent the region being
attended to currently would be boosted. However, even while attending to a specific region, if
we are actively looking for objects that shift upward, then all neurons that are tuned to respond
to upward movement for every region in the visual field will have boosted outputs (True et al.,
297).
You misspelled the author's name
+1.5
7. How can apparently nonlinear attentional modulation of a neural response arise from
multiplicative modulation in an earlier area?
Multiplicative modulation at one levels means an amplification or suppression of a neuron’s
output. If a neuron is tuned to respond to a preferred region, then a multiplicative modulation
could cause its response to increase and decrease according to how stimuli in that region
appear, but it could not, for instance, shift the boundaries of that region (True et al., 298).
However, if we consider the inputs at the level below this neuron, then the receptive field of
those lower level neurons tile the receptive field of the one they feed. We can imagine this
region to be a circular space, filled with lots of smaller circles. In the absence of attentional
modulation stimuli will elicit a response in the higher level neuron that is equal to a weighted
sum of the responses at the lower level. In this case a maximum response would be caused by
presenting a stimulus in the physical center of the lower level neurons combined receptive
fields. However, if attention is paid to the right side of this receptive field, then the lower level
neurons in that part of the region will have boosted responses. If the stimulus is shifted between
the left and right sides of the receptive field under this attending condition, and we compare the
response of the higher level neuron, it will respond greater when the stimulus is presented to
the right side, giving the appearance that it has contracted the boundaries of its receptive field
toward the right.
+1