Download Association Cortex and Smooth Pursuit Eye Movements

Perception of Motion, Depth
and Form
David M. Waitzman, M.D., Ph.D.
Discussion Question
• Motion in Depth lecture points:
– What are the two visual streams and what kinds of
tasks are common to both. What tasks are
different? If an object is moving toward you what
portion of the hemisphere would be activated? What
portion of the cortex would recognize the object?
– Dorsal and Ventral
– Common: Disparity Cells in visual (V1, V2, V4) AND
extrastriate (MT) cortex respond to targets closer
(orange) or further (blue) than fixation point (green)
– Different: Motion versus Object recognition
– Near: MST (looming objects)
– Recognition: Area IT
Phi or Beta Phenomenon
The POINT of this Demonstration: The brain is wired to
perceive motion even though in this demonstration none
of the objects were actually moving
Perceived Motion: Analysis of Motion
Occurs in a Separate Pathway
Movement of an image on the retina or
an eye movement (above)
True movement or apparent (flashed)
movement (right)
Receptive Fields in
the Retina
• Two types of ganglion
cells:
– on and off dependent
upon the bipolar neurons
• Center Surround
structure of the
receptive field
described by Kuffler
• Best activated by
central illumination
• Best inhibited by
annular illumination
Different View of Center-Surround
Organization: Parallel Pathways
• Transformation of
visual information is
evident in the ganglion
cells of the retina
• X cells – sustained
linear responses
• Y cells – transient,
excitatory non-linear
responses
P and M Projections to LGN:
Different Physiologic Channels
• P cells in the retina (also known as midget ganglion cells) project to
the parvocellular layers (3-6) of LGN
• M cells in the retina (also known as parasol cells) project to the
magnocellular (ventral most) layers (1-2) of the LGN
• Intercalated layers are termed koniocellular (dustlike or tiny cells)
Comparison of P and M Pathways
• P  Parvocellular
– Small Receptive Fields
– Slow conduction to LGN
– Sustained responses (X
Like)
– Sensitive to color
– High contrast
• M  Magnocellular
– Larger receptive fields
– Rapid conduction to LGN
– Transient responses (Y
like)
– Broadband color sensitivity
– Low Contrast
Anatomic Segregation of Motion
and Object Recognition Pathways
• parvocellular: 4C “high acuity”; to blobs; to thin stripes, to V4
(color)
• magnocellular: 4Ca “motion pathway”; to layer 4B of V1 and to
V2 (area 18) and hence to Area MT
MT Solves the Aperture Problem
Top : Grating moving in any of 3 apertures has the same apparent
motion
Bottom: In MT two edge detecting neurons are linked to
permit identification of the overall motion of the entire object
Plaid Motion: V1 versus Area MT
Neurons in V1 encode the direction of
individual components of motion (top)
MT neurons encode the perceived speed and
direction of the moving visual stimulus (below)
Confirmation of Different Pathways
from Clinical Syndromes
Type of Agnosia
• Object agnosia
• Agnosia for drawings
• Prosopagnosia
• Color Agnosia
•
•
•
•
Color anomia
Achromatopsia
Visual spatial agnosia
Movement agnosia
Symptoms
• Can’t name use or recognize
real objects
• Doesn’t recognize drawn
objects
• Can’t recognize faces
• Doesn’t associate a color with
an object
• Can’t name colors
• Can’t distinguish hues
• Loss of stereoscopic vision
• Can’t see objects moving
Dorsal and
Ventral Streams
• Dorsal Stream: Where: Motion
• MT projects to MST
• MST projects to inferior parietal
lobule 7a and to the FEF
• Ventral Stream: What : Form and Color
Smooth Pursuit: Characteristics
Purpose: Track visual
stimuli moving at <
100/s
Stimulus: Step-Ramp
Smooth Pursuit: Characteristics
Despite a target appearance on the contralateral
side, initial pursuit velocity is in the direction of
smooth motion
Area MT Physiology
• MT neurons
encode visual
stimulus motion
(discharge is
reduced when
target is blanked)
MT Lesion: Scotoma for Motion
• Scotoma (i.e., blindness)
for motion in a specific
portion of the contralateral
visual hemifield.
• Saccades to moving
targets in the blind area are
inaccurate, e.g., note the
overshoot of the leftward
saccade at down arrow
• Monkeys cannot estimate
the speed of the target until
it is outside the “motion
blind” area
• Saccades to stationary
targets are normometric
Area MST: Perception of Eye Motion
• Neurons in area MST
receive a signal of
current eye and head
velocity (A)
• large visual fields
• can estimate heading
and target motion by
eliminating self motion
• Discharge not reduced
when visual stimulus
is blanked or stabilized
(B)
MST Lesions: Loss of Eye Speed
Toward the Ipsilateral Side
Two deficits:
– retinotopic deficit: cannot
estimate speed in a
region of contralateral
visual hemifield
– unidirectional loss of
smooth pursuit toward
the side of the lesion
REGARDLESS of which
visual hemifield the
stimulus is presented
Monocular Cues for Depth Perception
1. Familiar Size
2. Occlusion
3. Linear
perspective
4. Size perspective
5. Distribution of
shadows and
illumination
6. Motion Parallax
7. And…..
Binocular Cues for Depth
Perception: Disparity Tuning
A binocular disparity
signal arises in V1
(Hubel and Wiesel)
Cells in visual (V1, V2,
V4) AND extrastriate
(MT) cortex respond to
targets closer (orange)
or further (blue) than
fixation point (green)
Both dorsal and
ventral pathways
have disparity tuning
Dorsal Pathway (Area MT):
Direction and Disparity Tuning
Tuning for targets closer or further
from the monkey (near and far
response patches)
Newsome et al, J. Neurosci. 1999
Ventral Pathway (V4):
Binocular Disparity
• Tuning in depth
• Dorsal and Ventral
Streams NOT
Segregated for
Disparity (a
common feature)
Color and Form Are Major
Differences in Ventral Path
• Human subjects
undergo PET scan to
demonstrate motion
regions (left column)
and color regions
(right column)
corresponding with
monkey area MT and
V4 respectively
Illusory Contours in V2 Neurons
• Receptive field of the
neuron is the ellipse
• Strong activation when
an edge passes over
the receptive field
• Persistent activation
when an illusory contour
passes over receptive
field (2)
• No response when the
edge is partial (3 and 4)
V4 Neurons Sense Gaze position
(not movement) and Relative Size
• View objects at 3 different distances and at 9
different gaze positions
• Spike rasters of the stimulus (S) period for each of the 27
experimental conditions
• The center figure represents the mean firing rates for this cell during
both the fixation period (FO, blue bars) and the stimulation period
(S, red bars)
• Leftmost graphs show the mean firing rate data during the
stimulation period using an interpolated color plot. Each of the
planes represents a monitor viewing distance.
V4 Neurons: Complex forms, size,
orientation, color, but not Faces
• On the right, contour plot of the S (size) regions (WHITE) is
superimposed on a color-coded map of orientation preference.
Color saturation within this map indicates orientation selectivity.
Unlike in V2, S regions in V4 are not limited to unoriented
regions, but can span iso-orientation domains.
Inferotemporal cortex respond to
form, color, and faces
Cells increase in complexity of object recognition,
but lose the ability to spatially localize objects (no
gaze direction sensitivity)
Face recognition is present
Temporal Lesions: Loss of Object
Recognition and Prosopagnosia
Regions for face recognition have been
demonstrated by fMRI studies include the
FFA and PPA (temporal lobe) and overlap
with object recognition regions Haxby et al,
Science 293: 2425-2430 , 2001.
Neural basis of prosopagnosia… N. Hadjikhani and B.
de Gelder, Human Brain Mapping, 16(3):176-182, 2002.
Visual Attention: Solves the binding
problem
• The activity of V4 neurons is greatly enhanced
by selective attention to the color of the
matching stimulus (red)
• Significance for the monkey is greatly
increased for the matched (above) then the
unmatched trials (below)
Attention causes activation of a
parieto-frontal pathway
• Lack of attention to visual stimuli produced NO
activation of the cortex (A)!
• With attention to the peripheral target with
discrimination, activation of a parieto-frontal
pathway either directly (B) or in anticipation of
target appearance (C).
An Example of Excellent Depth
and Motion Perception
alivsfrazier123.mpg