Visual streams and object identification

parvocellular

ventral(temporal) pathway

“what”

magnocellular

dorsal (parietal) pathway

“where”

see in V1 and V2?

continue to see separation of information

this pathway is

bidirectional

blobs

regions of the visual cortex where groups of neurons sensitive to color group together in cylindrical shapes

interblobs

receive same visual input, but process orientation instead of color

v1 → v2 → processing

blobs →

interblobs →

layer 4β →

blobs → thin stripes → color

interblobs → pale/inner stripes →spatial form

layer 4β → thick stripes → motion

what are v2 receptive fields (after v1)

extrastriate receptive fields which are more complex

striate cortex → V1

extrastriate cells

• receptive fields more sophisticated than striate cortex (V1)

• respond to visual properties important for perceiving objects

• boundary ownership → differentiate object and boundary

boundary ownership

v2 plays a role in determining what is part of an object and what is part of the background

Good continuation

two elements will tend to group together if they lie on the same contour

• neurons in v2 respond to contours not present

Gestalt

“the whole is greater than the sum of parts”

V2 neurons and Gestalt grouping rules

figure-ground segmentation - process of dividing an image into a foreground "figure" and a background "ground

Dorsal stream

• “Where” pathway aka “how” pathway

• spatiotemporal information

• stimulus location

• motion

• sends information to motor areas of the frontal cortex

• how to move body in space

Ventral Stream

• “what” pathway

• object identification

• color

• facial recognition

• object recognition

primary visual cortex may have information about a stimulus that is unable to be utilized if there is damage to one of the “higher order” or more specialized processing areas

True or False

True

What is prominently activated for color in humans? (PET)

v4

What is prominently activated for motion in humans? (PET)

MT/V5

What is prominently activated for color and motion in humans? (PET)

V1

V4 role

shape perception and color

damage to v4

problems with

• illusory contours

• complex shapes

• 2 paintings

  • couldn’t tell face of one where color and boundary were mixed

  • could tell face of painting with sharp edges

damage to V5/MT

• lesions in temporoparietal cortices

• very rare intact visual acuity, color vision

• view world as snapshots → “smooth motion” disrupted’

• Akinetopsia → “motion blindness”

Object constancy

• result of perception

• ability to recognize objects in multiple situations from

  • different viewing positions

  • different illuminations

  • different contexts

color constancy, shape constancy, size constancy

visual agnosia

failure to recognize something

damage to “what” pathway

can’t match but can perform action

lesions in lateral occipital cortex

lateral occipital cortex

critical for object shape perception

fusiform face area (FFA)

• facial recognition

• activated in people who are blind that touch faces

• location: fusiform gyrus

• “area of expertise”