Higher Visual Processing

Lecture 16

Center- Surround Receptive Field

A circular receptive field with opposing zones.

On-center: fires when light hits the center, inhibited by the surround.

Off-center: Fires when center is dark, excited by surround

Seen in retinal ganglion cells and LGN neurons. Best activated by spots of light, not diffuse illumination

Critical Period

Developmental window where visual experience permanently shapes cortical wiring.

Cats: 3 Weeks- 3 months

Non-human primates: Birth-6 months

Humans: 6 months-6 years

Monocular deprivation during this window causes lasting damage; after it, the brain is resistant to change

Synesthesia

A condition where stimulating one sense automatically and involuntarily triggers a perception in another (seeing colors when hearing music).

Thought to involve cross-activation between adjacent sensory cortices

Monocular v. Binocular cells in V1

Monocular cells (layer 4C): respond to input from ONE eye only, where LGN axons first terminate, keeping inputs separate

Binocular Cells (layers 2/3, 5/6, above/below 4C): respond to both eyes

Eye mixing first occurs in V1 after layer 4C. Most binocular cells still favor one eye

Monocular v. Binocular deprivation

Monocular: Devastating— open eye takes over; deprived eye loses all cortical response; no binocular cells remain

Binocular: much less damaging— body eyes equally deprived so no competition; LGN axons still form normal columns; cortical cells can still respond to both eyes when repopened

Dorsal v. Ventral Streams

Dorsal (where): V1—> MT—> posterior parietal cortex. Processes location, motion, spatial attention

Ventral (what): V1—> V4—> inferior temporal cortex. Processes object identity, color, faces

Parietal lesion impairs landmark task (where); temporal lesion impairs object discrimination (what)

Face Selective neurons (temporal) vs facial expression cells (anygdala)

Temporal (inferior temporal cortex): responds selectively to whole faces, not parts or similarly shaped objects. Very selective for face orientation/identity— the “who” system.

Amygdala: receives projections from temporal lobe; neurons fire preferentially to threatening expressions— the “threat” detector

Orientation Selective Cells in V1

V1 neurons that fire only when a bar of light is at a specific angle. Arranged in columns of similar orientation preference

Simple Cells: respond to stationary bars at one orientation

Complex cells: respond to moving bars at a preferred orientation.

First stage of edge/shape detection— not seen in LGN

Visual Agnosia vs prosopagnosia

Visual agnosia: inability to recognize objects generally despite intact vision— ventral stream/ temporal lobe damage. Can see but can not identify what something is.

Prosopagnosia: inability specifically to recognize faces— damage to the inferior temporal cortex (fusiform face area). Can recognize objects but not people by face

Lateral vs. medial geniculate nucleus

LGN: visual relay in the thalamus, has 6 monocular layers; processes color and movement; projects to V1.

MGN: auditory relay in the thalamus

Memory trick: Lateral=light=visual

Medial=music=auditory

Segregation of eye inputs: LGN layers and ocular dominance columns

In LGN: each of the 6 layers receives input from only one eye (monocular).

V1 Layer 4C: LGN axons form ocular dominance columns— alternating stripes of LRLR eye input. Binocular mixing only begins above.below layer 4C

Selective neurons in V1 vs temporal lobe

V1: orientation-selective cells (simple and complex)— respond to bars/edges at specific angles; arranged in columns

Temporal lobe: responds to complex, high-level stimuli— whole faces, specific orientations of faces, objects. Object identification uses population coding (many neurons), not single cells. Location of activity shifts systematically with face rotation

Neglect Syndrome

Caused by unilateral parietal lobe lesion (usually right hemisphere, posterior/inferior parietal cortex). Patient ignores all stimuli from the contralateral side of the lesion

Patients copy only the right half of a house. Demonstrates parietal lobe’s role in spatial attention

Right parietal monitors both visual fields; left parietal monitors only right