Lecture 14: Visual Cortex, Streams, and Perceptual Disorders

Hubel and Wiesel’s experiment and why it matters

An experiment that recorded cat visual cortex discovered the function of simple vs. complex cells on the retina

Difference between a simple and complex cell

Responds to a line or edge at one orientation, fed through a row of circular fields; complex cells respond to the edge that moves nearby, receive several simple cell signals to get this idea of movement

Orientation pinwheel

Across the cortex; neurons that respond to different edge orientations

An edge

an abrupt, high frequency change in brightness

Spatial frequency theory

Cortical cells are tuned to different frequencies

difference in high and low frequencies that the cortical cells recieve

High frequencies give borders and fine detail; while low frequencies give overall shape, shadow, and texture

Parvocellular cells

Small, numerous, and colorful opponents; focus on fine details and color

Magnocellular cells

Large brightness opponent fields, fast but brief. Brightness, contrast, movement, and depth

Ventral “what” stream

mostly parvocellular functions; V2, V4, then to inferior temporal cortex for object recogniton

Dorsal “Where” stream

Mostly magnocellular: V2, V5/MT for movement, then the posterior parietal cortex to track objects in space

Ventral damage result vs dorsal damage results

Can’t identify an object by sight but reaches for it accurately; Can identify an object by sight but cannot reach for it accurately

Agnosia

Inability to recognize objects, people, or sounds

Prosopagnosia

Can’t recognize familiar faces, even one’s own, but knows people by voices and mannerisms; caused by fusiform face area damage

Greebles usage in studies

Activate the FFA depending on those with knowledge of what the character is or not

Blindsight

Not being able to track motion and avoid obstacles; occurs when V1 is destroyed

The visual word form area

Recognizes written words as whole visual objects

color constancy

An object keeps its color when the lighting changes

How are wavelengths vs color codes coded;

V1 cells; V4 cell

Color agnosia

lost color perception after damage between V1 and the FFA(where V4 is)

Movement agnosia

Loss of radial movement ability (telling where you’re heading)

Posterior parietal cortex

Combines visual, auditory, and touch inputs to locate objects and oritent the body

The binding problem

How does the brain combine shape, color, and movement into one object

Synesthesia

One sense triggers an experience in another, like seeing numbers in color

Neglect

ignoring sights, touches, and sounds on the side opposite the damage, caused by right parietal cortex damage