Module 7: Visual Pathways, Cortex, and Streams

Topographic Map

A map in which there is a spatial correspondence between two structures

Magnification Factor

The apportioning of proportionally more space on the cortex (and LGN in vision) to the representation of specific sensory areas

Parvocellular

layers 3-6 of the LGN (Lateral Geniculate Nucleus) (top); small receptive field size relative to magno; slow, sustained temporal response; insensitive at low contrasts, barely responding until 25%; 90% highly wavelength sensitive (color sensitivity)

Magnocellular

layers 1 and 2 of the LGN (bottom); 2-3x larger than parvo receptive field size; fast, transient temporal response; responds at low contrasts, but saturates by 30% contrast; effectively colorblind

Equiluminance

differing in color, not luminance (also called isoluminance)

Heterochromatic Flicker Photometry (“Minimal Flicker Technique”)

Alternate between stimulus 1 and 2 at a flicker rate less than 12 Hz and adjust one of the stimulus until no flicker is perceived; psychophysically equiluminant

(e.g., set red luminance and adjust green until you perceive minimal flicker)

Striate Cortex

V1, Primary visual cortex); receives the sensory input (especially the LGN)

Extrastriate Cortex

V2-V7, VP, V4, V8; processes sensory input

Adaption

The diminishing response of a sense organ to a sustained stimuli

Tilt After-Effect

Perceived tilting of lines in opposite direction of test stimuli

Results after looking at tilted lines in two different directions (e.g., to the left at the top and to the right at the bottom)

Simple Cells

Neurons in V1 that respond best ti lines/bars of light or dark in a particular orientation and location

Complex Caells

Neurons in V1 that respond best tom moving bars of light or dark in a particular orientation (like in the Hubel & Wiesel Cat Experiment)

Hubel and Wiesel Cat Experiment

Found cats responded to the line of changing slides; showed orientation preference of neurons

End-Stopped Cells

Neurons in V1 that respond best to moving lines of a specific length

Location Column

Part of the hypercolumn; A column in the striate cortex that contains neurons with the same receptive field location on the retina (this is related to the retinotopic map from the eyes to V1 that also involves cortical magnification factor)

Orientation Column

A column in striate cortex that contains neurons with the same orientation preference; within location columns

Ocular Dominance Column

A column in visual cortex that contains neurons of the same ocular dominance

Hypercolumn

A 1mm block in striate cortex containing a location column for a particular area on the retina with a complete set of orientation columns (0-180 deg) for both left and right ocular dominance columns; contains all three types of columns

M system in Movement Perception

• Impaired using equiluminant gratings

• Relative motion as a depth cue is reduced at equiluminance

• Detail perception is impaired

M System in Depth Perception

• Difficult to see depth in equiluminant color stereograms

• Perception of shape/depth from shading disrupts equiluminance

• Size illusion from perspective cues lost at equiluminance

• Linking by collinearity and movement perception difficult at equiluminance

Ablation Study

Damage the brain of the subject (monkey), then allow recovery from surgery, then test to see damage (what is affected)

Landmark Task

a “where” task; find food in a bin closer to the landmark; with the parietal pathway, couldn’t retrain this task

Object Task

a “what” task; find food under square object; with temporal pathway, couldn’t retrain this task

Visual Form Agnosia

a condition where a person can see clearly but has difficulty recognizing what they see; makes it difficult to synthesize parts of an object into a whole; typically follows brain damage due to stroke, tumors, or traumatic head wounds

Prosopagnosia

A form of agnosia where a person can no longer recognize familiar faces

Achromatopsia

The complete loss of color vision following brain damage