1. In the human visual system, the shortest visible wavelengths (about 400 nm), are perceived as violet; progressively longer wavelengths are perceived as blue, green, yellow and red near 700 nm.

2. Trichromatic (Young-Helmholtz) Theory: According to this theory of color vision, humans have three different types of cones, each sensitive to a different set of wavelengths. This theory was based on older research using psychophysical observations (reports by observers concerning their perceptions).

3. Opponent-Process Theory: According to this theory we perceive color in terms of paired opposites: white-black, red-green and yellow-blue.

4. Negative afterimages: Result from fatiguing a response by opponent-process cells (e.g., a cell which responds to green light becomes fatigued after prolonged stimulation, which results in a red afterimage when the green light is removed).

5. Color Constancy: The ability to recognize the color of objects despite changes in lighting. This ability is not explained by the trichromatic theory or the opponentprocess theory.

6. Retinex Theory: Theory proposed to account for color constancy. When information from various parts of the retina reaches the cortex, the cortex compares each of the inputs to determine the brightness and color perception for each area.

7. Color Vision Deficiency (color blindness): The inability to perceive color

differences as most people do. Red-green color blindness is the most common

form of this disorder (primarily seen in males).

A. Rods and cones make synaptic connections with horizontal cells and bipolar cells. The horizontal cells make inhibitory contact onto bipolar cells, which in turn make synapses with amacrine cells and ganglion cells. All these cells are in the retina.

B. Axons of the ganglion cells from each eye form the optic nerves; the optic nerves from the left and right eyes meet at the optic chiasm before synapsing in the lateral geniculate nucleus (LGN) of the thalamus. Most axons from the LGN synapse in the visual areas of the cerebral cortex.

C. Visual Field: The whole area of the world that you can see at any time.

D. The portion of the visual field to which any neuron responds is that neuron's receptive field.

E. Lateral inhibition: Reduction of activity in one neuron by activity in a neighboring neurons (this process normally serves to heighten contrast at borders).

F. Most ganglion cells are either parvocellular neurons (small cell bodies and are located in or near the fovea), or magnocellular neurons (larger cell bodies and are distributed evenly throughout the retina).

1. Parvocellular neurons have small receptive fields and respond best to visual details and color. These cells synapse onto smaller cells of the LGN.

2. Magnocellular neurons have larger receptive fields and respond best to moving

stimuli. These cells synapse onto large cells of the LGN.

G. Most axons from the LGN go first to the primary visual cortex, (aka area V1 or striate cortex). This area of the cortex is responsible for the first stage of visual processing. Area V 1 sends information to the secondary visual cortex (area V2 ) which is responsible for the second stage of processing. The connections between V1 and V2 are reciprocal.

H. In the cortex, the parvocellular and magnocellular pathways split from two pathways to

the following three pathways:

1. A mostly parvocellular pathway sensitive to details of shape.

2. A mostly magnocellular pathway with a ventral branch sensitive to movement and

a dorsal branch that is important for integrating vision with action.

3. A mixed parvocellular and magnocellular pathway sensitive to brightness and

I. Ventral Stream: The parvocellular and magnocellular pathways sensitive to shape,

movement, and color-brightness that lead to the temporal cortex. These pathways are

also called the "what" pathways because there are specialized for identifying and

recognizing objects.

J. Dorsal Stream: The mostly magnocellular pathway associated with intergrating vision

and movement that leads to the parietal cortex. This pathway is the "where" or "how"

pathway as it helps the motor system find objects, move toward them, grasp them and

so forth.

K. David Hubel and Torsten Wiesel discovered three categories of neurons in the visual cortex: simple, complex and end-stopped or hypercomplex cells.

1. Simple cells: Neurons with fixed excitatory and inhibitory zones in their receptive

fields; these cells are found only in the primary visual cortex (V1).

2. Complex cells: Located in either V1 or V2, these neurons have receptive fields which respond to particular orientations of light but cannot be mapped into fixed excitatory and inhibitory zones.

3. End-stopped (hypercomplex) cells: Strongly resemble complex cells but in

addition have an inhibitory area at one end of its bar-shaped receptive field.

L. Cells in the visual cortex are grouped together in columns perpendicular to the surface according to their responsiveness to specific stimuli. For example, cells in a particular column may respond only to visual input from the left, right or both eyes about equally. Also, cells in some columns respond best to stimuli of a single orientation.

M. Feature Detectors: Neurons whose responses indicate the presence of a particular feature. For example, neurons in V 1 or V2 respond strongly to bar- or edge-shaped pattems.

N. Inferior Temporal Cortex: Neurons in this brain region provide information about complex shape stimuli (e.g., hands or face). The area is important for shape constancy (the ability to recognize an object's shape even as it approaches, retreats or rotates.

O. Visual agnosia: The inability to recognize objects despite otherwise normal vision;

prosopagnosia is the inability to recognize faces without an overall loss of vision or

memory.

P. Cerebral Cortex: The Color Pathway

1. Color perception depends mostly on the parvocellular pathway.

2. Blobs: Patches of cells highly sensitive to color in areas of V1. These cells include parvocellular neurons for color perception and magnocellular cells for brightness perception. The blobs send their output to areas V2, V4, and the posterior inferior temporal cortex.

3. Area V4 or other near by brain regions are believed to be important for color constancy. Area V4 also contributes to visual attention.

Q. Cerebral Cortex: The Motion and Depth Pathways

1. The magnocellular pathway appears specialized for stereoscopic depth perception

(the ability to detect depth by differences in what the two eyes see).

2. The magnocellular pathway that is specialized for motion perception projects to MT and area MST.

3. Area MT (middle-temporal cortex, also known as area VS) and area MST (medial

superior temporal cortex) are important for motion detection. Damage to or around

area MT results in people becoming motion blind (inability to determine if objects

are moving or are stationary).

R. The Binding Problem Revisited: Visual Consciousness

1. Some visual processing takes place without being conscious. Processing up to the level of the lateral geniculate is unconscious.

2. A limited amount of visual processing takes place without any of it being

conscious. Some people with extensive damage to area V 1 (i.e. cortical blindness)

have blindsight which means they can localize visual objects although they have a

blind visual field.

A. Human infants see better in their periphery than in the center of vision due to underdeveloped receptors in the fovea. Infants also have trouble shifting their gaze from one object to the next until approximately 6 months of age.

B. Effects of Experience on Visual Development

1. Most neurons in the visual cortex receive binocular input (stimulation from both

eyes).

2. If a mammal is deprived of light stimulation of one eye early in life, then the

thalamic axons representing the deprived eye lose most of their synapses onto

cortical cells. If both eyes are deprived of stimulation, cortical cells will remain

responsive (albeit sluggishly) to both eyes.

3. Sensitive or critical period: A stage of development when experiences have a

particularly strong and long-lasting influence. The effects of abnormal experiences

on cortical development are dependent on the length of the sensitive period.

4. Lazy Eye or Amblyopia ex anopsia: Condition in which a child ignores the vision

in one eye, sometimes even letting it drift a different direction from the other eye.

This disorder is treated by putting a patch over the active eye, forcing the child to use the ignored eye.

5. Retinal disparity: Discrepancy between what the left eye sees versus the right. Retinal disparity is necessary for stereoscopic depth perception. The fine-tuning of binocular vision depends on experience.

6. Strabismus: Condition in which the eyes do not point in the same direction. Individuals bom with this disorder cannot perceive depth better with two eyes as opposed to one.

7. Astigmatism: A blurring of vision for lines in one direction; this disorder is caused by an asymmetric curvature of the eyes. Corrective lenses during early childhood (before ages 3-4 years) improve visual capacity during adulthood.

8. If animals grow up without seeing movement they will become motion blind.

9. Certain portions of the visual cortex in people who become blind early in life,

become responsive to auditory or touch stimuli.

Study Questions