THE VISUAL SYSTEM

P h o t o n s a n d t h e R a n d o m n e s s o f L i g h t E n e r g y o f a p h o t o n :

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Eccentricity

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Midget ganglion cells (P cells)

Small bistratified ganglion cells (K cells)

Parasol ganglion cells (M cells)

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L and M cones Rods

L and M cones S cones

P cells (midget) M cells (diffuse)

K cells

Competing Goals for Visual System Design

Maximize spatial resolution

Maximize field of view

Minimize neural resources

Solution

High resolution foveal vision

Low resolution peripheral vision

Eye movement system

Types of Eye Movements

•Saccadic Movements:–Scanning movements where the gaze is abruptly shifted from one point to the next: conjugate, ballistic, no visual feedback

•Vergence Movements:–Cooperative movements that keep both eyes fixed on the target; converge or diverge

•Pursuit movements:–Smooth tracking movements that keep an object’s image fixed in place on the retina

•Vestibulo-Ocular Reflex (VOR)–Stabilize image during head and body movements

•Micro movements–Tremor, drift, microsaccades

Define the task and stimuli

Measure behavioral responses

Measure responses of single neurons or populations to same stimuli (preferably in same species and while behaving the task)

Test linking hypotheses; for example,Most sensitive neuron hypothesisOptimal pooling hypothesisSub-optimal pooling hypothesesInvariance hypothesis

(read Parker & Newsome 1998)

Relating Neuron Responses and Behavior

1. Optical point-spread function (Campbell & Gubisch, 1966)

2. Ganglion cell sampling lattice (Curcio & Allen, 1980)

3. Receptive field properties of P cells (Derrington & Lennie,1984; Croner & Kaplan, 1995):

a. Center diameter of one cone in fovea

b. Center diameter increasing in proportion to GC density

c. Surround diameter 4-6 times larger than center

d. Surround strength 50%-80% of the center

4. Response noise (Croner et al., 1993):

a. Constant additive noise

Retinal/LGN Model

Light and Dark Adaptation: Solving the Dynamic Range Problem

The pupil opens up at low light levels, closes down at high light levels.

There are two photoreceptor systems: the rod

system (for low light levels) and the cone system (for high light levels).

The photoreceptors adjust their individual

sensitivities based upon the ambient light level.

The other retinal neurons adjust their sensitivities based upon the ambient light level.