Ch 6: The Visual System pt 2

The RetinaAfter light passes through the pupil & lens, it

hits the retina & is converted to a neural signal

Has 5 layers of different types of neurons1. Receptors2. Horizontal cells3. Bipolar cells4. Amacrine cells5. Retinal ganglion cells

The RetinaThe layers of the retina are considered

inside-outLight must pass through the top 4 layers

before reaching the receptor layerOnce the receptors are

activated, the neural signal goes back through the layers

to the retinal ganglion cells (which then exits the eyeball)

The RetinaThere is a blind spot where the ganglion cell

axons leave the eyeFovea: an indentation at the center of the

retina specialized for high acuity vision (fine details)

CompletionYour visual system fills in the gaps in your

retinal image (created by the blind spot) by completionUses info from nearby receptors to “assume”

what receptors in the blind spot would be “seeing” if they were there

Completion used in other scenariosEx: key info will be gathered about edges & the

rest of the object will be filled in

Rods & Cones2 types of retinal

receptorsRods

Scotopic vision More sensitive to light Lacks detail & color 100s of rods converge on

each retinal ganglion cell Brain can’t “be sure”

where the light is coming from

None in the fovea

Cones Photopic vision Dominant in good lighting Provides hi-def color

vision A few cones converge on

each retinal ganglion cell Brain “knows” exactly

where the light is coming from

Animal Vision

Eye MovementEven though your cones are concentrated in

the fovea, you can see a whole view of colorDue to constant scanning of the eyes &

summation of that visual input informationTemporal integration

Essentially allows you to not notice when you blinkYour eyes continuously move and fixate on

one point to the next (fixational eye movements)Saccades

Eye MovementVisual neurons respond to changeIf your eyes were to stop moving, your vision

would fade out & stop working!!

Visual TransductionTransduction: the conversion of energy from

one form to anotherVisual transduction: conversion of light to

neural signals by the visual receptorsRhodopsin: the red pigment in rods that

absorb lightA G-protein coupled receptor that responds to

light (not NTs)Movement of Na+ ions & glutamate NTs allow

for transduction in rods

Retina to the BrainMain pathway is the retina-geniculate-

striate pathwaySends neural signals from each retina to the

primary visual cortex (AKA striate cortex) via the lateral geniculate nuclei (LGN) of the thalamusCalled striate cortex because the cortex is

layered; with stripes/striations

Retina to the BrainTerminology:

Ipsilateral: same sideContralateral: opposite side

All signals from the visual field of one side go to the primary visual cortex of the contralateral hemisphere

Seeing EdgesEdges are the most informative features of

visual stimuliSo our brains have become excellent at

detecting edgesEdges are just where 2 different areas of an

image meetSo our perception of an edge is a contrast

between 2 adjacent areas of the visual fieldMach bands: our brains enhance the contrast

at edges to make them easier to see(we see edges as more

highlighted than they are in the real world)