Ch 6: The Visual System pt 2

The Retina After light passes through the pupil & lens, it hits the retina & is converted to a neural signal Has 5 layers of different types of neurons Receptors Horizontal cells Bipolar cells Amacrine cells Retinal ganglion cells

The Retina The layers of the retina are considered inside-out Light must pass through the top 4 layers before reaching the receptor layer Once the receptors are activated, the neural signal goes back through the layers to the retinal ganglion cells (which then exits the eyeball)

The Retina There is a blind spot where the ganglion cell axons leave the eye Fovea: an indentation at the center of the retina specialized for high acuity vision (fine details)

Completion Your visual system fills in the gaps in your retinal image (created by the blind spot) by completion Uses info from nearby receptors to “assume” what receptors in the blind spot would be “seeing” if they were there Completion used in other scenarios Ex: key info will be gathered about edges & the rest of the object will be filled in

Rods & Cones 2 types of retinal receptors Rods Cones 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 Movement Even though your cones are concentrated in the fovea, you can see a whole view of color Due to constant scanning of the eyes & summation of that visual input information Temporal integration Essentially allows you to not notice when you blink Your eyes continuously move and fixate on one point to the next (fixational eye movements) Saccades

Eye Movement Visual neurons respond to change If your eyes were to stop moving, your vision would fade out & stop working!!

Visual Transduction Transduction: the conversion of energy from one form to another Visual transduction: conversion of light to neural signals by the visual receptors Rhodopsin: the red pigment in rods that absorb light A G-protein coupled receptor that responds to light (not NTs) Movement of Na+ ions & glutamate NTs allow for transduction in rods

Retina to the Brain Main pathway is the retina-geniculate-striate pathway Sends neural signals from each retina to the primary visual cortex (AKA striate cortex) via the lateral geniculate nuclei (LGN) of the thalamus Called striate cortex because the cortex is layered; with stripes/striations

Retina to the Brain Terminology: Ipsilateral: same side Contralateral: opposite side All signals from the visual field of one side go to the primary visual cortex of the contralateral hemisphere

Seeing Edges Edges are the most informative features of visual stimuli So our brains have become excellent at detecting edges Edges are just where 2 different areas of an image meet So our perception of an edge is a contrast between 2 adjacent areas of the visual field Mach 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)