Ch 6 the visual system pt 2
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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.

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Ch 6: The Visual System pt 2

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Ch 6 the visual system pt 2

Ch 6: The Visual System pt 2


The retina

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 retina1

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 retina2

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

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

Rods & Cones

  • 2 types of retinal receptors

    • Rods

      • 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

Animal Vision


Eye movement

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 movement1

Eye Movement

  • Visual neurons respond to change

  • If your eyes were to stop moving, your vision would fade out & stop working!!


Visual transduction

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

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 brain1

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

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)


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