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Ch 6: The Visual System pt 2

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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  1. Ch 6: The Visual System pt 2

  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

  3. 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)

  4. 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)

  5. 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

  6. 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

  7. Animal Vision

  8. 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

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

  10. 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

  11. 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

  12. 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

  13. 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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