2.15 Vision Flashcards
sensation
effect of a stimulus on sensory receptors w/n sensory organs
Vision: stimulus, sensory organ, sensory receptors
lightwaves, eye, cones and rods (photoreceptors)
Sensory Transduction
how stimulus is translated into neural signals that are transmitted to brain
Perception
processing, organizing, and interpretation of neural signals, result in mental picture/understanding of stimulus
Vision Process (light –> retina)
Light enters through cornea and pupil –> Iris controls how much light enters through pupil (dilation/constriction) –> lens focus/project light rays onto retina in the back of eyeball, where photoreceptors are located
-Image projected onto retina is inverted and reversed
pupillary light reflex
Bright light → restrict amount of light entering eye, iris constricts pupils → allows less light into eye
Dim light → increase amount of light entering eye, iris muscles dilate pupils → more light let into eye
visual gap
(blind spot) in optic disc b/c there are nerve bundles that lead to brain; no photoreceptors
Visual Process (retina –> optic nerve)
- Light projects to back of the retina, reaching rods and cones (densely packed)
- When photoreceptors absorb light, they convert light waves into electrical signals and stimulate/inhibit (+ or -) bipolar cells (medium layer)
- Bipolar cells, in turn, stimulate or inhibit retinal ganglion cells (neurons, top layer), that form bundles and ultimately generate action potentials through optic nerve to the brain
Rods
Highly sensitive to light Used at night/in low-light conditions Low resolution (acuity) Located in high density away from the fovea (center of visual field), concentrated in periphery (peripheral vision) Highest sensitivity to blue-green light
Cones
Less sensitive to light Used during the dat/in bright light High resolution (acuity) Concentrated at fovea (focal vision) S, M, L cones respond to short, medium, and long wavelengths of light (blue → red)
Receptive Field
regions of sensory space in which a stimulus can lead to a neuronal response
RF for neurons involved in vision
regions of space where when there is an appropriate visual stimulus (light wave) –> modifies neuron’s activity
-size and complexity of RF vary across layers in the retina and in different parts of the brain
RF of photoreceptor
small circular points of light in the center or periphery of visual field
- photoreceptors in fovea have small receptive fields that correspond to center of visual field
- photoreceptors outside of fovea have larger receptive fields that correspond to the peripheral part of the visual field
RF of retinal ganglion cells
larger and circular regions
-concentric circle (center-surround organization), where inner and outer part of circle oppose each other due to lateral inhibition of photoreceptors
on-center off-surround cells
When light hits inner circle → depolarization, neuron will fire
When light hits outer circle → inhibition (hyperpolarization)
Off-center on-surround cells
When light hits inner circle → inhibition (hyperpolarization)
When light hits outer circle → depolarization
LGN
retinal ganglion cells project to the lateral geniculate nucleus (LGN) of the thalamus
Lateral inhibition: center-surround organization
One ganglion cell receives inputs its own bipolar cells and photoreceptors as well as from surrounding ganglion cells and their corresponding bipolar cells and photoreceptors and transmits impulses (action potentials) to optic nerve
Lateral inhibition: on-center off-surround cell
-if light hits inner circle
photoreceptors in the inner circle will transmit sensory input → bipolar cells → excite ganglion cell → relay action potential via optic nerve to brain
Lateral inhibition: on-center off-surround cell
-if light hits outer circle
photoreceptors in the outer circle will relay sensory input → bipolar cells → ganglion cell layer
Ganglion cell layer will relay lateral inhibitory inputs to master ganglion cell → no action potential via optic nerve to brain
importance of center surround organization
better detect changes in contrast (edges)
Visual Process (optic nerve –> brain)
Retinal ganglion neurons exit eye, axons of which form optic nerve
Ganglion nerves synapse onto neurons in the thalamus (relay station) called the LGN (lateral geniculate nucleus - nucleus in thalamus that deals with vision)
Axons of LGN neurons project to the primary visual cortex (V1) when fired
-Neurons that relay info about right visual field map to the left V1
-Neurons that relay info about left visual field map to the right V1
Receptive fields of neurons in primary visual cortex (V1)
rectangular
simple cells
V1 neurons that respond to different orientations of rectangular bars of light of particular orientations
(e.g. diagonal bar of light –> more firing/action potentials, vertical bar of light –> fewer action potentials)
retinotopic mapping
V1 neurons are mapped in similar ways that ganglion neurons are mapped in retina
what is represented most heavily in V1 (primary visual cortex)
fovea
when primary visual cortex is damaged
the photoreceptors in retina that correspond with the damaged portion of the V1 would be unable to absorb light → cortical blindness (blindspots in areas of visual space)
blindsight
the ability to reflexively respond to visual information within the blindspot without being able to perceive the stimulus itself
