Transduction
The process in the retina’s photoreceptor cells that converts light into electrical signals
Law of Specific Nerve Energies
Nerves are specific to their senses
Coding
How the brain translates light signals into meaningful visual information for perception and memory. Aspects: which neurons are active, & how many action potentials a neurons sends per unit of time
Identify the visual receptors. What is the collective name for the receptors and what are the individual names?
Photoreceptors: rods (faint light, periphery of eye), & cones (color, fovea)
How does light stimulate receptors?
Strikes the 11-cis-retinal (the protein complex that activates when hit by light waves) and causes transduction
Anatomy of the eye
Iris & pupil (light enters; pupil can constrict and dilate), cornea & lens (light focused onto the retina), retina (lined w/receptors), fovea (center of the macula), macula (contains cones; sharp details, recognize colors, and read)
What is the pathway and direction of impulses after the receptors are stimulated?
Bipolar cells - ganglion cells - optic nerve - brain
Extra cells
Amacrine cells get info from bipolar cells and send it to other bipolar, amacrine, and ganglion cells; horizontal cells make inhibitory contact onto bipolar cells, which in turn make synapses onto amacrine and ganglion cells
What does the optic nerve consist of and what causes the blind spot?
Consists of the axons of the ganglion cells that travel to the brain; has no receptors as it is the point at which the optic nerve leaves the back of the eye
Describe receptive fields and state how the receptive field combines when multiple receptors are funneled to one ganglion cell.
The point in space from which light strikes it; small group of rods or cones connect to a bipolar cell, several of those connect to a ganglion cell (receptive field becomes larger, and include both excitation and inhibition)
What is the adaptive purpose of rod cell neural convergence?
It maximizes visual sensitivity in low-light conditions; the summation enables perception of fainter lights in the periphery
What does the fovea contain and what is the purpose? What cells are in the peripheral aspect of the retina?
Filled with cones which allows for acute & detailed vision, and allows for the registering of the exact location of input; rods which respond to faint light
Compare and contrast foveal and peripheral vision.
Foveal: cones, each ganglion cell excited by a single cone, distinguishes among bright lights & responds poorly to dim, good detail vision and good color vision
Peripheral: rods increase, each ganglion cell excited by many receptors, responds well to dim light, poor detail vision and color vision
What is the range of wavelengths that humans can perceive? What does this have to do with color vision?
380nm - 750 nm; they are called light because the receptors in our eyes are tuned to detecting them
What is the trichromatic theory of color vision? What does this have to do with cones?
Relative rates of response by 3 kinds of cones (i.e., long, medium, & short) - each is most sensitive to a diff set of wavelengths & the ratio of activity determines color
Why have other theories been proposed, and what was proposed to explain the negative color after image?
Opponent process theory: perceive color in terms of opposites (brain has a mechanism that perceives color on a continuum from red to green, yellow to blue, and white to black) - an increase in response produces one perception, and a decrease produces a different perception
State the most common type of color vision deficiency. What is the genetic link?
Red-green color deficiency; is on the X chromosome (either the lack of a type of cone, or a cone w/abnormal properties)
Trace the pathway for vision from the optic chiasm to the lateral geniculate nucleus and then to primary visual cortex. What happens to the visual field information at the optic chiasm? What parts of the visual
field does each eye see?
Information from the nasal half crosses to the contralateral hemisphere, info from the temporal half goes to the ipsilateral hemisphere - percentage of crossover varies depending on the location of the eyes
What happens to visual info after going to V1?
1st processing (occipital lobe) - as imagining seeing something, activity increases in area V1 in a pattern similar to actually seeing object; V1 to V2 to other areas
Define and describe blindsight.
Ability to respond in limited ways to visual information without perceiving it consciously - no awareness of visual input (could be that small island of healthy tissue remain, or thalamus sends visual input to several other brain areas besides V1, including parts of the temporal cortex)
What functions are the ventral and dorsal streams associated with? What parts of the brain are these streams associated with?
Ventral: through the temporal cortex, “what” pathway - specialized for identifying and recognizing objects
Dorsal: through the parietal cortex, “where” pathway - helps the motor system locate objects
Why is V1 called striate cortex?
Because of its striped appearance, line of Gennari (thick band of myelinated axons)
Define visual agnosia. What is prosopagnosia and what part of the brain is it related to?
Visual agnosia is the inability to recognize objects and is caused by damage to the pattern pathway usually in temporal cortex - V1 is okay; prosopagnosia is the inability to recognize faces and occurs after damage to the right fusiform gyro of the inferior temporal cortex
What is motion blindness and what area of the brain is it related to?
Refers to the inability to determine the movement of objects (which direction & how fast); area MT (middle temporal cortex) & MST (medial superior temporal cortex)
