Chapter 6
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sensory receptor
A specialized neuron that detects a particular category of physical events.
sensory transduction
The process by which sensory stimuli are transduced into slow, graded receptor potentials.
receptor potential
A slow, graded electrical potential produced by a receptor cell in response to a physical stimulus.
hue
One of the perceptual dimensions of color; the dominant wavelength.
brightness
One of the perceptual dimensions of color; intensity.
saturation
One of the perceptual dimensions of color; purity.
vergence movement
The cooperative movement of the eyes, which ensures that the image of an object falls on identical portions of both retinas.
saccadic movement
The rapid, jerky movement of the eyes used in scanning a visual scene.
pursuit movement
The movement that the eyes make to maintain an image on the fovea.
accommodation
Changes in the thickness of the lens of the eye, accomplished by the ciliary muscles, that focus images of near or distant objects on the retina.
retina
The neural tissue and photoreceptive cells located on the inner surface of the posterior portion of the eye.
rod
One of the receptor cells of the retina; sensitive to light of low intensity.
cone
One of the receptor cells of the retina; maximally sensitive to one of three different wavelengths of light and hence encodes color vision.
photoreceptor
One of the receptor cells of the retina; transduces photic energy into electrical potentials.
fovea
The region of the retina that mediates the most acute vision of birds and higher mammals. Color-sensitive cones constitute the only type of photoreceptor found in the fovea.
optic disk
The location of the exit point from the retina of the fibers of the ganglion cells that form the optic nerve; responsible for the blind spot.
bipolar cell
A bipolar neuron located in the middle layer of the retina, conveying information from the photoreceptors to the ganglion cells.
ganglion cell
A neuron located in the retina that receives visual information from bipolar cells; its axons give rise to the optic nerve.
horizontal cell
A neuron in the retina that interconnects adjacent photoreceptors and the outer processes of the bipolar cells.
amacrine cell
A neuron in the retina that interconnects adjacent ganglion cells and the inner processes of the bipolar cells.
lamella
A layer of membrane containing photopigments; found in rods and cones of the retina.
photopigment
A protein dye bonded to retinal, a substance derived from vitamin A; responsible for transduction of visual information.
opsin
A class of protein that, together with retinal, constitutes the photopigments.
retinal
A chemical synthesized from vitamin A; joins with an opsin to form a photopigment.
rhodopsin
A particular opsin found in rods.
transducin
A G protein that is activated when a photon strikes a photopigment; activates phosphodiesterase molecules, which destroy cyclic GMP and close cation channels in the photoreceptor.
dorsal lateral geniculate nucleus
A group of cell bodies within the lateral geniculate body of the thalamus; receives inputs from the retina and projects to the primary visual cortex.
magnocellular layer
One of the inner two layers of cells in the dorsal lateral geniculate nucleus; transmits information necessary for the perception of form, movement, depth, and small differences in brightness.
parvocellular layer
One of the four outer layers of cells in the dorsal lateral geniculate nucleus; transmits information necessary for perception of color and fine details.
calcarine fissure
A horizontal fissure on the inner surface of the posterior cerebral cortex; the location of the primary visual cortex.
striate cortex
The primary visual cortex.
optic chiasm
A cross-shaped connection between the optic nerves, located below the base of the brain, just anterior to the pituitary gland.
receptive field
That portion of the visual field in which the presentation of visual stimuli will produce an alteration in the firing rate of a particular neuron.
protanopia
An inherited form of defective color vision in which red and green hues are confused; "red" cones are filled with "green" cone opsin.
deuteranopia
An inherited form of defective color vision in which red and green hues are confused; "green" cones are filled with "red" cone opsin.
tritanopia
An inherited form of defective color vision in which hues with short wavelengths are confused; "blue" cones are either lacking or faulty.
negative afterimage
The image seen after a portion of the retina is exposed to an intense visual stimulus; consists of colors complementary to those of the physical stimulus.
complementary colors/complementary colours
Colors that make white or gray when mixed together.
simple cell
An orientation-sensitive neuron in the striate cortex whose receptive field is organized in an opponent fashion.
complex cell
A neuron in the visual cortex that responds to the presence of a line segment with a particular orientation located within its receptive field, especially when the line moves perpendicularly to its orientation.
sine-wave grating
A series of straight parallel bands varying continuously in brightness according to a sine-wave function, along a line perpendicular to their lengths.
spatial frequency
The relative width of the bands in a sine-wave grating, measured in cycles per degree of visual angle.
retinal disparity
The fact that points on objects located at different distances from the observer will fall on slightly different locations on the two retinas; provides the basis for stereopsis.
CO blob/cytochrome oxidase blob
The central region of a module of the primary visual cortex, revealed by a stain for cytochrome oxidase; contains wavelength-sensitive neurons; part of the parvocellular system.
ocular dominance
The extent to which a particular neuron receives more input from one eye than from the other.
blindsight
The ability of a person to reach for objects located in his or her "blind" field; occurs after damage restricted to the primary visual cortex.
extrastriate cortex
A region of visual association cortex; receives fibers from the striate cortex and from the superior colliculi and projects to the inferior temporal cortex.
color constancy/colour constancy
The relatively constant appearance of the colors of objects viewed under varying lighting conditions.
achromatopsia
Inability to discriminate among different hues; caused by damage to the visual association cortex.
inferior temporal cortex
In primates, the highest level of visual association cortex; located on the inferior portion of the temporal lobe.
visual agnosia
Deficits in visual perception in the absence of blindness; caused by brain damage.
apperceptive visual agnosia
Failure to perceive objects, even though visual acuity is relatively normal.
prosopagnosia
Failure to recognize particular people by the sight of their faces.
associative visual agnosia
Inability to identify objects that are perceived visually, even though the form of the perceived object can be drawn or matched with similar objects.
pulvinar
A large thalamic nucleus that projects to the visual association cortex and may play a role in compensating for eye and head movements.
Balint's syndrome
A syndrome caused by bilateral damage to the parieto-occipital region; includes optic ataxia, ocular apraxia, and simultanagnosia
optic ataxia
Difficulty in reaching for objects under visual guidance.
ocular apraxia
Difficulty in visual scanning.
simultanagnosia
Difficulty in perceiving more than one object at a time.