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Vision and the Visual System

Glossary :

abducens nucleus--nucleus in the brainstem that gives rise to the 6th cranial nerve that innervates the lateral rectus mucle of the eye.

afterimages--after exposure to a bright stimulus an image remains visible after its removal, most commonly producing reverse contrast.

amacrine cells--retinal cells that collect signals laterally and interconnect with bipolar cells and retinal ganglion cells.

arcuate--a sulcus posterior to which resides the frontal eye fields.

bipolar cells--retinal cells that receive input from photoreceptors and horizontal cells and connect with retinal ganglion cells.

conjugate eye movements--when the two eyes move in unison.

direction specificity--the disposition of neurons in the visual system to repond preferentially to some directions of stimulus motion.

express saccades--short-latency eye movements that form the first mode of a bimodal distribution of saccadic latencies, the second mode of which is called regular saccades. Express saccades occur after extensive training to singly appearing visual targets.

fovea--a small central area in the retina where the density of cells is the highest thereby affording high-acuity vision.

frontal eye fields (FEF)--a region of the frontal lobe involved in eye-movement control.

horizontal cells--retinal cells that collect signals laterally and interconnect with photoreceptors and bipolar cells.

horopter--a circle drawn through the nodal point of the eye and the point of fixation along which any point falls on corresponding points in the retinae of the two eyes.

lateral geniculate nucleus (LGN)--a thalamic nucleus that receives input from the retinal ganglion cells.

lunate--a sulcus posterior to which area V1 resides.

magnocellular--in the lateral geniculate nucleus of the thalamus the bottom two laminae that contain large cells.

medial eye fields (MEF)--a region of the frontal lobe involved in eye-movement and limb control.

microelectrode--a fine sharpened wire or drawn glass pipette that is used to record from single neurons in the brain or to electrically stimulate.

microstimulation--delivering small amounts of current through a microelectrode inserted into brain tissue that activates neuronal activity.

motion parallax--when in motion, objects at different distances from the observer move at different velocities on the retinal surface, which can be utilized to code relative depth.

MT--an extrastriate visual area extensively interconnected with other occipital, parietal and temporal visual areas.

oculomotor nucleus--nucleus in the brainstem that gives rise to the 3rd cranial nerve that innervates the superior, inferior, medial and superior recti as well as and the superior oblique mucles of the eye.

OFF-cells--cells in the retina and the lateral geniculate nucleus that respond vigorously to light decrement.

ON-cells--cells in the retina and the lateral geniculate nucleus that respond vigorously to light increment.

ON/OFF cells--cells in the retina and the lateral geniculate nucleus that respond vigorously but transiently to both light increment and light decrement.

orientation specificity--the disposition of neurons in the visual system to respond preferentially to some orientations of line segments.

parvocellular--in the lateral geniculate nucleus of the thalamus the top four laminae that contain medium size cells.

photoreceptor cells--cells in the retina that produce electrochemical signals when activated by photons.

principalis--a sulcus immediately anterior to the arcuate.

receptive field--a small area in the visual field which when stimulated with light elicits responses from a single cell in the visual system.

retina--a thin layer of neural tissue in the back of the eye that converts photons into electrochemical signals that are sent to the central nervous system.

retinal ganglion cell--the output cells of the retina that send action potentials to the central nervous system.

saccadic eye movements--rapid eye movements made to shift the center of gaze from one location to another.

saccadic vector--the direction and amplitude of a saccade.

smooth pursuit--tracking eye movements that keeps the center of gaze on a visual object while in motion.

spatial frequency selectivity--disposition on part of visual neurons to respond better to some spatial frequencies than others as commonly determined with sinusoidal gratings of different spatial frequencies.

stereopsis--a mechanism for computing depth on the basis of the disparity of the image on the retinal surface of the two eyes produced by objects at various distances from the observer.

striate cortex--primary visual cortex in the occipital lobe, same as V1.

superior colliculus--a neural structure on the roof of the midbrain that is involved in eye-movement control.

trochlear nucleus--nucleus in the brainstem that gives rise to the 4th cranial nerve that innervates the superior oblique mucle of the eye.

topographic order--the orderly fashion in which the visual field is laid out on in various visual structures.

unit recording--recording the electical activity of neurons in the brain.

V1--primary visual cortex in the occipital lobe.

V2--an extrastriate visual area extensively interconnected with other occipital, parietal and temporal visual areas.

V4--an extrastriate visual area extensively interconnected with other occipital, parietal and temporal visual areas.

vergence eye movements--when the two eyes move in opposition by either converging or diverging.