The Peripheral Neuropathy Solution

Peripheral Neuropathy Program By Dr. Randall Labrum

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Wertheimer, M. "Laws of Organization in Perceptual Forms." In A Sourcebook of Gestalt Psychology. Translated and edited by W. D. Ellis. New York: Humanities Press, 1950. 71-88.

Wheatstone, C. "Contributions to the Physiology of Vision. I. On Some Remarkable and Hitherto Unobserved Phenomena of Binocular Vision." Philosophical Transactions of the Royal Society B 128 (1838): 371-394.

Wiesel, T. N. "Postnatal Development of the Visual Cortex and the Influence of Environment." Nature 299 (1982): 583-591.

Wojtach, W. T., K. Sung, S. Truong, and D. Purves. "An Empirical Explanation of the Flash-Lag Effect." Proceedings of the National Academy of Sciences 105 (2008): 16338-16343.

Wuerger, S., R. Shapley, and N. Rubin. "'On the Visually Perceived Direction of Motion' by Hans Wallach 60 Years Later." Perception 25 (1996): 1317-1367.

Wundt, W. "Contributions to the Theory of Sensory Perception." In Classics in Psychology. Edited by T. Shipley. New York: Philosophical Library, 1961. 51-78.

Wyszecki, G., and W. S. Stiles. Color Science: Concepts and Methods, Quantitative Data and Formulae. Edited by G. Wyszecki and W. S. Stiles. New York: John Wiley, 1982.

Yang, Z., and D. Purves. "The Statistical Structure of Natural Light Patterns Determines Perceived Light Intensity." Proceedings of the National Academy of Sciences 101 (2004): 8745-8750.

Young, T. "An Account of Some Cases of the Production of Colours, Not Hitherto Described." Philosophical Transactions of the Royal Society B 92 (1802): 387-397.

Glossary absorption spectrum

The spectral distribution of light that has passed through a medium (or been reflected from a surface) that absorbs some portion of the incident light.


Pertaining to visual stimuli (or scenes) that are perceived only in shades of grays ranging from black to white.

action potential

The electrical signal conducted along neuronal axons by which information is conveyed from one place to another in the nervous system.


The ability of the visual system to accurately discriminate spatial detail, as in the standard Snellen eye chart exam. Usually tested by the spatial discrimination of two points.


Adjustment to different levels of stimulus intensity, which allows operation of a sensory system over a wide range.

afferent neuron

An axon that conducts action potentials from the periphery to more central parts of the nervous system. algorithm

A set of rules or procedures set down in logical notation, typically (but not necessarily) carried out by a computer.


Diminished visual acuity arising from a failure to establish appropriate visual cortical connections in early life, typically as a result of visual deprivation.

analytical (analyze)

To determine the nature of something according to a set of principles, such as the features of an image (contrasts with empirical).


An opening in the foreground through which one can view a scene (think of looking through a keyhole).

aperture problem

The challenge of explaining the different speed and direction of a moving line perceived through an aperture.

artificial intelligence

A phrase used to describe a computational approach to mimicking brain function that generally depends on algorithmic solutions.

artificial neural network

A computer architecture for solving problems by feedback from trial and error instead of by a predetermined algorithm.

association cortex

Regions of the cerebral neocortex defined by their lack of involvement in primary sensory or motor processing.

autonomic ganglia

Collections of autonomic motor neurons outside the central nervous system that innervate visceral smooth muscles, cardiac muscle, and glands.

autonomic nervous system

All the neural apparatus that controls visceral behavior. Includes the sympathetic, parasympathetic, and enteric systems.


Synonym for consciousness.


Referring to the part or parts of a scene that are farther away from an observer and/or less salient.

Bayesian decision theory

The application of Bayes's theorem to real-world problems.

Bayes's theorem

A theorem that formally describes how valid inferences can be drawn from conditional probabilities. binding problem

Understanding how perceptual qualities are brought together in the perception of objects.


Pertaining to both eyes.

binocular rivalry

See rivalry.

blind spot

The region of visual space that falls on the optic disk in the view generated by each eye. Because of the lack of photoreceptors in this part of the retina, objects that lie completely within the blind spot are not perceived in monocular view.


Iterated units of unknown function in the primary visual cortex of humans and most other primates.


A term that loosely refers to the flow of information from sensory receptors to the cerebral cortex.

bottom-up processing

Generally refers to peripheral sensory processing.


The cerebral hemispheres, cerebellum, and brainstem.


The portion of the brain that lies between the diencephalon and the spinal cord. Comprises the midbrain, pons, and medulla.


Technically, the apparent intensity of a source of light; more generally, a sense of the effective overall intensity of a light stimulus. See lightness.

Brodmann's Area 17

The primary visual cortex, also called the striate cortex.

calcarine sulcus

The major sulcus on the medial aspect of the human occipital lobe. The primary visual cortex lies largely within this sulcus.


The basic biological unit in plants and animals, defined by a cell membrane that encloses cytoplasm and (typically) the cell nucleus.

cell body

The portion of a neuron that houses the nucleus.

central nervous system

The brain and spinal cord of vertebrates (by analogy, the central nerve cord and ganglia of invertebrates).

central sulcus

A major sulcus on the upper and lateral aspect of the hemispheres that forms the boundary between the frontal and parietal lobes. The anterior bank of the sulcus contains the primary motor cortex; the posterior bank contains the primary somatic sensory cortex.


Prominent hindbrain structure concerned primarily with motor coordination, posture, and balance.

cerebral cortex

The superficial gray matter of the cerebral hemispheres.


The largest and most rostral part of the brain in humans and other mammals, consisting of the two cerebral hemispheres.


Pores in the membrane of neurons and other cells that allow the inward and outward movements of ions that underlie neural signaling.

chiasm (optic chiasm)

The crossing of optic nerve axons from the nasal portions of the retinas in humans and other mammals such that the temporal visual fields are represented in the opposite cerebral hemispheres.


In neurobiology, refers to the connections between neurons. Usually pertinent to some particular function (as in visual circuitry).


Referring to higher-order mental processes such as perception, attention, and memory.


The subjective sensations elicited in humans by different distributions of power in the spectra of light stimuli. color blind

Outmoded term for individuals who have abnormal or absent color vision. See color deficient.

color constancy

The similar appearance of surfaces despite different light spectra coming from them. Usually applied to the approximate maintenance of object appearances in different illuminants.

color contrast

The different appearance of surfaces despite similar spectra coming from them. color deficient

Term for individuals who have abnormal color vision as a result of the absence of (or abnormalities in) one or more of the three human cone types.

color opponent

Term used to refer to the experience of seeing red-green, blue-yellow, and black-white as opposites.

color-opponent cells

Cells whose receptive field centers and surrounds are sensitive to opposing spectral qualities.

color space

The depiction of a human color experience in diagrammatic form by a space with three axes representing the perceptual attributes of hue, saturation, and brightness.


Measurements of the human responses to uniform spectral stimuli presented in the laboratory with a minimum of contextual information.


In biology, the struggle for limited resources essential to survival or growth. complementary colors

The colors elicited by lights that, when mixed together, generate a neutral sensation of some shade of gray (often applied more loosely to colors that are oppositely disposed around the Newton color circle).

cone opsins

The distinct photopigment proteins found in human and other cones.


Photoreceptors specialized for high visual acuity and the perception of color. consciousness

A contentious concept that includes the ideas of wakefulness, awareness of the world, and awareness of the self as an actor in the world.


The information provided by the surroundings of a target. The division of a scene into target and surround is useful but arbitrary because any part of a scene provides contextual information for any other part.


On the opposite side.


The difference, usually expressed as a percentage, between the luminance (or spectral distribution, in the case of color) of two surfaces.

Cornsweet illusion

An edge effect elicited by opposing light gradients that meet along a boundary (sometimes called the Craik-O'Brien-Cornsweet illusion).

corpus callosum

The large midline fiber bundle that connects the two cerebral hemispheres.


The gray matter of the cerebral hemispheres and cerebellum, where most of the neurons in the brain are located.

cortical columns (cortical modules)

Vertically organized, iterated groups of cortical neurons that process the same or similar information. Examples are ocular dominance columns and orientation columns in the primary visual cortex.

critical period

A restricted developmental period during which the nervous systems of humans or other animals are particularly sensitive to the effects of experience.

cyclopean vision

The normal sense when looking at the world with both eyes that we see it as if with a single eye. dark adaptation

The adjustment of the sensitivity of the visual system to dim light conditions. degree

Unit in terms of which visual space is measured; 1° is approximately the width of the thumbnail held at arm's length and covers about 0.2mm on the retina.


A neuronal process extending from the cell body that receives synaptic input from other neurons. depth perception

General term used to indicate the perception of distance from the observer (can be either monocular or stereoscopic).


A nerve cell or other device that responds in the presence of some stimulus feature (luminance, orientation, and so on).


A color-deficient person (or the majority of mammals) whose color vision depends on only two cone types.


Having only two cone types, and thus different color perceptions. diencephalon

Portion of the brain that lies just rostral to the brainstem; comprises the thalamus and hypothalamus. direction

The course taken by something, such as a point moving within a frame of reference. Direction and speed define velocity.


The geometrical difference between the view of the left and right eye in animals with frontal eyes and stereoscopic depth perception.

dorsal lateral geniculate nucleus

The thalamic nucleus that relays sensory information from the retina to the cerebral cortex. Usually referred to as the lateral geniculate or just the geniculate.

dorsal stream

Pathway from the visual cortex in the occipital lobe to the parietal cortex. Thought to support attention and object location.


Away from the center. In vision, typically refers to the distance in degrees away from the line of sight centered on the fovea.

edge effects

Perceptual phenomena in which the qualities at an edge affect the perception of the qualities (for example, brightness of color) of the adjoining territory (or territories). See Cornsweet illusion.

efferent neuron

An axon that conducts information away from the central nervous system.

electromagnetic radiation

The full spectrum of radiation in the universe, of which light comprises only a tiny portion.


Study of the nervous system by means of electrical recording.


Derived from past experience, effectively by trial and error (the opposite of analytical).

excitatory neuron

A neuron whose activity depolarizes (excites) the target cells it contacts.


Referring to the regions of visual cortex that lie outside the primary (or striate) visual cortex.

extrastriate visual areas

See extrastriate. Includes areas V4, MT, and MST, which are taken to be particularly pertinent to the processing of a specific visual quality (for example, color in V4, and motion in MT and MST).


Physical characteristic of a stimulus. feature detection

The idea that the visual system (or other sensory systems) detects and represents the characteristics of stimuli and/or the objects and conditions that give rise to them.

filling in

The perceptual attribution of a property or properties to a region of visual space when the information from that space is either absent or degraded.


Looking steadily at a particular point in visual space; the fixation point is where the lines of sight from the left and right eyes intersect.

flicker-fusion frequency

The frequency at which alternating presentations of light and dark are seen as continuous light.


Area of the human retina specialized for high acuity. Contains a high density of cones and few rods. Most mammals do not have a well-defined fovea, although many have an area of central vision (called the area centralis) in which acuity is higher than in more eccentric retinal regions.


How often something occurs within a unit of time or space. frequency distribution

Histogram or other graphical representation showing the relative frequency of occurrence of some event or other phenomenon.

frontal lobe

One of the four lobes of the brain. Includes all of the cortex that lies anterior to the central sulcus and superior to the lateral fissure.

functional imaging

Technique of noninvasive brain imaging that depends on the metabolic activity of the brain tissue to reveal the location of a neural function (usually refers to positron emission topography and/or functional magnetic resonance imaging).

functional magnetic resonance imaging (fMRI)

A functional imaging technique that reveals relative brain activity based on paramagnetic differences between saturated and unsaturated blood oxygen levels.


Collections of neurons that reside in the peripheral nervous system. ganglion cell

An output neuron from the vertebrate retina whose axons form the vertebrate nerve. Also used to refer to neurons in autonomic ganglia.


A hereditary unit located on a chromosome that encodes the information needed to construct a particular protein.

genetic algorithm

A computer-based scheme for simulating the evolution of artificial neural networks.


The complete set of an animal's genes.


The genetic makeup of an individual.

geometrical illusions

Discrepancies between the measured geometry of a visual stimulus (measurements of length, angle, and so on) and the resulting perception.

gestalt (psychology)

A school of psychology founded by Max Wertheimer in the early twentieth century in which the overall qualities of a scene are taken to be determinants of its perception. A gestalt in German means "shape or form."

glial cell

See neuroglial cell.


Characteristic collections of neurons and their processes in the olfactory bulbs. Formed by dendrites of mitral cells and terminals of olfactory receptor cells, as well as processes from local interneurons.

gray matter

Term used to describe regions of the central nervous system rich in neuronal cell bodies. Includes the cerebral and cerebellar cortices, the nuclei of the brain, and the central portion of the spinal cord.

growth factors

Molecules that promote the survival and growth of nerve or other cells.


(pl. gyri) The ridges of the infolded cerebral cortex. The valleys between these ridges are called sulci. Hering illusion

A classical geometrical effect in which parallel lines placed on a background of radiating lines look bowed. heuristic

A rule of procedure derived from past experience that can be used to solve a problem when an algorithm for getting the answer is not known. In vision, such rules are often taken to be the determinants of perception.

hidden layer

Layer in an artificial neural network that lies between the input and output layers. hierarchy

A system of higher and lower ranks. In sensory systems, the idea that neurons in the input stages of the system determine the properties of higher-order neurons.


Neural processes and/or brain areas taken to be further removed from the input stages of a system; sometimes used as a synonym for cognitive processes.

higher-order neurons

Neurons that are relatively remote from peripheral sensory receptors or motor effectors.


A specialized cortical structure located in the medial portion of the temporal lobe. In humans, concerned with short-term declarative memory, among many other functions.

The aspect of color sensation (brightness and saturation being the others) that refers specifically to the qualities of red, green, blue, or yellow.

hypercomplex cells

Neurons in the primary visual cortex whose receptive field properties are sensitive to the length of the stimulus. Also called end-stopped cells, and originally thought to be determined by convergent innervation from complex cells.


A source of illumination.


The light that falls on a scene or surface.


A much-abused word that refers to discrepancies between the physically measured properties of a visual stimulus and what is actually seen. In fact, all percepts are illusory in this sense.


The representation on the retina or in perception of an external form and its characteristics. image formation

The process of focusing the light rays diverging from adjacent points on object surfaces onto another surface (such as a screen or the retina) to form a corresponding set of points on a two-dimensional plane.


The particulars gleaned when an observer (or other receiver) can extract a signal from the background noise. information theory

Theory of communication channel efficiency elaborated by Claude Shannon in the late 1940s. inhibition

Decrease in neuronal excitability or firing rate. inhibitory neuron

A neuron whose activity decreases the likelihood that the target cells it contacts will, in turn, be active. innervate

Establish synaptic contact with another neuron or target cell. innervation

Referring to the synaptic contacts made on a target cell or larger entity such as a muscle.


The information supplied to a neural processing system.


A neuron in the pathway between primary sensory and primary effecter neurons; more generally, a neuron that branches locally to innervate other neurons.

inverse optics problem

The impossibility of knowing the world directly through sense information because of the ambiguity of light patterns projected onto the retina.

inverse problem

The impossibility of knowing the world directly through the senses because of the conflation of information at the level of sensory receptors.


On the same side.


A machine consisting of a collection of parts cobbled together in a nonengineered way to accomplish a specific purpose.


(pl. laminae) One of the cell layers that characterize the neocortex, hippocampus, cerebellar cortex, spinal cord, or retina.



Land Mondrian

A collage of papers used by Edwin Land to explore lightness and color perception. Named after early-twentieth-century abstract artist Piet Mondrian, who produced many works of this sort.

lateral inhibition

Inhibitory effects extending laterally in the plane of the retina or visual cortex; widely assumed to play a major role in perceptual phenomenology.


The acquisition of novel information and behavior through experience.


The transparent and spherical part of the eye whose thickening or flattening under neural control allows the light rays emitting from objects at different distances to be focused on the retina. More generally, any object that refracts light.


The range of wavelengths in the electromagnetic spectrum that elicits visual sensations in humans (i.e., photons that have wavelengths of about 400-700nm).

light adaptation

See adaptation.


The apparent reflectance of a surface experienced as achromatic values ranging from white through grays to black. See brightness.

lightness constancy

The similar appearance of two or more surfaces despite differences in the overall intensity of the spectra coming from them (typically as a function of illumination). See color constancy.

line (linear)

A set of points connected by a common property (such as straightness); an extension in length without thickness.

line of sight

An imaginary straight line from the center of the fovea through the point of fixation.


The four major divisions of the cerebral cortex (frontal, parietal, occipital, and temporal).

long-term potentiation (LTP)

A particular kind of enhancement of synaptic strength as a result of repeated activity.


The physical (photometric) intensity of light returned to the eye or some other detector, adjusted for the sensitivity of the average human observer.

luminance gradients

Gradients of light intensity.

Mach bands

Perceptual bands of lightness maxima and minima that occur at the onset and offset of luminance gradients, described by physicist Ernst Mach in 1865.


Any man-made device—or, more broadly, any apparatus—that accomplishes a purpose through the operation of a series of causally connected parts.

magnocellular system

Component of the primary visual pathway specialized for the perception of motion. Named because of the relatively large cells involved.


An animal whose embryos develop in a uterus and whose young suckle at birth (technically, a member of the class Mammalia).

A systematic arrangement of information in space. In neurobiology, the ordered projection of axons from one region of the nervous system to another, by which the organization of a relatively peripheral part of the body (such as the retina) is reflected in the organization of the nervous system (such as the primary visual cortex).


The corresponding arrangement of the peripheral and central components of a sensory or motor system. medium

In the context of vision, a substance (such as the atmosphere or water) between the observer and the object or objects in a scene.


Light levels at which both the rod and cone systems are active. metamers

Two light spectra that have different distributions of wavelength but nonetheless elicit the same color percepts.


A recording device (typically made of wire or a glass tube pulled to a point and filled with an electrolyte) used to monitor electrical potentials from individual or small groups of nerve cells.


The full spectrum of consciousness at any point in time. Although used frequently in everyday speech (as in "This is what I have in mind" or "My mind is a blank"), it has little scientific meaning.


A category of function. For example, vision, hearing, and touch are different sensory modalities.


A general term used to refer to an iterated cortical unit (ocular dominance columns, orientation columns, or blobs) found in many regions of mammalian brains.

monochromatic light

Light nominally comprising a single wavelength. In practice, often a narrow band of wavelengths generated by an interference filter.


Color-deficient individuals who have only one or no cone opsins and, therefore, have no color vision.


Pertaining to one eye.


The changing position of an object defined by speed and direction in a frame of reference.


Pertaining to biological movement.

motor cortex

The region of the cerebral cortex in humans and other mammals lying anterior to the central sulcus concerned with motor behavior.

motor neuron

A nerve cell that innervates skeletal or smooth muscle.

motor system

Term used to describe all the central and peripheral structures that support motor behavior. MST/MT

Extrastriate cortical regions in the medial temporal lobe of primates specialized for motion processing. Muller-Lyer illusion

A geometrical effect in which the length of a line terminated by arrowheads appears shorter than the same line terminated by arrow tails. First described by nineteenth-century German philosopher and sociologist F. D. Mueller-Lyer.

muscle fibers

Cells specialized to contract when their membrane potential is depolarized. muscle spindles

Highly specialized sensory organs found in most skeletal muscles. The spindles provide mechanosensory information about muscle length.


The six-layered cortex that covers the bulk of the cerebral hemispheres in mammals.


A collection of peripheral axons that are bundled together and travel a common route in the body.

nerve cell

Synonym for neuron.

neural circuit

A collection of interconnected neurons dedicated to some neural processing goal.

neural network

Typically refers to an artificial network of interconnected nodes whose connections change in strength with experience as a means of solving problems.

neural plasticity

The ability of the nervous system to change as a function of experience; typically applied to changes in the efficacy or prevalence of synaptic connections.

neural processing

A general term used to describe operations carried out by neural circuitry.

neural system

A collection of peripheral and central neural circuits dedicated to a particular function (the visual system, the auditory system, and so on).

neuroglial cell (glial cell)

Several types of non-neural cells found in the peripheral and central nervous system that carry out a variety of functions that do not directly entail signaling.

neuromuscular junction

The synapse made by a motor axon on a skeletal muscle fiber.


Cell specialized for the conduction and transmission of electrical signals in the nervous system. neuronal receptive field

The area of the sensory periphery (such as the retina or skin) that elicits a change in the activity of a sensory neuron.

neuronal receptive fields properties

The specific response characteristics of a receptive field.


Study of the structure and function of the nervous system.


A chemical agent released at synapses that affects the signaling activity of the postsynaptic target cells.

neurotransmitter receptor

A molecule embedded in the membrane of a postsynaptic cells that binds neurotransmitter.


Random fluctuations that obscure a signal and do not carry information. objects

The physical entities that give rise to visual stimuli by reflecting illumination (or by emitting light if, as more rarely happens, they are themselves generators of light).

occipital cortex

Region of the cerebral cortex nearest the back of the head, containing mainly visual processing areas. occipital lobe

The posterior of the four lobes of the human cerebral hemisphere; primarily devoted to vision. occlusion

Blockage of the background in a visual scene by an object in the foreground.

ocular dominance columns

The iterated stripes in the primary visual cortex of some species of primates and carnivores, arising from segregated patterns of thalamic inputs representing the two eyes.

olfactory bulb

Olfactory relay station that receives axons from the nose via cranial nerve I and transmits this information via the olfactory tract to higher centers.


The developmental history of an individual animal. Sometimes used as a synonym for development.

opponent colors

Colors that appear as perceptual opposites around the Newton color circle (for example, red versus green or blue versus yellow).


Proteins in photoreceptors that absorb light (in humans, rhodopsin and the three specialized cone opsins).

optic chiasm

See chiasm.

optic disk

The region of the retina where the axons of retinal ganglion cells exit to form the optic nerve. optic nerve

The nerve (cranial nerve II) containing the axons of retinal ganglion cells; extends from the eye to the optic chiasm.

optic radiation

Contains the axons of lateral geniculate neurons that carry visual information to the primary visual cortex. optic tectum

The first central station in the visual pathway of many vertebrates (homologous to the superior colliculus in mammals).

optic tract

The axons of retinal ganglion cells after they have passed through the region of the optic chiasm en route to the lateral geniculate nucleus of the thalamus.


A subcellular component visible in a light or electron microscope (nucleus, ribosome, endoplasmic reticulum, and so on).


The arrangement of an object in a two- or three-dimensional space. orientation selectivity

Describing neurons that respond selectively to edges presented over a relatively narrow range of stimulus orientations.


Making a right angle with another line or surface. parallel processing

Processing information simultaneously in different components of a sensory (or other) system.

parietal lobe

The lobe of the human brain that lies between the frontal lobe anteriorly and the occipital lobe posteriorly.

parvocellular system

Referring to the component of the primary visual pathway in primates specialized for the detection of detail and color; named because of the relatively small size of the nerve cells involved.


The subjective awareness (typically taken to be conscious) of any aspect of the external or internal environment, including thoughts, feelings, and desires.

perceptual space

The organization of a perceptual quality in subjective experience.

peripheral nervous system

All the nerves and neurons that lie outside the brain and spinal cord (that is, outside the central nervous system).


The geometrical transformation of three-dimensional objects and depth relationships when projected onto a two-dimensional surface.


Word used to describe the observed behavior of something.


A device that measurers the physical intensity of light.


Referring to normal levels of light, in which the predominant information is provided by cones. See scotopic.

photopic vision

Vision at relatively high light levels.


Cells in the retina specialized to absorb photons, thus generating neural signals in response to light stimuli.


The evolutionary history of a species or other taxonomic category.

physiological blind spot

See blind spot.


Substances that both absorb and reflect light.


A discrete element in a digital image.


The geometrical concept of a dimensionless location in space. posterior

Toward the back. Sometimes used as a synonym for caudal.


The rate of energy generation. primary color(s)

The four categories of hue in human color vision (red, green, blue, and yellow). Each category is defined by a unique color perception.

primary motor cortex

A major source of descending projections to motor neurons in the spinal cord and cranial nerve nuclei. Located in the precentral gyrus and essential for the voluntary control of movement.

primary sensory cortex

Any one of several cortical areas in direct receipt of the thalamic or other input for a particular sensory modality.

primary visual cortex

The region of cortex in each occipital lobe that receives axonal projections from dorsal lateral geniculate nucleus of the thalamus. Also called Brodmann's Area 17, V1 or striate cortex. (The latter comes from the prominence of layer IV in myelin-stained sections, which gives this region a striped appearance.)

primary visual pathway

Pathway from the retina via the lateral geniculate nucleus of the thalamus to the primary visual cortex; carries the information that enables visual perception.


Order of mammals that includes lemurs, tarsiers, marmosets, monkeys, apes, and humans (technically, a member of this order).


The likelihood of an event, usually expressed as a value from 0 (will never occur) to 1 (will always occur).

probability distribution

Probability of a variable having a particular value, typically shown graphically.


A general term that refers to the neural activity underlying some function.


The study of mental processes in humans and other animals. psychophysics

The study of mental processes by quantitative methods, typically involving reports by human subjects of the perceptions elicited by carefully measured stimuli.


The distance of a point in space from an observer or a measuring device.


Location on a scale, often expressed as a percentile.

real world

Phrase used to convey the idea that there is a physical world even though it is directly unknowable through the senses.

receptive field

The area of a receptor surface (such as the retina or skin) whose stimulation causes a sensory nerve cell to respond by increasing or decreasing its baseline activity. See also neuronal receptive field.


Cell specialized to transduce physical energy into neural signals.

receptor cells

The cells in a sensory system that transduce energy from the environment into neural signals (such as photoreceptors in the retina, hair cells in the inner ear).


The percentage of incident light reflected from a surface. reflection

The return of light hitting a surface as a result of its failure to be absorbed or transmitted. reflex

A stereotyped response elicited by a defined stimulus. Usually taken to be restricted to involuntary actions. reflex arc

Referring to the circuitry that connects a sensory input to a motor output. refraction

The altered direction and speed of light as a result of passing from one medium to another (such as from air to the substance of the cornea).


The ability to distinguish two points in space. See acuity. retina

Neural component of the eye that contains the photoreceptors (rods and cones) and the initial processing circuitry for vision.

retinal disparity

The geometrical difference between the same points in the images projected on the two retinas, measured in degrees with respect to the fovea. See disparity.

retinal ganglion cells

The output neurons of the retina whose axons form the optic nerve.

retinal image

The image focused on the retina by the optical properties of the eye. retinex theory

Edwin Land's algorithm for explaining color contrast and constancy. retinotectal system

The pathway between retinal ganglion cells and the optic tectum in vertebrates such as frogs and fish. retinotopic map

A map in which neighbor relationships at the level of the retina are maintained at higher stations in the visual system.


The maintenance of the neighbor relationships at progressively higher stations in the visual system. rhodopsin

The photopigment found in vertebrate rods.

rivalry (binocular rivalry)

The unstable visual experience that occurs when the right and left eye are presented with incompatible or conflicting images.


System of photoreceptors specialized to operate at low light levels.


The ballistic, conjugate eye movements that change the point of binocular foveal fixation. These normally occur at about three or four per second.


The aspect of color sensation pertaining to the perceptual distance of a color from neutrality (thus, an unsaturated color is one that approaches a neutral gray).


An ordering of quantities according to their magnitudes.


Psychophysical technique for measuring the magnitude of a sensation.


The arrangement of objects and their illumination with respect to the observer; gives rise to visual stimuli.


A defect in the visual field as a result of injury or disease to some component of the primary visual pathway. scotopic

Referring to vision in dim light, where only the rods are operative. sensation

The subjective experience of energy impinging on an organism's sensory receptors (a word not clearly differentiated from perception).


The relative ability to respond to the energy in a sensory stimulus.


Pertaining to sensing the environment.

sensory neuron

Any neuron involved in sensory processing.

sensory stimuli

Any pattern of energy impinging on a sensory receptor sheet such as the retina, skin, or basilar membrane in the inner ear.

sensory system

All the components of the central and peripheral nervous system concerned with sensation in a modality such as vision or audition.

simple cell

A cell in visual cortex that receives direct input from the lateral geniculate nucleus of the thalamus; the center-surround receptive fields of these neurons are organized as if constructed from the characteristics of geniculate neurons.

simultaneous brightness contrast

The ability of contextual information to alter the perception of luminance (the lightness or brightness) of a visual target.

somatic sensory cortex

That region of the mammalian neocortex concerned with processing sensory information from the body surface, subcutaneous tissues, muscles, and joints; in humans, located primarily in the posterior bank of the central sulcus and on the post-central gyrus.

somatic sensory system

The components of the nervous system that process sensory information about the mechanical forces that act on both the body surface and deeper structures such as muscles and joints.


A taxonomic category subordinate to genus. Members of a species are defined by extensive similarities and the ability to interbreed.


Term applied to neural connections that entail specific discrimination by neurons of their targets. spectral differences

Differences in the distribution of spectral power in a visual stimulus that give rise to perceptions of color. spectral sensitivity

The sensitivity of a photoreceptor (or other detecting device) to light of different wavelengths.


A device for measuring the distribution of power across the spectrum of light.


A plot of the amplitude of a stimulus such as light or sound as a function of frequency over some period of sampling time.

spinal cord

The portion of the central nervous system that extends from the lower end of the brainstem (the medulla) to the cauda equina in the lower back.


The special sensation of depth that results from fusion of the two views of the eyes when they regard relatively nearby objects.


Misalignment of the two eyes (often congenital); compromises normal binocular vision unless corrected at an early age.

striate cortex

See primary visual cortex.


A generic term for a pattern of light, sound, or other energy from the environment that activates sensory receptor cells.


(pl. sulci) Valleys between gyral ridges that arise from infolding of the cerebral cortex.


Specialized apposition between a neuron and a target cell; transmits information by release and reception of a chemical transmitter agent.

synaptic potentials

Membrane potentials generated by the action of chemical transmitter agents.

synaptic vesicles

The organelles at synaptic endings that contain neurotransmitter agents.

temporal lobe

The lobe of the brain that lies inferior to the lateral fissure.


A presynaptic (axonal) ending.


A collection of nuclei that forms the major component of the diencephalon. Although its functions are many, a primary role of the thalamus is to relay sensory information from the periphery to the cerebral cortex.


The longer appearance of a vertically oriented line compared to a horizontally oriented line of the same length.


The cellular and molecular process by which energy is converted into neural signals.


The degree to which a substance allows light to pass through it (for example, the transmittance of the atmosphere).

trichromacy theory

The theory that human color vision generally is explained by the different properties of the three human cone types.


Referring to the three different cone types in the human retina that absorb long, medium, and short wavelengths of light, respectively.


The sustaining influence of one cell or tissue on another by the action of a trophic agent such as a growth factor.

tuning curve

Result of an electrophysiological test in which the receptive field properties of neurons are gauged. The maximum sensitivity (or responsiveness) is defined by the peak of the tuning curve.

unique hues

One of the four particular hues around the Newton color circle that are seen as having no admixture of another hue (unique red, green, blue, and yellow).

universal Turing machine

A computer that, using a series of steps, can solve any problem than can be formulated in logical terms.

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