Early Experience and Visual Information Processing in Perceptual and Reading Disorders: Proceedings of a Conference Held October 27-30, 1968, at Lake Mohonk, New York, in Association With the Committee on Brain Sciences, Division of Medical Sciences, National Research Council. Edited (1970)

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Glossary The jargon and technical terms of one discipline are deterrents to under- standing by readers trained in another discipline. This glossary is in- tended to relieve the irritation of reading material in an unfamiliar sub- ject and to promote comprehension. It consists of terms that are not explained by the speakers in context; the definitions are confined to the sense in which they are used. Terms in italics are defined elsewhere in the glossary. Specialists in the several disciplines have been consulted, as have the following authoritative sources: American Psychiatric Glossary, 3rd Edition. Washington, D.C.: American Psychiatric Association, 1969. C. H. Best and N. B. Taylor, Eds., Physiological Basis of Medical Practice, 8th Edi- tion. Baltimore: The Williams & Wilkins Company, 1966. F. P. Dinneen, Introduction to General Linguistics. New York: Holt, Rinehart and Winston, Inc., 1967. Borland's Illustrated Medical Dictionary, 24th Edition. Philadelphia: W. B. Saunders Company, 1965. P. Gray, Dictionary of the Biological Sciences. New York: Reinhold Publishing Corporation, 1967. D. V. Gunderson, Reading Problems; Glossary of Terminology. Reading Research Quarterly, 4(4), pp. 534-549, Newark, Delaware: International Reading Association, 1969. 493

Glossary C. A. Keele and E. Neil, Samson Wright's Applied Physiology, 11th Edition. London: Oxford University Press, 1965. S. Polyak, Vertebrate Visual System. Chicago: University of Chicago Press, 1957. Webster's Third International Dictionary, Unabridged. Springfield, Massachusetts: G. &C. Merriam Company, 1967. ACCOMMODATION: A change in thickness, curvature, and power of the lens of the eye through action of the ciliary muscle to adjust the eye for near vision. Accommodation is usually accompanied by changes in pupillary diameter and convergence of the eyes. "ACUTE" INVESTIGATIONS: In neurophysiology, usually investigations on anesthetized animals that are sacrificed at the termination of the experiment, in contrast to "chronic" animals, which are used more than once. AMACRINE CELLS: Intraretinal association neurons without long extensions, acting on synaptic junctions between bipolar and ganglion cells; one of several types of association cells facilitating or inhibiting retinal neural activity. See also horizontal cells. AMBLYOPIA: Reduction in visual acuity that cannot be improved with lenses or attributed to disease. AMETROPIA: An abnormality of the eye, such that when it is at rest, light rays are not focused on the retina, but rather in front of or behind it. Includes myopia, hyperopia, and astigmatism. ANTERIOR COMMISSURE: One of the transverse bands of nerve fibers that con- nect the two cerebral hemispheres. APHASIA: Impairment of the ability to use language because of defect or impair- ment of the brain, not sensory impairment of vision or hearing. APRAXiA: Inability to carry out purposeful movements in the absence of identi- fiable paralysis or other motor or sensory impairment. AREA 17: See striate area. AREA is: An area of the occipital lobe of the cerebral cortex, mainly on the dorsal surface of the lateral gyrus adjacent to area 17, showing electrical activity after stimulation of the visual tract. Also called "visual II" and "secondary visual cortical area." ASTIGMATISM : Lack of symmetry of curvature of the refractive surfaces of the eye as a result of which a ray of light is not focused on the retina sharply in all meridians. ATAXIA: Failure of muscular coordination. ATROPINE: A chemical compound found in plants that, among other uses, dilates the pupil and paralyzes the ciliary muscle controlling accommodation of the lens when applied locally to the eye. AUDIO GRAM: A plot of the acuteness of a subject's hearing at various sound frequencies. 494

Glossary AUDITORY BLENDING: The ability (1) to make sense out of the parts (phonemes) of a word pronounced with separations between phonemes and (2) to obtain an approximation of the intended word sufficient for recognition of the word. A-WAVE: Negative deflection of the electroretinogram occurring in the light- adapted eye following stimulation by a flash of light, or in the dark-adapted eye by a red light, and considered to be related to cone activity. AX ON : A nerve-cell expansion or process that is typically single and long, termi- nates in short branches relatively far from the cell body, and usually conducts impulses away from the cell body. AXOSPINODENDRITIC CONTACTS: Synapses between dendritic spines of one neuron and axons of another neuron. BENDER-GESTALT TEST: A performance test requiring reproduction of the con- figuration in line drawings. BENTON VISUAL RETENTION TEST: A test of visual perception that depends on visual-motor ability. BIPOLAR CELLS: Two types of connecting cells in the retina: monosynaptic bipolar cells, which connect cones to nerve fibers and may permit direct trans- mission to the brain; and polysynaptic bipolar cells, which connect both rods and cones and nerve fibers and permit interactions between receptors before transmission to the brain. BUCCAL SMEAR TEST: Microscopic examination of cells scraped from the inside of the cheek and stained to determine chromosomal irregularities. B-WAVE: Positive deflection of the electroretinogram following stimulation by a flash of light; largest in poor illumination and with blue light. Thought to be pro- duced mostly by rod activity. CEREBRAL AQUEDUCT: A narrow canal about 3/4 in. long extending from the third ventricle to the fourth ventricle in the midbrain region. CEREBRAL COMMISSURES: Neural tracts connecting the two sides of the cerebrum. Examples are the corpus callosum and anterior commissure. CEREBRAL DOMINANCE: A functional asymmetry in which one side of the brain assumes greater control of cerebral functions than the other side, demonstrated by laterality in voluntary motor acts. See also laterality and dominant laterality. CILIARY MUSCLE: The smooth (involuntary) muscle that affects the shape of the lens in visual accommodation. CLOSED COLONY: An inbred colony. CODE-EMPHASIS PROGRAMS: Programs of teaching reading that emphasize sound-symbol relationships. See also meaning-emphasis programs. COMMISSUROTOMY: Surgical sectioning of the connecting bands of a cerebral commissure. In the brain, surgical separation of the neural connections between the two hemispheres. CONE: A photoreceptor neuron characterized in part by being thicker than a rod, by the complexity of its bipolar connections, and by use chiefly in day vision; 495

Glossary occurs most densely in thefovea centralis area of the retina, where visual acuity is highest. CONSENSUAL PUPILLARY RESPONSE: Similar reaction of both pupils to a stimulus applied to only one eye. CONVERGENCE: In binocular vision, the coordinated movement of the two eyes toward fixation on the same near point. Neural convergence is the synapsing of many cells with fewer cells at successively lower or higher levels. CORPUS CALLOSUM: A large bundle of transverse nerve fibers connecting the cerebral hemispheres. COUNTERBALANCING: Technique used in experimental design for controlling continuation effects by presenting stimuli in balanced sequences (such as AB, BA). CRITERION: In experimental psychology, a predetermined level of performance usually less than a perfect score. d-c POTENTIALS: The so-called steady or slowly changing electrical potentials that can be observed in various parts of the brain under constant observation conditions. DECIBEL (db): Nontechnically, a unit for measuring the relative loudness of sounds approximately equal to the smallest degree of difference of loudness ordinarily detectable by the human ear, whose range includes about 130 db on a scale beginning with 1 for the faintest audible sound. DECUSSATioN: A crossing over in the visual system of optic fibers at the optic chiasma. DEEP STRUCTURE: Elements of a sentence that determine its semantic interpre- tation, including grammatical relations, functions, and categories. DENDRITE: A protoplasmic expansion or process of a neuron that usually termi- nates in a highly branched state relatively near the cell body and conducts im- pulses toward the cell body. Dendrites of pyramidal cells are termed apical dendrites or basilar dendrites depending on where they leave the cell body, which is oriented with the apex toward the cerebrocortical surface. DENDRITIC SPINE: A short bulbous expansion of the dendrite, with which the axon endings from another neuron make contact; spines are distributed along the dendrite according to distance from the cell body and other factors. DETOUR BEHAVIOR: A behavior pattern that seeks to reach a goal by an indirect route in order to avoid an unpleasant or apparently impassable barrier. DIOPTER: A unit of measurement of the refractive power of a lens; equal to the reciprocal of the focal length in meters. DIPLOPIA : Double vision—two simultaneous visual images of a single object. "DISABLED READER": See reading disability. DISH ABITU ATION : Restoration of a response that has weakened because of re- peated presentation of the stimulus. 496

Glossary DisiNHiBiTiON: Lessening the reciprocal inhibition caused by excitation of a neuron by means of excitation of another neuron. DOMINANT LATERALITY : The preferential use in voluntary motor acts of ip- silateral members of the different pairs of organs, as the right ear, eye, hand, leg (dextrality) or the left ear, eye, hand, leg (sinistrality). There is a general ten- dency to be consistently right-dominant or left-dominant, but mixed and crossed dominance are also found. DYSLEXIA: From "dys"—faulty, impaired—and "lexis"—speech, from legein (Greek) to speak; confused with legere (Latin) to read, the medically more common inter- pretation, as in alexia. See reading disability. DYSPRAXIA: Partial loss of ability to perform coordinated movements; nonspe- cific clumsiness; a sign considered indicative of possible neurologic involvement. EDGE,DETECTOR UNITS: Retinal rods or cones that respond to contrasting luminance boundaries such as that provided by the border between black and white areas. EDGE GRADIENT: The gradual variation in illumination at an edge between a bright field and a less bright field due to the optical-spread function and eye micromovements. ELECTROANATOMIC STUDIES: Deduction of probable anatomic pathways from the pattern of nerve impulses recorded at different sites. ELECTRORETINOGRAM (ERG): Record of changes in electrical potential in the retina after stimulation by light; may be obtained in man by placing one elec- trode on an anesthetized cornea and the indifferent electrode in the mouth. EMMETROPIA: A state of proper correlation between the refractive system of the eye and the axial length of the eyeball such that rays of light entering the eye parallel to the optic axis are brought to a focus on the retina. ENUCLEATION: Removal of the eyeball after the extraocular muscles and optic nerve have been severed. EVOKED POTENTIALS: Electrical signs of neuronal activity resulting from stimu- lation of nerves or sensory end-organs. EXTINCTION: Reduction or elimination of a conditioned response by not rein- forcing the response. EXTRAOCULAR MUSCLES: Three pairs of muscles (the superior and inferior recti, the internal and external recti, and the superior and inferior oblique muscles), arranged around the eyeball that act in a coordinated manner to rotate the eyeball and maintain appropriate convergence with the other eyeball. FACIAL DIPLEGIA: Paralysis of both sides of the face. FEATURE EXTRACTION: The process of separating from the total input the particular characteristics of a sensory stimulus that are recognized. FIXATION: The direction of gaze is such that the image of an object looked at falls on thefovea centralis. 497

Glossary FOOT LAM BE RT (fL): A unit of luminance equal to the luminance of a perfectly diffusing surface that emits or reflects one lumen per square foot. A lumen equals the light emitted (in a unit solid angle) by a uniform point source of one candle intensity. FOR MANX: The frequencies (or groups of frequencies) that characterize the timbre of a sound and distinguish it from sounds of different timbre. FOVEA CENTRALiS: A tiny depression, about 1 deg wide in man, in the center of the macula lutea retinae, where the layers of the retina are spread apart, per- mitting light to fall directly on the cones; the part of the retina that provides the best visual acuity and most sensitive visual discrimination. FOVEAL EXAMINATION: Examining an object with the most discriminative part of the retina, thefovea centralis; involves coordinated turning of the eyes so the optic axes converge on the fixation point. FUSIONAL MOVEMENTS: Movements of the eyes that bring the retinal images of the same part of a visual stimulus sufficiently close to corresponding points of the retina that a single image is perceived. GANGLION CELLS: In the retina, neurons representing the third link in the chain that carries visual stimuli to the brain, whose cell bodies and dendrites are in the retina and whose axons constitute the fibers of the optic nerve. Retinal ganglion cells differ greatly from one another in body size, location, and manner in which they make synaptic connections; they are activated by bipolar, horizontal, and amacrine cells. GATING FUNCTION: Selection on an arbitrary basis—for example, voltage of neuron spikes—in recording techniques; also applies to all kinds of neurophysio- logic phenomena that can be segregated. GENERALIZATION: In psychology, the ability of an associated or similar stimulus to cause a response produced by a particular stimulus, especially when of the same sense mode. GESTALT: A structure or configuration including shape, pattern, and time, of physical, biologic, or psychologic phenomena so integrated as to constitute a functional unit with properties not derivable from its parts in summation. GESTALT PSYCHOLOGY: The study of perception and behavior from the stand- point of an organism's response to configurational wholes, with stress on the identity of psychologic and physiologic events. GOLGI-COX PREPARATIONS: Preparation and staining of nerve tissues for his- tologic examination by the Cox modification of the first neurohistologic method, devised by Camillo Golgi 100 years ago. The method consists of staining suitably fixed and hardened slices of tissue with a weak silver nitrate solution; only nerve cells are blackened. The Cox modification results in improved staining of dendrites. GRAPHEME (syn. letter, phonogram): A functionally distinctive graphic symbol, used in the writing system of a particular language. 498

Glossary HABITUATION: The waning of a response as a result of repeated stimulation that is not followed by reinforcement; distinct from fatigue and sensory adaptation. HAPTIC SENSATIONS: Sensations arising from the touch receptors in the skin. HEMiPLEGiA: Paralysis of one side of the body. HERTZ (Hz): A unit of frequency of a periodic process equal to one cycle per second; replaces cycles per second (cps). HORIZONTAL CELLS: Inthe retina, one of the types of association cells whose function is to distribute photogenic impulses in the retinal tissue itself and by facilitation, inhibition, or some similar influence to modify the impulses trans- mitted to the cerebrum. The horizontal cells interconnect cone receptors with cones or rods and are so named because of the horizontal spreading of their axons. See also amacrine cells. HYPEROPIA (syn. hypermetropia, farsightedness): An optical abnormality of the eye in which, when it is at rest, rays of light entering the eye parallel to the optic axis are brought to a focus behind the retina. INITIAL TEACHING ALPHABET (ITA): A notation system using the conven- tional symbols of English plus some others designed to represent particular sounds that are variably represented in English. INNER PLEXIFORM LAYER: The relatively cell-free layer of the retina between the ganglion layer and the inner nuclear layer; consists mainly of synapses. INTRACORTICAL ASSOCIATION CELLS: Neurons carrying out an associative function between cortical neurons. KiNESTHESiA: A sense mediated by receptors that lie in the muscles, tendons, and joints and are stimulated by bodily movements and tensions. LABYRINTH: See vestibularapparatus. LATENCY: Time between application of a stimulus and the resulting response at some point in the neural system. LATERAL DOMINANCE: See dominant laterality. LATERAL GENICULATE NUCLEUS: A group of neuronal cell bodies arranged in six layers. Nerve fibers from the retina of the contralateral side are received in layers 1,4, and 6, and from equivalent spots of the ipsilateral retina, in layers 2, 3, and 5. LATERALITY : Awareness of the two sides of one's body and ability to identify each correctly as left or right. LATERAL SPECIALIZATION: See dominant laterality. LAYER IV: The fourth layer from the surface of the primate visual cortex charac- terized by relatively small, densely spaced cells, including stellate cells. LAYER V: The fifth layer from the surface of the primate visual cortex; charac- terized by large cells. LIGHT TRAP: A device that eliminates unwanted light to produce an area of uni- form illumination. 499

Glossary LONG-TERM MEMORY: Retention of information for longer than about a minute; usually, long-term memory refers to days, months, or years. LUMINANCE: Refers to the effectiveness of a given light intensity on the visual system. Formerly called "brightness," and frequently confused with the bright- ness or physical energy of a light source itself. See also Mach bands. MACH BANDS: Bright and dark rings noticed by Ernst Mach. When rapidly ro- tating disks of colored or black and white sectors are viewed, the successive sectors appear to fuse into a single color and intensity; the greater proportion of white to black (or one color to another), the brighter (or more like the better presented color) the rotating disk appears. When the proportions are not uni- form, Mach bands appear. The explanation of the brightness-contrast effect is based on the mutual dependence of neighboring retinal points due to the retinal neural network. MACROELECTRODE: A relatively large electrode (up to 0.5 mm in diameter) with low electrical resistance, used to record potentials accompanying reactions of aggregations of neurons. MACULA LUTEA RETINAE: The part of the retina distinguished from the remain- der of the retina by its yellow appearance; in the center lies ihefovea centralis, the area of greatest visual acuity. MEANING-EMPHASIS PROGRAMS: Programs of teaching reading that emphasize language meaning, as distinct from code-emphasis programs. MESENCEPH ALON (syn. midbrain): The middle division of the brain lying be- tween the forebrain and hindbrain. METHYLENE BLUE PREPARATIONS: Tissues prepared for study by a method devised by Paul Ehrlich (1886) consisting of injection of a weak solution of methylene blue into a living animal, whose nerve tissue decolorizes the blue dye. When the tissue is exposed to air, the nerve structures appear selectively stained in blue. MiCROELECTRODE: An electrode 1-3 p. in diameter used for intracellular recording of potentials accompanying reactions of single units. MOBIUS SYNDROME: Voluntary abstinence from motion owing to intense pain accompanying muscular movements. MOLAR BEHAVIOR: A unit of behavior, described in nonphysiologic, psychologic constructs and made up of smaller specific units elicited by specific stimuli. MOLECULAR BEHAVIOR: A small unit of behavior generally defined in terms of specific movements or of specific movements elicited by specific stimuli, or in physiologic terms. MYELINATION: The condition of some nerve fibers that are covered with a fat-like substance (myelin) as a sheath. MYOPIA (syn. nearsightedness): An optical abnormality of the eye in which, when it is at rest, light rays parallel to the optic axis are focused in front of the retina, rather than on it. 500

Glossary NEURAL GRADIENT; Variation in response of a neural system over a space distribution. NODAL POINT: One of two points on the axis of an optical system so situated that a ray falling on one will produce a parallel ray emerging through the other. NOISE: Interference of variable intensity that forms a diffuse background against which a specific stimulus effect occurs. Within neural systems, "noise" is the residual on-going activity, and any signal to be effective must be higher in inten- sity than this background noise. See also visual noise. NUCLEUS (pi. nuclei): In neuroanatomy, a general term used to designate a group of nerve-cell bodies usually within the central nervous system and bearing a direct relationship to the fibers of a particular nerve. OCCIPITAL LOBE: The posterior lobe of the cerebral hemisphere, merging laterally with the temporal lobe, and concerned primarily with visual input. OCULAR ASTHENOPIA: Weakness or rapid tiring of the eyes attended by pain in the eyes, headache, and dimness of vision. OCULOMOTOR (syn. ocular-motor): Pertaining to the movements of the eye. OCULOMOTOR APRAXiA: Loss of ability to move the eyes at will or to follow a moving object to either side. OCULOMOTOR COORDINATION: Reciprocal innervation of the pairs of extra- ocular muscles, resulting in coordination of movement of the eyeball on a point of fixation. OCULOMOTOR DYSLEXIA: Reading disability due to oculomotor dysfunction. OIL-IMMERSION PHOTOMICROGRAPH: A photomicrograph taken through a high-power lens having a clear oil phase instead of air between the lens and the object (or cover glass). The magnification is usually about 2,000 times, de- pending on the power of the objective and ocular lenses. OMMATIDIUM (pi. ommatidia): An elongated unit of a compound eye of an arthropod, consisting typically of an external corneal lens beneath which is a crystalline cone and below it a rod-shaped form enclosed in a sensitive retinal element protected by pigment. OPERANT CONDITIONING (syn. instrumental conditioning): The experimental procedure of presenting an animal with a reinforcing stimulus immediately following the occurrence of a certain response. Usually, these are responses oc- curring freely; when the correct response occurs, the animal is immediately re- inforced, resulting in an increased rate of responding; e.g., a pigeon may peck at many notes on a piano but when he pecks at the first note of "The Star Spangled Banner," he is immediately reinforced. OPERANT LEVEL: Base-line or pretesting or prereinforcing level of activity. OPTICAL-SPREAD FUNCTION: The total effect of the optical processes (dif- fraction, aberration, light scatter, accommodation errors, optical characteristics, and eye movements) by which rays from an external point of light are spread in a bell-shaped distribution curve to form the image on the retina. 501

Glossary OPTIC CHIASMA: The X-shaped junction of the optic nerves, a short stout bridge through which, in mammals, there is a partial exchange of fibers between the two nerves. In man and other primates, fibers that originate in the somewhat larger nasal half of the retina pass from one nerve through the chiasma and join fibers in the opposite optic tract, whereas the fibers arising from the temporal side of the retina proceed in the lateral portions of the chiasma, without decussation, from the optic nerve to the corresponding optic tract. (See Figure 1 in the Introduction.) OPTIC DISK: The "blind spot" where the layers of the retina are replaced by the optic nerve fibers leaving the eyeball. OPTIC NERVE: Nerve made up of fibers that originate in the ganglion cells of the retina and terminate in several paired nuclei (lateral geniculate nucleus, pregenicu- late gray nucleus) or subcortical stations (superior colliculus, pulvinar of the thalamus) in the brain. OPTIC RADIATION: See visual radiation. OPTIC TECTUM : Dorsal (roof-like) part of the midbrain containing the superior colliculi, which form part of the intermediary visual system. OPTIC TRACTS: The tracts by which the optic nerves continue from the optic chiasma to the brain. The tract on each side carries fibers conducting impulses set up by contralateral visual fields that fall on the ipsilateral retinas, the fibers from the opposite eye having crossed over at the chiasma. (See Figure 1 in the Introduction.) OPTOKINETIC NYSTAGMUS: Involuntary alternating rapid and slow movements of the eyeball induced by attempting to fixate on a point revolving around an axis. OPTOKINETIC TEST: A test in which the entire environment appears to move relative to the observer, so that the eyes are compelled to follow the moving object. See also optokinetic nystagmus. OPTOMOTOR NUCLEI: The nuclei of the third, fourth, and sixth cranial nerves, which innervate the extraocular muscles and are therefore concerned with the movements of the eyeball. ORIENTING REFLEXES: A variety of visceral, somatic, neural, and motor responses to initial presentation of a stimulus. ORTHOPTIC TRAINING: A technique of eye exercises designed to correct the visual axes of eyes not properly coordinated for binocular vision. OTOLITH ORGANS: Mechanoneural receptors in the inner ear that respond to gravitational changes, displacing calcium carbonate crystals (otoliths) attached to the ends of hair cells. OUTER GRANULAR LAYER: The second layer from the surface of the primate striate area, characterized by relatively small cells. PACIN G : Gearing of instruction to balance ease and difficulty for a subject learning a task. 502

Glossary PARAFOVEAL RETINA: The area of the retina surrounding thefovea centralis. PARALINGUISTICS: The study of patterned tone of voice. PATTERNED STIMULATION: Visual stimulation consisting of visual contrast, as opposed to diffuse light. PERCEPT: A perceived object. PERCEPTUAL PRETRAINING: Training in the awareness of objects, relationships, or qualities, especially visual, as a preliminary to reading. PHONEM E: The smallest unit of speech that distinguishes one utterance from another. PHONOLOGY: The science of speech sounds. PHOTON: A quantum of radiant energy. PHOTOPUPILLARY MOTION: Reaction of the pupil to light, resulting in changes in its diameter. PHOTOTONIC BALANCE: Balance in the tonic condition of the intrinsic eye musculature (ciliary muscle and pupillary muscle) resulting from exposure to particular conditions of lighting. PLASTICITY : Ability of one part of the brain as a result of experience and train- ing to modify its function to some extent so that eventually it appears to take over the function of a damaged part. Anatomic plasticity refers to the ability of neurons and nerve-tissue aggregations to change by extensions of dendrites and interconnections. PLEOPTics: A technique of eye exercises designed to develop fuller vision of an amblyopic eye and ensure proper binocular coordination. POLYGRAPH: An instrument for recording tracings of several different events simultaneously. PONS: A discernible enlargement of the lower brain lying immediately below the midbrain. POSTSYNAPTIC STRUCTURES: Neurons and their dendrites with which efferent terminals of presynaptic neurons make functional contact. PRAXIS: Motor performance or action, especially that controlled by the cerebral cortex. PRETECTAL SYSTEM : The system of nerve fibers from the optic tract and superior colliculus that reach the pretectal nuclei of the midbrain; thought to provide feedback pathways for regulation of eye movements and accommodation. PRIMARY HEALTH SERVICES: First point of contact for medical care or health supervision. In the case of children, these services are provided by pediatricians, general practitioners, public-health or school nurses, or hospital emergency staffs. PRIMARY STRIATE CORTEX: See striate area. PRIMARY VISUAL SYSTEM: The nerve system that conducts impulses from the retina to the cortical, thalamic, and collicular regions of the brain. PROPRIOCEPTIVE END ORGANS: Sensory nerve receptors that give information concerning movements and position of the body; found chiefly in muscles, ten- dons, and the vestibular apparatus. 503

Glossary PULVINAR SYSTEM: One of several nuclei in the thalamus, a main relay area in the midbrain for sensory input, and having projections to an important associa- tion area of the cerebral cortex. PUPILLARY MUSCLE: The smooth (involuntary) muscle of the iris that controls the size of the pupil. PYRAMIDAL CELL LAYER: The third layer from the surface of the primate striate area, characterized by relatively large pyramid-like cells. PYRAMIDAL DISCHARGES: Impulses discharged down the pyramidal nerve tracts from the motor cortex and concerned with voluntary motor activities. REACQUisixiON: Regaining, by reinforcement, a conditioned response that has been extinguished. READING DISABILITY (syn. reading deficiency, dyslexia): A condition in a person whose reading age is below 90% of expectation with respect to his intel- ligence and educational opportunity. See also specific reading disability. READING RETARDATION: A behavioral term to describe a person who does not score at or above the norm for his age on valid tests of reading. RECEPTIVE FIELD: A small region of the retina where a spot of light will cause impulses in a single optic-nerve or optic-tract fiber, in a single ganglion cell, or in a single nerve cell in the visual cortex. Other single units in these structures may also be activated by the same receptive field or by a different or overlapping retinal receptive field. RECEPTOR : A sensory nerve terminal that responds to stimuli; in the eye, refers to the rods and cones in the retina. RECTi: See extraocular muscles. REDUNDANCY: Superfluous linguistic elements or repetition of cues or informa- tion that do not need to be recognized in order to perceive the Gestalt. REFLECTANCE: The reflected fraction of the total luminous flux incident on a surface. RETICULAR FORMATION (syn. reticular system): A diffuse network or reticulum of nerve cells and fibers, forming the core of the brain stem, that when stimulated produce "arousal." This system is concerned with such general effects as alerting the entire organism, maintaining attention, controlling muscular activity, and regulating receptivity of peripheral sensory end organs. RETINAL CONTRAST: Physiologic processes in the retina that produce noticeable subjective differences between objects of different luminosities placed side by side. RETINAL RIVALRY: A condition in which one area of the retina is dominant and the corresponding retinal area of the other eye, recessive; spontaneous reversal of the condition is implied. ROD: A photoreceptor neuron characterized in part by an elongated shape that synapses with some bipolar cells and is used chiefly in night vision; anatomically 504

Glossary absent from the fovea centralis; in man, a fairly uniform cylinder 24 ^ long and 3 n wide. RUBIN'S GOBLET FIGURE: An ambiguous figure that can be seen either as a goblet or as two facial profiles. SACCADE: A jerk-like movement of the eyes horizontally in either direction as the fixation point changes, interspersed with pauses. A skilled reader will demon- strate smooth movements in one direction with few pauses and a regressive move- ment at the end of a line. SCHEMA (pi. schemata): A plan, outline, or arrangement. SCOTOM A: A blind spot; an area of depressed vision within the visual field, due to retinal damage or to specific lesion within the visual pathways or visual cortex and surrounded by an area of less depressed or normal vision. SEGMENT (syn. chunk): To break up continuous spoken sound into segments on the basis of knowledge of the speaker's language. SEMICIRCULAR CANALS: Within the vestibular apparatus, three canals each in a different plane at right angles to each other, and sensitive to movement of the endolymph contained within them due to change in position, acceleration, or convection. SEPARATION ANXIETY: The fear and apprehension noted in infants when re- moved from their mothers (or surrogates) or when approached by strangers. SHADOWING: Repetition of an auditory stimulus slightly after the original presentation. SHORT-TERM MEMORY: Retention of information for about a minute or less. SNELLEN FRACTION: A measure of visual acuity based on Snellen's test type; the numerator is the subject's distance from the test type, and the denominator, the distance at which the letters could be read by a normal eye. Thus 20/20 means normal vision in the eye being tested. SOMADENDRITIC FACTOR: In the neuronal tissue (soma), an undefined factor that determines the distribution of dendritic spines along the dendrites. SOMATESTHESIA (syn. somesthesia): The consciousness of having a body. SOUND SPECTOGRAPH: An instrument that portrays in graphic form the time variations of the frequency spectrum of the speech (sound) wave. SPECIFIC READING DISABILITY (syn. developmental reading disability, con- genital reading disability, strephosymbolia, specific dyslexia, congenital dys- lexia): A condition in which the reading age is below 90% of expectation with respect to measured intelligence and educational opportunity, but with no evi- dence of structural defect of the central nervous system and with peripheral sensory apparatus sufficiently intact for the recognition of sensory stimuli in- volved in reading. There is evidence that this is due to an inherited lag in the maturation of neurophysiologic functions (as yet unidentified) involved in reading. 505

Glossary SPECIFIC VISUAL AFFERENTS: Afferent fibers conducting nerve impulses from the visual photoreceptors (rods and cones) to the brain. S-R: Stimulus-response; usually refers to theories of the acquisition of information and learning. STABILIMETER: Apparatus for recording the amplitude and frequency of the motions of a subject. STELLATE CELLS: Star-shaped cells found chiefly in layer IV of the striate area and characterized by the horizontal arrangement of their processes. STEREOGNOSIS: Ability to perceive and recognize shape or form qualities of an object by handling it; tactile recognition of form. STEREOPSIS: The sensation of relative visual depth that results from the neural integration of dissimilarities in the images seen by the two eyes. STRABISMUS: Deviation of the eye that a subject cannot overcome; the visual axes of the two eyes assume a position relative to each other different from that required by the visual stimulus conditions. STRATIFIED RANDOM SAMPLING: Random drawing of individuals from each of several strata of the population, in contrast to sampling by chance alone. STREPHOSYMBOLIA: Perception of visual stimuli, especially words,in reverse order but without the reversal of individual letters characteristic of mirror perception. STRIATE AREA (syn. area 17, area striata, visual I, primary striate cortex): The primary visual receiving area or highest brain center of vision. A portion of the cerebral cortex in the inner, medial space of the occipital lobe of each cerebral hemisphere, where the fibers of the visual radiation terminate. SUBJECTIVE CONTRAST: The subjective experience produced by physiologic retinal and brain contrast mechanisms, distinguished from contrast in the visual stimulus itself. SUPERIOR COLLICULAR SYSTEM: The nerve system in the forward or rostral pair of the two pairs of rounded eminences (colliculi) in the roof of the mid- brain concerned primarily with regulation of ocular reflexes and adjustment of head and body position relative to visual orientation. SUPPRESSION : Overriding of one response by another, as distinct from inhibition of a response. In visual suppression, the specific mechanisms are varied and un- known, and the visual input in an eye may be entirely or partially blocked. SUPRAGRANULAR LAYERS: Layers I-III lying above or toward the surface from layerIV of the primate striate area, which is characterized by granular cells. SUPRANUCLEAR LESIONS: Lesions of higher (i.e., more centrally located) struc- tures than the nucleus associated with the nerve pathway involved. SURFACE STRUCTURE (syn. surface form): Elements of a sentence that deter- mine its phonetic interpretation, including the actual formatives, phonetic sig- nals, and combination of perceived utterances. 506

Glossary SURROUND FIELD: An area surrounding a center or receptive field where both on and off responses are obtained to light stimulation. SYNAPSE: Locus at which a nerve impulse passes from the axon of one neuron to the dendrites of another; forms boundary between the two nerve fibers or pos- sibly only a surface of contact. SYNTACTIC REDUNDANCY: Repetition of elements of syntax, the rules by which sentences are constructed. TACHiSTOSCOPE: An instrument for the very brief presentation of visual stimuli, such as dot patterns, words, numbers, or pictures of objects; it is capable of pre- senting stimuli to either eye at intervals of less than 0.001 sec. TEMPORAL LOBE: The lower lateral lobe of the cerebral hemisphere, merging behind with the occipital lobe and subserving the hearing and equilibration senses. TRANSNEURONAL DEGENERATION: Degeneration or atrophy of a neural pathway or structure when the neurons with which they make synaptic connec- tion are cut, or in some cases when the end organs and their afferent nerves are not used. TRIGGER FEATURE: A particular set of stimulus conditions capable of eliciting activity in a single unit in the retina or higher visual center. TROLAND: A unit of intensity of light at the retina equal to the illumination received per square millimeter of a pupillary area from a surface having a brightness of 1 candle per square meter. VESTIBULAR APPARATUS: The system of intercommunicating semicircular canals and the otolith organ within the inner ear (filled with endolymph) that forms a sense organ stimulated by gravity and rotational movements. VESTIBULAR MOVEMENTS: Reflex movements of the eyes initiated by im- pulses from the vestibular apparatus in the inner ear due to rotation of the head. VISUAL CLIFF: A sharp vertical discontinuity in visual space, such as the edge of a desk, either real or contrived by optical illusion. VISUAL CORTEX: See striatearea. VISUAL DISCRIMINATION: The differentiation between visual patterns. VISUAL FIELD: What a fixating eye can see. Visual fields for the two eyes are normally somewhat ovoid and overlap. VISUAL-MOTOR: Refers to the coordination of vision with movements of the body or of parts of the body. VISUAL NOISE: A visual stimulus having no recognizable organization, such as scattered parts of letters or numerals. VISUAL PLACING: A visually directed response consisting of placing of one or more of the extremities upon an object. VISUAL PROJECTIONS: Nerve fibers carrying visual impulses that originate in the retina and are directed or relayed to other parts of the brain. 507

Glossary VISUAL PROJECTION AREAS: Cortical areas, such as the striate area, to which fibers from the lateral geniculate nucleus project and transmit impulses. VISUAL RADIATION (syn. optic radiation): The paired nerve tracts or fiber sys- tem carrying visual impulses from the lateral geniculate nucleus to the highest brain center of vision, the striate area. VISUAL RESOLVING POWER: A measure of visual acuity or the ability of the eye to discriminate or resolve small visual stimulus differences. VISUAL II AREA: See area 18. 508