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PARALLAX. The difference in the apparent direction or position of an object when viewed from different points. (See BINOCULAR PARALLAX.)
PART TASK SIMULATION. That type of simulation which gives the subject an opportunity to learn selected aspects of the total task.
PATTERN RECOGNITION. A human or machine process associated with identifying or classifying a set of input stimuli.
PERCENTILE THRESHOLD. Relates to the collective visual acuity of a group, stating the percentile probability of detection of a small object. Object size is expressed by its subtended angle at the eye. An important parameter in visual inspection for quality control. (See VISUAL ACUITY.)
PERCEPTION. (1) Perception is the process by which sensations are interpreted. (2) The awareness of external objects, qualities, or relations, which ensues directly upon sensory processes.
PERCEPTIVE DEAFNESS. Deafness from conditions involving the cochlear structures or auditory nerve. Persons so affected may be deaf only to high frequency sounds, or, alternatively, only to low frequencies. Perceptive deafness is considered to be occupational since it often occurs in persons working in noisy environments. Also called boilermaker’s deafness. (See CONDUCTIVE HEARING LOSS.)
PERCEPTUAL AUGMENTATION. A type of display aiding in which enriched perceptual information is provided to a performer.
PERCEPTUAL OVERLOAD. Input of too much information. The condition in which a person receives too much sensory information. The nervous system, on becoming saturated, is unable to process or understand the information received. The result of a perceptual overload is no reaction or response.
PERCEPTUAL-MOTOR TASK. A task involving an overt movement response to a nonverbal stimulus situation; the response is determined by the organization of sensory cues within the individual.
PERCEPTUAL SKILL. Relative ability to detect and interpret information received through the sensory channels; contrast with motor skill.
PERFORMANCE DECREMENT. A decrease in human proficiency often attributable to operator overload, stress, or fatigue, and characterized by increasing errors, misjudgments, omission of task elements, reduced intensity of effort, etc.
PERFORMANCE MEASURES. Objective and subjective measures developed to evaluate personnel effectiveness; objective measures include productivity, job knowledge tests, job performance samples, proficiency tests and checklists; subjective techniques include peer ratings, supervisory ratings, and self-ratings.
PERIPHERAL VISION. “Side” vision, primarily sensed by the rod cells. May produce an orienting reflex whereby the head turns and the visual object is focused on the fovea for optimal perception.
PERSONNEL SUBSYSTEM (PS). The segment of system management which provides the human performance necessary to operate, maintain, support, and control the system in its operational environment.
PHON. The unit of loudness level of sound, numerically equal to the sound pressure level in decibels, relative to 20µ N/m2 x (20µ Pa) of a simple 1000 Hz tone judged by listeners to be equivalent in loudness. Compare sone.
PHOTOPIC ADAPTATION. The decreased visual sensitivity to light, sometimes manifested by decreased perceived brightness of a fixed stimulus.
PHOTOPIC VISION. Vision associated with levels of illumination (luminance) about 30 cd/m2 or higher, characterized by the ability to distinguish colors and small details. Also called foveal vision.
PHOTORECEPTORS. Sensory end organs (q.v.) of the optic nerve. Elements of the retina called rods and cones (q.v.). They contain biochemicals which operate in the interpretation of perceived light. The industrial designer should be aware of the differences in these end organs. Rods are black/white discriminating, cones are color sensitive. Rods are effective in the peripheral visual field and color perception is limited to the central visual field.
PIE CHART. A circular, radially divided, graphical display indicating proportional data.
PILOT STUDY. An initial evaluation of the procedures of an experimental study prior to conducting the full study.
PIXEL. The smallest element of an image that can be individually processed in a video display system.
POPULATION STEREOTYPE. A consistent set of expectations within a specific social or cultural group. Distinguished from compatibility in that stereotypes are learned expectations, such as read meaning danger, up meaning more.
POP UP MENU. An overlay menu in a software application, activated by a command input.
POSITIONING MOVEMENT, PRIMARY. The first movement an operator makes in positioning a control; movement which carries the body member approximately to the point of aim; also called “gross adjustment” or “slewing.”
POSITIONING MOVEMENT, SECOND CORRECTIVE. That part of the positioning movement made to bring the body member into exact relation with the point of aim; also called “fine adjustment.”
POSITION, TRACKING. A tracking task in which movement of the operator’s control is associated with a direct displacement or movement of the tracking indicator, a linear relationship is implied between system error and control movement.
PRACTICE EFFECT. A change in performance due to repetitive experience at a task.
PREDICTOR DISPLAY. Shows the operator some aspect of what will happen to the system in the future. Such displays may be pursuit or compensatory, symbolic or pictorial; and they may predict the vehicle output, the system input, or both. The distinguishing feature is that they provide advanced information that allows the operator to anticipate future requirements for control movements.
PRESENCE. The degree to which experience in a virtual environment is felt as real.
PREVIEW CONTROL. Control involving the use of a predictor display.
PROACTIVE INHIBITION. The state or process hypothesized to account for the lessened ease of learning of the later members of a series following learning of an earlier member.
PROBLEM SOLVING. The generation of a set of actions to accomplish a goal.
PROBLEM SPACE. A representation of a problem solving task indicating the set of problem states and the actions which cause transitions between states.
PROCESS CONTROL. The control of a continuous state production system, such as industrial, chemical, and energy conversion processes.
PROPRIOCEPTION. The sensing of one’s location relative to the external environment. An important sense for maintenance of balance and for orienting one’s self for performing work tasks. An impaired proprioceptive sense can cause industrial accidents or faulty performance where controls are located outside the visual field.
PROPRIOCEPTOR. Any receptor sensitive to the position and movement of the body and its members, including (1) receptors in the vestibule of the inner ear and in the semicircular canals sensitive to the orientation of the body in space, and to bodily rotation, and (2) receptors in the muscles, tendons, and joints giving rise to kinesthetic sensations.
PROTOTYPING TOOLS. Software applications allowing rapid construction and testing of an initial design for an interface.
PSYCHOMETRICS. The measurement of psychological processes through application of mathematical and statistical techniques.
PSYCHOMOTOR ABILITY. Of or pertaining to muscular action ensuing directly from a mental process, as in the coordinated manipulation of aircraft or spacecraft controls.
PSYCHOMOTOR TASK. A task which involves precise coordination of a sensory or ideational process and a motor activity, e.g., a tracking task.
PSYCHOPHYSICAL METHODS. Standardized procedures for presenting stimulus material to a subject for judging and for recording the results. Originally developed for determining functional relations between physical stimuli and correlated sensory responses, but now used more widely.
PSYCHOPHYSICAL QUANTITY. A physical measurement, as a threshold; dependent on human attributes or perception.
PSYCHOPHYSICS. A branch of psychology that deals with the quantitative relationship between physical and psychological events.
PULL DOWN MENU. A menu at the top of a computer display, extending vertically, activated by a command input.
PURKINJE EFFECT. The response of the human eye which makes it less sensitive to lights of longer wavelengths under conditions of decreased illumination, e.g., red appears darker at night than blue having the same brightness under photopic conditions.
PURSUIT TRACKING. A task in which the subject is required to keep a response marker in line with a moving target.
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