- 1. The point at which a stimulus first produces a sensation. 2. The lower limit of perception of a stimulus. 3. The minimal stimulus that produces excitation of any structure; e.g., the minimal stimulus eliciting a motor response. SYN: limen (2) [TA]. [A.S. therxold]- brightness difference t. the smallest difference that can be perceived as a difference in brightness. SYN: light difference (2).- convulsant t. the smallest amount of stimulation, electric current, or drug required to induce a convulsion.- differential t. the lowest limit at which two stimuli can be differentiated. SYN: t. differential.- double-point t. the least degree of separation of two points applied to the body surface that permits their being felt as two.- erythema t. the dose at which erythema of the skin is produced by irradiation with ultraviolet, gamma, or x-rays.- t. of nose SYN: limen nasi.- phenotypic t. a quantitative genetic trait with a continuous distribution termed its liability; may generate two kinds of phenotype, according to whether the liability lies above or below some critical t. at about which a radical change in behavior occurs. For instance, blood uric acid level is a liability with an approximately gaussian distribution. At a critical point of chemical saturation (the t.), crystallization occurs and the resulting gout or nongout is a t. trait.- relational t. the smallest degree of difference between two stimuli that permits them to be perceived as different.- speech awareness t. the lowest sound intensity at which speech can be detected. SYN: speech detection t..- speech reception t. the intensity at which speech is recognized as meaningful symbols; in speech audiometry, it is the decibel level at which 50% of spondee words can be repeated correctly by the subject.- swallowing t. 1. the moment that the act of swallowing begins after the mastication of food; 2. the critical moment of reflex action initiated by minimum stimulation, prior to the act of deglutition.- visual t., t. of visual sensation the minimal light intensity evoking a visual sensation. SYN: achromatic t., minimum light t..
* * *thresh·old 'thresh-.(h)ōld n the point at which a physiological or psychological effect begins to be produced (as the degree of stimulation of a nerve which just produces a response or the concentration of sugar in the blood at which sugar just begins to pass the barrier of the kidneys and enter the urine) <below the \threshold of consciousness> <the \threshold of pain> <a high renal clearance \threshold> called also limen
* * *n.(in neurology) the point at which a stimulus begins to evoke a response, and therefore a measure of the sensitivity of a system under particular conditions. A thermoreceptor that responds to an increase in temperature of only two degrees is said to have a much lower threshold than one that will only respond to a change in temperature of ten degrees or more. In this example the threshold can be measured directly in terms of degrees.
* * *thresh·old (threshґōld) 1. the minimum level of input required to cause some event to occur. 2. limen; that value at which a stimulus just produces a sensation, is just appreciable, or comes just within the limits of perception; see also absolute t. and differential t. 3. renal t. 4. in anatomy, limen (def. 2).
Medical dictionary. 2011.