Infrathreshold

Infrathreshold refers to stimuli whose intensity lies below the perceptual threshold of a given sensory modality. The perceptual threshold is the minimum stimulus level at which a typical observer can reliably detect or consciously report the stimulus under defined conditions. By definition, infrathreshold stimuli do not produce conscious percepts under ordinary testing, though they may interact with sensory pathways and evoke subcortical or cortical activity.

Thresholds are modality- and context-specific and can vary with stimulus duration, adaptation, attention, background noise, and individual differences. For example, an infrathreshold flash of light, a subthreshold tone, or a barely perceptible touch may be indistinguishable from noise in simple detection tasks but can influence processing in priming paradigms or be detected by more sensitive physiological measures. Thresholds are estimated with psychophysical methods such as the method of limits, staircase procedures, or adaptive titration, and analyses often employ signal detection theory to separate sensory sensitivity from response bias.

Terminology varies: infrathreshold is often used to denote subperceptual stimuli, while subthreshold and subliminal have related but not identical usages in different fields. Infrathreshold research contributes to understanding unconscious processing, sensory integration, and clinical assessment of sensory function. See also perceptual threshold, detection threshold, suprathreshold, subliminal perception, psychophysics, signal detection theory.