Stimuli

A stimulus is any detectable change in the environment or within an organism that can elicit a physiological or behavioral response. In biology, stimuli activate sensory receptors and trigger neural signals; in psychology, stimuli are used to study perception, attention, and learning.

Stimuli are commonly categorized as external or internal. External stimuli include light, sound, temperature changes, pressure, and chemical cues from the environment. Internal stimuli arise from the body's own states, such as hunger, thirst, pain, and visceral signals like heart rate or muscle stretch. Sensory modalities include visual, auditory, olfactory, gustatory, tactile, vestibular, proprioceptive, and interoceptive cues.

Receptors specialized for each modality transduce stimuli into electrical signals that travel to the brain for processing. The absolute threshold is the minimum stimulus needed for detection, while the difference threshold (the just noticeable difference) is the smallest detectable change in a stimulus.

Adaptation may reduce responsiveness to a constant stimulus, while attention and prior experience shape perception and reaction. Stimuli can produce reflexive responses or learned behaviours, depending on neural pathways and context.

In research and applied contexts, stimuli are the controllable inputs used to study perception, learning, and decision making. The term also covers conditioned stimuli in classical conditioning and subliminal or salient stimuli that influence behaviour, sometimes without conscious awareness.

Overall, stimuli are fundamental elements through which organisms detect and interpret their environment, guiding behavior and physiological regulation.