Post-stimulatory suppression, facilitation and tuning for delays shape responses of inferior colliculus neurons to sequential pure tones.

Temporal changes in the excitability of inferior colliculus (IC) neurons will shape their responses to complex stimuli. Single-unit responses of rat IC neurons to the second (probe) of a pair of tones exhibited suppression, facilitation and delay tuned effects. Responses to probe tones were markedly suppressed (by 76% for contralateral stimulation with equal intensity tone pairs) during contralateral and binaural stimulation in 60% of IC neurons. Suppression developed rapidly as a function of the duration of the initial tone, and approached maximum for tones of less than 200 ms. Suppression decreased as the interval between tones increased, and this recovery of responsiveness was often exponential (time constants: mean: 271.4 ms; median: 72.8 ms; n = 47), and independent of the duration and intensity of preceding stimulation. Facilitation of responses to probe tones was observed chiefly in neurons with 'pauser/buildup' response patterns, and decreased as the intertone interval increased. The greatest suppression of responses to probe tones occurred only after intertone intervals of 32 ms (delayed minimum; n = 8) in 11% of IC neurons. Other IC neurons exhibited an increased excitability to probe tones presented 128 ms after stimulation (delayed maximum; n = 7). The latencies of the later neurons' responses were longer (mean: 29.5 ms) than other IC neurons. The role of suppression in sound localization and echo suppression, and the relationship between 'delay tuning' effects and encoding of complex stimuli are discussed.