The responses of single units in the inferior colliculus of the guinea pigto damped and ramped sinusoids

Temporal asymmetry can have a pronounced effect on the perception of a sinu soid. For instance, if a sinusoid is amplitude modulated by a decaying expo nential that restarts every 50 ms, (a damped sinusoid) listeners report a t wo-component percept: a tonal component corresponding to the carrier and a drumming component corresponding to the envelope modulation period. When th e amplitude modulation is reversed in time (a ramped sinusoid) the percepti on changes markedly, the tonal component increases while the drumming compo nent decreases. The long-term Fourier energy spectra are identical for damp ed and ramped sinusoids with the same exponential half-life. Modelling stud ies suggest that this perceptual asymmetry must occur central to the periph eral stages of auditory processing (Patterson and Irino, 1998). To test thi s hypothesis, we have recorded the responses of single units in the inferio r colliculus of the anaesthetised guinea pig. We divided single units into three groups: onset, on-sustained and sustained, based on their temporal ad aptation properties to suprathreshold tone bursts at the unit's best freque ncy. The asymmetry observed in the neural responses of single units was qua ntified in two ways: a simple total spike count measure and a ratio of the tallest bin of the modulation period histogram to the total number of spike s. Responses were more diverse than those observed with similar stimuli in a previous study in the ventral cochlear nucleus (Pressnitzer et al., 2000) . The main results were: (1) The shape of the responses of on-sustained uni ts to ramped sinusoids resembled the shape of the responses to damped sinus oids. This is in contrast to the response shapes in the VCN, which were alw ays similar to the stimulating sinusoid. (2) Units classified as onsets oft en responded only to the damped stimuli. (3) All units display significant asymmetry in discharge rate for at least one of the half-lives tested and 2 0% showed significant response asymmetry over all of the half-lives tested. (4) A summary population measure of temporal asymmetry based on total spik e count reveals the same pattern of results as that obtained psycho physica lly using the same stimuli (Patterson, 1994a). (C) 2001 Elsevier Science BN . All rights reserved.