Representation of the purr call in the guinea pig primary auditory cortex.

Guinea pigs produce the low-frequency purr or rumble call as an alerting signal. A digitised example of the call was presented to anaesthetised guinea pigs via a closed sound system while recording from the primary auditory cortex. The exemplar used in this study had 9 regular phrases each spaced with their centres about 80 ms apart. Low-frequency (1.1 kHz) units responded best to the call but within this population there were four separate groups: (1) cells that responded vigorously to many or all of the 9 phrases; (2) cells that gave an onset response; (3) cells that only responded to a click embedded in the call; (4) cells that did not respond. Particular response types were often grouped together. Thus when orthogonal electrode tracks were used most units gave a similar response. There was no correlation between the type of response and the cortical depth. A similar range of response types was also found in the thalamus and there was no evidence of a distinct response in the cortex that was due to intracortical processing. Cells in the cortex were able to represent the temporal structure of the purr with the same fidelity as cells in the thalamus.