Auditory responses from the cortex of the anaesthetized tree shrew

Species‐specific vocalizations, pure tones, and white noise were used to elicit responses in the primary auditory cortex of tree shrews (Tupaia) anaesthetized with urethane. Action potentials were recorded with 25 μ platinum iridium wire from small clusters of neurons. Almost all the cell groups responded to white noise and pure tones over a wide range of frequencies, and fewer responded to vocalizations. In contrast, studies on awake, unrestrained tree shrews using the same type of electrodes [J. A. Manley and P. M. Milner, J. Acoust. Soc. Am. 57, S54(A) (1975)] showed that almost all cell groups respond to vocalizations and considerably fewer to tones and noise. In both studies the cell groups responding to vocalizations responded to all or several calls and no real specificity was seen. Response patterns in the anaesthetized animal consisted mainly of phasic “onset,” “on‐off,” or “off” although sustained responses also occurred, whereas the awake animals gave almost entirely sustained responses. A shorter latency and less intertrial variability were seen in the anaesthetized preparation. Occasional cell groups exhibited reverberating bursts of activity. [Supported by a National Research Council of Canada grant to P. M. Miller.]