On the mechanisms of call coding through auditory neurons in the squirrel monkey.

The main goal of the study was to investigate the neural processing of those acoustic signals through auditory neurons whose relevance for communication is either obvious or has been tested by psychoacoustic or behavioral experiments. Thus the activity of cortical, thalamic (MGB) and midbrain (IC) neurons of the auditory pathway were studied with periodically amplitude-modulated (AM) sounds, species-specific AM vocalizations and self-produced vocalizations. With regard to the processing of AM stimuli, there is evidence of a neural correlate to the psychoacoustic phenomenon "fluctuation strength": maximum of the Best Modulation Frequency (BMF) for the cortex was registered at 4 Hz. Furthermore, a relatively large number of units within the IC and the MGB can encode such amplitude changes which have been shown to be of communicative function; here too a neural correlate to the encoding processes of species-specific calls was indicated. Self-produced vocalizations do not seem to underlie a specific processing except that in higher auditory structures, they evoke quantitatively lower responses. In the midbrain, such less active areas are rare and were localized in regions belonging more to secondary auditory structures than primary ones.