[Coding of the acoustic information in the superior auditory centers]

The internal ear may be considered analysing acoustical signals in the frequency domain. This spectral analysis appears in the auditory pathways as a place code, each neuron being activated for a narrow and well defined frequency band. But in addition to this place code, temporal information on the phase and the period of low frequency signals is preserved in the low auditory centers. In the medial geniculate body, the last relay before the cerebral cortex, the place code shows the same properties as in lower centers but with a greater diversity in the response patterns and tuning properties. The tonotopic organization is less precise and, for the pars lateralis, follows the histological lamellar organization of this region. The most lateral laminae are composed of cells responding to low frequencies, the most medial ones of high frequency cells. In the auditory cortex intracellular recordings confirm the importance of an active inhibition underlying the diverse response patterns observed. Persistance of a time code is shown by certain cells presenting responses precisely time-locked to individual clicks in a train for rates ranging from 50 to 1000 Hz. Other cells respond selectively for certain click train frequencies without marking the temporal structure of the stimuli. Thus a temporal and a place code are still both present at the cortical level for this particular kind of signals.