TEMPORAL CODING OF AMPLITUDE AND FREQUENCY-MODULATION IN THE RAT AUDITORY-CORTEX

The rat primary auditory cortex was explored for neuronal responses to pure tones and sinusoidally amplitude-modulated (SAM) and frequency-m odulated (SFM) stimuli. Units showed phase-locked responses to SAM sti mulation (55%) and SFM stimulation (80%), with modulation frequencies up to 18 Hz. Tuning characteristics to the modulation frequency were m ainly band-pass with best modulation frequencies (BMFs) between 4 and 15 Hz. Units with synchronized activity to SFM stimulation showed thre e response types with respect to the direction of the frequency modula tion: 52% were selective to the upward direction, 30% to the downward direction, and 18% had no preference. Triangular frequency modulations were used to test if units were tuned to specific modulation frequenc ies or to specific rates of frequency change. In the vast majority of units tested the response characteristics were strongly influenced by varying the modulation frequency, whereas varying the rate of frequenc y change had little effect in the stimulus range used. Units that show ed phase-locked responses to SAM and SFM stimulation had similar activ ity patterns in response to both types of stimuli. BMFs for SAM and SF M stimulation were significantly correlated. Intrinsic oscillations of up to 20 Hz could be seen in the spontaneous activity and after the s timuli independent of the stimulus type. Oscillation frequencies were significantly correlated with the BMFs of the respective units. The re sults are discussed in terms of a mechanism for periodicity detection based on a temporal code. This could be important for the recognition of complex acoustic signals.