Rate and synchronization measures of periodicity coding in cat primary auditory cortex

Periodicity coding was studied in primary auditory cortex of the ketamine anesthetized cat by simultaneously recording with two electrodes from up to 6 neural units in response to one second long click trains presented once per 3 s. Trains with click rates of 1, 2, 4, 8, 16 and 32/s were used and the responses of the single units were quantified by both rate measures (entrainment and rate modulation transfer function, rMTF) and synchronization measures (vector strength VS and temporal modulation transfer functions, tMTF). The rate measures resulted in low-pass functions of click rate and the synchrony measures resulted in band-pass functions of click rate. Limiting rates (-6 dB point of maximum response) were in the range of 3-24 Hz depending on the measure used. Best modulating frequencies were in the range of 5-8 Hz again depending on the synchrony measure used. It appeared that especially the VS was highly sensitive to spontaneous firing rate, duration of the post click suppression and the size of the rebound response after the suppression. These factors were dominantly responsible for the band-pass character of the VS-rate function and the peak VS frequency was nearly identical to the inverse of the suppression period. It is concluded that the use of the VS and to a lesser extent also the tMTF as the sole measure for the characterization of periodicity coding is not recommended in cases where there is a strong suppression of spontaneous activity. The combination of entrainment and tMTF appeared to characterize the periodicity coding in an unambiguous way.