Maturational aspects of periodicity coding in cat primary auditory cortex

The click-following responses for single units in the primary auditory cortex of the cat were explored as a function of age. Recordings were obtained in kittens from 9-53 days of age and assembled in four age groups; 10-15 days, 16-21 days, 22-27 days and 30-60 days. Age group means were compared to results obtained in adult cats. The stimulus consisted of one second long click trains presented every three seconds with click rates ranging from 1-32 clicks per second. The response was characterized by entrainment, rate Modulation Transfer Function (rMTF), vector strength (VS) and temporal Modulation Transfer Function (tMTF). Maturational effects on periodicity coding comprised changes in overall responsiveness as well as click-rate dependent changes. The number of spikes elicited by single stimuli increased on average 3-fold between the second post-natal week and adulthood, probably as a result of more efficient synapses in the central auditory pathway and some improvement in thresholds. Adaptation became less pronounced with age; neurons started to respond to the later clicks in the 8/s and 16/s click trains from the third post natal week on. By the end of the first post-natal month the click following responses resembled the adult ones qualitatively, however, increased firing rates and spontaneous rates together with rebound responses continued to produce quantitative differences between the 30-60 days olds and the adults. Limiting rates for the tMTF (50% of the response at 1/s) increased from 6 Hz in the 10-15 day old to 12 Hz in adults. The decrease in the duration of the post-activation suppression coupled with the increased response with age to trains with higher click rates suggested that the maturation of inhibitory processes in the cortex play a major role in this rate dependence.