EFFECTS OF ADAPTATION ON NEURAL CODING BY PRIMARY SENSORY INTERNEURONS IN THE CRICKET CERCAL SYSTEM

Methods of stochastic systems analysis were applied to examine the eff ect of adaptation on frequency encoding by two functionally identical primary interneurons of the cricket cereal system. Stimulus reconstruc tions were obtained from a Linear filtering transformation of spike tr ains elicited in response to bursts of broadband white noise air curre nt stimuli (5-400 Hz). Each linear reconstruction was compared with th e actual stimulus in the frequency domain to obtain a measure of wavef orm coding accuracy as a function of frequency. The term adaptation in this paper refers to the decrease in firing rate of a cell after the onset or increase in power of a white noise stimulus. The increase in firing rate after stimulus offset or decrease in stimulus power is ass umed to be a complementary aspect of the same phenomenon. As the spike rate decreased during the course of adaptation, the total amount of i nformation carried about the velocity waveform of the stimulus also de creased. The quality of coding of frequencies between 70 and 400 Hz de creased dramatically. The quality of coding of frequencies between 5 a nd 70 Hz decreased only slightly or even increased in some cases. The disproportionate loss of information about the higher frequencies coul d be attributed in part to the more rapid loss of spikes correlated wi th high-frequency stimulus components than of spikes correlated with l ow-frequency components. An increase in the responsiveness of a cell t o frequencies >70 Hz was correlated with a decrease in the ability of that cell to encode frequencies in the 5-70 Hz range. This nonlinear p roperty could explain the improvement seen in some cases in the coding accuracy of frequencies between 5 and 70 Hz during the course of adap tation. Waveform coding properties also were characterized for fully a dapted neurons at several stimulus intensities. The changes in coding observed through the course of adaptiation were similar in nature to t hose found across stimulus powers. These changes could be accounted fo r largely by a change in neural sensitivity. The effect of adaptation on the coding of stimulus power was examined by measuring the response curves to steps in stimulus power before and after exposure to an ada pting stimulus. Adaptation caused a loss of information about the mean stimulus power but did not cause any improvement in the coding of cha nges in stimulus power. The unadapted response of the cells did not sh ow any saturation even at the highest powers used in these experiments .