INFLUENCE OF ADAPTATION ON NEURAL SENSITIVITY TO TEMPORAL CHARACTERISTICS OF SOUND IN THE DORSAL MEDULLARY NUCLEUS AND TORUS-SEMICIRCULARIS OF THE GRASSFROG

The influence of long-term (~ minutes) adaptation on the responses of 15 dorsal medullary nucleus (DMN) and 36 torus semicircularis (TS) neurons of the grassfrog to temporally structured stimuli has been investigated. The neurons were stimulated with periodic click trains and sine amplitude modulated (AM) tone bursts presented in isolation or embedded in a randomly structured background. A tone burst with the envelope of the conspecific mating call was presented in addition. The stimulus-response relation has been analyzed by peri-stimulus time histograms (PSTH), average rate and synchronization index histograms. Global response types based on average rate and synchronization index were altered by adaptation in about half of the units. The trend was to enhance selectivity for temporal frequency and to increase response synchronization. Long-term adaptation had a moderate to dramatic effect on detailed response structure, as expressed by the PSTH, in 86% of the DMN and 96% of the TS units. In general the adaptational influence was somewhat larger for AM tones than for clicks. The observed effects could not be explained by a simple neural threshold shift. In almost all units the response to periodic sound bursts can be regarded as a modulation below and above an average response level induced by the random background. The response to the mating call resemblant tone burst was clearly distinct from the responses to any of the sine AM tone bursts and to the background in 20% of the units. The dependency of functional neural characteristics on long-term adaptation, has important implications for system-theoretical stimulus-response analysis procedures.