TIME-COURSE OF FORWARD MASKING TUNING CURVES IN CAT PRIMARY AUDITORY-CORTEX

Nonsimultaneous two-tone interactions were studied in the primary audi tory cortex of anesthetized cats. Poststimulatory effects of pure tone bursts (masker) on the evoked activity of a fixed tone burst (probe) were investigated. The temporal interval from masker onset to probe on set (stimulus onset asynchrony), masker frequency, and intensity were parametrically varied. For all of the 53 single units and 58 multiple- unit clusters, the neural activity of the probe signal was either inhi bited, facilitated, and/or delayed by a limited set of masker stimuli. The stimulus range from which forward inhibition of the probe was ind uced typically was centered at and had approximately the size of the n euron's excitatory receptive field. This ''masking tuning curve'' was usually V shaped, i.e., the frequency range of inhibiting masker stimu li increased with the masker intensity. Forward inhibition was induced at the shortest stimulus onset asynchrony between masker and probe. W ith longer stimulus onset asynchronies, the frequency range of inhibit ing maskers gradually became smaller. Recovery from forward inhibition occurred first at the lower- and higher-frequency borders of the mask ing tuning curve and lasted the longest for frequencies close to the n euron's characteristic frequency. The maximal duration of forward inhi bition was measured as the longest period over which reduction of prob e responses was observed. It was in the range of 53-430 ms, with an av erage of 143 +/- 71 (SD) ms. Amount, duration and type of forward inhi bition were weakly but significantly correlated with ''static'' neural receptive field properties like characteristic frequency, bandwidth, and latency. For the majority of neurons, the minimal inhibitory maske r intensity increased when the stimulus onset asynchrony became longer . In most cases the highest masker intensities induced the longest for ward inhibition. A significant number of neurons, however, exhibited l ongest periods of inhibition after maskers of intermediate intensity. The results show that the ability of cortical cells to respond with an excitatory activity depends on the temporal stimulus context. Neurons can follow higher repetition rates of stimulus sequences when success ive stimuli differ in their spectral content. The differential sensiti vity to temporal sound sequences within the receptive field of cortica l cells as well as across different cells could contribute to the neur al processing of temporally structured stimuli like speech and animal vocalizations.