Effect of tone-pulse rise time on rate-level functions of cat auditory cortex neurons: excitatory and inhibitory processes shaping responses to tone onset.

1. The responses of cat auditory cortex neurons are largely dominated by transient stimulus events, including tone-pulse onset. In addition, these neurons often receive sensitive inhibitory inputs in tone frequency-intensity domains flanking the excitatory one centered at characteristic frequency (CF). These observations suggest that auditory cortex neurons might be sensitive to the spectral splatter that occurs at tone onset due to the tone-pulse envelope shape. 2. To investigate this hypothesis, single neurons in the primary auditory cortex of anesthetized cats were studied for the form of their spike-rate versus tone-level functions using CF tone pulses of different rise times. Stimuli were presented to the contralateral ear using a calibrated, sealed stimulus delivery system. 3. Some neurons with monotonic rate-level functions for conventional (5-10 ms) rise-time tones were relatively insensitive to variations in tone-pulse rise time. Other monotonic neurons showed rate-level functions that became increasingly bell shaped for shorter rise-time stimuli. All neurons with bell-shaped, nonmonotonic rate-level functions for conventional rise-time tones became increasingly nonmonotonic for shorter rise-time signals. In the same neurons, lengthening of tone rise times typically reduced the slope of the high-intensity, descending limb of the rate-level function, in some cases to zero. 4. This pattern of rise-time effects is consistent with previous evidence on the association between rate-level function shape and the presence of inhibitory tone response areas flanking the excitatory one at CF. The present data suggest that cortical neurons are sensitive to the gross shape of the short-term stimulus spectrum at tone onset, and that for many neurons, the nonmonotonic form of CF tone rate level functions may be configured as much by the rate of tone onset as by the plateau amplitude of a tone pulse.