Pulse repetition rate and duration affect the responses of bat auditory cortical neurons

Under free field stimulation conditions, we studied the effect of pulse repetition rate and duration on the responses of auditory cortical neurons of the big brown bat, Eptesicus fuscus. The best frequency (BF) and minimum threshold (MT) of each recorded neuron were first determined with a 4 ms pulse delivered from a specific point of the bat's frontal auditory space at which the neuron had maximal spatial sensitivity. Then a 4 ms pulse was delivered repetitively at different rates with an intensity in 10 dB increments above the neuron's MT and its number of impulses and response latency to each combination of repetition rate and intensity were recorded. From the series of isorepetition rate-intensity functions, we identified the neuron's best intensity (BI) which elicited a maximal number of impulses. By using a 4 ms BF and BI pulse delivered at a wide range of repetition rates, we then determined the neuron's best repetition rate (BRR) to which the neuron discharged maximally. The effect of pulse duration on the neuron's responses was subsequently studied by recording its number of impulses and latency to a BF, BI and BRR pulse delivered at a wide range of durations. A neuron's best duration (BD) to which the neuron discharged maximally was also identified. We found that cortical neurons sequentially isolated with an orthogonally penetrated electrode have comparable BFs, MTs and latencies. In addition, high BF neurons tend to have higher MTs and shorter latencies than low BF neurons. Most intensity-rate functions of cortical neurons were nonmonotonic but they can be affected by pulse repetition rate. Under repetitive single pulse stimulation, all cortical neurons show low pass characteristic to pulse repetition rate such that their number of impulses reduced drastically with increasing pulse repetition rate regardless of pulse intensity. The BRRs of most cortical neurons studied are between 1 and 3 pulses/s and their BDs are between 0.5 and 10 ms. Response latencies of cortical neurons generally shortened to a plateau level with increasing pulse intensity. In contrast, they increased with pulse repetition rate and varied in different manner with pulse duration. Nevertheless, a cortical neuron generally responds with a shortest latency to a pulse delivered at its BF, BI, BRR and BD. Thus in auditory signal processing, a neuron's responses to a wide combination of pulse intensity, duration and repetition rate enhance its versatility in signal detection. Its maximal response to a specific combination of signal duration, repetition rate and intensity provides a neural basis for fine analysis of signal features.