Effect of signal temporal shaping on the frequency specificity of the action potential in gerbils

We examined the effects of temporal shaping of a tone burst on the frequency specificity of evoked responses in gerbils. Frequency specificity was assessed by means of whole-nerve action potential (AP) tuning curves (APTCs). The envelopes of the onset and offset portions of the AP-eliciting tone burst (or probe) were either a linear or nonlinear (1/4 sine) function of time. At probe frequencies of 1, 4 and 8 kHz, APTCs in the two envelope conditions were similar in shape and had equally narrow tips. This was true at both low (40-50 dB SPL) and high (70-75 dB SPL) probe levels. It suggests that tone bursts in the two envelope conditions evoke AP responses that are equally frequency specific. Despite these similarities, a notable difference in the two conditions was found at high probe levels. Specifically, raising the level of an 8-kHz probe from 40 to 70 dB SPL resulted in a downward shift of the APTC tip frequency, and this shift was greater for probes with linear on-off envelopes than for those with nonlinear on-off envelopes. This result suggests that of the two waveform shapes, the nonlinear envelope is likely to produce responses at frequencies closer to the nominal signal frequency. Reducing the rise-fall time of the probe from 2.0 to 0.75 ms caused the tip of the 8-kHz APTC to undergo an even greater displacement in frequency, indicating a greater 'off-frequency' response for briefer rise times.