Temporal integration of sound pressure determines thresholds of auditory-nerve fibers

Current propositions of the quantity of sound driving the central auditory system, specifically around threshold, are diverse and at variance with one another. They include sound pressure, sound power, or intensity, which are proportional to the square of pressure, and energy, i.e., the integral of sound power over time. Here we show that the relevant sound quantity and th e nature of the threshold can be obtained from the timing of the first spik e of auditory-nerve (AN) fibers after the onset of a stimulus. We reason th at the first spike Is triggered when the stimulus reaches threshold and occ urs with fixed delay thereafter. By probing cat AN fibers with characterist ic frequency tones of different sound pressure levels and rise times, we sh ow that the differences in relative timing of the first spike (including la tencies > 100 msec of fibers with low spontaneous rates) can be well accoun ted for by essentially linear integration of pressure overtime. The inclusi on of a constant pressure loss or gain to the integrator improves the fit o f the model and also accounts for most of the variation of spontaneous rate s across fibers. In addition, there are tight correlations among delay, thr eshold, and spontaneous rate. First-spike timing cannot be explained by mod els based on a fixed pressure threshold, a fixed power or intensity thresho ld, or an energy threshold. This suggests that AN fiber thresholds are best measured in units of pressure by time. Possible mechanisms of pressure int egration by the inner hair cell-AN fiber complex are discussed.