In quiet, peripheral auditory neurons discharge spontaneously at various rates [1, 30]. When a sound breaks the silence, the auditory neurons start to discharge specifically to the sound. The duration of the active discharge pattern usually represents the duration of the stimulus as well. However, a click that has an extremely short duration such as 25 to 100 microseconds ( s) can be presented in the auditory nerve fibers for as long as 25 milliseconds ( s) (e.g., [121]). The big duration discrepancy is remarkable.
6 10 3
10
As can be summarized from background information presented earlier, the post-stimulus neural latency pattern to a click is generally thought to be due to a resonant response of the sound transducer, filtered by the combined outer and middle ear, but thereafter dependent upon the
basilar membrane at the neuron’s characteristic frequency that is half-wave rectified by the inner-hair-cell and synapse system (e.g., [85, 93, 118]). Therefore, the click responses in peristimulus-time histogram show multiple peaks separated by the inverse of the neuron’s characteristic frequency, and the peaks from rarefaction and condensation clicks neatly interleave [85, 86, 117-121] (Figure 24A).
Figure 24. A single auditory neuron continues to respond specifically to the stimulus after the stimulus ends. Arrows indicate the time when the stimulus is gone. The latency pattern changes when the stimulus varies. A. Compound peristimulus-time histogram to rarefaction and condensation clicks. CF: 1.40 kHz. The inter-peak interval is 1/CF. Reprinted with permission from Figs. 1 and 2, Lin, T. and J. J. Guinan, Jr. (2000). "Auditory- nerve-fiber responses to high-level clicks: interference patterns indicate that excitation is due to the combination of multiple drives." J Acoust Soc Am 107(5 Pt 1): 2615-2630. Copyright 2000, Acoustic Society of America. B. Peristimulus-time histograms of a cochlear-nucleus neuron to two 50-ms tone bursts, 400 and 800 Hz, respectively. The inter-peak interval is the inverse of the stimulus frequency. CF: 4.3 kHz. Threshold: 10 dB SPL Spontaneous rate: 16.5 spikes/second. Reprinted from Fig. 7, Neuroscience 154(1): 87-98. Rhode, W. S. "Response patterns to sound associated with labeled globular/bushy cells in cat", Copyright (2008), with permission from Elsevier. C. Compound peristimulus-time histogram of a single auditory nerve fiber to click-pair stimuli (a complex sound) with click separation of 1.96 ms. The responses to both click pairs are different from those to single clicks and are different in between the polarities. The inter- peak interval is irregular after the stimulus ends (when the second click is gone). CF: 260 Hz. Reprinted with permission from Fig. 6, Goblick, T. J., Jr. and R. R. Pfeiffer (1969). "Time- domain measurements of cochlear nonlinearities using combination click stimuli." J Acoust Soc Am 46(4): 924-938. Copyright 1969, Acoustic Society of America.
However, the duration discrepancy also can be found in auditory neural responses to other stimuli such as tone bursts [112, 116] or complex sounds (e.g., click pairs [118]). The duration of active neural responses is longer than that of the stimulus.
To tone bursts, most studies in the literature used a tone burst of the same frequency as the neuron’s characteristic frequency to obtain the best synchronization of responses. It is impossible to interpret whether the post-stimulus responses are specific to the stimulus or not, because the stimulus frequency is the same as the neuron’s characteristic frequency.
There are not many studies focusing on and demonstrating post-stimulus neural latency pattern in various units to an identical tone burst. Carney and Yin transformed a tone burst into a click in several steps and recorded the latency patterns to these stimuli in a single auditory nerve fiber in cats [94]. Their results showed when the stimulus was varied from a tone burst to a click, the dominant component of the synchronized response moved from the frequency of the stimulus toward the characteristic frequency of the fiber.
Rhode recorded a cat’s single cochlear-nucleus neuron to various 50-ms tone bursts. The duration of active responses after the stimulus ended was about 5 ms (Figure 24B). The results suggested that the neuron was more likely to respond to the tone burst’s frequency after the stimulus ends than to its characteristic frequency [116] (which is 4.3 kHz).
To complex sounds, taking the study of Goblick and Pfeiffer [118] for an example, their results showed that a single auditory neuron responded to a click-pair stimulus (two same- polarity clicks with the same intensity, separated by 1.96 ms) with a special latency pattern. Although the single unit responded to a single click similarly with the classical pattern in Figure 24A, it responded to the click pair in a specific pattern (Figure 24C) in which the peaks are not separated by the inverse of the neuron’s characteristic frequency. The responses to the rarefaction-click pair were also different from those to the condensation-click pair. The auditory neuron continued to respond with stimulus-specific latencies after the second click ends, lasting for at least about 10 ms.
The examples in Figure 24 together show that an auditory neuron may continue to respond with different patterns to different stimuli, for about 5 to 10 ms (in these examples) after the stimulus ends. Thus in conventional view, the post-stimulus vibration of basilar membrane in
mammals must have different modes for different stimuli. But the underlying mechanism is unclear.
All of the above unique latency patterns come from the total response of pinna, middle ear, cochlear mechanics, and maybe the contribution from synaptic and neural transmission. Next, this final step will be reviewed. Synaptic and neural transmission may change the contribution proportion of parts of the pathway in the first spike latency.