When the horseshoe crab is kept in constant darkness, the lateral eye produces larger electroretinographic and optic nerve responses at night than during the day. These circadian rhythms are mediated by synchronous bursts of efferent impulses in the optic nerve trunk. The endogenous efferent activity appears to increase both the gain and the quantum catch of the photoreceptors.

Frequency of vibration has not been widely used as a parameter for encoding speech-derived information on the skin. Where it has been used, the frequencies employed have not necessarily been compatible with the capabilities of the tactile channel, and no determination was made of the information transmitted by the frequency variable, as differentiated from other parameters used simultaneously, such as duration, amplitude, and location. However, several investigators have shown that difference limens for vibration frequency may be small enough to make stimulus frequency useful in encoding a speech-derived parameter such as the fundamental frequency of voiced speech. In the studies reported here, measurements have been made of the frequency discrimination ability of the volar forearm, using both sinusoidal and pulse waveforms. Stimulus configurations included the constant-frequency vibrations used by other laboratories as well as frequency-modulated (warbled) stimulus patterns. The frequency of a warbled stimulus was designed to have temporal variations analogous to those found in speech. The results suggest that it may be profitable to display the fundamental frequency of voiced speech on the skin as vibratory frequency, thought it might be desirable to recode fundamental frequency into a frequency range more closely matched to the skin's capability.

Adaptation of firing rates in auditory-nerve fibers appears to reflect two distinct processes. Rapid adaptation occupies the first few milliseconds of response and is superimposed upon short-term adaptation which has a time constant of about 40 ms. The properties of the two processes are reviewed and compared, and a phenomenological model is developed that successfully accounts for them. The model consists of several stages which have been tentatively associated with underlying physiological processes. In the first stage stimulus intensity is transformed by a static nonlinearity, followed by a low-pass filter. The filtered output may correspond to the hair-cell receptor potential. It modulates the release of a substance that possibly represents synaptic transmitter. Adaptation is produced by the depletion of transmitter which is located in three stores in cascade. A global store with fixed concentration controls the steady-state response and replenishes a local store which is responsible for short-term adaptation. The local store replenishes a rapidly depleted immediate store. Flow between stores is proportional to concentration gradients with the following exceptions. The immediate store is subdivided into independent volumes or sites and there is no flow among sites or back to the local store. A given site becomes activated only when the receptor potential exceeds its particular activation value and the number of activated sites is proportional to the receptor potential. The flow of transmitter from the immediate store is assumed to be proportional to neural firing rate, with some minor modifications described in the text. The properties of the model are determined from the underlying equations and from a computer simulation. The model produces realistic response properties including PST histograms, onset and steady-state rate-intensity functions, incremental and decremental responses, response modulation for amplitude modulated stimuli, and period histograms for low-frequency tones.

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Adaptation of auditory-nerve responses was investigated by applying increments and decrements in intensity to an ongoing tonal background. The change in firing rate produced by a change in intensity was obtained as a function of the time delay from the onset of the background to the onset of the change in intensity. The initial change in firing rate was measured using both small (1 ms) and large (10 ms) time intervals in order to evaluate properties of rapid and short-term adaptation, respectively. Consistent with previous results, the incremental and decremental responses measured with large windows were independent of time delay and the amount of prior adaptation. A similar additivity was observed for the incremental response measured with a small time window. In contrast, the decremental response measured with a small window decreased with increasing time delay and in proportion to the decrease in firing rate produced by the background. A similar decrease was observed in the response modulation produced by sinusoidal amplitude modulation. It was concluded that sensitivity to decrements in intensity decreases during adaptation, so that this response component does not reflect the additivity inherent in other aspects of adaptation.

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