No abstract

Human subjects were suspended in a safety harness 28 cm above the floor by a steel cable connected to a computer controlled force generator (electromagnetic brake). After the subjects were unexpectedly released, various controlled patterns of downward acceleration (less than 1 g) could be produced. During the falls, EMG activity was recorded simultaneously from the gastrocnemius, soleus, tibialis anterior, rectus femoris, and biceps femoris, along with knee and ankle joint angle in one leg. Subjects were tested eyes closed and also eyes open, both in darkness and in light using a wide field visual display. The display scene could be moved downwards at exactly the same velocity as the moving subject, left fixed with respect to the laboratory ("normal" visual field), or moved upwards at a speed equal to the subject's falling speed (upward moving visual field). Ten vestibularly normal subjects each underwent a total of 45 drops, experiencing three replications of each vision/motion combination used. Under normal visual field conditions, both short and long latency postural responses were seen, which were dependent on the magnitude of the acceleration stimulus. Several of the visual conditions significantly altered both the short and the long latency responses in most of the muscles tested. Effects were particularly prominent in the gastrocnemius and soleus, and were also more pronounced during slow (0.5 g) falls. The upward moving visual field condition increased the short latency EMG reaction in gastrocnemius and soleus for 0.5 g falls. A preliminary scheme for visual-vestibular interaction in short latency EMG responses is presented. Long latency responses are more variable and not conducive to a simple interpretation.

Pitch phenomena related to the periodic interruption rate of white noise have traditionally been regarded as evidence for time domain pitch processing in the auditory system since spectra of these signals are flat. Short-term spectra, however, contain information about the interruption rate. Melodic interval identification experiments were performed with sounds comprising low-pass filtered noise modulated by either a sine wave, a square wave, or a periodic narrow pulse wave. Melodic intervals were generated by varying the modulation frequency fm while the low-pass cutoff frequency fco of the noise was an experimental variable. The correct identification score shows a particular dependence on the ratio fco/fm for each type of modulation signal. Shapes and relative positions of performance functions are compared with predictions derived from an energy detector (time domain) model and a short-term power spectrum correlation (frequency domain) model, and are found to support the former more strongly than the latter. There is some evidence, however, for the existence of both types of processing in the auditory system.

No abstract

The pitch of periodically chopped white noise has been the object of several investigations in the literature. Because of the essentially flat spectrum of such a sound, any musical pitch effects are difficult to account for with a place model. Previous investigations, mostly involving pitch matching experiments, have left some doubt whether observed pitch effects were indeed factual or artifactual [Pollack, J. Acoust. Soc. Am. 45, 237 (1969)]. A series of musical interval identification experiments was performed using interrupted gaussian wide-band noise whose interruption rate was given by the notes to be presented. Intensities were 30 dB SPL, the noise on-time fraction was 0.5 and gating frequencies varied between 100 and 300 Hz. In this range, the pitch confusion data are well described by a simple Gaussian decision model whose variances are somewhat larger than the data scatter in previously reported pitch matching experiments. However, pitch identification performance with chopped noise is much worse than the same subjects' identification performance with pure tones or other periodic signals, which seems to indicate that, at least from a musical standpoint, we are dealing with a rather weak effect. [Work supported by the National Institutes of Health, grant 1RO1 NS 11680-01.]