Evidence for a behavioral significance of saccular acoustic sensitivity in humans

In this article the results are reported of an experiment to provide direct evidence for a perceptual and behavioral significance of human saccular acoustic sensitivity. Ten human subjects were stimulated monaurally with 100-ms trains of 10-ms tone pulses with pulse repetition rate of 40 Hz, and were required to rate the pleasantness of the stimuli on a nine-point scale. The design included three within-subject factors: carrier frequency (two levels, 200 and 4,000 Hz), intensity [13 levels from 55 to 115 dB(A) in 5-dB steps] and ear (left and right). For intensities above 90 dB myogenic vestibular evoked potentials (MVEP) were also obtained from the ipsilateral sternocleidomastoid muscle from which it was possible to obtain thresholds by linear regression of MVEP amplitudes against intensity. A further between-subjects factor was added which assessed subjects' attitude to vestibular sensations. The results indicate that across subjects there is a general trend of decreasing pleasantness with increasing intensity, but for the 200-Hz condition there is a significant positive departure from monotonicity in pleasantness (p<0.05) above the mean saccular threshold. However, when split by the between-subjects factor, the positive departure was only evident for those subjects who have a positive attitude to vestibular sensations (p < 0.01). Implications of these results for human responses to loud sound and the possible evolutionary significance of saccular acoustic sensitivity are discussed.