Human Skin Perception of Traveling Waves Similar to Those on the Cochlea

Abstract

The membrane of a dimensional model of the mechanical parts of the human cochlea (30 cm in over-all length) showed under stroboscopic illumination traveling waves similar to those on the cochlea and a flat maximum of vibration which shifted from one end of the model to the other when the frequency was increased from 40 cps to 160 cps. When the skin of the arm is placed in close contact with the membrane, the vibrations are localized in a quite narrow section of the membrane, so that any change in frequency can be detected by a change in the locus of stimulation. The localization is as sharply defined for pure tone pulses of the length of two sine waves as for continuous tones. The localization of pure tones along the membrane is practically instantaneous. The sharp localization in spite of the flat distribution of vibrations along the membrane is produced by the inhibitory effects of the skin. Their influence and magnitude are demonstrated by various experiments. Some basic relations between nerve density on the cortex, threshold, and loudness increase on the skin are pointed out in the hope that they will later enable us to determine the differences between the organ of Corti and the simple skin membrane. This would be helpful to our understanding of the nature and function of the complicated structure of the organ of Corti. On the whole, in the observation of pitch of pure tones, the simple skin-model is as useful as the organ of Corti itself. The differences become apparent only in the observation of more complex sounds. Two models were used to investigate the possibility of obtaining effects similar to the stereophonic effects in hearing.