First-order and second-order spectral 'motion' mechanisms in the human auditory system

Light energy displaced along the retinal photoreceptor array leads to a perception of visual motion. In audition, displacement of mechanical energy along the cochlear hair cell array is conceptually similar but leads to a perception of 'movement' in frequency space (spectral motion)--a rising or falling pitch. In vision there are other types of stimuli that also evoke a percept of motion but do not involve a displacement of energy across the photoreceptors (second-order stimuli). In this study, we used psychophysical methods to determine if such second-order stimuli also exist in audition, and if the resulting percept would rival that of first-order spectral motion. First-order auditory stimuli consisted of a frequency sweep of sixteen non-harmonic tones between 297 and 12123 Hz. Second-order stimuli consisted of the same tones, but with a random subset turned on at the beginning of a trial. During the trial, each tone in sequence randomly changed state (ON-to-OFF, or OFF-to-ON). Thus, state transitions created a 'sweep' having no net energy displacement correlated to the sweep direction. At relatively slow sweep speeds, subjects readily identified the sweep direction for both first-order and second-order stimuli, though accuracy decreased for second-order stimuli as the sweep speed increased. This latter characteristic is also true of some second-order visual stimuli. These results suggest a stronger parallelism between auditory and visual processing than previously appreciated.