Academic journal article Perception and Psychophysics

Vibrotactile Intensity and Frequency Information in the Pacinian System: A Psychophysical Model

Academic journal article Perception and Psychophysics

Vibrotactile Intensity and Frequency Information in the Pacinian System: A Psychophysical Model

Article excerpt

The objective of the study was to characterize the Pacinian representation of stimulus waveform. Subjects were presented with pairs of high-frequency vibrotactile stimuli that varied in intensity and/or frequency content and made same-different judgments under conditions of low-frequency adaptation designed to minimize the contribution of the RA system. We wished to infer the nature of the information conveyed by the Pacinian system about the stimuli from measured sensitivity (d') to stimulus differences. We first tested the hypothesis that the Pacinian system conveys only intensive information about vibratory stimuli and found that intensive cues could not account for much of the variance in the discrimination data. We then proposed a model characterizing the Pacinian-mediated representation of an arbitrary stimulus as a pattern of activation in a set of frequency-tuned minichannels. The model was shown to predict the discriminability of the stimulus pairs presented in the psychophysical experiments. Furthermore, the model parameters, optimized to fit the discrimination data, were compatible with analogous values obtained in other experimental contexts. One of the assumptions underlying the model is that information about individual spectral components is conveyed in parallel and quasi-independently. By simulating the response of a population of Pacinian afferents to a polyharmonic stimulus, we demonstrated that such a population can simultaneously convey information about multiple frequency components, despite having a homogeneous spectral profile.

The Pacinian system has been found to mediate the tactile perception of high-frequency vibrations (Bolanowski, Gescheider, Verrillo, & Checkosky, 1988; Horch, 1991; Hyvärinen, Sakata, Talbot, & Mountcastle, 1968; Mountcastle, Talbot, Darian-Smith, & Kornhuber, 1967; Mountcastle, Talbot, Sakata, & Hyvärinen, 1969; Ochoa & Torebjörk, 1983; Talbot, Darian-Smith, Kornhuber, & Mountcastle, 1968; Verrillo, 1966). However, despite the abundance of studies on this sensory channel, few have investigated its ability to convey information about nonsinusoidal waveforms (see Bensmaïa & Hollins, 2000; Formby, Morgan, Forrest, & Raney, 1992; Horch, 1991; Lamoré, Muijser, & Keemink, 1986; Morley, Archer, Ferrington, Rowe, & Turman, 1990; Weisenberger, 1986). In a previous study (Bensmaïa & Hollins, 2000), we found that subjects could not use Pacinian signals to discriminate between diharmonic stimuli that differed only in the phase angle of the high-frequency component with respect to the low-frequency component. The fact that phase relations among frequency components are ignored suggests that spectral composition, rather than waveform per se, plays a primary role in shaping the Pacinian representation of a complex wave. Beyond that, the degree to which the Pacinian system can convey information about complex vibratory stimuli is as yet unspecified.

Even with regard to the discriminability of sinusoidal waveforms within the Pacinian range, the literature is inconclusive. An early study by Goff (1967) showed frequency discrimination to be rather poor and to grow worse (i.e., Weber fractions increased) as the standard frequency increased; once vibrotactile channel specificity had been established (Verrillo, 1968), Goff's results seemed to suggest that the Pacinian channel, when isolated, was capable of mediating only rudimentary frequency discrimination. In later work, using improved stimulus control, the Weber fraction was found to remain roughly constant across the high-frequency portion of the spectrum (Mountcastle, Steinmetz, & Romo, 1990; Mountcastle et al., 1969). Perhaps the most remarkable report of frequency discrimination was that of Franzén and Nordmark (1975), who found Weber fractions as low as 0.03 at frequencies up to 384 Hz; but their stimuli were brief, half-wave rectified blips, rather than sinusoids.

In the present study, we measured subjects' ability to distinguish between pairs of high-frequency simple and polyharmonic stimuli. …

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