Temporal factors in tactile spatial acuity: evidence for RA interference in fine spatial processing

We investigated the extent to which subjects' ability to perceive the fine spatial structure of a stimulus depends on its temporal properties (namely the frequency at which it vibrates). Subjects were presented with static or vibrating gratings that varied in spatial period (1-8 mm) and vibratory frequency (5-80 Hz) and judged the orientation of the gratings, presented either parallel or perpendicular to the long axis of the finger. We found that the grating orientation threshold (GOT)-the spatial period at which subjects can reliably discriminate the orientation of the grating-increased as the vibratory frequency of the gratings increased. As the spatial modulation of SA1 and RA afferent fibers has been found to be independent of vibratory frequency, the frequency dependence of spatial acuity cannot be attributed to changes in the quality of the peripheral signal. Furthermore, we found GOTs to be relatively independent of stimulus amplitude, so the low spatial acuity at high flutter frequencies does not appear to be due to an inadequacy in the strength of the afferent response at those frequencies. We hypothesized that the RA signal, the strength of which increases with vibratory frequency, interfered with the spatially modulated signal conveyed by SA1 fibers. Consistent with this hypothesis, we found that adapting RA afferent fibers improved spatial acuity, as gauged by GOTs, at the high flutter frequencies.