Abstract
When we touch an object, the skin copies its surface shape/texture, and this deformation pattern shifts according to the objects movement. This shift pattern directly encodes spatio-temporal “motion” information of the event, and has been detected in other modalities (e.g., inter-aural time differences for audition and first-order motion for vision). Since previous studies suggested that mechanoreceptor-afferent channels with small receptive field and slow temporal characteristics contribute to tactile motion perception, we tried to tap the spatio-temporal processor using low-frequency sine-waves as primitive probes in our previous study. However, we found that asynchrony of sine-wave pair presented on adjacent fingers was difficult to detect. Here, to take advantage of the small receptive field, we investigated within-finger motion and found above threshold performance when observers touched localized sine-wave stimuli with one finger. Though observers could not perceptually discriminate rightward from leftward motion, the adaptation occurred in a direction-sensitive way: the motion/asynchronous detection was impaired by adapting to asynchronous stimuli moving in the same direction. These findings are consistent with a possibility that human can directly encode short-range spatio-temporal patterns of skin deformation by using phase-shifted low-frequency components, in addition to detecting short- and long-range motion using energy shifts of high-frequency components.
Highlights
Tactile signals are sensed by mechanoreceptors distributed over the elastic surface of the body, i.e., the skin
We found that the performance of detecting asynchronous sine-wave stimuli from synchronous stimuli was significantly improved when the stimuli were presented on a single finger, but the performance was not as good as that with high-frequency components
The first experiment measured the threshold for discriminating an asynchronous stimulus from synchronous stimuli presented on the finger(s) and examined effects of the number of stimulated fingers and stimulators
Summary
Tactile signals are sensed by mechanoreceptors distributed over the elastic surface of the body, i.e., the skin. This shift is the source of the brain’s ability to know the location changes or movements of an object on the skin (Fig. 1B) In other modalities, this type of spatiotemporal input pattern (e.g., inter-aural time differences in audition; motion in vision) is considered to be detected by coincidence detectors with delay lines[1,2] or spatiotemporal energy detectors[3]. The frequency of vibration is within the low-frequency range preferred by www.nature.com/scientificreports/ We used this type of stimulus to psychophysically examine the human ability to detect spatiotemporal shifts of tactile inputs, and tactile movements, between adjacent fingers[28]. We tried to gain insight into the motion detection mechanism that takes low-frequency inputs (Fig. 1C) by presenting the sine-wave motion stimuli on a single finger. We conjecture that the tactile motion system may have a long-range motion mechanism and a short-range motion mechanism[29] (see e.g.,30 for a similar idea in vision), and that the non-PC channels with small receptive fields may feed inputs to the short-rage (within-finger) motion mechanisms but not to the long-range ones
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