Scinob Kuroki

Directional remapping in tactile inter-finger apparent motion: a motion aftereffect study

Tactile motion provides critical information for perception and manipulation of objects in touch. Perceived directions of tactile motion are primarily defined in the environmental coordinate, which means they change drastically with body posture even when the same skin sensors are stimulated. Despite the ecological importance of this perceptual constancy, the sensory processing underlying tactile directional remapping remains poorly understood. The present study psychophysically investigated the mechanisms underlying directional remapping in human tactile motion processing by examining whether finger posture modulates the direction of the tactile motion aftereffect (MAE) induced by inter-finger apparent motions. We introduced conflicts in the adaptation direction between somatotopic and environmental spaces by having participants change their finger posture between adaptation and test phases. In a critical condition, they touched stimulators with crossed index and middle fingers during adaptation but with uncrossed fingers during tests. Since the adaptation effect was incongruent between the somatotopic and environmental spaces, the direction of the MAE reflects the coordinate of tactile motion processing. The results demonstrated that the tactile MAE was induced in accordance with the motion direction determined by the environmental rather than the somatotopic space. In addition, it was found that though the physical adaptation of the test fingers was not changed, the tactile MAE disappeared when the adaptation stimuli were vertically aligned or when subjective motion perception was suppressed during adaptation. We also found that the tactile MAE, measured with our procedure, did not transfer across different hands, which implies that the observed MAEs mainly reflect neural adaptations occurring within sensor-specific, tactile-specific processing. The present findings provide a novel behavioral method to analyze the neural representation for directional remapping of tactile motion within tactile sensory processing in the human brain.

Apparent time interval of visual stimuli is compressed during fast hand movement.

The influence of body movements on visual time perception is receiving increased attention. Past studies showed apparent expansion of visual time before and after the execution of hand movements and apparent compression of visual time during the execution of eye movements. Here we examined whether the estimation of sub-second time intervals between visual events is expanded, compressed, or unaffected during the execution of hand movements. The results show that hand movements, at least the fast ones, reduced the apparent time interval between visual events. A control experiment indicated that the apparent time compression was not produced by the participants’ involuntary eye movements during the hand movements. These results, together with earlier findings, suggest hand movement can change apparent visual time either in a compressive way or in an expansive way, depending on the relative timing between the hand movement and visual stimulus.

Contribution of within- and cross-channel information to vibrotactile frequency discrimination

Vibrotactile stimuli normally activate multiple information-processing channels starting from different types of mechanoreceptors, and the most sensitive channel alternates depending on the range of vibration frequency. How the tactile system encodes vibration frequency using within-channel information (e.g., the temporal pattern of neural activity of each channel) and/or cross-channel information (the relationship between activities of channels) has been examined, and the usefulness of the former has been evidently shown, while that of the latter remains controversial. To see the contribution of within- and cross-channel information to vibrotactile frequency encoding, we investigated frequency discrimination for near-threshold vibration, which can activate the channels more selectively than conventional supra-threshold vibration. At near-threshold intensity, the contribution of cross-channel information, if it exists, would be observed only when two vibrations activate different channels. In the first experiment, we examined the frequency discrimination thresholds for a wide range of frequencies. The results did not show a clear contribution of the cross-channel information, though that of the within-channel information was evident. In the second experiment, we compared the signal detection threshold and the frequency identification threshold between frequency pairs. We found that for certain pairs of stimuli whose frequencies were far enough apart from each other to activate different channels, each stimulus was identified as soon as it was detected. This suggests that each channel is a labeled line as expected from cross-channel encoding of vibration frequency, but not all data were consistent with this idea. We conclude that though a labeled-line structure might exist as a basis of cross-channel encoding, the cross-channel information does not play a dominant role in frequency discrimination.

Physical-Perceptual Correspondence for Dynamic Thermal Stimulation

Thermal displays have been applied in various haptic applications, from material simulation to interpersonal communication; however, there is insufficient knowledge about the temporal processing in human thermal sense to provide a　knowledge basis for thermal display design. In this study, we investigated the physical-perceptual correspondence for dynamic thermal stimulation to shed a light on the temporal processing of human thermal sense. In the experiments, participants reported subjective timings of the temperature onset and temperature peak of continuous temperature changes applied to the thenar eminence. We found that the physical-perceptual correspondence was not consistent for warm and cold stimulations. For warm stimulation, the subjective experience always came after the corresponding physical event. On the other hand, for cold stimulation, while the subjective onset always lagged the physical onset, the subjective temperature peak preceded the physical temperature peak. We analyzed these results in the framework of linear systems theory. The results suggest that the senses of warmth and cold have distinct temporal filtering properties, with the sense of cold being more transient than the sense of warmth. These findings advance our knowledge regarding temporal processing in human thermal sense and serve as a basis for thermal display design.

Sanshool on The Fingertip Interferes with Vibration Detection in a Rapidly-Adapting (RA) Tactile Channel

An Asian spice, Szechuan pepper (sanshool), is well known for the tingling sensation it induces on the mouth and on the lips. Electrophysiological studies have revealed that its active ingredient can induce firing of mechanoreceptor fibres that typically respond to mechanical vibration. Moreover, a human behavioral study has reported that the perceived frequency of sanshool-induced tingling matches with the preferred frequency range of the tactile rapidly adapting (RA) channel, suggesting the contribution of sanshool-induced RA channel firing to its unique perceptual experience. However, since the RA channel may not be the only channel activated by sanshool, there could be a possibility that the sanshool tingling percept may be caused in whole or in part by other sensory channels. Here, by using a perceptual interference paradigm, we show that the sanshool-induced RA input indeed contributes to the human tactile processing. The absolute detection thresholds for vibrotactile input were measured with and without sanshool application on the fingertip. Sanshool significantly impaired detection of vibrations at 30 Hz (RA channel dominant frequency), but did not impair detection of higher frequency vibrations at 240 Hz (Pacinian-corpuscle (PC) channel dominant frequency) or lower frequency vibrations at 1 Hz (slowly adapting 1 (SA1) channel dominant frequency). These results show that the sanshool induces a peripheral RA channel activation that is relevant for tactile perception. This anomalous activation of RA channels may contribute to the unique tingling experience of sanshool.

Neural timing signal for precise tactile timing judgments

The brain can precisely encode the temporal relationship between tactile inputs. While behavioural studies have demonstrated precise interfinger temporal judgments, the underlying neural mechanism remains unknown. Computationally, two kinds of neural responses can act as the information source. One is the phase-locked response to the phase of relatively slow inputs, and the other is the response to the amplitude change of relatively fast inputs. To isolate the contributions of these components, we measured performance of a synchrony judgment task for sine wave and amplitude-modulation (AM) wave stimuli. The sine wave stimulus was a low-frequency sinusoid, with the phase shifted in the asynchronous stimulus. The AM wave stimulus was a low-frequency sinusoidal AM of a 250-Hz carrier, with only the envelope shifted in the asynchronous stimulus. In the experiment, three stimulus pairs, two synchronous ones and one asynchronous one, were sequentially presented to neighboring fingers, and participants were asked to report which one was the asynchronous pair. We found that the asynchrony of AM waves could be detected as precisely as single impulse pair, with the threshold asynchrony being ∼20 ms. On the other hand, the asynchrony of sine waves could not be detected at all in the range from 5 to 30 Hz. Our results suggest that the timing signal for tactile judgments is provided not by the stimulus phase information but by the envelope of the response of the high-frequency-sensitive Pacini channel (PC), although they do not exclude a possible contribution of the envelope of non-PCs.

Roughness Perception of Micro-particulate Plate: A Study on Two-Size-Mixed Stimuli

Tactile roughness perception of fine-texture has been investigated by using non-uniform surface materials (e.g., polishing papers), and therefore it was difficult to quantitatively discuss the relationship between physical surface property and perceived roughness. To solve this issue, we made a surface plate that has close-packed structure with micro-particles of single size (uniform surface), and a surface plate that has pseudo-close-packed structure with micro-particles of two sizes (mixed surface). In this paper, we estimated subjective equality of perceived roughness of the mixed surfaces in relative to the uniform surfaces, and compared the physical properties of the mixed and uniform surfaces that had perceptually equal roughness. We found that perceived roughness cannot be explained only by the physical distance between the micro-particles.

Dissociation of vibrotactile frequency discrimination performances for supra-threshold and near-threshold vibrations

For the wide range of vibration frequencies, the human capacity for vibrotactile frequency discrimination has been reported constant. However, vibrotactile detection depend on two different receptors, one is Meissner corpuscle for low frequencies and the other is Pacinian corpuscle for high frequencies. To examine the impact of input pathway on frequency discrimination task, discrimination capacity has been compared directly by using supra-threshold and near-threshold stimuli since near-threshold stimuli mainly activate one input pathway. Each standard frequencies 15, 30, 60, 120, 240 and 480 Hz at amplitude 6dB and 16 dB detection threshold, was paired with a series of comparison frequencies, and discrimination capacity was quantified by the discriminable frequency increment (Δf) and the Weber Fraction (Δf/f). The result revealed constant and good discrimination capacities for strong stimulus conditions but discrete and bad capacities for weak stimulus conditions. Near-threshold stimuli produced a marked impairment in vibrotactile discrimination at the high standard frequencies around 240 Hz, probably detected by Pacinian corpuscle, but relatively little effect at lower frequencies, mainly detected by Meissner corpuscle.

Integration of vibrotactile frequency information beyond the mechanoreceptor channel and somatotopy

A wide variety of tactile sensations arise from the activation of several types of mechanoreceptor-afferent channels scattered all over the body, and their projections create a somatotopic map in the somatosensory cortex. Recent findings challenge the traditional view that tactile signals from different mechanoreceptor-channels/locations are independently processed in the brain, though the contribution of signal integration to perception remains obscure. Here we show that vibrotactile frequency perception is functionally enriched by signal integration across different mechanoreceptor channels and separate skin locations. When participants touched two sinusoidal vibrations of far-different frequency, which dominantly activated separate channels with the neighboring fingers or the different hand and judged the frequency of one vibration, the perceived frequency shifted toward the other (assimilation effect). Furthermore, when the participants judged the frequency of the pair as a whole, they consistently reported an intensity-based interpolation of the two vibrations (averaging effect). Both effects were similar in magnitude between the same and different hand conditions and significantly diminished by asynchronous presentation of the vibration pair. These findings indicate that human tactile processing is global and flexible in that it can estimate the ensemble property of a large-scale tactile event sensed by various receptors distributed over the body.

Linkage between Free Exploratory Movements and Subjective Tactile Ratings

We actively move our hands and eyes when exploring the external world and gaining information about object’s attributes. Previous studies showing that how we touch might be related to how we felt led us to consider whether we could decode observers’ subjective tactile experiences only by analyzing their exploratory movements without explicitly asking how they perceived. However, in those studies, explicit judgment tasks were performed about specific tactile attributes that were prearranged by experimenters. Here, we systematically investigated whether exploratory movements can explain tactile ratings even when participants do not need to judge any tactile attributes. While measuring both hand and eye movements, we asked participants to touch materials freely without judging any specific tactile attributes (free-touch task) or to evaluate one of four tactile attributes (roughness, hardness, slipperiness, and temperature). We found that tactile ratings in the judgment tasks correlated with exploratory movements even in the free-touch task and that eye movements as well as hand movements correlated with tactile ratings. These results might open up the possibility of decoding tactile experiences by exploratory movements.