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Feeling a texture typically involves sliding the fingers of a hand across that surface or rubbing the surface between the thumb and another digit. Texture signals appear to be integrated across the digits of a hand with perceived roughness at one finger swayed in the direction of texture touched by another finger of the same hand. To date, one study has reported similar integrative effects when the pairs of digits belong to different hands. This contrasts with observations from studies of tactile detection and tactile attention where the patterns of interactions between the digits depend on whether the digits belong to the same hand or not. The present experiments revisit the question of hand identity on multidigit roughness perception using two interval forced choice (2IFC) discrimination and single interval absolute magnitude estimation (AME). Pairs of tactile gratings were actively touched using the thumb and index fingers from the same or different hands. Attention was directed towards roughness at the thumb and index finger sensations were to be ignored. For both discrimination and ratings tasks, roughness perceived at the thumb varied with the textures touched by the index finger suggesting integration of roughness cues from the two digits. This integration occurred despite differences in the two tasks such as working memory requirements. Notably, roughness signals were integrated when originating in pairs of digits on the same hand but not when from different hands. These findings add to a body of evidence based on experiments using different stimuli and tasks, suggesting that hand identity affects interactions across the digits., Competing Interests: Declarations. Competing interests: The author declares no competing interests., (© 2024. The Author(s).)

How does vision affect active touch in judgments of surface roughness? We contrasted direct (combination of visual with tactile sensory information) and indirect (vision alters the processes of active touch) effects of vision on touch. Participants judged which of 2 surfaces was rougher using their index finger to make static contact with gratings of spatial period 1580 and 1620 μm. Simultaneously, they viewed the stimulus under one of five visual conditions: No vision, Filtered vision + touch, Veridical vision + touch (where vision alone yielded roughness discrimination at chance), Congruent vision + touch, Incongruent vision + touch. Results from 32 participants showed roughness discrimination for touch with vision was better than touch alone. The visual benefit for touch was strongest in a filtered (spatially non-informative) vision condition, thus results are interpreted in terms of indirect integration. An indirect effect of vision was further indicated by a finding of visual benefit in some but not all visuo-tactile congruency conditions., (© 2024. The Author(s).)

It has previously been shown that the perceived roughness of a surface touched by one digit is influenced by the roughness of a different surface touched simultaneously by another digit on the same hand. The present study was designed to examine whether this is the case when surfaces of varying roughness are touched using digits on separate hands. Participants touched pairs of sandpaper surfaces, in sequence, using the same digit, and identified which of the two was rougher. Roughness discrimination was measured in the presence of distractor surfaces touched simultaneously with the target surface, but using a different digit either on the same or on the other hand. The overall perception of roughness of the attended surfaces was better on the left than on the right hand. Perceived roughness also varied systematically with the roughness of the distractor surfaces. Attended surfaces were more likely to be perceived as smoother when they were paired with smooth rather than rough distractors. Likewise, attended surfaces tended to be perceived as rougher with rough distractors. This pattern of results occurred whether the attended and distractor digits were on the same hand or different hands. These data confirm that it is difficult to restrict tactile attention for roughness to a single digit and show that this difficulty extends to restricting attention to a single hand. Furthermore, the effect of a stimulus at an unattended body location was not simply to impair perception in general, but to bias it in the roughness direction of the distractor surface.

It has often been shown that people experience difficulty restricting their tactile attention to one hand when both hands are touched simultaneously, especially if the hands are in close spatial proximity. Taken in conjunction with a growing body of evidence that the sensations arising from stimulation of the skin are referred to locations in external space rather than their positions on the body, these tactile attention findings provide evidence for the spatial distribution of attention in touch. However, the effect of spatial distance between the arms on the distribution of attention has been shown in only a limited number of studies and in single testing sessions. In a set of experiments we examined the stability of the arm separation effect by measuring performance over two sessions and with varying visual information about arm location. Tactile selection was more efficient when the stimulated hands were placed far apart compared with when they were adjacent but the effect was short lived (session 1 only) and only occurred when participants were tested with their eyes open. These findings suggest that somatosensory attention is a flexible, dynamic process, based on coordinates that can vary over time., (2010 Elsevier B.V. All rights reserved.)

In two experiments, we investigated the automatic integration of touch across the fingers. Participants made judgments about the roughness of sequences of textures presented to one finger while simultaneously feeling textures of varying roughness with another finger (the distractor digit) on the same hand. Integration across digits was evident when the thumb and index finger were used together, whereas there was a general disruption of attention when the thumb and little finger were used together. In addition, interference was greater when the distractor digit was above the touched surface than when it was below the touched surface. These results suggest that there is integration of information across fingers when people feel textures, but that this integration pattern does not conform to a spread of activation across somatosensory maps.

The ability to report the temporal order of 2 tactile stimuli (1 applied to each hand) has been shown to decline when the arms are crossed over compared with when they are uncrossed. However, these effects have only been measured when temporal order was reported by stimulus location. It is unknown whether this spatial manipulation of the body affects all tactile temporal order judgments (TOJs) or only those judgments that are spatially defined. The authors examined the effect of crossing the arms on tactile TOJs when stimuli were identified by either spatial (location) or nonspatial (frequency or duration) attributes. Spatial TOJs were significantly impaired when the arms were in crossed compared with uncrossed postures, but there was no effect of posture when order was judged by nonspatial attributes. Task-dependent modulation of the effects of posture was also evident when response complexity was reduced to go/no-go responses. These results suggest that crossing the arms impairs tactile localization and thus spatial TOJs. However, the data also suggest that localization is not a necessary precursor when temporal order can be computed by nonspatial means.

We investigated the effect of correlation in background noise on the ability of participants to detect a kinaesthetic target on the index fingers of their hands. Participants had to judge whether the target (a smoothed ramp with quarter-sine onset and offset), was on the left or right finger (experiments 1 and 2). These targets were embedded in noise generated by pseudo-random up-down movements of both the left and right fingers. Positive correlation between the noise on the left and right fingers aided discrimination of the target signal relative to when the noise was uncorrelated. However, this benefit of positive correlation was subject to temporal limitations and diminished with increasing lag between the noise on the fingers (experiment 2). Tests of explicit detection of correlation in kinaesthetic noise (experiment 3) showed a similar pattern, with detection of correlation decreasing with the temporal lag between the noise on the fingers. The results suggest that kinaesthetic signal detection involves sensory integration across the fingers within a limited temporal window.