Matthew Longo

Several studies have shown the presence of large anisotropies for tactile distance perception across several parts of the body. The tactile distance between two touches on the dorsum of the hand is perceived as larger when they are oriented mediolaterally (across the hand) than proximodistally (along the hand). This effect can be partially explained by the characteristics of primary somatosensory cortex representations. However, this phenomenon is significantly attenuated relative to differences in acuity and cortical magnification, suggesting a process of tactile size constancy. It is unknown whether the same kind of compensation also takes place when estimating the size of a continuous object. Here, we investigate whether the tactile anisotropy that typically emerges when participants have to estimate the distance between two touches is also present when a continuous object touches the skin and participants have to estimate its size. In separate blocks, participants judged which of two tactile distances or objects on the dorsum of their hand felt larger. One stimulation (first or second) was aligned with the proximodistal axis (along the hand) and the other with the mediolateral axis (across the hand). Results showed a clear anisotropy for distances between two distinct points, with across distances consistently perceived as larger than along distances, as in previous studies. Critically, however, this bias was significantly reduced or absent for judgments of the length of continuous objects. These results suggest that a tactile size constancy process is more effective when the tactile size of an object has to be approximated compared to when the distance between two touches has to be determined. The possible mechanism subserving these results is described and discussed. We suggest that a lateral inhibition mechanism, when an object touches the skin, provides information through the distribution of the inhibitory subfields of the RF about the shape of the tactile RF itself. Such a process allows an effective tactile size compensatory mechanism where a good match between the physical and perceptual dimensions of the object is achieved.

Psychophysical experiments have demonstrated large and highly systematic perceptual distortions of tactile space. Such a space can be referred to our experience of the spatial organisation of objects, at representational level, through touch, in analogy with the familiar concept of visual space. We investigated the neural basis of tactile space by analysing activity patterns induced by tactile stimulation of nine points on a 3 × 3 square grid on the hand dorsum using functional magnetic resonance imaging. We used a searchlight approach within pre-defined regions of interests to compute the pairwise Euclidean distances between the activity patterns elicited by tactile stimulation. Then, we used multidimensional scaling to reconstruct tactile space at the neural level and compare it with skin space at the perceptual level. Our reconstructions of the shape of skin space in contralateral primary somatosensory and motor cortices reveal that it is distorted in a way that matches the perceptual shape of skin space. This suggests that early sensorimotor areas critically contribute to the distorted internal representation of tactile space on the hand dorsum.

Hands are commonly held up as an exemplar of well-known, familiar objects. However, conceptual knowledge of the hand has been found to show highly stereotyped distortions. Specifically, people judge their knuckles as farther forward in the hand than they actually are. The cause of this distal bias remains unclear. In Experiment 1, we tested whether both visual and tactile information contribute to the distortion. Participants judged the location of their knuckles by pointing to the location on their palm directly opposite each knuckle with: 1) a metal baton (using vision and touch) 2) a metal baton while blindfolded (using touch), 3) a laser pointer (using vision). In Experiment 2, we investigated whether judgments are influenced by visual landmarks such as the creases at the base of each finger on the palm. Participants localized their knuckles on either a photograph or a silhouette of their hand. In both experiments, clear distortions were found across conditions, of generally similar magnitude. These results show that distal bias is resistant to changes in the stimulus information and does not rely on any specific stimulus cue or single sensory modality, suggesting that such mislocalisations reflect a conceptual misrepresentation of hand structure.

The perception of limb position in space relies on sensory signals and motor commands. Another potential source of input is standard representations of body posture, which may bias perceived limb position towards more common positions. We recently found that tactile stimuli are processed more efficiently when they are delivered to a thumb in a relative low or to an index finger in a relative high position, supporting the existence of a standard posture of those two fingers. However they may have specific features because of their special roles in development. We investigated the existence of a standard posture of the entire hand testing: a) if similar associations are detectable for different finger contrasts; b) if associations are properties of each specific digit ; c) the role of intermanual competition, and d) the neurophysiological features of standard posture by means of somatosensory evokedpotentials (SEP). Results showed that the thumb is associated with low position, while the other fingers are associated with upper locations configuring the hand as a plier in a position ready to grasp. SEP analysis showed that preferential postures impact the P50 a component typically associated with the activity of primary somatosensory cortex.

Recent research has shown systematic patterns of mislocalisations and confusions between digits. These distortions appear highly stereotyped, and consistent across individuals. The present study addressed whether such mislocalisations arise from early somatosensory maps or from higher-level body representations. We obtained confusion matrices showing pattern of mislocalisation between toes and fingers on the glabrous and hairy surface of the foot and hand. As the two skin surfaces of the hands and feet have distinct representations in somatosensory cortex, an effect arising from early somtotopic maps may show distinct patterns on each skin surface. In contrast, if the effect arises from higher-level body representations which represent the digits as complete, volumetric units, similar patterns should be apparent regardless of which side of the digit is touched. Our results clearly replicated the distinct pattern of mislocalisations on the glabrous skin of the fingers and toes. Critically, these effects were highly similar on the hairy skin. Despite the pattern of mislocalisations being highly stereotyped across participants, there were consistent individual differences in task performance across the two skin surfaces. These results suggest that mislocalisations occur at the level of complete toes, rather than individual skin surfaces, consistent with their resulting from higherlevel body representations.

Knowledge of knuckle location shows highly stereotyped distortions: people judge their knuckles as farther forward in the hand than they actually are. This distal bias does not rely on any specific stimulus cue or single sensory modality, but rather reflects a conceptual misrepresentation of hand structure (Ambroziak, in press; Longo, 2015). Here, in 2 experiments, we investigated whether distal bias can be eliminated by careful inspection of participant’s own hand in movement. In Experiment 1, participants performed a knuckle localization task both before and after being allowed to explore the actual configuration of their hand. During the task, participants judged the location of their knuckles by pointing to the location on the palm directly opposite their knuckle. During the intervention, participants were inspecting their hand while bending their fingers. In Experiment 2, participants performed the same task three times: before any intervention, after inspecting the hand while splaying the fingers and after inspecting the hand while bending the fingers. In both experiments, after the interventions participants judged their knuckles to be closer to the actual location, however, significant distal bias still remained. These results showed that distal bias can be reduced but not eliminated by inspection of participant’s own hand.

Several studies have shown the presence of large anisotropies for tactile distance perception across several parts of the body. Tactile distance between two touches on the dorsum of the hand is perceived as larger when they are oriented mediolaterally than proximodistally. This effect can be partially explained by a process of tactile size constancy. It is unknown whether the same kind of compensation is taking place also when objects size has to be estimated. We investigated whether tactile anisotropy that typically emerges when participants have to estimate the distance between two touches is also present when a continuous object touch the skin and participants have to estimate its size. In separate blocks, participants judged which of two tactile distances or objects on the dorsum of their hand felt larger. One stimulation was aligned with the proximodistal axis, the other with the mediolateral axis. Results showed that across distances were consistently perceived as larger than along ones, whereas the size of the objects was not. These results suggest that a processing of tactile size constancy is more effective when the tactile size of an object has to be estimated compared to when the distance between two touches has to be judged.

Spatial distortions in touch have been investigated since the 19th century. For example, two touches applied to the hand dorsum feel farther apart when aligned with the mediolateral axis (i.e., across the hand) than when aligned with the proximodistal axis (along the hand). Stimulations to our sensory receptors are usually dynamic, where spatial and temporal inputs closely interact to establish our percept. For example, physically bigger tactile stimuli are judged to last longer than smaller stimuli. Given such links between space and time in touch, we investigated whether there is a tactile anisotropy in temporal perception analogous to the anisotropy described above. In this case, the perceived duration between the onset of two touches should be larger when they are aligned with the mediolateral than with the proximodistal axis of the hand dorsum. To test this hypothesis, we asked participants to judge which of two tactile temporal sequences, having the same spatial separation along and across the dorsum, felt longer. A clear anisotropy of the temporal perception was observed: temporal intervals across the hand were perceived as longer than those along the hand. Consistent with the spatial anisotropy, the temporal anisotropy did not appear on the palm side of the hand, indicating that the temporal anisotropy was based on perceptual processes rather than top-down modulations such as attentional or decisional/response biases. Contrary to our predictions, however, we found no correlation between the magnitudes of the temporal and spatial anisotropies. Our results demonstrated a novel type of temporal illusion in touch, which is strikingly similar in nature to the previously reported spatial anisotropy. Thus, qualitatively similar distorted somatosensory representations appear to underlie both temporal and spatial processing of touch.

Should political theorists engage in ethnography? In this letter, we assess a recent wave of interest in ethnography among political theorists and explain why it is a good thing. We focus, in particular, on how ethnographic research generates what Ian Shapiro calls “problematizing redescriptions”—accounts of political phenomena that destabilize the lens through which we traditionally study them, engendering novel questions and exposing new avenues of moral concern. We argue that (1) by revealing new levels of variation and contingency within familiar political phenomena, ethnography can uncover topics ripe for normative inquiry; (2) by shedding light on what meanings people associate with political values, it can advance our reflection on concepts; and (3) by capturing the experience of individuals at grips with the social world, it can attune us to forms of harm that would otherwise remain hidden. The purchase for political theory is considerable. By thickening our understanding of institutions, ethnography serves as an antidote to analytic specialization and broadens the range of questions political theorists can ask, reinvigorating debates in the subfield and forging connections with the discipline writ large.

Plasticity of body representation fundamentally underpins human tool use. Recent studies have demonstrated remarkably complex plasticity of body representation in humans, showing that such plasticity (1) occurs flexibly across multiple time scales and (2) involves multiple body representations responding differently to tool use. Such findings reveal remarkable sophistication of body plasticity in humans, suggesting that Vaesen may overestimate the similarity of such mechanisms in humans and non-human primates