Matthew Longo

Patients with lesions of the left posterior parietal cortex commonly fail in identifying their fingers, a condition known as finger agnosia, yet are relatively unimpaired in skilled action. Several studies have shown that non-informative vision of the body enhances performance in numerous tactile tasks. However, it is unknown whether body structural representations are also affected by vision, given that finger agnosia is typically assessed while patients are blindfolded. Here, we investigated whether structural body representations are modulated by non-informative vision of the body. We tested healthy participants in a classic task used to assess finger agnosia, the “in-between” test, to determine whether tactile fingers identification vary when seeing the stimulated hand (non-informative vision). Across blocks, we used three different visual conditions: (1) viewing the stimulated hand, (2) viewing an object, and (3) viewing the other hand. Participants judged the numbers of unstimulated fingers “in between” the two touched fingers and responded vocally as quickly and accurately as possible. Critically, numerosity estimates were less accurate when they were seeing the stimulated hand compared to when an object or the unstimulated hand was visible. Our results demonstrate that vision of the body modulates body structural representations impairing tactile fingers identification.

Patients with lesions of the left posterior parietal cortex commonly fail in identifying their fingers, a condition known as finger agnosia, yet are relatively unimpaired in skilled action. Such dissociations have classically been taken as evidence that representation of body structure is distinct from sensorimotor representations, such as the body schema. Here, we investigated whether the representations of finger numerosity is modulated by the internal posture of the hand. We used the ‘in between’ test in which participants estimate the number of unstimulated fingers between two touched fingers. Across blocks, the fingers were placed in three postures: (1) with fingers touching each other, (2) fingers separated by one centimetre, (3) fingers spread to the maximum comfortable splay. Participants judged the number of unstimulated fingers ‘in between’ the two touches and responded vocally as quickly and accurately as possible. Critically, participants gave larger numerosity estimates when the fingers were positioned far apart compared to when they were close together or touching. Our results demonstrate that increasing the spatial distance between the fingers makes participants experience the fingers as more numerous. These results show that there are dynamic functional links between offline representations of body structure and online representations of body posture.

Tactile distance perception is believed to require that immediate afferent signals be referenced to a stored representation of body size and shape (the body model). For this ability, recent studies have reported that the stored body representations involved are highly distorted, at least in the case of the hand, with the hand dorsum represented as wider and squatter than it actually is. Here, we aim to define the neural basis of this phenomenon. In a behavioural experiment participants estimated the distance between touches on two points by adjusting the length of a visually-presented line on the screen. The technique of multidensional scaling (MDS) was used to reconstruct a perceptual map of tactile space. Analysis of spatial distortion using Procrustes alignment showed that maps were stretched in the mediolateral hand axis. In order to determine the neural correlates of these body distortions, we performed an fMRI study. For each participant, we used a searchlight pattern classifier with Euclidean distance on pre-defined regions of interests (ROIs). In order to relate the representations between the different points and to computational models, we compare response-pattern dissimilarity matrices in these ROIs. Similar to the behavioural experiment, we used MDS to reconstruct maps of the neural representation of tactile space using the values from the dissimilarities matrices. We were able to reconstruct the perceptual map of tactile space in the contralateral primary somatosensory and motor cortices. This suggests that these areas are critical to generate the tactile representations of the dorsum of the hand.

Introduction: In an fMRI study, we explored the population receptive eld (pRF; Dumoulin & Wandell, 2008) properties of the voxels in the dorsum and palm area of the primary somatosensory region (area 3b). Behavioural studies adopting tactile discrimination tasks suggested that these two skin surfaces are represented differently, with the dorsum representation being more distorted than the palm representation (Longo & Haggard, 2011). Longo and Haggard (2011) explained these results by suggesting that the receptive elds of the dorsum surface are more elongated along the proximodistal hand axis than the ones of the palm – the pixel model. The authors based this idea mostly on animal studies (e.g. Alloway et al, 1989; Brooks et al, 1961; Brown et al, 1975). However, it is not clear if this is also the case for humans as, to the best of our knowledge, no study so far has investigated the pixel hypothesis systematically. Methods: The same participant was tested in two fMRI experiments (12 runs for experiment 1; 6 runs for experiment 2). In separate runs, the experimenter manually stimulated either the participant's dorsum or palm with an object (a circle covering 4 grid cells when centred at one intersection in experiment 1, a square covering one cell in experiment 2). A 4x4 grid (7 cm per side) was drawn on the participant's palm and dorsum (Figure 1). In experiment 1, the intersections of the grid lines were numbered from 1 to 25, whereas in experiment 2 each cell of the grid was numbered from 1 to 16. An acoustic cue instructed the experimenter on which skin region to stimulate and for how long. In experiment 1, we employed a random sequence in which each grid intersection was stimulated once per run interleaved with 7 randomly presented null trials. Each trial was 4 seconds and included the delivery of the touch on the instructed location plus small rotatory movements in anti-clockwise and clockwise directions. In experiment 2, we used an ordered sequence from 1 to 16 and back which was repeated 4 times in each run (2 null trials were presented every 4 trials). A tactile stimulation lasted for 2 seconds and consisted of 4 tactile events (~ 2Hz) in which the object gently touched the target skin location. We estimated the shape of the pRFs in anatomically and functionally predened regions of interests (see Figure 2A). To t the data, we used an oriented elliptical Gaussian with four parameters: the stimulated skin location (x and y); the long and short axes of the ellipse (σ1 and σ2), and the orientation (ϑ). We dened the anisotropy as the absolute value of the logarithm of the ratio between the horizontal and vertical axis. Orientations were transformed such that ellipses with the long axis running along the proximodistal orientation had an angle of 90 degrees. Results: Figure 2B shows the results for the voxel with the highest goodness of t for each of the skin surfaces stimulated in experiment 2. The green dot indicates the centre of the elliptical shape. The estimated pRF on the dorsum has a higher anisotropy than the one on the palm. Figure 2C shows the locations of all the estimated pRFs. In Figure 2D, we plotted each semi-major axis angle as an oriented line with the length proportional to the level of anisotropy. Ellipses oriented along the proximodistal axis of the hand appear vertical in the plot. The thick line indicates the resultant vector which gives an indication of the general orientation and anisotropy within the area. In both experiments, the dorsum showed higher anisotropy and both skin surfaces showed a slight deviation from the vertical orientation. Conclusions: In this study, we were able to estimate the pRF locations and shapes of the hand dorsum and palm. Our results suggest that many pRFs of the dorsum are oval-shaped and oriented along the proximodistal axis of the hand; the pRFs on the palm also show a similar characteristic, but less than dorsum. This provides some support for the pixel model (Longo & Haggard, 2011).

In vision, a target stimulus presented on aa apparent motion trajectory becomes undetectable. In the present study, we investigated whether this perceptual masking phenomenon also occurs in tactile perception. Three vibrotactile stimulators were placed along the hand-to-elbow axis on the medial side of the participant's left arm. When the vibrations were presented alternately from the upper and lower ends, an apparent motion was perceived (apparent motion condition). During the apparent motion presentation, a trial with or without a target stimulus was presented at random timing from the central stimulator. A control condition was also introduced in which the apparent motion was not perceived because the vibrations were presented simultaneously from the upper and lower oscillators. We calculated an index of sensitivity to the target stimulus based on the signal detection theory, and found that the sensitivity was lower in the apparent motion condition than in the control condition. On the other hand, there was no difference in the index of judgment criterion (bias). These results indicate that a perceptual masking effect, i.e., inhibition of tactile input detection, occurs on the tactile apparent motion trajectory.

It has been shown the existence of representational stages of touch that distinguish between body-regions more than body-sides with different interactions between homologous compared to non-homologous fingers of the two sides of the body. However, it is unknown whether such interactions are also present across different limbs that are morphologically similar such as hands and feet. Here, we investigated the effect of tactile double simultaneous stimulation (DSS) between the fingers and toes to explore the within between limbs interactions of touch. Participants were instructed to perform a go/no-go task to detect a tactile stimulus on a target digit (e.g., left index) in isolation or with a simultaneous stimulus distractor on a non-target digit, either on the hand (e.g., left middle) or on another limb (e.g., left big toe = homologous; left second toe = non-homologous). In different blocks the target digit could be a digit on the left hand (i.e., index finger, middle finger) or on the left foot (i.e., big toe, second toe). Results show that the DSS interference is reduced when homologous digits are stimulated on different limbs. However, this pattern is more prominent when the target is presented on the hand compared to the foot.

We examined the neural basis of tactile distance perception by analyzing activity patterns induced by tactile stimulation of nine points on a 3 x 3 square grid on the hand dorsum using functional magnetic resonance (fMRI). We used a searchlight approach within pre-defined regions of interests (ROIs) to compute the pairwise Euclidean distances between the activity patterns elicited by tactile stimulation. Then, we used multidimensional scaling (MDS) to reconstruct skin 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 (SI) and motor (M1) cortices reveal that it is distorted in a way that matches the perceptual shape of skin space. This suggests that early sensorimotor areas are critical to processing tactile distance perception.

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.

Borders sit at the center of global politics. Yet they are too often understood as thin lines, as they appear on maps, rather than as political institutions in their own right. This book takes a detailed look at the evolution of border security in the United States after 9/11. Far from the walls and fences that dominate the news, it reveals borders to be thick, multi-faceted and binational institutions that have evolved greatly in recent decades. The book contributes to debates within political science on sovereignty, citizenship, cosmopolitanism, human rights and global justice. In particular, the new politics of borders reveal a sovereignty that is not waning, but changing, expanding beyond the state carapace and engaging certain logics of empire.

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.