A neural surveyor to map touch on the body

Perhaps the most recognizable sensory map in all of neuroscience is the somatosensory homunculus. Although it seems straightforward, this simple representation belies the complex link between an activation in a somatotopic map and the associated touch location on the body. Any isolated activation is spatially ambiguous without a neural decoder that can read its position within the entire map, but how this is computed by neural networks is unknown. We propose that the somatosensory system implements multilateration, a common computation used by surveying and global positioning systems to localize objects. Specifically, to decode touch location on the body, multilateration estimates the relative distance between the afferent input and the boundaries of a body part (e.g., the joints of a limb). We show that a simple feedforward neural network, which captures several fundamental receptive field properties of cortical somatosensory neurons, can implement a Bayes-optimal multilateral computation. Simulations demonstrated that this decoder produced a pattern of localization variability between two boundaries that was unique to multilateration. Finally, we identify this computational signature of multilateration in actual psychophysical experiments, suggesting that it is a candidate computational mechanism underlying tactile localization.

A somatosensory computation that unifies bodies and tools

It is often claimed that tools are embodied by the user, but whether the brain actually repurposes its body-based computations to perform similar tasks with tools is not known. A fundamental body-based computation used by the somatosensory system is trilateration. Here, the location of touch on a limb is computed by integrating estimates of the distance between sensory input and its boundaries (e.g., elbow and wrist of the forearm). As evidence of this computational mechanism, tactile localization on a limb is most precise near its boundaries and lowest in the middle. If the brain repurposes trilateration to localize touch on a tool, we should observe this computational signature in behavior. In a large sample of participants, we indeed found that localizing touch on a tool produced the signature of trilateration, with highest precision close to the base and tip of the tool. A computational model of trilateration provided a good fit to the observed localization behavior. Importantly, model selection demonstrated that trilateration better explained each participant's behavior than an alternative model of localization. These results have important implications for how trilateration may be implemented by somatosensory neural populations. In sum, the present study suggests that tools are indeed embodied at a computational level, repurposing a fundamental spatial computation.

Validation of Spanish Erasmus-Modified Nottingham Sensory Assessment Stereognosis Scale in Acquired Brain Damage

Acquired brain injury (ABI) is the third leading cause of death in Spain. The disability derived from ABI can include considerable difficulties in upper-limb use due to somatosensory deficits. One of the assessments most commonly used to evaluate ABI is the Nottingham Sensory Assessment (NSA); however, there is no complete psychometric analysis or standardized version in Spanish. We aimed to develop and validate a Spanish version of the stereognosis component of the NSA for evaluating Spanish adults with ABI via a single-center, observational, cross-sectional study. The Spanish version of the NSA was developed in two steps. The first was based on the standardization and collection of normative data in 120 asymptomatic participants. For the second, we recruited 25 participants with ABI to establish concurrent criterion-related validity, internal consistency, and floor/ceiling effects. Criterion validity was assessed against two-point discrimination and tactile-localization tests. Our normative data showed significant differences among the various age groups (p < 0.05), supporting the validity of the Spanish-version assessment. For the ABI sample, we also found further evidence of validity with Spearman’s rho coefficient between the total scores and the two-point discrimination and tactile-localization tests, which showed low and moderate correlations (rho = 0.50–0.75, p < 0.05). Internal consistency was excellent, with a Cronbach’s alpha of 0.91. No ceiling or floor effects were found. We conclude that the stereognosis component of the NSA in its Spanish version is a valid scale that can be used to comprehensively and accurately assess stereognosis capacity in adults with ABI. As a low-cost evaluation, this assessment has great potential to be widely used in clinical practice and research settings.

Imagine Your Crossed Hands as Uncrossed: Visual Imagery Impacts the Crossed-Hands Deficit

Successful interaction with our environment requires accurate tactile localization. Although we seem to localize tactile stimuli effortlessly, the processes underlying this ability are complex. This is evidenced by the crossed-hands deficit, in which tactile localization performance suffers when the hands are crossed. The deficit results from the conflict between an internal reference frame, based in somatotopic coordinates, and an external reference frame, based in external spatial coordinates. Previous evidence in favour of the integration model employed manipulations to the external reference frame (e.g., blindfolding participants), which reduced the deficit by reducing conflict between the two reference frames. The present study extends this finding by asking blindfolded participants to visually imagine their crossed arms as uncrossed. This imagery manipulation further decreased the magnitude of the crossed-hands deficit by bringing information in the two reference frames into alignment. This imagery manipulation differentially affected males and females, which was consistent with the previously observed sex difference in this effect: females tend to show a larger crossed-hands deficit than males and females were more impacted by the imagery manipulation. Results are discussed in terms of the integration model of the crossed-hands deficit.

Upper Limb Sensory Evaluations in Women With Breast Cancer Related Lymphedema Receiving Complex Decongestive Physiotherapy: An Ultrasonographic Study

Purpose: The aim of this study was to investigate if/how the presence of lymphedema affected the sensation of the upper limb, and to assess whether complex decongestive physiotherapy (CDP) had a favorable impact on the sensory testings.Methods: A total of 27 patients with unilateral stage 2 breast cancer related lymphedema (BCRL) were included in the study. Bilateral ultrasonographic (epidermis, dermis, and subcutaneous fat thicknesses) and circumferential measurements were peformed at 10 cm distal to the elbow crease. Semmes-Weinstein monofilament (SWM), static and moving two-point discrimination, pressure pain threshold (PPT) and tactile localization tests were also applied at the same site. After their initial evaluation, all patients underwent CDP phase 1 program (five times a week, for three weeks). All the evaluations were repeated at the end of their treatment as well.Results: Before CDP, affected sides had significantly higher values than the unaffected sides in terms of SWM (p<0.001), static (p=0.002) and moving two-point discrimination (p=0.011), PPT (p=0.001), and tactile localization (p<0.001) values. After CDP, SWM (p=0.002), static (p=0.009) and moving two-point discrimination (p=0.024), PPT (p=0.014) and tactile localization (p<0.001) values decreased significantly on the affected sides.Conclusion: BCRL seem to reduce light touch, static and moving two-point discrimination, PPT and tactile localization sensations whereas CDP seems to improve these sensory perceptions in women with BCRL. Of note, ultrasonographic measurements also appear to be promising for prompt and convenient follow up in the management of these patients. Clinical Trial Registration Number: NCT04296929Date of registration: March 5, 2020