Cutaneous Sensitivity Across Regions of the Foot Sole and Dorsum are Influenced by Foot Posture

Understanding the processing of tactile information is crucial for the development of biofeedback interventions that target cutaneous mechanoreceptors. Mechanics of the skin have been shown to influence cutaneous tactile sensitivity. It has been established that foot skin mechanics are altered due to foot posture, but whether these changes affect cutaneous sensitivity are unknown. The purpose of this study was to investigate the potential effect of posture-mediated skin deformation about the ankle joint on perceptual measures of foot skin sensitivity. Participants (N = 20) underwent perceptual skin sensitivity testing on either the foot sole (N = 10) or dorsum (N = 10) with the foot positioned in maximal dorsiflexion/toe extension, maximal plantarflexion/toe flexion, and a neutral foot posture. Perceptual tests included touch sensitivity, stretch sensitivity, and spatial acuity. Regional differences in touch sensitivity were found across the foot sole (p < 0.001) and dorsum (p < 0.001). Touch sensitivity also significantly increased in postures where the skin was compressed (p = 0.001). Regional differences in spatial acuity were found on the foot sole (p = 0.002) but not dorsum (p = 0.666). Spatial acuity was not significantly altered by posture across the foot sole and dorsum, other than an increase in sensitivity at the medial arch in the dorsiflexion posture (p = 0.006). Posture*site interactions were found for stretch sensitivity on the foot sole and dorsum in both the transverse and longitudinal directions (p < 0.005). Stretch sensitivity increased in postures where the skin was pre-stretched on both the foot sole and dorsum. Changes in sensitivity across locations and postures were believed to occur due to concurrent changes in skin mechanics, such as skin hardness and thickness, which follows our previous findings. Future cutaneous biofeedback interventions should be applied with an awareness of these changes in skin sensitivity, to maximize their effectiveness for foot sole and dorsum input.

Bax Contributes to Retinal Ganglion Cell Dendritic Degeneration During Glaucoma

The BCL-2 (B-cell lymphoma-2) family of proteins contributes to mitochondrial-based apoptosis in models of neurodegeneration, including glaucomatous optic neuropathy (glaucoma), which degrades the retinal ganglion cell (RGC) axonal projection to the visual brain. Glaucoma is commonly associated with increased sensitivity to intraocular pressure (IOP) and involves a proximal program that leads to RGC dendritic pruning and a distal program that underlies axonopathy in the optic projection. While genetic deletion of the Bcl2-associated X protein (Bax-/-) prolongs RGC body survival in models of glaucoma and optic nerve trauma, axonopathy persists, thus raising the question of whether dendrites and the RGC light response are protected. Here, we used an inducible model of glaucoma in Bax-/- mice to determine if Bax contributes to RGC dendritic degeneration. We performed whole-cell recordings and dye filling in RGCs signaling light onset (αON-Sustained) and offset (αOFF-Sustained). We recovered RGC dendritic morphologies by confocal microscopy and analyzed dendritic arbor complexity and size. Additionally, we assessed RGC axon function by measuring anterograde axon transport of cholera toxin subunit B to the superior colliculus and behavioral spatial frequency threshold (i.e., spatial acuity). We found 1 month of IOP elevation did not cause significant RGC death in either WT or Bax-/- retinas. However, IOP elevation reduced dendritic arbor complexity of WT αON-Sustained and αOFF-Sustained RGCs. In the absence of Bax, αON- and αOFF-Sustained RGC dendritic arbors remained intact following IOP elevation. In addition to dendrites, neuroprotection by Bax-/- generalized to αON-and αOFF-Sustained RGC light- and current-evoked responses. Both anterograde axon transport and spatial acuity declined during IOP elevation in WT and Bax-/- mice. Collectively, our results indicate Bax contributes to RGC dendritic degeneration and distinguishes the proximal and distal neurodegenerative programs involved during the progression of glaucoma.

Spatial Acuity of the Nociceptive System and Spatial Summation of Pain: Potential Implications for the Clinic

Previous studies investigating spatial acuity measured by two-point discrimination threshold concluded that the nociceptive system is less accurate than the innocuous tactile system. In the discussed article, the authors point out that the nociceptive system is more accurate than the tactile system when controlling for the stimulus modality and intensity in healthy pain-free individuals. Furthermore, this article shows that the pattern of distance-based and areabased spatial summation of pain is modality independent.

Tactile perception by mouth: Perceiving properties of objects when vision is impaired

The lips and tongue demonstrate similar or greater spatial acuity than the fingertips. Indeed, infants use the mouth to perceive properties of objects such as hardness, texture, and shape. In normal development, it is assumed that mouthing decreases in favour of increasingly advanced hand exploration patterns. However, anecdotal reports suggest that mouthing continues to serve a perceptual function when a person’s vision is abnormal. This study explored blind or visually impaired (BVI) adults’ self-reported use of mouthing to perceive properties of objects. We conducted semi-structured interviews with 20 BVI adults with visual acuities ranging from no light perception to 20/40. Data were analysed using content analysis to identify specific properties perceived by the mouth. Despite social norms that discourage mouthing, some BVI adults use oral tactile perception of texture, shape, temperature, and taste to better characterize objects. These findings suggest that compensatory behaviours using the mouth can support the rehabilitation of individuals with abnormal vision.

Visual acuity performance level is independent of locomotion

ABSTRACTLocomotion has a global impact on circuit function throughout the cortex, including regulation of spatiotemporal dynamics in primary visual cortex (V1). The mechanisms driving state-changes in V1 result in a 2-3 fold gain of responsiveness to visual stimuli. To determine whether locomotion-mediated increases in response gain improve the perception of spatial acuity we developed a head-fixed task in which mice were free to run or sit still during acuity testing. Spatial acuity, ranging from 0.1 to 0.7 cycles/°, was assessed before and after 3-4 weeks of reward-based training in adult mice. Training on vertical orientations once a day improved the average performance across mice by 22.5 ± 0.05%. Improvement transferred to non-trained orientations presented at 45°, indicating that the improvement in acuity generalized. Furthermore we designed a second closed-loop task in which acuity threshold could be directly assessed in a single session. Using this design, we established that acuity threshold matched the upper limit of the trained spatial frequency; in two mice spatial acuity threshold reached as high as 1.5 cycles/°. During the 3-4 weeks of training we collected a sufficient number of stimulus trials in which mice performed above chance but below 100% accuracy. Using this subset of stimulus trials, we found that perceptual acuity was not enhanced on trials in which mice were running compared to trials in which mice were still. Our results demonstrate that perception of spatial acuity is not improved by locomotion.