Hand posture, but not vision of the hand, affects tactile spatial resolution in the grating orientation discrimination task

The grating orientation discrimination task (GOT) is a sensitive and reliable measure of tactile spatial resolution, or acuity. We used the GOT in three experiments to investigate the effects of hand posture and hand visibility on spatial acuity. Participant sex and experimental design were also manipulated. Healthy adult participants received brief touches to their index fingertips of grated, domed objects. Their task was to decide whether the gratings ran 'along' or 'across' their finger. Measures of the smallest grating width for which participants could reliably discriminate between orientations were recorded as threshold. Experiment 1 evaluated the effect of two- versus one-interval discrimination, hand used and participant sex. Experiments 2 and 3 evaluated the effects of hand visibility (visible or covered) and hand posture (in front or to the side). Females were better than males; the two-interval task resulted in lower thresholds than the one-interval task; and left and right hand thresholds were not significantly different. Most importantly, while hand visibility did not have a significant effect on the task, thresholds were affected by hand posture—worse when the hand was oriented to the side of the body than in front. These results replicate previously reported effects of sex (or finger size), but failed to replicate the so-called ‘visual enhancement of touch’ (VET) effect. We also report a meta-analysis of 27 VET studies, finding a significant effect of ‘non-informative’ vision on tactile perception. Our novel finding is that hand posture affects tactile acuity.

Transcranial direct current stimulation ameliorates tactile sensory deficit in multiple sclerosis

BACKGROUND

Deficit of tactile sensation in patients with MS is frequent and can be associated with interference with daily life activities. Transcranial direct current stimulation (tDCS) showed to increase tactile discrimination in healthy subjects.

OBJECTIVE

In the present study, we investigated whether tDCS may be effective in ameliorating tactile sensory deficit in MS patients.

METHODS

Patients received sham or real anodal tDCS of the somatosensory cortex for 5 consecutive days in a randomized, double blind, sham-controlled study. Discrimination thresholds of spatial tactile sensation were measured using the grating orientation task (GOT). As secondary outcomes we also measured subjective perception of tactile sensory deficit through a visual analog scale (VAS), quality of life and overall disability to evaluate the impact of the treatment on patients daily life. Evaluations were performed at baseline and during a 4-week follow-up period.

RESULTS

Following anodal but not sham tDCS over the somatosensory cortex, there was a significant improvement of discriminatory thresholds at the GOT and increased VAS for sensation scores. Quality of life, and disability changes were not observed.

CONCLUSION

Our results indicate that a five day course of anodal tDCS is able to ameliorate tactile sensory loss with long-lasting beneficial effects and could thus represent a therapeutic tool for the treatment of tactile sensory deficit in MS patients.