Tactile acuity in the blind: a closer look reveals superiority over the sighted in some but not all cutaneous tasks.

Previous studies have shown that blind subjects may outperform the sighted on certain tactile discrimination tasks. We recently showed that blind subjects outperformed the sighted in a haptic 2D-angle discrimination task. The purpose of this study was to compare the performance of the same blind (n=16) and sighted (n=17, G1) subjects in three tactile discrimination tasks dependent solely on cutaneous inputs from the fingertip of the index finger, D2. A second group of sighted subjects (n=30, G2) were also tested. Texture discrimination thresholds were 0.62 (G1)-0.80 mm (G2) for the sighted subjects, and 0.64 mm for the blind (standard, 2mm spatial period). Grating orientation thresholds were 0.99 (G1)-1.12 mm (G2) for the sighted subjects, and 0.96 mm for the blind. Finally, vibrotactile frequency discrimination thresholds (100 Hz standard) were 19.5 (G2) and 20.0 Hz (G1) for the sighted, and 16.5 Hz for the blind subjects. There were no significant differences in performance between the blind and the sighted subjects for the grating orientation or vibrotactile frequency discrimination tasks. In contrast, blind subjects outperformed the sighted for the texture discrimination task (G2 only), possibly reflecting the fact that the raised dot surfaces were similar to the dots forming Braille characters (all were fluent Braille readers).

Brain activity associated with the electrodermal reactivity to acute heat pain.

Pain is associated with the activation of many brain areas involved in the multiple dimensions of the experience. Several of those brain areas may also contribute to the monitoring and regulation of autonomic activity but this aspect of pain responses has been largely overlooked in human imaging studies. This functional magnetic resonance imaging (fMRI) study relied on blood-oxygen level dependent (BOLD) signal to investigate subject-related differences in brain activity associated with the individual differences in electrodermal responses evoked by 30 s noxious (pain) and innocuous (warm) thermal stimuli. Pain-related activity (pain-warm) was found in the thalamus, somatosensory cortices (leg area of SI/MI, SII, and insula), the anterior cingulate cortex (ACC), and the amygdala. Brain activation related to stimulus-evoked electrodermal activity was identified by modeling the predicted BOLD responses with the magnitude of each subject's skin conductance reactivity. Subjects showing larger skin conductance reactivity to the innocuous and/or noxious stimuli displayed larger stimulus-evoked brain responses in the somato-motor cortices (SI/MI, SII, and insula), the perigenual and supracallosal ACC, the orbitofrontal cortex and the medulla. Further analyses revealed brain activation more specifically associated with the pain-related skin conductance reactivity in the supracallosal ACC, amygdala, thalamus, and hypothalamus. These findings demonstrate that individual differences in electrodermal reactivity partly reflect differences in pain-evoked brain responses, consistent with a role of these structures in the monitoring/regulation of pain-related autonomic processes.

Ipsilateral cortical representation of tactile and painful information in acallosal and callosotomized subjects.

Previous studies have shown that unilateral painful but not tactile stimulation produces ipsilateral cortical activation in callosotomized patients. Here, we used functional magnetic resonance imaging (fMRI) to compare activation evoked by tactile and thermal pain stimulation in two individuals with callosal agenesis, one callosotomized patient, and six control subjects. Bilateral tactile activation was found in S1 and/or S2 of both hemispheres in control and acallosal subjects whereas no ipsilateral activation was detected in these structures in the callosotomized participant. In contrast, although there was some inter-individual variability in the pattern of responses to pain, all subjects including the callosotomized patient showed ipsilateral responses in at least two of the target pain-related areas (S1, S2, insula and/or cingulate cortex). These findings are consistent with the plasticity of the touch system in callosal agenesis and further confirm that ipsilateral activation of pain-related regions does not require the integrity of the corpus callosum.

Tactile acuity in the blind: a psychophysical study using a two-dimensional angle discrimination task.

Growing evidence suggests that blind subjects outperform the sighted on certain tactile discrimination tasks depending on cutaneous inputs. The purpose of this study was to compare the performance of blind (n = 14) and sighted (n = 15) subjects in a haptic angle discrimination task, depending on both cutaneous and proprioceptive feedback. Subjects actively scanned their right index finger over pairs of two-dimensional (2-D) angles (standard 90 degrees ; comparison 91-103 degrees ), identifying the larger one. Two exploratory strategies were tested: arm straight or arm flexed at the elbow so that joint movement was, respectively, mainly proximal (shoulder) or distal (wrist, finger). The mean discrimination thresholds for the sighted subjects (vision occluded) were similar for both exploratory strategies (5.7 and 5.8 degrees , respectively). Exploratory strategy likewise did not modify threshold in the blind subjects (proximal 4.3 degrees ; distal 4.9 degrees ), but thresholds were on average lower than for the sighted subjects. A between-group comparison indicated that blind subjects had significantly lower thresholds than did the sighted subjects, but only for the proximal condition. The superior performance of the blind subjects likely represents heightened sensitivity to haptic inputs in response to visual deprivation, which, in these subjects, occurred prior to 14 years of age.