P 30. The effects of transcranial direct current stimulation on conscious perception of sensory inputs from hand palm and dorsum

Abstract

Conscious perception of sensory inputs is dependent on the salience and relevance given to them. As a consequence, there can be differences in subjective awareness (AW) of the timing of sensory perception according to site and type of the stimulus. Since this is a cortical function, it might be susceptible to modification through brain stimulation. To characterize differences in AW and investigate the effects of transcranial direct current stimulation (tDCS) on the conscious perception of sensory inputs from glabrous and hairy skin. In 14 healthy volunteers, we applied heat pain and weak electrical stimuli to the hand palm and dorsum while subjects were assessing the time of sensory perception through the Libet’s clock (Libet et al., Brain 1983;106:623–42). In brief, subjects were requested to state the position of the clock’s hand at the time they felt the stimulus. Latency of AW was calculated as the difference between the actual time at which the stimulus was issued and its subjective assessment. In order to assess whether the cortex’s role in processing sensory inputs differs for stimulus site and type, we examined the effects on AW of cathodal (inhibitory) tDCS over the parietal cortex contralateral to the hand receiving thermal and electrical stimuli. Latency of AW was significantly longer for thermal than for electrical stimuli, in correspondence with the different conduction velocity of the axons involved. It was also longer for palm than for dorsum for thermal stimulation ( p < 0.01) but no differences between stimulation site were observed for the electrical stimuli. Furthermore, tDCS induced a significant shortening of AW to thermal stimuli in the palm but not in the dorsum and no effects were seen with electrical stimuli. Longer latency of AW in the palm than in the dorsum is attributable to the thickness of the skin in the hand palm, which leads to a deeper location of the receptors. These differences are not seen with electrical stimuli because they activate directly the subcutaneous myelinated axons. The finding that tDCS caused a change in latency of AW exclusively to thermal stimulation of the glabrous skin indicates a cortical control of nociceptive inputs that is specific for hand palm and could be related to the need to hold potentially harmful objects.