Haggarty CJ, Malinowski P, McGlone F, Walker SC

An ultra-late potential (ULP) evoked by robotically delivered touch, specifically targeted to activate C-tactile afferents (CTs), has previously been identified over frontal and central electrodes (Ackerley et al 2013). Here, using a manually delivered tactile stimulus, we aimed to replicate this finding and compare the time course of the response to that evoked by a faster, non-CT optimal stimulus, which preferentially activates myelinated Aβ afferents. Seventeen participants received brush strokes to the dorsal surface of their right forearm. The strokes were delivered manually by a researcher, using a soft cosmetic brush, in a proximal-distal direction, over 10cm of skin. The velocity of stroke was guided by a visual metronome. The experiment was divided into twenty blocks of five trials. In order to maintain vigilance, participants were asked to decide whether the stroke they felt was delivered at the target speed (CT-optimal 3cm/sec or non-CT-optimal 30cm/sec depending on the block) or at an oddball speed of 15cm/sec. Each block began with the target velocity of 3cm/sec or 30cm/sec and included between zero and two oddball strokes. A total of 43 strokes were delivered for each of the target stimuli. ERPs were time-locked to the brush breaking a laser beam located over the participant's arm at the start of each trial. Analysis revealed that stimuli delivered at 30cm/sec stimuli elicited a robust ERP peak over electrodes Cz & Pz, between 200-600 ms after stimulus onset. This P300 was of significantly higher amplitude than that evoked by CT-optimal 3cm/sec strokes. In addition, consistent with the previous report of Ackerley et al (2013), a ULP was measured for CT-optimal touch over frontal electrodes 2500-3500 ms after stimulus onset. This study adds further support for an ERP response evoked by activation of CT afferents, that can be differentiated from earlier brain potentials seen in response to Aβ activation, that occur less than 1 second after stimulus onset.