Exp Brain Res (2011) 212:279–291 DOI 10.1007/s00221-011-2728-6 123 RESEARCH ARTICLE The impact of light Wngertip touch on haptic cortical processing during a standing balance task David A. E. Bolton · William E. McIlroy · W. Richard Staines Received: 7 October 2010 / Accepted: 5 May 2011 / Published online: 17 May 2011  Springer-Verlag 2011 Abstract Availability of Wngertip touch onto a stable surface reduces body sway for subjects standing with eyes closed. This is largely associated with sensory feedback from the Wngertip when mechanical load is limited. Here, it is possible that the central nervous system facilitates corti- cal sensory processing to augment feedback to control upright stance. To test this, we compared cortical sensory excitability between tasks with and without light Wnger touch while standing. Subjects stood in tandem on a force plate with eyes closed while lightly touching a stable sur- face with the index Wnger. This was, in two diVerent stud- ies, compared to: (1) no haptic contact or (2) light touch on an object not referenced to balance. Throughout testing, the median nerve was stimulated and electroencephalography was used to measure somatosensory evoked potentials (SEPs). As expected, availability of stable light touch reduced medial–lateral COP sway. Peak amplitudes for SEP components revealed reduced P100 (48%), but increased P50 (31%), N140 (80%), and P200 (20%) during stable touch versus no touch. The modulation of P50 and N140 was no longer present when comparing stable to con- trol (touch), which suggested that attending to touch on either surface, regardless of stability reference, accounted for these changes. Conversely, P200 was increased (19%) when touching the stable surface. Our data show SEP modulation during a standing balance task related to hand contact. Facilitation of P200 in particular may indicate task-speciWc regulation of the cortical representation of Wngertip aVerent input when it is relevant to providing stable cues for static balance control. Keywords Balance · Haptic · EEG · SEP · Somatosensation · EMG Introduction Sensory feedback is necessary to control postural stability, and this feedback is primarily derived from visual, vestibu- lar, and somatosensory cues (Day and Cole 2002; Day et al. 2002; Mergner et al. 2003, 2006). Any of these sensory modalities can provide signals relevant to control stability, making the sensory regulation of posture quite robust. However, as sources of feedback are progressively removed, markers of reduced stability can be observed. For example, subjects standing with their eyes closed demon- strate greater sway compared to when their eyes are open (Jeka and Lackner 1994). Similarly, ischemic block of the lower limb during quiet stance results in greater sway and therefore demonstrates the contribution of somatosensation to postural control (Mauritz and Dietz 1980). Although many sensory sources can be exploited to control stability, somatosensory cues, especially those from the support base (typically the feet), provide a particularly quick and important D. A. E. Bolton (&) · W. E. McIlroy · W. Richard Staines Department of Kinesiology, University of Waterloo, 200 University Avenue W, Waterloo, ON N2L 3G1, Canada e-mail: dbolton@uwaterloo.ca D. A. E. Bolton · W. E. McIlroy · W. Richard Staines Heart and Stroke Foundation Centre for Stroke Recovery, Toronto, ON, Canada W. E. McIlroy Sunnybrook Health Sciences Centre Research Institute, Toronto, ON, Canada W. E. McIlroy Department of Medicine (Neurology), University of Toronto, Toronto, ON, Canada