Processing from Fingertips in Touchscreen Phone Users

Institute of Cognitive Neuroscience, University CollegeLondon, 17 Queen Square, London WC1N 3AR, UKSummaryCortical activity allotted to the tactile receptors on fingertipsconforms to skilful use of the hand [1–3]. For instance, instring instrument players, the somatosensory cortical activ-ity in response to touch on the little fingertip is larger thanthat in control subjects [1]. Such plasticity of the fingertipsensory representation is not limited to extraordinary skillsand occurs in monkeys trained to repetitively grasp andrelease a handle as well [4]. Touchscreen phones alsorequire repetitive finger movements, but whether and howthe cortex conforms to this is unknown. By using electroen-cephalography(EEG),wemeasuredthecorticalpotentialsinresponsetomechanicaltouchonthethumb,index,andmid-dle fingertips of touchscreen phone users and nonusers(owning only old-technology mobile phones). Although thethumb interacted predominantly with the screen, the poten-tials associated with the three fingertips were enhanced intouchscreen users compared to nonusers. Within thetouchscreen users, the cortical potentials from the thumband index fingertips were directly proportional to the inten-sity of use quantified with built-in battery logs. Remarkably,the thumb tip was sensitive to the day-to-day fluctuations inphone use: the shorter the time elapsed from an episode ofintense phone use, the larger the cortical potential associ-ated with it. Our results suggest that repetitive movementson the smooth touchscreen reshaped sensory processingfrom the hand and that the thumb representation was up-dated daily depending on its use. We propose that corticalsensory processing in the contemporary brain is continu-ously shaped by the use of personal digital technology.ResultsCortical Fingertip Representations in Touchscreen PhoneUsers Differ from Those Found in NonusersWe analyzed 37 right-handed volunteers, 26 of whom usedtouchscreen phones and 11 of whom used old-technologymobile phones. Questionnaires provided few key insightsinto how the more modern phones were used. First,touchscreen users primarily used their right thumb on thescreen as opposed to other fingers (Figure 1A), and none ofthemusedastylus.Thethumbpreferencewasexpectedgiventhat hand-held phones were designed as such [5]. Second, inagreement with a US national survey on smartphone use,80% of the touchscreen users in our study mainly used theirphone for receiving and sending text messages or e-mails,as opposed to passively listening to music, watching videos,or making calls [6]. Finally, according to the self-reports,touchscreen users spent noticeably more time with theirphones than did the nonusers (Figure 1B).Weinvestigatedwhetherthesomatosensorycorticalelectri-cal activity evoked from the fingertips differed betweentouchscreen phone users and nonusers. Sixty-two surfaceelectrodes distributed over the entire scalp were used todetect cortical potentials evoked by touch on the thumb, in-dex, and middle fingertips of the right hand. Each tactile stim-ulus consisted of a light mechanical contact that lasted for2 ms, and event-related potentials (ERPs) were based on1,250 stimulations on each fingertip. For all three fingertipstested both in touchscreen users and nonusers, the touch re-sulted in a dipole field around the contralateral (to stimulation)somatosensory cortex with signal onset at 32 ms and peak at55 ms (on grand mean traces). The positive ERPs were de-tectedinthecontralateralparietalelectrodes,andthenegativesignalsweredetectedmoremediallyinthecontra-andipsilat-eral frontal electrodes (Figures 1C–1H). Based on the latencyand signal topology, we could assert that these signals origi-nated from the primary somatosensory cortex [7–9]. Weanalyzed the signal differences between the touchscreenusers and nonusers across all time points (50 ms prestimula-tionto120mspoststimulation)andforeachelectrodebyusingtwo-sample t tests corrected for multiple comparisons using2D spatiotemporal clustering [10]. Interestingly, for all of thetested fingertips, the amplitude of the positive ERP was largerin touchscreen users compared to nonusers (Figures 1C–1H).Temporally,thepositivesignalsdifferedbetween39and68msforthethumbtip, between 38and60msfor theindex fingertip,andbetween48and66msforthemiddlefingertip(Figures1C,1E, and 1G). Spatially, the statistical maps revealed that thedifferences were clustered on the contralateral parietal scalpfor all the three fingertips (Figures 1D, 1F, and 1H). However,the spatial extent of these differences was the smallest forthe middle finger (Figure 1H).In short, touchscreen users relied mostly on their thumb tointeract with the screen, but the cortical potentials associatedwith the first three fingertips were enhanced in comparison tononusers. However, the spatiotemporal impact of phone usewas the least prominent for the middle fingertip.The Amount of Touchscreen Phone Use InfluencesCortical ActivityTheincreasedcorticalactivityintouchscreen userscomparedtononuserscouldbeduetoamoreintenseusageofthehand,in the sense that the former group used the right thumb morethan the latter group did. Alternatively, it could be due tothe development of touchscreen-specific motor routines or‘‘skills’’ as the movements associated with push buttons (innonusers, who used only old-technology mobile phones)