The definitive version of this post was originally published on February 4, 2015 on the PLOS Neuroscience Community website, where I serve as an editor.
Twenty years ago, very few among us had ever used touchscreens; now, there is one in every pocket. Their ease of use provides a seamless interface with mobile phones, tablets and computers, and we spend literally hours every day interacting with those devices through touchscreens. How does all that tapping and swiping influence the brain? Quite a bit, according to scientists at the University of Fribourg and the Swiss Federal Institute of Technology of Zurich, both in Switzerland, who recently reported their findings in Current Biology. Using EEG and somatosensory evoked potentials (an EEG measure of the brain’s response to tactile stimulation), they found that users of smartphones had increased responses to stimulation of their thumb. This increase was directly proportional to both the average intensity of smartphone use and its day-to-day fluctuations. The researchers conclude that sensory processing in our brains is continuously updated by our use of modern technology.
Previous research had shown that musicians had increased cerebral responses to finger touch, as did blind people who read Braille. Here, Anne-Dominique Gindrat and her colleagues recruited 38 university students, 27 of whom had a smartphone, the remaining 11 still using old-technology mobile phones (those with numbered buttons on them, if you remember). They first confirmed that the owners of touchscreen smartphones used their devices for a much longer time each day, and that they predominantly manipulated them with their right thumb. Then, while recording their EEG, the researchers delivered tactile stimulation to the tip of each of the first three fingers. They found a considerable increase in the magnitude of the brain responses, most importantly for the thumb, but also for the index and middle fingers. Based on the location and timing of the responses, they could ascertain that the changes involved the representation of the fingers in the primary somatosensory cortex.
Updating cortical maps
Spectacularly, the changes in the brain’s response to touch correlated with each participant’s recent history of smartphone use. In order to estimate that use over a 10-day period, Gindrat and colleagues cleverly turned to battery logs: they had the participants install an app that would record battery usage every ten minutes when the phone was in use. Hourly phone usage as well as the time elapsed since the period of most intense use during the last 10 days were extracted from those logs and used as regressors on the brain responses. The researchers found that the more the volunteers had used their smartphone in the days before the EEG recording session, the more intense their brain responses to tactile stimulation of the thumb. Similarly, the closer the period of most intense touchscreen use from the recording session, the more intense the changes in brain responses. Results for the index finger were along the same lines, although less pronounced. By contrast, the researchers found that the total time of smartphone ownership (a measure that they termed “age of inception”) did not meaningfully impact brain tactile responses. These findings strongly suggest that the brain continuously updates its sensory representations of the environment to reflect day-to-day variations in sensory inputs.
No loss of lateral inhibition
Turning to the potential mechanisms for this striking plasticity, Gindrat and colleagues explored whether it could be due to a loss of lateral inhibition. Simply put, brain responses to simultaneous stimulation of the thumb and index fingers are normally smaller than what would be expected by summing the brain responses to either finger in isolation. This phenomenon is presumably explained by lateral inhibitory interactions between the cortical representations of neighboring fingers. Here, the researchers found that responses to combined thumb and index finger stimulation were indeed smaller than expected in smartphone users; in fact, that reduction was even more pronounced than in non-smartphone users.
Overall, the results of this study suggest that, in smartphone users, the representations of the thumb and index fingers in the somatosensory cortex are both enhanced and better individualized. This likely reflects the importance of somatosensory inputs and feedback for the fine motor behaviors that are required to efficiently control smartphones and other computers using a touchscreen.
A caveat regarding waveform-based EEG analysis
No scientific study is perfect, and the major weakness of the present work, in my opinion, is the fact that all the analyses were based on the EEG measurements themselves. Furthermore, some of the statistical tests were performed directly on the individual EEG waveforms, which are not entirely adequate indexes of the underlying neural activity. Modern EEG analysis incorporates information from every EEG electrode into whole-head maps which better reflect cerebral activity (much more on this technical but important subject in this review article). These maps can then be used to estimate the active sources in the brain that generated them (EEG source imaging). Such an approach could have better revealed the cerebral sites where plasticity took place, as well as the neural correlates of the intensity of recent smartphone use.
Nevertheless, this study is a beautiful experimental confirmation of the notion that the brain dynamically and continuously adapts to changes in the environment and in sensorimotor experience.
Gindrat, A., Chytiris, M., Balerna, M., Rouiller, E., & Ghosh, A. (2015). Use-Dependent Cortical Processing from Fingertips in Touchscreen Phone Users Current Biology, 25 (1), 109-116 DOI: 10.1016/j.cub.2014.11.026