One of the most remarkable things about our brains is how organized they are. Sensory information from our eyes, mouth, skin, nose and ears goes to different locations in the brain. For example, visual signals are processed first in the very back of the brain, whereas sensations of touch and pain activate the middle region of the brain called the somatosensory cortex.
Functional organization of brain cortex. (Source: imgarcade.com/1/sensorycortex)
Remarkably, the brain gets even more organized from there. Within the visual cortex, there are columns of neurons that only respond to light that is horizontal and others that only respond to lines that are tilted 45 degrees. The somatosensory cortex is also highly organized, with different parts of the body represented by specific sets of neurons. If you were to send electrical shocks into one specific area of the somatosensory cortex to activate those neurons, you may elicit feelings of touch from the right thumb, even though the subject is not being touched at all. Move those electrical signals over slightly to another area, and the subject may feel touch instead coming from the palm of their hand.
Wow, right? But here’s the real mind blower: this organization can change over time as the person experiences different sensory inputs. If you are a violin player, you feel the strings with your fingertips a lot, so the fingertip part of the somatosensory cortex is super active. This extra activity allows the fingertip representation in the brain to grow and recruit nearby neurons to also respond to touch in the fingertips. The cortical representations are “plastic” and always changing with use.
A violin player may practice this one particular skill a lot, but what about other activities we do everyday with less intensity, like using smart phones? Think about how often you are swiping the screen with your thumb. That’s a lot of sensory information being sent to the thumb part of your somatosensory cortex. Would this increase the thumb representation in your brain? A recent paper by Gindrat et al. addressed this exact question using EEG to record brain activity in smart phone users versus people with the old-style cell phones.
Electroencephalography
How can you actually measure the area of body representations in the somatosensory cortex? You could stick electrodes into people’s brains and record the activity in their neurons, but that’s a little invasive. You could put them into a MRI machine and measure brain activity when you touch their thumbs, but that is time consuming for so many subjects (37 total). Instead, the authors used a method known as electroencephalography, or EEG, which consists of 62 surface electrodes placed on the scalps of the subjects. Each electrode records the summed electrical activity from all the neurons positioned right under the electrode. Before an experiment, all the electrodes would be picking up a baseline of activity from lots of different neurons firing asynchronously. However, during an experiment, there is a single stimulus (like touching the subject’s thumb), which elicits activity in a lot of neurons all at the same time. This activity summates to give one large response called the event related potential (ERP), which is recorded by the nearest electrodes.
EEG electrodes record brain activity (source: Wikimedia commons)
Finger representations in smart phone users
The ERPs for the thumb, index finger and middle finger were larger for the smart phone users than for the non-touchscreen users. There was a correlation between the amount of phone use per hour and the ERP, so the more use, the greater the ERP, which is to say the more activity in the somatosensory cortex. The number of electrodes recording the ERP was greater in the touchscreen users, so when you touch the thumb of a touchscreen user, a larger part of the somatosensory cortex responds. In other words, the thumb representation was larger in smartphone users who use their thumbs more often.
The more recently the subjects had used their phones intensely, the larger the ERP for the thumb, which indicates that brain remodeling occurs on a very short time scale (within 10 days in this experiment). Interestingly, there was no correlation between ERPs and the age at which the subject started using a touchscreen. This is in contrast to the previous experiments done with trained violin players, which did show a correlation between the size of the finger representations and the age at which they first started playing. The authors suspect that a trained violinist develops a more stable sensory representation than touchscreen users who are casually using their phones (as opposed to years of disciplined practice).
So the take-home message is that normal day-to-day activities can influence brain plasticity and the way our sensory representations are organized in our brains. This could be a good thing, because subjects develop better touchscreen skills. On the other hand, the enlarged thumb representation could cause focal dystonia, which is characterized by involuntary muscle contractions and sometimes pain, as the various body part representations lose their distinct boundaries and start to overlap. This probably won’t be a problem for most phone users, but be forewarned all you smart phone addicts out there.

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