When we are touched on the skin, sensory receptors in that location fire and send a signal via nerves to the brain, but this is not enough to let us know where on our body the touch occurred. To decipher where the touch is on the body, the brain needs some kind of map of the locations of the skin’s sensory endings in relation to one another. Such maps of the skin, or other aspects of the body, seem to be learned and can change in response to sensory input (Buonomano & Merzenich 1998). For example, when two sites on the hand are stimulated simultaneously, perception of the relative locations of the sites can start to be confused (Braun et al 2000). However, it is not clear how somatotopic maps are created and maintained.
One way that our brains could organize a map of the skin is through identifying the neighbours of each patch of skin. We postulated that the brain might identify skin patches as neighbours if the patches were often stimulated in sequence, as would happen naturally if an object moved along the skin. To test this idea, we used a novel skin stimulus that was designed to convince the brain that two patches of skin were neighbours although they were actually 10 cm apart.
WHAT DID WE FIND?
A brush moved back and forth along the forearm. The brush started near the elbow, brushed ~8 cm down the arm, skipped a 10-cm patch of skin and then resumed brushing for ~8 cm to end near the wrist. The path was then reversed. Crucially, the time taken to skip the 10 cm of skin in the middle of the arm was very short so that the timing of stimulation of the skin on either side was the same as if these patches of skin were next to each other on the arm.
Participants did not feel the 10-cm gap in the brush path – they reported that brushing was continuous along the whole forearm. Furthermore, locations on either side of the 10-cm gap were perceived to be >3 cm closer together than in a control condition. Thus, the skin stimulus resulted in “abridging” of the perceived motion path, and went part way towards convincing the brain that non-neighbouring areas of skin were adjacent.
SIGNIFICANCE AND IMPLICATIONS
The findings suggest that motion is an important influence on our perceptions of skin location, and that our perceptions of skin location depend on dynamic maps that quickly adjust to current patterns of sensory input. Furthermore, the patterns of skin stimulation that occur with natural movements of objects across the skin may provide a powerful organizing principle for spatial maps.
Seizova-Cajic T, Taylor JL (2014). Somatosensory space abridged: rapid change in tactile localization using a motion stimulus. PLoS One, March DOI: 10.1371/journal.pone 0090892
Braun C, Schweizer R, Elbert T, Birbaumer N, Taub E (2000). Differential activation in somatosensory cortex for different discrimination tasks. J Neurosci 20: 446-450.
Buonomano DV, Merzenich MM (1998). Cortical plasticity: from synapses to maps. Ann Rev Neurosci 21: 149–86.