When we notice a mosquito alight on our forearm, we direct our gaze in order to find its exact position and quickly try to swat it or brush it away to prevent it bite us. This apparently simple, instantaneous reaction is the result of a mental process that is much more complex than it may seem. It requires the brain to align the tactile sensation on the skin with spatial information about our surroundings and our posture.
For the first time, a study done by the Cognitive Neuroscience Research Group (GRNC), attached to the Barcelona Science Park, has shown how this process unfolds over time, examining the conflicts posed by the coexistence of differing spatial maps in the brain. GRNC researchers Salvador Soto-Faraco (ICREA research professor) and Elena Azañón conducted the new study.
“The main finding of the study is that it has enabled us to confirm that tactile sensations are initially located unconsciously in anatomical coordinates, but they reach our awareness only when the brain has formed an image of their origin in the spatial coordinates, external to the body,” explained Salvador Soto-Faraco. The coexistence of different spatial reference frames in the brain has been known for some time. So has the fact that confusions between them may result in some cases, such as when we invert the usual anatomical position of some body parts (e.g. when crossing our arms over the body midline). “The brain sorts out problems of this kind rapidly, in a matter of tenths of a second. To do so, however, it has to integrate information arriving in formats that are quite disparate”, Sotoa-Faraco added. “Our research has helped us understand how this process works and how the brain manages spatial realignment when faced with conflict”, he concluded.
A simple example serves to illustrate the confusion that can occur when different spatial reference frames are set in conflict: cross one of your arms over the other, then interleave the fingers of both hands together, palms touching, and turn your hands towards your body so that the left hand is on the right side and vice versa. While holding this position, if you receive an instruction but no direct physical contact that you are to move one of your fingers, you will most likely move the equivalent finger of the opposite hand.
In order to determine how long it takes for the brain to realign these conflicting spatial reference frames, the GRNC researchers devised a specific methodology that enabled indirect measurement of the location of a tactile sensation on the skin. To do this, they measured response times to a brief flash (produced with an LED light emitting diode) appearing near one of the observer’s hands. The researchers then compared the reaction times to the flash when it had appeared near a hand that had previously received a tactile stimulus, versus when the flash had appeared near the opposite hand. In the main study, the participants (a group of 32 university students) were asked to cross their arms so that their right hand lay in their left-hand visual field and vice versa. The purpose of this procedure was to ensure that the actual external position of the hands was in conflict with their anatomical location.
Each participant underwent roughly 600 essays of this sort. The time between the tactile sensation and the appearance of the target visual stimulus, as well as their realtive locations, were varied at random. It was observed that the participants’ responses to the flash changed dramatically as a function of the time elapsed between receiving the tactile sensation and the presentation of the visual stimulus. In the initial phase (60 ms or earlier), the brain tended to locate the tactile sensation in anatomical terms, i.e. if it received the sensation on the left hand, even though it was crossed over to the right- visual field, the sensation was processed as though it had happened on the left-hand side of the body. However, only a few tenths of a second later (roughly 200 ms), compensation occurred and the tactile sensation was determined to arise from the right-hand side.
Curiously, when participants in the study were asked to locate the tactile stimulus explicitly, they always referred their response to its external source. This reveals that, although a transition occurs from an initial anatomically-based reference frame towards a visually or externally-based reference frame, we apparently become aware of the tactile sensation in the latter phase.
“The study’s results have allowed us to deepen our understanding of how tactile information is located, suggesting that our brain avoids confusions among the various spatial reference frames by keeping the initial part of the process below the threshold of awareness”, explained Soto-Faraco. “Put simply, it could be said that this system of spatial transformation works much as when we hastily jot down some rough notes and later copy them out into final form, discard the original draft,” he concluded.
- Azañón et al. Changing Reference Frames during the Encoding of Tactile Events. Current Biology, 2008; DOI: 10.1016/j.cub.2008.06.045
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