Brandes, J. & Heed, T. (2015). Reach Trajectories Characterize Tactile Localization for Sensorimotor Decision Making, Journal of Neuroscience 35(40): 13648-13658; doi: 10.1523/JNEUROSCI.1873-14.2015
I’m very happy that this paper is now out. It is the first paper of Janina’s PhD work. She spent a ton of time to optimize the experimental paradigm and, even more so, the analysis.
The research question
Imagine you feel an itch on your foot and want to scratch it. Where your hand has to go depends on where you’ve placed your foot. The brain merges body posture and the skin location of the itch so that your hand goes to the right place.
But how exactly the brain localizes the touch is debated. The origin of the debate are results from experiments with crossed limbs. In a number of tasks involving touch, people are much worse with crossed than with uncrossed hands.
When you cross your hands, your right hand (that is, the skin location) lies in left space. So the two pieces of information – skin and space – are in conflict. But what does that tell us about how the brain actually processes touch location?
One hypothesis is that hand crossing makes it difficult for the brain to compute the touch location in space. According to this idea, the skin location is known fast, but (with crossed limbs) the spatial location becomes available late. Another hypothesis is that the computation is actually not at all difficult. Rather, the brain remembers the skin location, and integrates skin and spatial locations. With crossed hands, the two are in conflict (right vs. left), and this conflict must be resolved. Our experiment sought to dissociate these two accounts.
What we show
In our experiment, participants received a touch on their crossed feet, and they had to reach to the touched location. If only skin location was available at first then people should initially reach to the wrong foot, because the skin location is opposite to the foot’s actual spatial location (Hypothesis 1).
In contrast, if the brain computes the spatial location fast but then tries to resolve the conflict then people should take a while to reach (longer than when the feet are not crossed and there isn’t any conflict), but the reach should go directly to the correct location (Hypothesis 2).
The second option is basically what we found. Overall, it’s not quite that simple so read about the details in the paper…
Why the results are important
We show that the transformation from skin location to the 3D location in space is not difficult. With this finding, we refute a common idea about how touch is located.
We demonstrate experimentally that the brain integrates information from different kinds of spatial representations to localize touch. We already had found modeling evidence for this in a study by Steph Badde (recently came out, see here), and had also presented this hypothesis in our recent TiCS paper. Finally, our results provide a direct link from spatial integration in touch to decision making models such as drift diffusion models (more on that in the paper!).