Thesis defense Frank Leoné (Donders Series 171)
24 October 2014
Promotors: Prof.dr. P. Medendorp, Prof.dr. I. Toni
Mapping sensorimotor space: Parieto-frontal contributions to goal-directed movements
Imagine how your brain processes the world. Imagine a red line, from the center of your gaze, to your cup of coffee. Imagine a second, blue, line, from the tip of your right index finger, to the same coffee cup. Last, imagine a third, green, line, from the tip of your right big toe, again to the coffee cup. Now, move your eyes. See the red line change length and direction. Notice how your coffee cup moves from the right side of your gaze to the left side and back. Now do the same with your hand. And with your foot. Imagine the lines changing, independently of each other, each line keeping track of the location of your cup of coffee, even though the cup is continually at a single position. The brain however codes targets relative to body parts, and transform positions from perception and action. For example, from the red line (gaze) to the blue line (hand) to actually grasp your cup of coffee.
We studied how the brain performs such sensorimotor transformations: the transformation of sensory information to motor actions. Specifically, we studied how the brain represents motor coordinates, how it transforms coordinates from sensory perception to action and how it selects the appropriate body part to respond with. All studies were performed used fMRI, applying newly developed analysis methods. The methods allowed, respectively, to test the shape of the cortical response fields, take both negative and positive evidence for a hypothesis into account (what should be the same vs. different), and combine measures of both average activation and detailed pattern of activation in a region. Using these methods, we could shed more light on the dynamics of parieto-frontal sensorimotor mappings.
What we found is the following. Visual information enters the posterior parietal cortex in gaze-centered coordinates (relative to gaze: red line). The posterior parietal regions code the direction and amplitude of visual targets (respectively the direction and length of the line) for saccade planning. Medial regions encode large amplitude, while lateral regions code short amplitudes. More anterior parietal regions are involved in effector (e.g., hand or foot) selection. Medial anterior regions code movement planning for the limbs, lateral regions for the fingers. The type of movements controlled determines the cortical location where reference transformations are performed, the modality of the target (visual or proprioceptive) the dominant reference frame. Finally, frontal regions are involved in the actual execution of the movements.
Future studies are needed to solidify these conclusions. To illustrate, compare cognitive neuroscience to the exploration by Christopher Columbus. Columbus sailed out to find the west sea route to the Indies. After finding land, he assumed his assumptions were correct and named the native inhabitants 'Indians'. This assertion was however not based on discoveries (the 'data') per se, but rather on the all too strong projections of his assumptions on the data. The same might be true for cognitive neuroscience. Young as we are as a science, we might be studying the mirror image of our assumptions, rather than the data. More research on the validity of these assumptions, more data-driven studies, and data aggregation could help bring the field forward. Methodological developments, in this thesis and elsewhere, can help us to see how.