Étude du cortex prémoteur et préfrontal lors de la prise de décision pendant l'intégration temporelle des informations

A variety of models of the decision-making process in many different contexts suggest that subjects sample, accumulate and integrate sensory evidence for and against different alternative choices, until one of those signals exceeds a decision criterion threshold. Early models assumed that this process is static and does not change during a trial or even between trials, but only between blocks of trials when task demands such as speed versus accuracy change. However, newer models suggest that the decision-making process is dynamic and factors that influence the evidence accumulation process might change both between trials in a block and even during a trial.  This thesis project aims to demonstrate that decisions about reaching movements emerge from a mechanism of integration of sensory evidence to a decision criterion threshold. We developed a paradigm for decision-making about reach direction based on ambiguous sensory input to search for neural correlates of the decision-making process in primary motor cortex (M1), premotor cortex (PMd) and dorsolateral prefrontal cortex (DLPFc).  We first tested several versions of the task with human subjects before developing a task (“Choose and Go”) that showed ideal behavior from the subjects to test our hypothesis. The task required subjects to choose between two color-coded targets in different spatial locations by deciding the predominant color of a central “decision cue” that contained different amounts of colored squares of the two target colors. The strength of the evidence was manipulated by varying the relative numbers of squares of the two colors. The response times and error rates both increased in parallel as the strength of the sensory evidence in the decision cue (its color bias) became increasingly weaker. Computational modelling showed that the choice behaviour of the subjects could be captured by different variants of the drift-diffusion model for accumulation of sensory evidence to a decision threshold. We then recorded cells from M1, PMd and DLPFc in 2 macaques while they performed the task. Behavioral data showed that response times and error rates increased with the amount of ambiguity of the decision cues. M1 cells discharged in correlation with movement onset and were not influenced by the ambiguity of the decision cues. In contrast, the discharge of PMd cells increased more slowly with increased ambiguity of the decision cues and took increasingly more time to signal the movement direction chosen by the monkeys. The changes in activity reflected the monkeys’ reach choices. These data support a role for PMd in the choice of reach direction. DLPFc data are preliminary but reveal a stronger effect of the color-location conjunction rule in the neuronal discharge than in PMd.  Our conclusion is that PMd is involved in the evaluation of evidence for and against different alternatives and about target spatial location independent of the color of the targets. DLPFC neurons play a greater role in processing information about the color and location of the spatial targets and decision cue to resolve the color-location conjunction rule required to decide on the reach target direction.

Keyphrases: human, monkey, decision making process, movement, reaching task, primary motor cortex, premotor cortex, prefrontal cortex