Eye-movements as a signature of age-related differences in global planning strategies for spatial navigation

The ability to efficiently find alternatives routes when faced with unexpected obstacles along our path is among the most compelling evidence of the flexibility of human behaviour. Although a plethora of plausible computations have been put forward to elucidate how the brain accomplishes efficient goal-oriented navigation, the mechanisms that guide an effective re-planning when facing obstructions are still largely undetermined. There is a fair consensus in postulating that possible alternatives routes are internally replayed sampling from past experiences, however, there is currently no account of the criterion according to which those memories are replayed. Here, we posit that paths, which are expected to be more rewarding are replayed more often and that eye movements are the explicit manifestation of this re-planning strategy. In other words, the visual sampling statistics reflects the retrieval of available routes on a mental representation of the environment. To test our hypothesis, we measured the ability of both young and old human subjects to solve a virtual version of the Tolman maze, while we recorded their eye movements. We used reinforcement learning (RL) to corroborate that eye movements statistics was crucially subtending the decision making process involved in re-planning and that the incorporation of this additional information to the algorithm was necessary to reproduce the behavioral performance of both screened populations.