Visual imagery is a basic form of cognition central to activities such as problem solving or creative thinking. Phenomena such as mental rotation, in which mental images undergo spatial transformations, and motion imagery, in which we imagine objects in motion, are very elusive. For example, although several aspects of visual imagery and mental rotation have been reconstructed through mental chronometry, their instantaneous evolution has never been directly observed. We paired mental chronometry to eye movement recording in subjects performing a visuospatial mental rotation task and an instructed circular motion imagery task. In both tasks, sequences of spontaneous saccades formed curved trajectories with a regular spatio-temporal evolution. In the visuospatial mental rotation task, saccadic amplitude decreased progressively within each sequence, resulting in an average gaze rotation with a bell-shaped asymmetrical angular velocity profile whose peak and mean increased with the amount of the to-be-performed rotation, as in reaching movements. In the second task, the average gaze rotation reproduced faithfully the to-be-imagined constant-velocity circular motion, thus excluding important distortions in the oculomotor performance. These findings show for the first time the instantaneous spatio-temporal evolution of mental rotation and motion imagery. Moreover, the fact that visuospatial mental rotation is modeled as a reaching act suggests that reaching pertains to the realm of visuospatial thinking, rather than being restricted to the motor domain. This approach based on eye movement recording can be profitably coupled to methods such as event-related potentials, transcranial magnetic stimulation, or functional magnetic resonance to study the precise neuronal dynamics associated with an ongoing mental activity.

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