Does eye vergence dissociate between covert and overt attention?

Abstract

The neural mechanisms of attention are closely related to oculomotor control of saccadic eye movements and vergence eye movements. Visual covert attention is a mechanism for mentally scanning the visual field to enhance the sensory signal. This shift in covert attention is linked to eye movement circuitry that prepares a saccadic eye movement to a particular location. Overt attention is believed to direct the saccade towards that location. Currently, it is unclear whether and how covert and overt attention influences vergence responses. To test this idea, we used a visual task in which subjects focused on a central fixation spot surrounded by an array of 8 letters. After one second of fixation one of the letters flashed. After additional fixation period an identical or a different letter briefly appeared at the fixation spot. The observer responded by making a saccade towards the flashed letter in the case it was the same as the central letter. Otherwise the observer remained its gaze at the fixation spot. In addition a button press was requested. Our findings show that eye vergence changes during the task. During the initial period, where covert attention was required, eyes converge in a plane further away than the physical depth (screen) plane. During the period a saccade was planned (overt attention) eyes converged back to the depth plane of the screen. From our observation we conclude that eye vergence may serve not only for depth perception but also have a role in covert and overt attention. The findings are interpreted in terms of relaxation during covert attention, perceived depth, and that during covert attention the visual system benefits from a wider view of field.