Effect of Saccadic Adaptation on Sequences of Saccades

Our movements allow us to interact with the surrounding world. The contribution of the motor system in acquisition of sensory information is particularly critical for vision. Because high visual acuity is restricted to the central part of the retina, saccadic eye movements are necessary for collecting detailed visual information. Thus, a series of different snapshots of the visual scene is provided by retinal input following each saccade. There is a large consensus about the brain capability to predict the retinal input sampled after each saccade, helping both visual exploration and the generation of subsequent saccades (Wurtz, 2008). Regarding saccade production, this prediction capability has been demonstrated in double-saccade tasks. These tasks have indeed shown that both monkey and human subjects can generate two accurate saccades in response to the rapid succession of two flashed targets, even when the second target disappears during the latency period of the first saccade (Hallett & Lightstone, 1976; see review in Pisella et al., 2011). The accurate production of the second saccade without direct visual input relies on brain mechanisms that compensate for the eye displacement achieved during the first saccade and keep track of this nonvisual eye displacement information. In principle, this extra-retinal signal of eye displacement could be provided by ocular proprioceptive information or by a copy of the motor command sent to the extraocular muscles (termed efference copy or corollary discharge). However, empirical studies in the monkey have refuted the contribution of ocular proprioception (Guthrie, Porter, & Sparks, 1983; Lewis, Zee, Hayman, & Tamargo, 2001; Sparks & Mays, 1983) and it is now widely accepted that the extra-retinal signals used to accurately plan subsequent saccades are based on an efference copy signal (Moschovakis, Scudder, & Highstein, 1996; Robinson, 1975; Keller, 1981).