Oculomotor plasticity: are mechanisms of adaptation for reactive and voluntary saccades separate?

Saccadic eye movements are permanently controlled and their accuracy maintained by adaptive mechanisms that compensate for physiological or pathological perturbations. In contrast to the adaptation of reactive saccades (RS) which are automatically triggered by the sudden appearance of a single target, little is known about the adaptation of voluntary saccades which allow us to intentionally scan our environment in nearly all our daily activities. In this study, we addressed this issue in human subjects by determining the properties of adaptation of scanning voluntary saccades (SVS) and comparing these features to those of RS. We also tested the reciprocal transfers of adaptation between the two saccade types. Our results revealed that SVS and RS adaptations disclosed similar adaptation fields, time course and recovery levels, with only a slightly lower after-effect for SVS. Moreover, RS and SVS main sequences both remained unaffected after adaptation. Finally and quite unexpectedly, the pattern of adaptation transfers was asymmetrical, with a much stronger transfer from SVS to RS (79%) than in the reverse direction (22%). These data demonstrate that adaptations of RS and SVS share several behavioural properties but at the same time rely on partially distinct processes. Based on these findings, it is proposed that adaptations of RS and SVS may involve a neural network including both a common site and two separate sites specifically recruited for each saccade type.