Characteristics of simian adaptation fields produced by behavioral changes in saccade size and direction.

The gain of saccadic eye movements can be altered gradually by moving targets either forward or backward during targeting saccades. If the gain of saccades to targets of only one size is adapted, the gain change generalizes or transfers only to saccades with similar vectors. In this study, we examined the spatial extent of such saccadic size adaptation, i.e., the gain adaptation field. We also attempted to adapt saccade direction by moving the target orthogonally during the targeting saccade to document the extent of a direction or cross-axis adaptation field. After adaptive gain decreases of horizontal saccades to 15 degrees target steps, >82% of the gain reduction transferred to saccades to 25 degrees horizontal target steps but only approximately 30% transferred to saccades to 5 degrees steps. For the horizontal component of oblique saccades to target steps with 15 degrees horizontal components and 10 degrees upward or downward vertical components, the transfer was similar at 51 and 60%, respectively. Thus the gain decrease adaptation field was quite asymmetric in the horizontal dimension but symmetric in the vertical dimension. Although gain increase adaptation produced a smaller gain change (13% increase for a 30% forward adapting target step) than did gain decrease adaptation (20% decrease for a 30% backward adapting target step), the spatial extent of gain transfer was quite similar. In particular, the gain increase adaptation field displayed asymmetry in the horizontal dimension (58% transfer to 25 degrees saccades but only 32% transfer to 5 degrees saccades) and symmetry in the vertical direction (50% transfer to the horizontal component of 10 degrees upward and 40% transfer to 10 degrees downward oblique saccades). When a 5 degrees vertical target movement was made to occur during a saccade to a horizontal 10 degrees target step, a vertical component gradually appeared in saccades to horizontal targets. More than 88% of the cross-axis change in the vertical component produced in 10 degrees saccades transferred to 20 degrees saccades but only 12% transferred to 4 degrees saccades. The transfer was similar to the vertical component of oblique saccades to target steps with either 10 degrees upward (46%) or 10 degrees downward (46%) vertical components. Therefore both gain and cross-axis adaptation fields have similar spatial profiles. These profiles resemble those of movement fields of neurons in the frontal eye fields and superior colliculus. How those structures might participate in the adaptation process is considered in the DISCUSSION.