Characteristics of Simian Adaptation Fields Produced by Behavioral Changes in Saccade Size and Direction

Abstract

Characteristics of 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° target steps, >82% of the gain reduction transferred to saccades to 25° horizontal target steps but only ∼30% transferred to saccades to 5° steps. For the horizontal component of oblique saccades to target steps with 15° horizontal components and 10° 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° saccades but only 32% transfer to 5° saccades) and symmetry in the vertical direction (50% transfer to the horizontal component of 10° upward and 40% transfer to 10° downward oblique saccades). When a 5° vertical target movement was made to occur during a saccade to a horizontal 10° 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° saccades transferred to 20° saccades but only 12% transferred to 4° saccades. The transfer was similar to the vertical component of oblique saccades to target steps with either 10° upward (46%) or 10° 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.