Comparison of adaptation characteristics between visually and memory-guided saccades.

Saccade adaptation plays a crucial role in maintaining saccade accuracy. The behavioral characteristics and neural mechanisms of saccade adaptation for an externally cued movement, such as visually guided saccades (VGS), are well studied in nonhuman primates. In contrast, little is known about the saccade adaptation of an internally driven movement, such as memory-guided saccades (MGS), which are guided by visuospatial working memory. As the oculomotor plant changes because of growth, aging, or skeletomuscular problems, both types of saccades need to be adapted. Do both saccade types engage a common adaptation mechanism? In this study, we compared the characteristics of amplitude decrease adaptation in MGS with VGS in nonhuman primates. We found that the adaptation speed was faster for MGS than for VGS. Saccade duration changed during MGS adaptation, whereas saccade peak velocity changed during VGS adaptation. We also compared the adaptation field, that is, the gain change for saccade amplitudes other than the adapted. The gain change for MGS declines on both smaller and larger sides of adapted amplitude, more rapidly for larger than smaller amplitudes, whereas the decline in VGS was reversed. Thus, the differences between VGS and MGS adaptation characteristics support the previously suggested hypothesis that the adaptation mechanisms of VGS and MGS are distinct. Furthermore, the result suggests that the MGS adaptation site is a brain structure that influences saccade duration, whereas the VGS adaptation site influences saccade peak velocity. These results should be beneficial for future neurophysiological experiments. Plasticity helps to overcome persistent motor errors. Such motor plasticity or adaptation can be investigated with saccades. Thus far our knowledge is primarily about visually guided saccades, an externally cued movement, which we can make only when the object is visible at the time of saccade. However, as the world is complex, we can make saccades even when the object is not visible. Here, we investigate the adaptation of an internally driven movement: the memory-guided saccade.