Adaptation to rotated visual feedback depends on the number and spread of target directions.

It has been proposed in the past that adaptation to rotated visual feedback is based on directionally tuned modules. Here, we investigate whether adaptation depends on the number of modules that are concurrently activated. To disambiguate the number of modules and their spatial overlap, we decided to vary the number of target directions and their spacing independently. In light of recent work on the existence of fast and slow adaptive processes, we analyzed the role of target number and spacing separately for the first eight movements under rotated visual feedback and for later movements. We found that during the first eight movements, adaptation progressed three times faster when targets were spaced across a 42° rather than across a 360° range, irrespective of their number. During the subsequent movements, adaptation progressed 1.66 times faster with two than with eight targets, irrespective of their spacing. This differential dependence of early and late adaptation on target metrics confirms the existence of adaptive processes with different time scales and suggests that those processes differ not only by their clocking speed, but also by their functional properties. Specifically, the speed of fast processes seems to be constrained by the directional tuning width of adaptive modules, and the speed of slow processes by the number of presentations per target direction.