Goal-directed reaching: the allocentric coding of target location renders an offline mode of control

Abstract

Reaching to a veridical target permits an egocentric spatial code (i.e., absolute limb and target position) to effect fast and effective online trajectory corrections supported via the visuomotor networks of the dorsal visual pathway. In contrast, a response entailing decoupled spatial relations between stimulus and response is thought to be primarily mediated via an allocentric code (i.e., the position of a target relative to another external cue) laid down by the visuoperceptual networks of the ventral visual pathway. Because the ventral stream renders a temporally durable percept, it is thought that an allocentric code does not support a primarily online mode of control, but instead supports a mode wherein a response is evoked largely in advance of movement onset via central planning mechanisms (i.e., offline control). Here, we examined whether reaches defined via ego- and allocentric visual coordinates are supported via distinct control modes (i.e., online versus offline). Participants performed target-directed and allocentric reaches in limb visible and limb-occluded conditions. Notably, in the allocentric task, participants reached to a location that matched the position of a target stimulus relative to a reference stimulus, and to examine online trajectory amendments, we computed the proportion of variance explained (i.e., R² values) by the spatial position of the limb at 75% of movement time relative to a response's ultimate movement endpoint. Target-directed trials performed with limb vision showed more online corrections and greater endpoint precision than their limb-occluded counterparts, which in turn were associated with performance metrics comparable to allocentric trials performed with and without limb vision. Accordingly, we propose that the absence of ego-motion cues (i.e., limb vision) and/or the specification of a response via an allocentric code renders motor output served via the 'slow' visuoperceptual networks of the ventral visual pathway.

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