Prism adaptation effects are not limited to dorsal visual processing: evidence from pro-pointing and anti-pointing

Prism adaptation (PA) offers a promising rehabilitation tool for stroke patients with neglect (a lack of awareness of the contralesional side of space), yet its underpinning mechanisms remain poorly understood. One hypothesis suggests that PA relies heavily on dorsal, rather than ventral, visual stream processing thus affecting visuomotor rather than perceptual tasks (1). According to the perception-action model (2) the two visual streams code information about visual targets in different frames of reference, an aspect so far unexplored in the PA literature. Specifically, the dorsal stream provides visual representations of targets in an egocentric (viewer-based) reference frame (e.g. reaching for a mug) while the ventral stream codes visual targets in an allocentric (scene-based) reference frame (e.g. drawing or sketching). We hypothesised that if PA effects are achieved largely via dorsal processing, greater after-effects should be present during tasks that rely on an egocentric (i.e. pro- pointing) rather than an allocentric (i.e. anti-pointing) coding. To this end 24 participants performed blocks of pro- and antipointing after sham- and prism-adaptation to either leftward- or rightward-shifting prisms. In line with previous research prism-adaptation induced pointing after-effects in the opposite direction to the prismatic shift. However, no differences between pro- and anti-pointing tasks were observed after prisms suggesting that the effects of PA arise via the influence of both dorsal and ventral stream processing.