Abstract
Prism adaptation (PA) is both a model for visuomotor learning and a promising treatment for visuospatial neglect after stroke. The task involves reaching for targets while prism glasses horizontally displace the visual field. Adaptation is hypothesized to occur through two processes: strategic recalibration, a rapid self-correction of pointing errors; and spatial realignment, a more gradual adjustment of visuomotor reference frames that produce prism aftereffects (i.e., reaching errors upon glasses removal in the direction opposite to the visual shift). While aftereffects can ameliorate neglect, not all patients respond to PA, and the neural mechanisms underlying successful adaptation are unclear. We investigated the feedback-related negativity (FRN) and the P300 event-related potential (ERP) components as candidate markers of strategic recalibration and spatial realignment, respectively. Healthy young adults wore prism glasses and performed memory-guided reaching toward vertical-line targets. ERPs were recorded in response to three different between-subject error feedback conditions at screen-touch: view of hand and target (Experiment 1), view of hand only (Experiment 2), or view of lines to mark target and hand position (view of hand occluded; Experiment 3). Conditions involving a direct view of the hand-produced stronger aftereffects than indirect hand feedback, and also evoked a P300 that decreased in amplitude as adaptation proceeded. Conversely, the FRN was only seen in conditions involving target feedback, even when aftereffects were smaller. Since conditions producing stronger aftereffects were associated with a phase-sensitive P300, this component may index a “context-updating” realignment process critical for strong aftereffects, whereas the FRN may reflect an error monitoring process related to strategic recalibration.
Highlights
Prism adaptation (PA) is a visuomotor task that demonstrates the brain’s adaptation to changes in visually perceived coordinates of objects in space (Redding et al, 2005)
Post hoc analyses showed that big misses’’ had more positive amplitudes than ‘‘hits’’ or ‘‘small misses’’ (p < 0.05), and the contrast analysis suggested that voltage increased with error size, F(1, 21) = 13.61, p = 0.001, partial η2 = 0.39 (Figure 5B)
Post hoc analyses showed that all three accuracy levels differed from each other (p < 0.05); namely, contrast analyses suggested that amplitudes became increasingly negative as error size increased, F(1, 21) = 40.40, p = 0.017, partial η2 = 0.66 (Figure 7A)
Summary
Prism adaptation (PA) is a visuomotor task that demonstrates the brain’s adaptation to changes in visually perceived coordinates of objects in space (Redding et al, 2005). Upon glasses removal, they make errors in the direction opposite to the preceding prismatic displacement (i.e., aftereffects), providing a measure of the strength of adaptation (Redding et al, 2005). The leftward aftereffects following right-shifting PA can improve performance on visual scanning and reaching tasks (e.g., Rossetti et al, 1998; Nys et al, 2008), as well as in daily activities such as postural balancing or wheelchair driving (Tilikete et al, 2001; Jacquin-Courtois et al, 2008). Acquiring a better understanding of the neural processes that give rise to PA aftereffects, and presumably contribute to improvements in neglect symptoms, might help optimize the therapy for clinical use
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