Dissociating perceptual and motor effects of prism adaptation in neglect

Abstract

Prism adaptation reduces some symptoms of neglect; however the mechanisms underlying such changes are poorly understood. We suggest that prisms influence neglect by acting on dorsal stream circuits subserving visuomotor control, with little influence on perceptual aspects of neglect. We examined prism adaptation in three neglect patients and a group of healthy controls on line bisection and landmark tasks. Neglect patients showed a dramatic reduction in the rightward bias for line bisection, but absolutely no change in their leftward bias on the landmark task, which is a perceptual equivalent to bisection. However, in controls, prisms produced 'neglect-like' deficits on both the line bisection and landmark tasks. These data suggest that prisms influence visually guided actions more so than perception in neglect.

Prism adaptation power on spatial cognition: Adaptation to different optical deviations in healthy individuals

“…Nevertheless, it should be noticed here that, although the majority of studies examining cognitive after-effects in healthy subjects have used 15 • leftward optical deviations e.g., [6][7][8][9], at least two studies have shown after-effects on the landmark task following adaptation to only a 10 • leftward optical deviation [14,20]. It is likely that the experimental conditions (e.g., different line locations presented on a computer screen [20]) or alternate data analyses (e.g., considering only the percentage of 'left responses' without determining the subjective center of the line [14]) led to a more sensitive method for expressing the perceptual bias in the landmark task. Concerning manual bisection, it is worth noting that the slow movement executions may exclude open-loop control processes which were mainly assessed in the sensorimotor open-loop pointing [7].…”

“…With respect to the cognitive after-effects, the 10 • deviation seems to be a threshold for influencing manual line bisection, whereas the 15 • deviation represents a threshold for producing cognitive aftereffects in both the line bisection and landmark tasks. Because the manual bisection task combines both perceptual/representational and intentional processes, both of which are affected by prism adaptation [7,14,22], it is likely that this task is an appropriate tool to show cognitive after-effects arising from a moderate optical deviation (10 • ). A perceptual bias may only be expressed following adaptation to a 15 • leftward optical deviation (i.e., when the strength of the optical deviation is stronger).…”

“…At a sensorimotor level, after-effects reach 75% of the optical deviation in neglect patients [1], whereas they typically don't exceed 50% of the optical deviation in healthy individuals e.g., [12]. At a cognitive level, the reduction of rightward neglect biases in line bisection reach a magnitude of several centimetres [13,14], whereas the neglectlike rightward bias in healthy subjects reaches a magnitude of only several millimeters e.g., [7,14]. It is likely that the greater amplitude for both sensorimotor and cognitive after-effects in neglect patients reflects the exaggerated sensitivity of a damaged brain to sensorimotor plasticity.…”

Modulation of visual attention by prismatic adaptation

“…The SHD-VAS model offers a parsimonious explanation for both the positive effects and for absence of effects. The key feature of tasks that are improved by rightward PA is the involvement of the dorsal attentional system (Striemer and Danckert, 2010).…”

“…It was repeatedly shown to decrease lateralized deficits in neglect and is believed to alleviate attentional bias in neglect; however, its underlying neural mechanisms remain poorly understood (Barrett et al, 2012;Newport and Schenk, 2012;Redding and Wallace, 2006;Rode et al, 2003). The overall effect of rightward PA in neglect was interpreted as modulation of visual spatial attention, modification of spatial representations and/or changes in visually guided motor behaviours Striemer and Danckert, 2010;Jacquin-Courtois et al, 2013).…”