Spatial working memory deficits represent a core challenge for rehabilitating neglect

Abstract

Left neglect following right hemisphere injury is a debilitating disorder that has proven extremely difficult to rehabilitate. Traditional models of neglect have focused on impaired spatial attention as the core deficit and as such, most rehabilitation methods have tried to improve attentional processes. However, many of these techniques (e.g., visual scanning training, caloric stimulation, neck muscle vibration) produce only short-lived effects, or are too uncomfortable to use as a routine treatment. More recently, many investigators have begun examining the beneficial effects of prism adaptation for the treatment of neglect. Although prism adaptation has been shown to have some beneficial effects on both overt and covert spatial attention, it does not reliably alter many of the perceptual biases evident in neglect. One of the challenges of neglect rehabilitation may lie in the heterogeneous nature of the deficits. Most notably, a number of researchers have shown that neglect patients present with severe deficits in spatial working memory (SWM) in addition to their attentional impairments. Given that SWM can be seen as a foundational cognitive mechanism, critical for a wide range of other functions, any deficit in SWM memory will undoubtedly have severe consequences. In the current review we examine the evidence for SWM deficits in neglect and propose that it constitutes a core component of the syndrome. We present preliminary data which suggest that at least one current rehabilitation method (prism adaptation) has no effect on SWM deficits in neglect. Finally, we end by reviewing recent work that examines the effectiveness of SWM training and how SWM training may prove to be a useful avenue for future rehabilitative efforts in patients with neglect.

Keywords: neglect; parietal lobe; prism adaption; rehabilitation; spatial working memory.

Figure 1

The upper panel depicts the prism adaptation procedure used in neglect. Left: prior to adaptation the patient is blindfolded and asked to point straight ahead of their body midline. Owing to an altered egocentric reference frame, patients typically point far to the right. Middle: during the adaptation procedure patients wear prisms that shift their vision 10° to the right. When asked to point to targets to the left and right they initially miss to the right because of the visual shift induced by the prisms. Right: following ∼5 min of prism adaptation, when the patient is again asked to close their eyes and point straight ahead, they now point much closer to true center. The middle panel depicts typical performance on a cancelation test. Specifically, in addition to missing numerous targets on the left side of the page the patient has also missed a target on the right side of the page. Note that the patient is also demonstrating “revisiting” behavior (highlighted by gray circles) by re-canceling previously canceled items as if they were new, indicative of impaired spatial working memory. The lower panel depicts an example of how prism adaptation improves performance on clinical tests of neglect. Prior to prism adaptation the patient misses targets on the left side of the page. However, following adaptation the patient now cancels many more targets on the left side of the page.

Figure 2

Schematics of the spatial and verbal working memory tasks and results from 2 of the 4 patients tested by Ferber and Danckert (2006). The upper left panel depicts the layout of the spatial working memory task. Three squares were presented vertically aligned in right space for 2 s. Patients had to remember these locations over a 3-s delay. Following the delay a probe stimulus (a circle) appeared and the patient had to decide whether it was in a position previously occupied by one of the three squares. The lower panel depicts the layout for the verbal working memory task. Essentially the verbal working memory task used the same layout as the spatial working memory task. However, instead of remembering target locations patients had to remember three digits over a 3-s delay. Following the delay, the patient had to decide whether the probe digit was the same as one of the three previously presented digits. The right panel depicts the results of the spatial working memory task in a subset of two patients studied by Ferber and Danckert (2006). Specifically, both neglect patients performed extremely poorly on the spatial working memory task compared to right brain damaged controls (n = 4) without neglect (mean performance and standard deviation represented by the dotted line and gray bar). However, both neglect patients performed at ceiling on the verbal working memory task.

Figure 3

Data from the single case study of patients NS, an 80-year-old right-handed female. The upper panel depicts NS’s lesions to the parietal white matter (left) and thalamus (right) of the right hemisphere. To the right of these images are her subjective straight ahead (SSA) judgments made prior to prisms (left panel shows a 4.14° rightward bias) and after prism adaptation (SS = 0.08 degrees – not different from true center relative to her own body midline). The lower panels depict NS’s performance on line bisection, two cancelation tests, and the spatial working memory task prior to (pre-prisms; open bars), and following (post prisms; black bars) prism adaptation. Note that NS demonstrated a significant reduction in her rightward bias in line bisection, and a reduction in the number of items missed on the left in both cancelation tasks, but no change in her spatial working memory performance following prism adaptation. Note that for the spatial working memory data the dotted line and gray bar represent the mean performance (and standard deviation) of a group of right brain damaged controls without neglect tested in a previous study (Ferber and Danckert, 2006). We have since found a similar failure to improve SWM following prism adaptation in a group of six additional right brain damaged patients (Locklin and Danckert, in preparation).