Christopher L. Striemer

Neglected Time: Impaired Temporal Perception of Multisecond Intervals in Unilateral Neglect

Recent neuroimaging and neuropsychological studies have suggested that the right hemisphere, particularly frontal regions, is important for the perception of the passage of time. We examined the ability to estimate durations of up to 60 sec in a group of eight patients with unilateral neglect. When estimating multisecond intervals, neglect patients grossly underestimated all durations. On average, healthy controls (HC) demonstrated reasonably accurate estimates of all durations tested. Although the right hemisphere lesioned control patients without neglect also tended to underestimate durations, these underestimations were significantly better than the performance of the neglect group. These findings suggest a pivotal role for a right hemisphere fronto-parietal network in the accurate perception of multisecond durations. Furthermore, these findings add to a growing body of literature suggesting that neglect cannot be understood simply in terms of a bias in orienting attention to one side of space. Additional deficits of the kind demonstrated here are likely to be crucial in determining the nature and extent of the loss of conscious awareness for contralesional events.

Through a prism darkly: re-evaluating prisms and neglect

Many studies have demonstrated that prism adaptation can reduce several symptoms of visual neglect: a disorder in which patients fail to respond to information in contralesional space. The dominant framework to explain these effects proposes that prisms influence higher order visuospatial processes by acting on brain circuits that control spatial attention and perception. However, studies that have directly examined the influence of prisms on perceptual biases inherent to neglect have revealed very few beneficial effects. We propose an alternative explanation whereby many of the beneficial effects of prisms arise via the influence of adaptation on circuits in the dorsal visual stream controlling attention and visuomotor behaviors. We further argue that prisms have little influence on the pervasive perceptual biases that characterize neglect.

Dissociating perceptual and motor effects of prism adaptation in neglect.

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.

“Real-time” obstacle avoidance in the absence of primary visual cortex

When we reach toward objects, we easily avoid potential obstacles located in the workspace. Previous studies suggest that obstacle avoidance relies on mechanisms in the dorsal visual stream in the posterior parietal cortex. One fundamental question that remains unanswered is where the visual inputs to these dorsal-stream mechanisms are coming from. Here, we provide compelling evidence that these mechanisms can operate in “real-time” without direct input from primary visual cortex (V1). In our first experiment, we used a reaching task to demonstrate that an individual with a dense left visual field hemianopia after damage to V1 remained strikingly sensitive to the position of unseen static obstacles placed in his blind field. Importantly, in a second experiment, we showed that his sensitivity to the same obstacles in his blind field was abolished when a short 2-s delay (without vision) was introduced before reach onset. These findings have far-reaching implications, not only for our understanding of the time constraints under which different visual pathways operate, but also in relation to how these seemingly “primitive” subcortical visual pathways can control complex everyday behavior without recourse to conscious vision.

Differential influences of prism adaptation on reflexive and voluntary covert attention

Recent research has demonstrated some beneficial effects in patients with neglect using rightward shifting prismatic lenses. Despite a great deal of research exploring this effect, we know very little about the cognitive mechanisms underlying prism adaptation in neglect. We examined the possibility that prism adaptation influences visual attention by having healthy participants complete either a reflexive or a voluntary covert visual attention cuing paradigm before and after adaptation to leftward, rightward, or sham (no shift) prisms. The results for reflexive orienting demonstrated that a subset of participants with large cuing effects before prism adaptation were faster to reorient attention away from an invalid cue on the side of space opposite the prismatic shift post adaptation. For voluntary orienting, left prisms increased the efficiency of voluntary attention in both left and right visual space in participants with a small cuing effect before prism adaptation. In contrast, right prisms decreased the efficiency of voluntary attention in both left and right space for participants with a large cuing effect before prism adaptation. No significant effects were observed in the sham prism groups. These results suggest that prism adaptation may exert a variety of influences on attentional orienting mechanisms.

Deficits in peripheral visual attention in patients with optic ataxia.

Earlier research has suggested that optic ataxia, a deficit in reaching in peripheral vision, can be isolated from Balints syndrome as it is primarily a visuomotor disorder, independent of perceptual or attentional deficits. Yet almost no research has examined the attentional abilities of these patients. We examined peripheral visual attention in two patients with unilateral optic ataxia. Results indicated that both patients were slower to respond to targets in their ataxic visual field, irrespective of cuing condition (i.e. validly, invalidly, and no cue conditions), consistent with an overall decrease in the salience of stimuli in the ataxic field. Attentional deficits in peripheral vision are therefore an important factor to consider when examining visuomotor control deficits in optic ataxia.

Programs for action in superior parietal cortex: a triple-pulse TMS investigation.

Converging evidence from neurological patients and functional brain imaging studies strongly supports the notion that the posterior parietal cortex (PPC), especially in the left hemisphere, plays a critical role in both the programming (i.e., setting the initial movement parameters of the reach) and the online control of goal-directed reaching movements. Importantly, however, there is no clear consensus on how different subregions within the PPC contribute to the programming and online control of reaching. In the current study, we investigated the role of the inferior (IPL) and superior (SPL) parietal lobules in reach programming using MRI-guided event-related transcranial magnetic stimulation (TMS). Specifically, we applied triple-pulse (tp) TMS to either the left IPL or the left SPL at different time points during reaching movements either at target onset (programming) or at movement onset (online control) while participants (n=16) made pointing movements to targets in the periphery without visual feedback of the moving hand. Stimulating the SPL but not the IPL resulted in a significant increase in endpoint errors when tp-TMS was applied during the programming phase compared to the online control phase. In short, these data demonstrate that the SPL plays a critical role in real-time movement programming.

Spatial working memory deficits represent a core challenge for rehabilitating neglect.

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.

Can intention override the “automatic pilot”?

Previous research has suggested that the visuomotor system possesses an “automatic pilot” which allows people to make rapid online movement corrections in response to sudden changes in target position. Importantly, the automatic pilot has been shown to operate in the absence of visual awareness, and even under circumstances in which people are explicitly asked not to correct their ongoing movement. In the current study, we investigated the extent to which the automatic pilot could be “disengaged” by explicitly instructing participants to ignore the target jump (i.e., “NO-GO”), by manipulating the order in which the two tasks were completed (i.e., either “GO” or NO-GO first), and by manipulating the proportion of trials in which the target jumped. The results indicated that participants made fewer corrections in response to the target jump when they were asked not to correct their movement (i.e., NO-GO), and when they completed the NO-GO task prior to the task in which they were asked to correct their movement when the target jumped (i.e., the GO task). However, increasing the proportion of jumping targets had only a minimal influence on performance. Critically, participants still made a significant number of unintended corrections (i.e., errors) in the NO-GO tasks, even under explicit instructions not to correct their movement if the target jumped. Overall these data suggest that, while the automatic pilot can be influenced to some degree by top-down strategies and previous experience, the pre-potent response to correct an ongoing movement cannot be completely disengaged.

Deficits in reflexive covert attention following cerebellar injury.

Traditionally the cerebellum has been known for its important role in coordinating motor output. Over the past 15 years numerous studies have indicated that the cerebellum plays a role in a variety of cognitive functions including working memory, language, perceptual functions, and emotion. In addition, recent work suggests that regions of the cerebellum involved in eye movements also play a role in controlling covert visual attention. Here we investigated whether regions of the cerebellum that are not strictly tied to the control of eye movements might also contribute to covert attention. To address this question we examined the effects of circumscribed cerebellar lesions on reflexive covert attention in a group of patients (n = 11) without any gross motor or oculomotor deficits, and compared their performance to a group of age-matched controls (n = 11). Results indicated that the traditional RT advantage for validly cued targets was significantly smaller at the shortest (50 ms) SOA for cerebellar patients compared to controls. Critically, a lesion overlap analysis indicated that this deficit in the rapid deployment of attention was linked to damage in Crus I and Crus II of the lateral cerebellum. Importantly, both cerebellar regions have connections to non-motor regions of the prefrontal and posterior parietal cortices—regions important for controlling visuospatial attention. Together, these data provide converging evidence that both lateral and midline regions of the cerebellum play an important role in the control of reflexive covert visual attention.