Alison R. Lane

Visual Exploration Training Is No Better than Attention Training for Treating Hemianopia.

Patients with homonymous visual field defects experience disabling functional impairments as a consequence of their visual loss. Compensatory visual exploration training aims to improve the searching skills of these patients in order to help them to cope more effectively. However, until now the efficacy of this training has not been compared to that of a control intervention. Given that exploration training uses the visual search paradigm, which is known to require visual attention, in this study the efficacy of the technique was compared with training that requires visual attention but not exploration. Participants completed either exploration training (n = 21), or attention training followed by exploration training (n = 21). Assessment of the visual field, visual search, reading and activities of daily living were performed before and after each intervention that the participants completed. The results revealed that both the exploration training and the attention training led to significant improvements in most of the visual tasks. For most of the tasks exploration training did not prove superior to attention training, and for reading both types of intervention failed to yield any benefits. The results indicate that attention plays a large role in the rehabilitation of homonymous visual field defects.

Near and far space: Understanding the neural mechanisms of spatial attention

Visuospatial neglect is a multicomponent syndrome, and one dissociation reported is between neglect for near (peripersonal) and far (extrapersonal) space. Owing to patient heterogeneity and extensive lesions, it is difficult to determine the precise neural mechanisms underlying this dissociation using clinical methodology. In this study, transcranial magnetic stimulation was used to examine the involvement of three areas in the undamaged brain, while participants completed a conjunction search task in near and far space. The brain areas investigated were right posterior parietal cortex (rPPC), right frontal eye field (rFEF), and right ventral occipital cortex (rVO), each of which has been implicated in visuospatial processing. The results revealed a double dissociation, whereby rPPC was involved for search in near space only, whilst rVO only became necessary when the task was completed in far space. These data provide clear evidence for a dorsal and ventral dissociation between the processing of near and far space, which is compatible with the functional roles previously attributed to the two streams. For example, the involvement of the dorsal stream in near space reflects its role in vision for action, because it is within this spatial location that actions can be performed. The results also revealed that rFEF is involved in the processing of visual search in both near and far space and may contribute to visuospatial attention and/or the control of eye‐movements irrespective of spatial frame. We discuss our results with respect to their clear ramifications for clinical diagnosis and neurorehabilitation. Hum Brain Mapp, 2013.

Clinical treatment options for patients with homonymous visual field defects.

The objective of this review is to evaluate treatments for homonymous visual field defects (HVFDs). We distinguish between three treatments: visual restoration training (VRT), optical aids, and compensatory training. VRT is both the most ambitious and controversial approach, aiming to restore portions of the lost visual field. While early studies suggested that VRT can reduce the visual field defect, recent studies using more reliable means of monitoring the patients’ fixation could not confirm this effect. Studies utilizing modern optical aids have reported some promising results, but the extent to which these aids can reliably reduce the patients’ visual disability has yet to be confirmed. Compensatory approaches, which teach patients more effective ways of using their eyes, are currently the only form of treatment for which behavioral improvements have been demonstrated. However, with the exception of one study using a reading training, placebo-controlled clinical evaluation studies are lacking. It is also not yet clear whether the training benefits found in laboratory tasks lead to reliable improvements in activities of daily living and which of the various forms of compensatory training is the most promising. It is therefore too early to recommend any of the currently available treatment approaches.

The involvement of posterior parietal cortex and frontal eye fields in spatially primed visual search

BACKGROUND Right posterior parietal cortex (rPPC) and frontal eye fields (FEF) are known to be involved in processing visuospatial attention. However, the functional involvement of these areas in spatial priming in complex conjunction visual search has yet to be determined. OBJECTIVE This study aimed to examine the roles of rPPC and bilateral FEF in conjunction search when spatial ambiguity was reduced by priming the target location. METHODS Participants completed a conjunction search task whereby the target location was random or else repeated from the previous trial. Transcranial magnetic stimulation was delivered to each one of the three sites of interest at a time, and task performance was compared with a sham condition. RESULTS Spatial priming occurred for all conditions: search times were faster for primed relative to nonprimed trials. When the target appeared at a nonprimed location, stimulation over any of the three sites increased reaction times relative to the sham condition. However, when the target location was repeated, reaction time was only significantly increased by stimulation over the right FEF. CONCLUSIONS rPPC and left FEF are only involved when the target location is random, suggesting that these areas are essential for resolving spatial ambiguity to localize targets. Conversely, right FEF contributes equally to visual search regardless of spatial priming. We propose that right FEF has a role in the integration of bottom up saliency and top down expectancy signals and is the node at which rPPC and/or left FEF is either recruited or not.

The involvement of posterior parietal cortex in feature and conjunction visuomotor search

Successful interaction with the environment often involves the identification and localization of an item. Right posterior parietal cortex (rPPC) is necessary for the completion of conjunction but not feature visual search, regardless of the attentional requirements. One account for this dissociation is that the rPPC is primarily involved in processing spatial information. For target identification, conjunction tasks require that spatial information is used to determine if features occur at the same location, whereas feature search does not require such a process. This account suggests that if the requirement to localize the target is made explicit, then rPPC may also be necessary for feature search. This was examined using TMS and by manipulating the response mode: Participants were either required to press a button indicating the presence/absence of the target or else had to point to the target. TMS over rPPC did not disrupt performance of the feature task when a button press was required but significantly increased response time and movement time for the same task in the pointing condition. Conjunction search in both response conditions was significantly impaired by TMS. Performance on a task that required pointing to a target in the absence of distractors and thus did not involve visual search was unaffected by rPPC stimulation. We conclude that rPPC is involved in coding and representing spatial information and is therefore crucial when the task requires determining whether two features spatially co-occur or when search is combined with explicit target localization via a visuomotor transformation.

Functional Interaction between Right Parietal and Bilateral Frontal Cortices during Visual Search Tasks Revealed Using Functional Magnetic Imaging and Transcranial Direct Current Stimulation

The existence of a network of brain regions which are activated when one undertakes a difficult visual search task is well established. Two primary nodes on this network are right posterior parietal cortex (rPPC) and right frontal eye fields. Both have been shown to be involved in the orientation of attention, but the contingency that the activity of one of these areas has on the other is less clear. We sought to investigate this question by using transcranial direct current stimulation (tDCS) to selectively decrease activity in rPPC and then asking participants to perform a visual search task whilst undergoing functional magnetic resonance imaging. Comparison with a condition in which sham tDCS was applied revealed that cathodal tDCS over rPPC causes a selective bilateral decrease in frontal activity when performing a visual search task. This result demonstrates for the first time that premotor regions within the frontal lobe and rPPC are not only necessary to carry out a visual search task, but that they work together to bring about normal function.

Efficacy and Feasibility of Home-Based Training for Individuals With Homonymous Visual Field Defects

Background. Homonymous visual field defects (HVFDs) are one of the most common consequences of stroke. Compensatory training encourages affected individuals to develop more efficient eye movements to improve function. However, training is typically supervised, which can be time consuming and costly. Objective. To develop and evaluate the efficacy and feasibility of an unsupervised reading and exploration computer training for individuals with HVFDs. Methods. Seventy individuals with chronic HVFDs were randomly assigned to 1 of 2 groups: intervention or control. The former received 35 hours of reading and exploration training, and the latter received 35 hours of control training. Visual and attentional abilities were assessed before and after training using perimetry, visual search, reading, activities of daily living, the Test of Everyday Attention, and a Sustained Attention to Response task. Results. Eighteen individuals failed to complete the training; analyses were conducted on the remaining 28 intervention and 24 control group participants. Individuals in the intervention group demonstrated significant improvements in the primary outcomes of exploration (12.87%, 95% confidence interval [CI] = 8.44% to 17.30%) and reading (18.45%, 95% CI = 9.93% to 26.97%), which were significantly greater than those observed following the control intervention (exploration = 4.80%, 95% CI = 0.09% to 9.51%; reading = 1.95%, 95% CI = −4.78% to 8.68%). Participants in the intervention group also reported secondary subjective improvements, although these were not matched by objective gains in tasks simulating activities of daily living. Conclusions. Home-based compensatory training is an inexpensive accessible rehabilitation option for individuals with HVFDs, which can result in objective benefits in searching and reading, as well as improving quality of life.

Arm position does not attenuate visual loss in patients with homonymous field deficits

One of the most common and debilitating consequences of stroke is the loss of vision in the contralesional visual field. Clinicians typically regard this loss as irreversible and attempts at visual restoration have delivered only small and unreliable improvements. However, Schendel and Robertson [Schendel, K., & Robertson, L. C. (2004). Reaching out to see: Arm position can attenuate human visual loss. Journal of Cognitive Neuroscience, 16(6), 935-943] reported that the visual abilities of a hemianopic patient (WM) were significantly improved when the left arm was extended into the blind field. They suggest that visual stimuli near the arm recruited bimodal visual-tactile neurons, and this activity was sufficient to bring the stimulus into awareness. This result has enormous potential therapeutic value, but given that it is a single case study there are a number of reasons to be cautious about interpreting the data. Here, we investigate the effects of manipulating arm position on visual loss in a sample of five patients with homonymous field deficits. None of our patients showed any evidence of improved implicit or explicit visual ability in the blind field as a consequence of moving the arm. We suggest that WMs improvement was the consequence of a spatial bias towards the space containing his extended arm rather than the recruitment of bimodal neurons, and conclude that manipulating arm position is of little therapeutic value to patients with dense hemianopia.

Pathways involved in human conscious vision contribute to obstacle-avoidance behaviour

Human patients with visual field defects following damage to their primary visual cortex (V1) will often misperceive the midpoint of a horizontal line. They tend to shift the midpoint away from the real position towards their blind field. In patients with unilateral neglect, where midpoint shifts can also be observed, these perceptual errors do not lead to errors in an obstacle‐avoidance task, which also requires the ability to find the midpoint between two obstacles. This dissociation in neglect patients was taken as evidence that obstacle‐avoidance performance is guided by visual information from the dorsal visual stream. Recently it was shown that a patient with hemianopia could avoid an obstacle presented in his blind field. This suggests that obstacle‐avoidance behaviour can be guided by subconscious vision alone involving a direct route from extrageniculate structures in the brain to dorsal stream areas. To investigate whether obstacle avoidance relies only on this subconscious route or also uses information from pathways involved in conscious vision, we examined the effect of the hemianopic shift on obstacle‐avoidance behaviour. This shift is found in tasks where a conscious visual judgement is required and presumably arises in pathways underlying conscious vision (V1 and ventral stream areas). We compared the performance of six patients with left hemianopia with the performance of six patients with right hemianopia. We found a clear bias in both groups, which also affected obstacle‐avoidance performance. It is thus concluded that obstacle avoidance does not bypass the system for conscious vision completely.

Spatial priming in visual search: memory for body-centred information

Spatial priming allows memory for target locations to be evaluated, whereby when a target appears in the same location across trials, participants become more efficient at locating it and consequently their search times decrease. Previously, we reported priming effects when the location of a target was repeated with respect to the participant’s body but not when it was repeated relative to their eye position; therefore, suggesting that body-centred information is available after a delay of at least a few seconds (Ball et al. in Exp Brain Res 204:585–594, 2010). However, we were unable to rule out the possibility that stable allocentric cues within the room may have contributed to the priming effects that we observed. In this current study, we introduced a condition where despite participants moving to a new location between trials, their position relative to the target was maintained. This movement disrupted any potential room-based cues about the target location. While we replicated our previous finding of priming when the location of the target was repeated relative to the viewer when no movement was required, we also found robust priming effects when participants moved to a new location in between trials. Thus, we provide clear evidence that in our spatial priming task, the location of the target was being coded in a body-centred reference frame and that this information is available after a delay.