Search

The loss of visual function is a common and debilitating effect of brain injury. Such effects include the inability to attend to the contralesional part of space (visual neglect) and loss of vision in part of the visual field in both eyes (homonymous visual field defects). The underlying mechanism of these functional deficits are varied which explains why no one intervention is effective in their treatment. In this chapter, we examine the cognitive neuroscience that underlies visual performance and attention with particular focus on what we know about visual exploration using search tasks in the laboratory. We then discuss how this knowledge has informed the development of interventions for poor visual function and how these can be maximized., (© 2020 Elsevier B.V. All rights reserved.)

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., (Copyright © 2012 Elsevier Inc. All rights reserved.)

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.

Visual search is a task often used in the rehabilitation of patients with cortical and non-cortical visual pathologies such as visual field loss. Reduced visual acuity is often comorbid with these disorders, and it remains poorly defined how low visual acuity may affect a patient's ability to recover visual function through visual search training. The two experiments reported here investigated whether induced blurring of vision (from 6/15 to 6/60) in a neurotypical population differentially affected various types of feature search tasks, whether there is a minimal acceptable level of visual acuity required for normal search performance, and whether these factors affected the degree to which participants could improve with training. From the results, it can be seen that reducing visual acuity did reduce search speed, but only for tasks where the target was defined by shape or size (not colour), and only when acuity was worse than 6/15. Furthermore, searching behaviour was seen to improve with training in all three feature search tasks, irrespective of the degree of blurring that was induced. The improvement also generalised to a non-trained search task, indicating that an enhanced search strategy had been developed. These findings have important implications for the use of visual search as a rehabilitation aid for partial visual loss, indicating that individuals with even severe comorbid blurring should still be able to benefit from such training. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]