Although the spatial deficit of unilateral neglect tends to receive much attention, it is evident that also non-spatial components play central roles in this syndrome. The present dissertation is centered on the problem of resource allocation in neglect as well as considering the functioning of specific attention components in this syndrome. The three studies conducted, each had a different approach in the investigation of attentional resource utilization in neglect patients. Eye-tracking measures during multiple object tracking (MOT), pupillary responses during MOT, as well as binocular rivalry alternation rates, were obtained in right hemisphere stroke patients with neglect, and in right hemisphere stroke patients without neglect as well as healthy control participants for comparisons.
In Article I eye fixation frequencies were considered as an index of efficiency in visual processing (Goldberg & Kotval, 1999) and deployment of resources (Kasarskis, Stehwien, Hickox, Aretz, and Wickens, 2001). Results revealed reduced visual processing efficiency in both patient groups, with equally increased fixation frequencies. Fixation frequencies were also investigated in relation to task demands (in correct trials). Increases in fixation rates were found across all groups when the target load was incremented from one to multiple targets. However, no further increase was detected when load went up from two to three targets. Intuitively, the increase in fixation rate may reflect the added component of divided attention in trials with multiple targets.
In addition to providing an index of the general deployment of visual resources fixations were also assessed in relation to the spatial utilization of the visual field in Article I. The proportion of fixations in left and right fields were investigated in relation to the patients’ scoring on neglect tests, to assess whether neglect severity would predict the spatial distribution of attention and whether compensational mechanisms would be present in less severe neglect. Results revealed that the spatial distribution of fixations was in fact predicted by the severity of neglect. Patients with severe neglect made more fixations in their right field, while patients with less severe neglect made more fixations in their left field. The leftward bias shown in some patients could be reflecting compensational mechanisms for neglect, as they appear to be allocating additional attention resources into the left (problematic) field. Eye movement rates were also investigated, taking into account whether they were directed leftwards or rightwards, in order to evaluate whether neglect patients would reveal a direction-specific bias and confirm a directional hypokinetic component in neglect (Heilman, Bowers, Coslett, Whelan, & Watson, 1985). Indeed, the frequencies of saccades in left and right directions revealed an expected direction-specific bias in neglect patients, in line with a directional hypokinesia account.
In Article II pupillary responses and performance accuracy on the MOT task were used to estimate possible limitations in patients’ available attention resources. It has recently been demonstrated that mental workload, as operationalized in the MOT task by the number of targets, is reflected in stepwise, proportional, increases in pupillary dilations (Alnaes et al., 2014; Wahn, Ferris, Hairston, & Koenig, 2016). Both the patient groups revealed reduced pupillary dilations compared to the control participants, suggesting they had less available attention resources and could not elevate their attention arousal levels to the same degree as controls could. Notably, while pupillary dilations increased with enhanced load in the patients without neglect, no such increase could be detected in the neglect patients. The neglect patients’ results could indicate that their mechanisms for regulating arousal in accordance with the varying task demands may have been deficient.
In addition to the MOT task, participants were presented with a static visual search (VS) task, and their performance measures on these tasks were compared in Article II. Both tasks were shown to detect spatial neglect symptoms through reduced performance in the left compared to the right field. In addition, the MOT task revealed reduced ipsilesional performance in neglect patients. This confirms that attention deficits in neglect are not limited to the contralesional space (Weintraub & Mesulam, 1987). That neglect patients showed markedly reduced accuracy scores with higher load even in the right hemispace provides an indication that these patients have comprehensive problems with sustained and divided attention, and it fits well with the findings of bilaterally reduced arousal.
Differently from the previous two articles, which were based on a dynamic displacement of stimuli, Article III presented participants with a binocular rivalry task, where the stimuli dynamically replace one another, without any spatial change. Rates of perceptual alternations were taken to reflect the degree of available neural processing resources in that, reduced resources should result in slower conscious updating of the rival images. Article III assessed whether the degree of attention impairment (as indicated by BIT-scores) would influence the non-spatial mechanisms of rivalry dynamics. Results revealed severely reduced rivalry alternation rates in neglect patients, suggesting a limitation in their available neural processing resources, whereas the patients without neglect were not as slowed in their perceptual alternations. Still, BIT-scores explained only a small part of the variance, suggesting that BIT-scores may not fully capture the attention pathologies that are involved in neglect.
In total, results suggest that not only spatial attention deficits are involved in the neglect syndrome, but that more general mechanisms of attention can be affected as well. Our findings show that both right hemisphere stroke patients with and without neglect may suffer from lowered arousal and reduced visual processing efficiency. Since only the neglect patient group failed to show signs of adjusting their arousal levels to meet with increasing task demands, this suggests that neglect may involve malfunctioning mechanisms for arousal regulation. Our group of neglect patients additionally showed more extreme slowing of rivalry alternations than the patients without neglect, providing another indication that their neural mechanisms were more negatively affected than those of the non-neglect patients. With the notion that attention is a limited resource that should be spent wisely (Kastner & Ungerleider, 2000), the ability to regulate the expenditure of attention resources in accordance with task demands should be of utmost importance. The amount of available resources and the ability to allocate them may also be imperative with regards to making use of compensational mechanisms for spatial deficits.