Ian G. M. Cameron

Executive impairment in Parkinson's disease: Response automaticity and task switching

Patients with Parkinsons disease (PD) show slowed movement initiation and can have deficits in executive function, leading to impairments in controlling involuntary behavior. This results in difficulties performing an antisaccade, which requires one to suppress an automatic eye movement (a prosaccade) to a visual stimulus, and execute a voluntary eye movement in the opposite direction. Antisaccade deficits are similar to those seen in task switching, whereby one is required to change a response after performing a different behavior. Both antisaccade (Hood et al., 2007) and task switching (Cools, Barker, Sahakian, & Robbins, 2001) deficits in PD have been attributed to fronto-basal ganglia (BG) dysfunction. Previously, we demonstrated with functional magnetic resonance imaging that BG circuitry is important to both task switching and voluntary saccade generation, as greater caudate activation was seen when healthy young adults first prepared a prosaccade, but then switched to an antisaccade (Cameron, Coe, et al., 2009). Therefore, we hypothesized that PD patients would have difficulty switching from one saccade response to the other, with particular impairment in switching from a pro to an antisaccade. Here, we not only confirmed this prediction, but also showed that PD patients performed better than controls in switching from an anti to a prosaccade. This suggests that task switching deficits in PD are particularly pronounced when more automatic behavior needs to be overridden with alternative behavior. We suggest that this occurs primarily at the level of establishing the appropriate task set, which is an internalized rule that governs how to respond.

Role of the basal ganglia in switching a planned response

The ability to perform an appropriate response in the presence of competing alternatives is a critical facet of human behavioral control. This is especially important if a response is prepared for execution but then has to be changed suddenly. A popular hypothesis of basal ganglia (BG) function suggests that its direct and indirect pathways could provide a neural mechanism to rapidly switch from one planned response to an alternative. However, if one response is more dominant or ‘automatic’ than the other, the BG might have a different role depending on switch direction. We built upon the pro‐ and antisaccade tasks, two models of automatic and voluntary behavior, respectively, and investigated whether the BG are important for switching any planned response in general, or if they are more important for switching from a more automatic response to a response that is more difficult to perform. Subjects prepared either a pro‐ or antisaccade but then had to switch it unexpectedly on a subset of trials. The results revealed increased striatal activation for switching from a pro‐ to an antisaccade but this did not occur for switching from an anti‐ to a prosaccade. This activation pattern depended on the relative difficulty in switching, and it was distinct from frontal eye fields, an area shown to be more active for antisaccade trials than for prosaccade trials. This suggests that the BG are important for compensating for differences in response difficulty, facilitating the rapid switching of one response for another.

Contrasting instruction change with response change in task switching.

Switching between two tasks results in switch costs, which are increased error rates and response times in comparison to repeating a task. Switch costs are attributed to a change in task set, which is the internalized rule of how to respond to a stimulus. However, it is not clear if this is because the instruction about which task to perform has changed, or because a programmed response has changed. We examined this question by changing the instruction about whether to perform a pro or an antisaccade to a stimulus, before or after the stimulus was presented. As a saccade response is specified by instruction plus stimulus position, changing the instruction after the stimulus was present resulted in a change in the specified response, whereas changing the instruction beforehand did not. Three experiments investigated; (i) if changing instruction alone or changing the specified response produced switch costs; (ii) if predictability of switching instruction influenced switch costs; and (iii) if predictability of stimulus position influenced switch costs. Regardless of instruction or stimulus predictability, switch costs for both pro and antisaccades consistently resulted if the specified response switched. This suggests that a pro or antisaccade motor program was automatically programmed based on a presented instruction and stimulus position. Therefore, the given physical information drove switch costs, even if subjects could predict a change in task. This study demonstrates that switch costs result if changing an instruction changes a programmed response.

Occipital–Parietal Network Prepares Reflexive Saccades

The reaction times of saccadic eye movements vary considerably even when they are initiated in response to identical visual stimuli. This variability of saccade reaction times has been studied extensively as a probe for cognitive brain functions because it depends on cognitive demands imposed by

Changes to Saccade Behaviors in Parkinson’s Disease Following Dancing and Observation of Dancing

Background: The traditional view of Parkinson’s disease (PD) as a motor disorder only treated by dopaminergic medications is now shifting to include non-pharmacologic interventions. We have noticed that patients with PD obtain an immediate, short-lasting benefit to mobility by the end of a dance class, suggesting some mechanism by which dancing reduces bradykinetic symptoms. We have also found that patients with PD are unimpaired at initiating highly automatic eye movements to visual stimuli (pro-saccades) but are impaired at generating willful eye movements away from visual stimuli (anti-saccades). We hypothesized that the mechanisms by which a dance class improves movement initiation may generalize to the brain networks impacted in PD (frontal lobe and basal ganglia, BG), and thus could be assessed objectively by measuring eye movements, which rely on the same neural circuitry. Methods: Participants with PD performed pro- and anti-saccades before, and after, a dance class. “Before” and “after” saccade performance measurements were compared. These measurements were then contrasted with a control condition (observing a dance class in a video), and with older and younger adult populations, who rested for an hour between measurements. Results: We found an improvement in anti-saccade performance following the observation of dance (but not following dancing), but we found a detriment in pro-saccade performance following dancing. Conclusion: We suggest that observation of dance induced plasticity changes in frontal-BG networks that are important for executive control. Dancing, in contrast, increased voluntary movement signals that benefited mobility, but interfered with the automaticity of efficient pro-saccade execution.

Effects of development on low-level feature processing during natural viewing of dynamic scenes

Children need more time for visual processing, deciding where to look next, and/or initiating a saccade As the brain matures, faster visual processing time, faster decision, and/or faster saccade initiation resulting in shorter inter-saccade interval. young adults look more toward highly salient locations possibly due to (1) the balance between top-down and bottom-up attention and/or (2) learning to extract visual information effectively. As the brain ages, bottom-up attention of the elderly is as effective as that of young adults during free viewing (no cognitive tasks) . overall attention allocation changes significantly (presumably by top-down attention), but young adults are more similar to the elderly than to children.