Attention and novelty processing in stroke and Parkinson's disease

The ventral fronto-parietal network has been considered to play a crucial role in reorienting attention towards significant environmental events, while the dorsal system is thought to be dominant in controlling goal-directed behaviour (Corbetta and Shulman 2002). I begin by reviewing literature which suggests this distinction may not be so clear cut and suggest my own scheme which takes into account this evidence (Singh-Curry and Husain 2009). Specifically, ventral areas, particularly the right inferior parietal lobe (IPL), appear to be activated by tasks involving sustained attention, responding to salient taskrelevant events, detecting novel stimuli and switching between tasks. Accordingly, I hypothesise that the right IPL may play a crucial role in reconfiguring behaviour between a task-engaged state and a more exploratory mode of functioning, which permits the identification of potentially important novel events. The first few chapters of my thesis aimed to test this hypothesis by examining attention deficits in stroke patients with hemispatial neglect, the syndrome which frequently occurs following damage to the right IPL. These patients were shown to have difficulty sustaining attention over time, even when no spatial shifts of attention were required. This deficit in sustained attention was particularly evident for stimuli of lower perceptual salience. More importantly, however, these deficits were found to interact with each other, as well as the direction of spatial attention, suggesting that these functions may be dependent on an interrelated brain network. Consistent with this notion, the results of lesion-symptom analysis indicated that the Right IPL and ventral attention network appears to be crucial in the mediation of all of these processes, including the processing of novel stimuli, supporting my hypothesis. The detection of novel events has also been found to activate the midbrain dopaminergic system (Bunzeck and Duzel 2006), while the principal pathological feature of Parkinson’s disease (PD) is degeneration of these neurons (Hornykiewicz 1998). Although PD is traditionally considered a disorder of movement, more recently it has been recognised that there may be associated cognitive deficits, including disorders of impulse control (Weintraub 2008). At present, however, the factors which predispose some individuals with PD to develop such problems are unclear. Accordingly, in the second part of my thesis, I examined novelty processing and risktaking behaviour in PD in order to identify subgroups which may be particularly vulnerable to developing impulse control problems. In addition to PD patients with impulse control disorders (ICD), those who were classified as akinetic-rigid, as opposed to tremor dominant – without ICD – were found to process novelty more quickly than nonnovel perceptually salient stimuli, unlike tremor dominant PD patients. Novelty seeking was found to be associated with relative preservation of the mesolimbic dopaminergic system in patients without ICD, while increased risk-taking was associated with preservation of the mesolimbic system in ICD patients. Mesolimbic sparing, in addition to the akinetic-rigid motor phenotype of PD may therefore increase susceptibility to impulse control problems in PD.