Recent imaging studies in healthy controls with a conditional stop signal reaction time (RT) task have implicated the subthalamic nucleus (STN) in response inhibition and the pre-supplementary motor area (pre-SMA) in conflict resolution. Parkinson's disease (PD) is characterized by striatal dopamine deficiency and overactivity of the STN and underactivation of the pre-SMA during movement. We used the conditional stop signal RT task to investigate whether PD produced similar or dissociable effects on response initiation, response inhibition and response initiation under conflict. In addition, we also examined inhibition of prepotent responses on three cognitive tasks: the Stroop, random number generation and Hayling sentence completion. PD patients were impaired on the conditional stop signal reaction time task, with response initiation both in situations with or without conflict and response inhibition all being significantly delayed, and had significantly greater difficulty in suppressing prepotent or habitual responses on the Stroop, Hayling and random number generation tasks relative to controls. These results demonstrate the existence of a generalized inhibitory deficit in PD, which suggest that PD is a disorder of inhibition as well as activation and that in situations of conflict, executive control over responses is compromised.

Background: Stereotactic thermocoagulative lesions of the subthalamic nucleus (STN) have been shown to induce significant motor improvement in patients with Parkinson's disease (PD). Patients and methods: 89 patients with PD were treated with unilateral subthalamotomy. 68 patients were available for evaluations after 12 months, 36 at 24 months and 25 at 36 months.

The subthalamic nucleus (STN) is hypothesized to play a central role in the rapid stopping of movement in reaction to a stop signal. Single-unit recording evidence for such a role is sparse, however, and it remains uncertain how that role relates to the disparate functions described for anatomic subdivisions of the STN. Here we address that gap in knowledge using non-human primates and a task that distinguishes reactive and proactive action inhibition, switching and skeletomotor functions. We found that specific subsets of STN neurons have activity consistent with causal roles in reactive action stopping or switching. Importantly, these neurons were strictly segregated to a ventromedial region of STN. Neurons in other subdivisions encoded task dimensions such as movement per se and proactive control. We propose that the involvement of STN in reactive control is restricted to its ventromedial portion, further implicating this STN subdivision in impulse control disorders.

Changes of cognitive function in PD have been extensively documented and defined as a ‘frontal’ type executive dysfunction. One of the main components of this executive dysfunction is the impairment of verbal fluency. The aim of the present study was to assess semantic and phonemic fluency in a large sample of PD patients and to investigate the effect of clinical and sociodemographic variables on verbal fluency in this patient group.Three hundred patients with idiopathic Parkinson's disease who were consecutive referrals to our clinic and 50 age and education matched healthy controls completed the phonemic and semantic verbal fluency tasks. Both phonemic and semantic verbal fluency were significantly impaired in PD patients relative to matched controls. Stage of illness, presence of depression, education and age influenced verbal fluency measures. Regression analyses established that global measures of cognitive ability (MMSE) and executive function (FAB) and side of onset of motor symptoms predicted 36–37% of variance of phonemic or semantic verbal fluency measures. Thus, future studies aimed at assessing cognitive functioning in PD patients treated by deep brain stimulation (DBS) should adequately take into account several factors (stage of illness, depression, executive functioning) which may potentially influence performance on verbal fluency tasks.

Unilateral subthalamotomy is an effective treatment for the cardinal motor features of Parkinson's disease (PD). However, non-motor changes possibly associated with right or left subthalamotomy remain unknown. Our aim was to assess cognitive, psychiatric and neuroimaging changes after treatment with unilateral subthalamotomy. Fourteen medicated patients with PD were evaluated before and after (mean 6 months after operation) unilateral subthalamotomy (5 right, 9 left). In addition to motor assessments, cognitive (global cognition and executive functions), psychiatric (apathy, depression, anxiety, mania, hypo- and hyperdopaminergic behaviours, impulsivity), quality of life evaluations and volume of lesions were obtained. After surgery, significant improvement of motor signs was observed. Unilateral subthalamotomy improved general cognitive status, but left subthalamotomy reduced semantic verbal fluency compared to the pre-operative state. Depression and quality of life were improved with both right and left subthalamotomy. However, hyper-emotionality was present after surgery and right subthalamotomy increased impulsivity and disinhibition (on NeuroPsychiatric Inventory and Ardouin Scale for Behaviour in PD), a result linked to larger lesion volumes. We conclude that unilateral subthalamotomy is effective for treating the cardinal motor features of PD and improves mood. Right subthalamotomy is associated with greater risk of impulsivity and disinhibition, while left subthalamotomy induces further impairment of semantic verbal fluency.