Parkinson Disease: Horvitz JC

Horvitz JC, Choi WY, Morvan C, Eyny Y, Balsam PD. · Department of Psychology, Boston College, Chestnut Hill, MA 02467, USA. · Ann N Y Acad Sci. · Pubmed #17360799 No free full text.

Abstract: While extracellular dopamine (DA) concentrations are increased by a wide category of salient stimuli, there is evidence to suggest that DA responses to primary and conditioned rewards may be distinct from those elicited by other types of salient events. A reward-specific mode of neuronal responding would be necessary if DA acts to strengthen behavioral response tendencies under particular environmental conditions or to set current environmental inputs as goals that direct approach responses. As described in this review, DA critically mediates both the acquisition and expression of learned behaviors during early stages of training, however, during later stages, at least some forms of learned behavior become independent of (or less dependent upon) DA transmission for their expression.

Horowitz TS, Choi WY, Horvitz JC, Côté LJ, Mangels JA. · Brigham & Women's Hospital and Harvard Medical School, MA, USA. · Neuropsychologia. · Pubmed #16580700 No free full text.

Abstract: Patients with Parkinson's disease (PD), a degenerative disorder primarily affecting the nigrostriatal dopamine system, exhibit deficits in selecting task-relevant stimuli in the presence of irrelevant stimuli, such as in visual search tasks. However, results from previous studies suggest that these deficits may vary as a function of whether selection must rely primarily on the "bottom-up" salience of the target relative to background stimuli, or whether "top-down" information about the identity of the target is available to bias selection. In the present study, moderate-to-severe medicated PD patients and age-matched controls were tested on six visual search tasks that systematically varied the relationship between bottom-up target salience (feature search, noisy feature search, conjunction search) and top-down target knowledge (Target Known versus Target Unknown). Comparison of slope and intercepts of the RT x set size function provided information about the efficiency of search and non-search (e.g., decision, response) components, respectively. Patients exhibited higher intercepts than controls as bottom-up target salience decreased, however these deficits were disproportionately larger under Target Unknown compared to Target Known conditions. Slope differences between PD and controls were limited to the Target Unknown Conjunction condition, where patients exhibited a shallower slope in the target absent condition, indicating that they terminated search earlier. These results suggest that under conditions of high background noise, medicated PD patients were primarily impaired in decision and/or response processes downstream from the target search itself, and that the deficit was attenuated when top-down information was available to guide selection of the target signal.

Stern ER, Horvitz JC, Côté LJ, Mangels JA. · Department of Psychology, Columbia University, New York, NY 10027, USA. · Neuropsychology. · Pubmed #15656763 No free full text.

Abstract: The authors explored the effect of Parkinson's disease (PD) on the generation and maintenance of response readiness in a simple reaction time task. They compared performance of idiopathic PD patients without dementia, age-matched controls, and younger controls over short (1-, 3-, and 6-s) and long (12- and 18-s) foreperiod intervals. After each trial, the authors probed memory for visual information that also had to be maintained during the trial interval. Patients and controls did not differ overall in their ability to maintain readiness over long delays. However, within the PD group only, errors in maintaining visual information were correlated with difficulty in maintaining readiness, suggesting that systems impaired in PD may facilitate the maintenance of processing in both motor and cognitive domains.

Horvitz JC, Eyny YS. · Department of Psychology, Columbia University, New York, New York 10027, USA. · Behav Neurosci. · Pubmed #11085607 No free full text.

Abstract: Parkinsonian behavioral deficits are reduced in the presence of strong eliciting stimuli and are most pronounced when the response requires internal generation. In the present study, rats' head entries into a food compartment were measured in the presence and absence of an eliciting stimulus. The D2 receptor blocker raclopride suppressed the emission of spontaneous head entries but did not slow head entries emitted in response to a food cue. Rats subjected to a pharmacological disruption in dopamine (DA) transmission show response impairments that are reduced, and in this case eliminated, in the presence of strong eliciting stimuli. The present results support the view that neuroleptic-induced reductions in DA transmission do not produce an absolute limit on the speed with which an individual response can be generated but that they reduce the likelihood of response generation in the absence of strong eliciting stimuli.