Does dopamine depletion trigger a spreader lexical-semantic activation in Parkinson’s disease? Evidence from a study based on word fluency tasks.

It has been hypothesised that, in Parkinson’s disease (PD), dopamine might modulate spreading activation of lexical-semantic representations. In this study, we aimed to investigate this hypothesis in individuals with PD without dementia by assessing word frequency and typicality in verbal fluency tasks. We predicted that the average values of both of these parameters would be lower in PD patients with respect to healthy controls (HC). We administered a letter-cued and a category- cued fluency task to PD patients in the early stage of the disease in two experimental conditions: the tasks were administered both after 12-18 hours of dopaminergic stimulation withdrawal (“off” condition) and after the first daily dose dopaminergic therapy (“on” condition). HC were also administered the two tasks in two conditions with the same inter-session delay as PD patients, but without taking drugs. Results showed that in both off and on treatment conditions PD patients did not differ from HC for word frequency or word typicality. Moreover, in the PD group no significant difference was found between the off and on conditions. Our results show that semantic spreading was not altered in the PD sample examined; this suggests that in the early stages of PD the functioning of the semantic system is relatively independent from the activity of dopamine brain networks.