Analyses of correct responses and errors on measures of verbal fluency among Parkinson's disease and essential tremor patients.

Parkinson's disease (PD) and essential tremor (ET) involve neuroanatomical circuitry that impact frontal lobe functioning, via the striatum and cerebellum, respectively. The aim of this exploratory study was to investigate quantitative and qualitative performance between and within these groups on measures of verbal fluency. Sixty-three PD and 53 ET patients completed neuropsychological testing. Linear regression models with robust variance estimation compared verbal fluency performance between groups related to correct responses and errors. Paired -tests investigated within group error rates. PD patients gave more correct responses for phonological (=5.3, =.01) and category fluency (=4.1, =.01) than ET patients; however, when processing speed was added as a covariate, this attenuated performance on both measures and only phonological fluency remained significant (=4.0, =.04). There were no statistical differences in error scores between groups. Error rates within groups suggested that PD patients had higher error rates in total errors and perseveration errors on phonological fluency ( = 2.6, =.00;  = 1.6, =.00) and higher total errors and set-loss error rates on category switching ( = 5.1, <.001;  = 4.1, <.001). ET patients had higher error rate with relation to total errors and set-loss errors on phonological fluency ( = 2.5, =.00;  = 1.5, =.02) and category switching ( = 3.9, =,00;  = 3.9, <.001). PD patients performed better than ET patients on phonological fluency. PD patients appear to make more perseveration errors on phonological fluency, while ET patients made more set-loss errors. Implications for frontal lobe dysfunction and clinical impact are discussed.