As shown in Table 1, the STN and GPi DBS patient groups did not significantly differ in terms of age, gender split, or premorbid level of intellectual functioning, as estimated by the NART.
At baseline, there were no significant differences between the STN and GPi DBS patient groups in UPDRS-III scores off or on medication, nor in baseline levodopa-equivalent dosage (as shown in Tables 2 and 3).
The GPi DBS group also demonstrated frequent impairments in the additional domains of cognitive screen and speed.
When investigated further, we found that the GPi DBS patients obtained lower baseline scores on tests of general intellectual functioning (VIQ: t(38) = 4.24, p < 0.001; PIQ: t(38) = 2.33, p< 0.05), recognition memory (RMT words: U = 65.5, p < 0.05; RMT faces: t(37) = 3.74, p < 0.01), attention (TEA EC with distraction: t(37) = 2.76, p < 0.05), and executive functioning (category fluency: t(37) = 2.75, p < 0.05; Stroop: t(35) = 3.49, p < 0.01; Brixton: t(33) = 4.12, p < 0.01).
As shown in Table 5, there was a significant drop in phonemic and category fluency following both STN and GPi DBS. In the STN patients, there was also a significant decline in performance on Symbol Search, and in the GPi patients, there was also a decline in PIQ.
Case note review revealed mention of decline in cognitive function in 15% (n = 6) of patients after DBS (4 STN DBS, 14.3%; 2 GPi DBS, 16.7%).
Studies were reviewed if they were published in the English language and met our minimum inclusion criteria: (1) patients with idiopathic PD who underwent STN or GPi DBS, (2) reporting neuropsychological data after DBS surgery, (3) using at least one standardized neuropsychological instrument, and (4) including at least five subjects followed for a mean of at least 3 months postoperatively.
72 studies totaling 2,410 STN DBS patients and 702 GPi DBS patients were reviewed (Tables 1 and 2).
There were 62 such studies, totaling 1,913 STN DBS patients and 165 GPi DBS patients.
Decline in one or more measures of language, most often fluency, was reported in 3 studies totaling 36 patients followed up to 21 months after GPi DBS [25, 50, 66].
No statistically significant change in any measure of executive function up to 21 months after GPi DBS was reported in 7 studies, while one study reported improvement of at least one measure of executive function at 6 months  (Table 2).