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. Author manuscript; available in PMC: 2008 Feb 12.
Published in final edited form as: J Med Speech Lang Pathol. 2006;14(4):323–334.

Hemisphere-Specific Effects of Subthalamic Nucleus Deep Brain Stimulation on Speaking Rate and Articulatory Accuracy of Syllable Repetitions in Parkinson’s Disease

Emily Q Wang 1, Leo Verhagen Metman 1, Roy A E Bakay 1, Jean Arzbaecher 2, Bryan Bernard 3, Daniel M Corcos 4
PMCID: PMC2239259  NIHMSID: NIHMS26516  PMID: 18270553

Abstract

This study tested the hypothesis that left versus right deep brain stimulation (DBS) of the subthalamic nucleus (STN) would have differential effects on speech. Twenty right-handed individuals with advanced Parkinson’s disease (PD) underwent unilateral STN DBS. Ten were operated on the right and 10 on the left hemisphere as indicated by severity of nonspeech motor function. Speech was evaluated before surgery and 3 to 6 months after surgery with stimulator-off and with stimulator-on, with all participants off anti-parkinsonian medication for 12 hours before evaluation. Evaluators and patient speakers were blinded to the stimulator status at the postsurgery evaluations. Motor performance was assessed with UPDRS-III. Each participant produced three samples of diadochokinetic syllables. Syllable rate, syllable and vowel duration, VOT, and F0 were obtained. The diadochokinetic syllables were rated for articulatory accuracy and speaking rate. Twenty graduate clinicians served as judges. The samples were randomly presented via headphones. A mixed ANOVA with repeated measures was used to assess the significance of the changes in UPRS-III scores and speech measures. The results indicated that unilateral STN DBS produced improvement in nonspeech motor function regardless of the side of stimulation. In contrast, the changes in articulatory accuracy and syllable rate associated with the STN DBS were hemisphere specific.

Individuals with Parkinson’s disease (PD) frequently experience problems in their voice and speech (Cummings, 1992; Logemann, Fisher, Bashes, & Blonsky, 1978). Deep brain stimulation (DBS) of the subthalamic nucleus (STN) has been shown to be an effective treatment for advanced PD (e.g., Ashkan, Wallace, Bell, & Benabid, 2004; Olanow, Brin, & Obeso, 2000). However, results from available reports indicate that its effectiveness in alleviating speech and voice symptoms is far less substantial and negative outcomes have been reported in some cases (Dromey, Kumar, Lang, & Lozano, 2000; Gentil, Chauvin, Pinto, Pollak, & Benabid, 2001; Hoffman-Ruddy, Schulz, Vitek, & Evatt, 2001; Pinto, Gentil, Fraix, Benabid, & Pollak, 2003; Rousseaux 2004). One possible contributing factor to the mixed findings is that studying the influence of bilateral STN DBS on speech is potentially confounded by the inability to separate hemisphere-specific effects that could be different and even opposite (Santens, Van Borsel, DeReuck, & Caemaert, 2003; Wang, Verhagen Metman, Bakay, Arzbaecher, & Bernard, 2003). Turning off a system on one side in patients who underwent bilateral STN DBS is not a “pure” method because of the microlesion effect caused by multiple microelectrode insertions and DBS lead placement. Unilateral STN DBS circumvents this confound and thus provides a better model to study the effects of left versus right STN DBS on speech.

Previous studies investigating the outcome of unilateral and bilateral pallidotomy in the treatment of PD found that simultaneous bilateral pallidotomy worsened speech function in all studied patients even though it was more effective than unilateral pallidotomy in alleviating symptoms of tremor, rigidity and dyskinesias (Favre, Burchiel, Taha, & Hammerstad, 2000). Santens and his colleague examined bilateral STN DBS in seven patients by turning the STN DBS off, one side at a time, for 10 minutes before testing (Santens et al., 2003). They found that a negative effect on speech intelligibility was only associated with active left STN stimulation but not with active right STN stimulation (Santens et al., 2003). We also reported that left STN DBS versus right STN DBS had a differential impact on the respiratory/phonatory function in the six patients we studied in a pilot project, although we attributed the difference to the microlesion effect (Wang et al., 2003). Clearly, given the importance and success of the surgical procedure in treating late stage PD, we need to systematically test the hypothesis that left and right STN DBS will affect speech performance differentially.

METHODS

Participant: Production

Twenty six consecutive right-handed patients with a clinical diagnosis of idiopathic PD, who received unilateral STN DBS at Rush University Medical Center, were recruited. Six patients were excluded from the study: two had previous brain surgery, two had profound hearing loss in one ear, one had stuttered since childhood, and one was an eastern Indian-English speaker. The remaining 20 patients (11 males, 9 females) participated in this study. The protocol was approved by the Rush University Medical Center Institutional Review Board. All participants gave informed consent. The participants met the strict inclusion criteria for the surgical intervention program at Rush University Medical Center that included significant levodopa responsiveness and the presence of motor fluctuations. All participants were right handed, American-English-speaking individuals with functional hearing and without dementia or depression. Their mean age was 56.15 (SD 10.6) years and the mean duration of disease 12.35 (SD 6.03) years. Ten received right and 10 left STN DBS. The side of STN DBS was selected based on motor asymmetry in each patient. Detailed participant information is reported in Table 1.

TABLE 1.

Participant characteristics at baseline evaluation in the medication-off state.

S no. Age (Years) Gender Side of STN H & Y* “off” UPDRS-III “off” Disease Duration Speech*
1 59 M Right 3 46 15 1
2 38 F Right 4 47 12 1
3 52 M Right 4 25 11 3
4 59 M Left 4 42 16 2
5 63 M Left 2 19 16 0
6 58 M Left 4 62.5 16 2
7 60 F Left 4 30 24 0.5
8 63 F Left 4 63 17 1
9 61 F Left 4 46 15 1.5
10 62 M Left 4 42.5 20 1.5
11 33 M Left 4 23.5 4 0
12 73 F Right 4 33 17 .5
13 65 F Right 4 35 10 .5
14 43 F Right 4 39.5 7 1
15 46 M Right 4 54 16 1
16 62 M Right 4 33.5 2 .5
17 57 F Right 3 43.5 14 1
18 43 M Left 2 24.5 3 0
19 55 M Right 3 31.5 5 0
20 71 F Left 3 21.5 7 .5
Mean 56.15 3.6 38.13 12.35 0.93
SD (10.60) (.68) (12.80) (6.03) 0.79
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*

H & Y “off”, Hoehn and Yahr Scale, off-medication; Speech, UPDRS rating (item 18), coded as follows: 0 = none; 1 = slight loss of expression diction or volume; 2 = monotone, slurred but understandable, moderately impaired; 3 = marked impairment, difficult to understand; 4 = unintelligible

Participant: Perception

Twenty graduate student clinicians in speech-language pathology, all with normal speech and hearing, from Rush University Medical Center, served as judges in the perception portion of the study.

Procedure

All speakers with PD were tested in the medication-off state (12 hours off medication) at three time points: within 1 month before surgery (Baseline), and 3 to 6 months after surgery, with the STN stimulator-off (Stim-off) and with the STN stimulator-on (Stim-on). The two postsurgery testing sessions were scheduled within 1 to 2 weeks apart. For the postsurgery testing, the order of the stimulator conditions was randomly assigned, and the internal pulse generator or IPG activated or deactivated by the third author who was not involved in the testing. Speech testing took place at least 30 minutes later. Motor function assessment took place immediately after the speech testing. The evaluators (the first and the second authors), and the participants were blinded to the patients’ stimulator condition until after the data were analyzed. No discussion regarding the participants’ stimulator condition was allowed during the postsurgery testing sessions.

Hearing Screening

Hearing in all participants was screened twice for monitoring purposes, once at Baseline and again at the first postsurgery visit. The hearing screening was completed with a Beltone audiometer (Model 112) in a sound treated room. Pure tone signals were presented to the participants via headphones at 500, 1k, 2k, 4k, and 8 kHz. Hearing thresholds for these frequencies were obtained. Of the 20 participants, 6 had normal hearing, 8 had a mild to moderate loss at 8kHz, and the remaining 6 had a mild to moderate sloping loss from 4 to 8 kHz. However, all participants demonstrated functional hearing in the one-on-one testing session. The results from the postsurgery testing did not differ from the baseline testing for any of the participants.

Cognitive Testing and Depression Screening

The cognitive testing and depression screening were completed by the fifth author, BB, a neuropsychologist, before the surgery to determine if a patient met the inclusion criteria of cognitive status for the surgical protocol. All participants were free from dementia and clinical depression.

Motor Testing

Participants’ performance on nonspeech motor tasks was rated by the second author, LV, a movement disorders neurologist, using the motor section of the Unified Parkinson’s Disease Rating Scale (UPDRS-III) (Fahn, Elton, & UPDRS Development Committee, 1987; Langston et al., 1992).

Speech Testing: Production

Normal speech production requires that all speech subsystems interact with each other in a highly coordinated manner. To monitor changes in response to STN DBS in different speech subsystems that are known to be impaired in patients with PD (Kompoliti, Wang, Goetz, Leurgans, Ramen, 2000; Ramig, Titze, Scherer, & Ringel, 1988; Solomon & Hixon, 1993; Wang, Kompoliti, Jiang, & Goetz, 2000), several speech tasks were carried out. They included six maximally sustained “ah” vowel phonations (MSVPs), three repetitions of diadochokinetic syllables (i.e., fast repetitions of syllables “pah,” “tah,” and “kah”), reading of the Rainbow passage, and a structured monologue. Speech signals were recorded in a sound-treated room onto a digital audiotape deck (Sony 60ES) via a preamplifier (Symetrics SX202) and a SHURE (SM10A) professional unidirectional head-worn dynamic microphone with a mouth-microphone distance of 5 cm. The speech signals were digitized at 22 kHz into a Power Macintosh 8100 computer using the SoundEdit program (Version 2.0.7). This article reports on findings from the diadochokinetic task only.

Speech Testing: Perception

The perceptual stimuli were prepared as follows: The first 4 seconds of each syllable repetition were extracted and saved, totaling 540 unique syllable trains. Next, each unique syllable train was repeated twice totaling 1,080 syllable train samples. Twenty graduate student clinicians served as judges. The syllable repetitions were rated for articulatory accuracy using UPDRS-III item 18 (0 = none; 1 = slight loss of expression diction or volume; 2 = monotone, slurred but understandable, moderately impaired; 3 = marked impairment, difficult to understand; 4 = unintelligible), and perceived articulation rate (“slow, normal, and fast”). The 1,080 samples were randomly presented to each judge over headphones, totaling 21,600 responses on articulatory accuracy and 21,600 on speaking rate. Presentation and response collection were completed using a computer procedure written in Praat (Boersma & Weeink, 2006).

Speech Testing: Acoustic Analysis

The same 540 unique syllable trains were analyzed acoustically. The following acoustic events were labeled by a research assistant, who was blind to the study conditions, by identifying various acoustic markers (such as the release of stop consonants and the first vocal pulse for vowels) in both waveform and spectrographic views for each syllable in each syllable train: the syllable onset and offset, and the vowel onset and offset in each syllable. Figure 1 provides an example of the labeling. The following was obtained for each syllable train using a procedure written in Praat (Boersma & Weeink, 2006): the mean syllable rate (number of syllables per second), the mean syllable duration, the mean vowel duration, the mean VOT, and the mean F0. When a clear release of a stop consonant was missing (e.g., when two adjacent syllables ran together without a stop closure), the onset of the syllable and the onset of the vowel were identical, the VOT was counted as missing. Five percent of the data were randomly selected and relabeled and the intrajudge reliability was calculated. It was 99.5% for mean syllable rate, 98.1% for mean syllable duration, 99.3% for mean vowel duration, 98.5% for mean VOT, and 99.9% for mean F0.

Figure 1.

Figure 1

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The 3rd syllable from a syllable train of “pah” demonstrating the labeling: l3 = syllable onset/VOT onset, v3 = vowel onset/VOT offset, e3 = syllable offset/vowel offset.

STATISTICAL ANALYSIS

Two types of statistical analyses were used: Analysis of Variance and Correlation analysis. A mixed two-factor ANOVA (i.e., Side of STN DBS and Testing Conditions) with repeated measures was used to assess the significance of the changes in the UPDRS-III scores for nonspeech motor tasks. A mixed three-factor ANOVA (adding factor Syllable Type) with repeated measures was used to assess the changes in perceived articulatory accuracy and speaking rate, as well as in the measured syllable rate, syllable duration, vowel duration, VOT, and F0. The between factors were Side of STN DBS (i.e., Left STN DBS, and Right STN DBS) and Syllable Type (i.e., “pah,” “tah,” and “kah”), and the within factor was Testing Condition (i.e., Baseline, Stimulation-off, and Stimulation-on). If our hypothesis that right and left STN DBS have differential effects on speech was correct, the independent factor of Side of STN DBS would be significant for speech measures only but not so for the limb motor scores of the UPDRS. A significance level of .05 was used for all F-tests. The Scheffé test was used for post-hoc testing and the significance level was corrected for all comparisons (Keppel, 1982). Pearson’s correlation and Fisher’s r to z test were used to assess the correlations between the means of the measured syllable rate, the perceived syllable rate, and the perceived articulatory accuracy score.

RESULTS

Motor Testing: The UPDRS Scores

The mean UPDRS-III score improved from 37.4 (SD 12.04) at Baseline to 30.7 (SD 14.39) at Stimoff to 24.9 (SD 12.06) at Stim-on. A lower score indicates a lesser motor deficit. The main effect of Testing Condition was significant (p < .0001) while the main effect of Side of STN DBS was not. The amount of improvement was similar for the two STN-groups (Figure 2), and the interaction between the two main factors was not significant. These results indicate that STN DBS is effective regardless the side of treatment.

Figure 2.

Figure 2

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Group means (right-STN DBS vs. left STN DBS) of UPDRS-III scores under three testing conditions in the medication-off state.

The Articulatory Accuracy Rating Scores

The diadochokinetic syllables were rated for articulatory accuracy using UPDRS-III item 18. The articulatory accuracy rating ranged from 0 (no deficits or normal) to 4 (unintelligible), that is, the higher the rating was, the less intelligible the speech or the more impaired the speech was. Figure 3 shows the articulatory accuracy rating of all three syllable types separated by the Side of STN DBS and Testing Condition. The articulatory accuracy was more impaired in the left STN DBS group for “pah” and “tah” in comparison to the right STN DBS group. The syllable “kah” was more impaired than “pah” and “tah” in both STN groups. Active stimulation in the left STN had a greater negative impact on the articulatory accuracy for “tah” and ‘kah“ than that in the right STN when the Baseline condition was compared to the Stim-on condition. All these observed changes were statistically significant: Side of STN DBS (p = .0026), Syllable Type (p < .0001), and Testing Condition, (p < .0001). The two-way interactions between the Side of STN DBS and Testing Condition, and between the Syllable Type and the Testing Condition were significant (all p < .0001). The three-way interaction of the Side of STN DBS, the Syllable Type and the Testing Condition was significant (p < .0001). The post-hoc testing of means by Scheffé test showed that the observed difference between the two STN groups was significant regardless of Testing Condition and Syllable Type (p = .0034). While the syllables “pah” and “tah” had similar level of articulatory accuracy, both were significantly more accurate than the syllable “kah” (both p < .0001). Lastly, the articulatory accuracy was generally worse in the Stim-on condition than the Stim-off and Baseline conditions. Pairwise comparisons showed that these were statistically significant differences (p < .0001, p < .0001, p = .03 for Baseline vs. Stim-off, Baseline vs. Stim-on, and Stim-off vs. Stim-on, respectively).

Figure 3.

Figure 3

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Group means (right-STN DBS vs. left STN DBS) of articulatory accuracy rating separated by Syllable Type and Testing Condition in the medication-off state.

The Perceived Speaking Rate Scores

The diadochokinetic syllables were rated for speaking rate as slow, normal, and fast. The responses were then converted by a computer procedure into 1 (slow), 2 (normal), and 3 (fast). Figure 4 shows the perceived speaking rate of all three syllable types separated by the Side of STN DBS and Testing Condition. The speaking rate was generally slower for the Left STN DBS group in comparison to the Right STN DBS group. This difference was statistically significant (p = .0042). The speaking rate was perceived faster for syllable “pah” in comparison to the other two syllables with “kah” being the slowest, regardless of the Testing Condition. This difference of Syllable Type was also statistically significant (p = .0004). The overall effect of the Testing Condition was significant (p < .0001). It is worth noting that the micro lesion effect (i.e., Stim-off in comparison to Baseline) decreased the speaking rate while the active stimulation (i.e., Stim-on in comparison to Stim-off) had the opposite effect on the Left STN DBS group versus the Right STN DBS group: decreasing speaking rate in the Left STN DBS group and increasing it in the Right STN DBS group. This two-way interaction between the Side of STN DBS and Testing Condition was significant (p < .0001). The three-way interaction of the Side of STN DBS, the Syllable Type and the Testing Condition was significant (p < .0001) as well. Generally, the speaking rate between the two STN DBS groups did not respond to active STN DBS uniformly. It was either increased or unchanged for the Right STN DBS group, while it was decreased or remained unchanged for the Left STN DBS group.

Figure 4.

Figure 4

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Group means (right-STN DBS vs. left STN DBS) of perceived speaking rate scores separated by Syllable Type and Testing Condition in the medication-off state.

The Acoustically Measured Syllable Rate

The diadochokinetic syllables were measured acoustically. The means and standard deviations were shown in Figure 5. The overall pattern (see Figure 5) was quite similar to that of the perceived speaking rate (see Figure 4). For all three syllable types, the mean syllable rates were generally higher in the right STN DBS group than in the left STN DBS group. This difference between the Left versus Right STN groups was approaching significance (p = .062). All mean syllable rates were within the range reported on normal adult speakers (Duffy, 2005). However, it was observed that the actual production of the syllables was frequently impaired, especially for the syllable “kah.” The syllable rate was fast for the syllable “pah,” slower for “tah,” and slowest for “kah,” regardless of the Testing Condition. This difference of Syllable Type was significant (p < .0001). The factor of Testing Condition was not significant. However, comparing condition Stim-off to Stim-on, active stimulation decreased the syllable rate for “tah” and “kah” in the Left STN group while it increased the rate of all three syllables in the Right STN group. The two-way interaction between the Side of STN DBS and the Testing Condition was significant (p = .042).

Figure 5.

Figure 5

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Group means (right-STN DBS vs. left STN DBS) of measured syllable rates separated by Syllable Type and Testing Condition in the medication-off state.

The Correlations

Table 2 shows the results of Pearson’s correlation and Fisher’s r to z test for the articulatory accuracy rating score, perceived speaking rate, and measured syllable rate. The measured syllable rate was highly correlated with the perceived speaking rate (p < .0001). Both the perceived speaking rate and the measured syllable rate were negatively correlated with the articulatory accuracy rating because a lower rating score in articulatory accuracy indicates better articulation. The range of the syllable rates reported here was 5.2 to 6.9 repetitions per second, which was within the range reported for normal speakers (Duffy, 2005). Within this range, the higher the syllable rate, the better the perceived articulatory accuracy. In fact, if one combines the findings in Figure 3 and Figure 5, it is clear that for the syllables “tah” and “kah” the syllable rate reduction was associated with decreased articulatory accuracy.

TABLE 2.

Pearson’s Correlation and Fisher’s r to s test results.

Pearson’s Correlation and Fisher’s r to s Test Results
Correlation Pair Correlation Z-Value P-Value 95% Lower 95% Upper
A vs. MSR: Overall -0.887 -5.457 <.0001 -0.958 -0.718
A vs. MSR: Left -0.808 -2.747 0.006 -0.958 -0.311
A vs. MSR: Right -0.909 -3.721 0.0002 -0.981 -0.616
A vs. PSR: Overall -0.796 -4.209 <.0001 -0.921 -0.523
A vs. PSR: Left -0.666 -1.969 0.0489 -0.922 -0.004
A vs. PSR: Right -0.806 -2.732 0.0063 -0.958 -0.305
MSR vs PSR: Overall 0.921 6.172 <.0001 0.796 0.97
MSR vs PSR: Left 0.942 4.293 <.0001 0.741 0.988
MSR vs PSR: Right 0.922 3.928 <.0001 0.666 0.984
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A = Articulatory Accuracy Rating, PSR = Perceived Speaking Rate, MSR = Measured Syllable Rate

The Syllable Duration, Vowel Duration, VOT Duration, and F0

The overall findings of the four acoustical parameters are summarized in Table 3.

TABLE 3.

Summary of statistical findings on the acoustic parameters of syllable duration, vowel duration, VOT, and F0 of “pah”, “tah”, and “kah”.

Acoustic Parameter Variable P-Value
Mean syllable duration Side of STN DBS ns
Syllable Type *
Testing Condition *
Mean vowel duration Side of STN DBS ns (p = .06)
Syllable Type *
Testing Condition ns
Testing Condition × Side of STN DBS *
Mean VOT Side of STN DBS ns
Syllable Type *
Testing Condition ns (p = .06)
Mean F0 without the main factor of “Gender” Side of STN DBS ns
Syllable Type ns
Testing Condition ns
Mean F0 with the main factor of “Gender” Side of STN DBS *
Syllable Type ns
Testing Condition ns
Gender *
Gender × Side of STN DBS *
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*

indicates significance with p < .05

There were no significant changes associated with the Side of STN DBS for any of the four acoustic parameters listed above. As expected, there were significant differences in the syllable duration, the vowel duration, and VOT that were associated with the three different types of syllables. For mean syllable duration, “pah” was the shortest and “kah” the longest. For mean vowel duration, “kah,” was the shortest and “pah” the longest. The mean VOT was the shortest for “pah” and the longest for “kah,” which is consistent with the findings in normal speech production. The patterns remained the same for both STN DBS groups. Since F0 is expected to have gender-based differences, the between factor of Gender was added and the ANOVA was rerun as a four-factor model. The new results showed a significant effect of Side of STN DBS and that of Gender as well as a significant interaction of these two main factors. For the Left STN group, the mean F0 was 200 Hz (SD 21) for the four female speakers and 168 Hz (SD 32) for the six male speakers. For the Right-STN group, the mean F0 was 212 Hz (SD 17) for the five female speakers and 127 Hz (SD 15) for the five male speakers. Similar lateralized changes in F0 were previously found in the sustained vowel phonation in the same group of participants (Wang, Verghagen, Bakay, Arzbaecher, & Bernard, 2004). It was found that the F0 of sustained vowel phonations was higher than normal for male speakers and lower than normal for female speakers in those who received left STN DBS; in contrast, the F0 remained at the normal range for both female and male speakers in those who received right STN DBS (Wang et al., 2004).

DISCUSSION

The findings reported here for the diadochokinetic syllables are consistent with reports for vocal intensity and fundamental frequency in the same series of patients (Wang et al., 2003, 2004). That is, while the unilateral STN DBS produced improvement in nonspeech motor movement regardless of the side of STN DBS, its effect on articulatory accuracy and rate of the diadochokinetic syllables seemed to be associated with the side of STN that was stimulated. The articulatory accuracy decreased with active left STN stimulation, but improved or remained unchanged with active right STN stimulation. The syllable rate was in general decreased with active left STN stimulation, but increased or remained unchanged with active right STN stimulation. Further, the correlation analyses show that the slower syllable rate was associated with decreased articulatory accuracy perceptually. These findings support our hypothesis that right and left STN DBS may have differential effects on speech. It was also found that in general the articulatory accuracy was more affected for “kah” than “pah” and “tah.” This is consistent with some early observations that the tongue dorsum was more affected than the tongue blade and lips in patients with PD (Logemann et al., 1978). In addition, both articulatory accuracy and speaking rate were more negatively affected for the Left STN DBS group even at the Baseline, suggesting that the disease process of PD may affect speech differently depending on the motor asymmetry, that is, which side of the basal ganglia (BG) is more affected. Note that all 20 participants in this study were right handed and likely to have left cerebral dominance of linguistic function since 98% of the right-handed general population has left cerebral dominance of language (Hund-Georgiadis, Lex., & von Cramon, 2001; Knecht et al., 2000). The 10 patients received left STN DBS because their disease was more involved in the left BG resulting in more impaired body function on their right side, while the 10 patients who had right STN DBS had more involved disease in their right BG. It appears that when the disease was in the same hemisphere where language dominance is, the speech seemed to be more affected. The deterioration of speech at baseline as well as that associated with active STN stimulation in the Left STN DBS patient group suggests a close but complex relationship between the function of BG and the cortical control of speech production.

CONCLUSION

In conclusion, unilateral STN DBS produced a 33% improvement in nonspeech motor function. In contrast, the effect of STN DBS on articulatory accuracy and speaking rate of syllable repetitions was much less impressive, and the left versus right STN DBS produced changes that were hemisphere specific. The data reported here and from some other studies (Santens et al., 2003; Wang et al., 2003, 2004) indicate that the interaction between the disease process of PD and the cerebral dominance of linguistic function may shape what is behaviorally observed in terms of speech deterioration in PD and may also affect the therapeutic effect of STN DBS on speech. Future studies should address and carefully document how motor asymmetry of PD interacts with the cortical control of speech production and cerebral laterality of language control. Understanding these interactions will undoubtedly help in the development of more effective treatment of speech impairment in patients with PD.

Acknowledgments

The authors wish to thank Maureen Bradley, M.S., for her assistance in data analysis, and all patients who willingly served as participants on this study. This study was supported in part by grant NS 40902-03S1 from the National Institute of Neurological Disorders and Stroke.

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