The effects of Thalamic Deep Brain Stimulation on speech dynamics in patients with Essential Tremor: An articulographic study

Abstract

Acoustic studies have revealed that patients with Essential Tremor treated with thalamic Deep Brain Stimulation (DBS) may suffer from speech deterioration in terms of imprecise oral articulation and reduced voicing control. Based on the acoustic signal one cannot infer, however, whether this deterioration is due to a general slowing down of the speech motor system (e.g., a target undershoot of a desired articulatory goal resulting from being too slow) or disturbed coordination (e.g., a target undershoot caused by problems with the relative phasing of articulatory movements). To elucidate this issue further, we here investigated both acoustics and articulatory patterns of the labial and lingual system using Electromagnetic Articulography (EMA) in twelve Essential Tremor patients treated with thalamic DBS and twelve age- and sex-matched controls. By comparing patients with activated (DBS-ON) and inactivated stimulation (DBS-OFF) with control speakers, we show that critical changes in speech dynamics occur on two levels: With inactivated stimulation (DBS-OFF), patients showed coordination problems of the labial and lingual system in terms of articulatory imprecision and slowness. These effects of articulatory discoordination worsened under activated stimulation, accompanied by an additional overall slowing down of the speech motor system. This leads to a poor performance of syllables on the acoustic surface, reflecting an aggravation either of pre-existing cerebellar deficits and/or the affection of the upper motor fibers of the internal capsule.

Fig 1. Projection of all active contacts (blue) onto a neuroanatomical atlas [19].

Green: VIM, red: red nucleus. Note that most active contacts lay inside the VIM proper or at its ventral border while two contacts lay slightly more ventral in the zona incerta.

Fig 2. Articulograph (left) and location of sensors in the sagittal plane (right).

The person on the left side on the picture (by Fabian Stürtz) is neither a patient nor a subject of the study.

Fig 3

(a) Lower lip trajectory for fast syllable repetition task /pa/ for ET patient (black = DBS-ON; red = DBS-OFF;) and (b) schematized articulatory gesture with relevant landmarks.

Fig 4. Schematised modification of articulatory control parameters.

Fig 5

(a) TRS (A+B) scores for patients with inactivated (DBS-OFF) and activated stimulation (DBS-ON). (b) Mean VHI scores for control speakers and patients with DBS-ON according to VHI norm ranges. (c) VAS scores (“ability to speak”) for controls and patients with DBS-OFF and DBS-ON. Error bars represent standard deviation.

Fig 6. Acoustic measurements across subjects (mean and SD) for control speakers and ET patients with DBS-OFF and DBS-ON, shown separately for place of articulation (/pa, ta, ka/).

The display of frication-during-closure and voicing-during-closure refers to the proportion of cases of which frication and/or voicing during the consonantal constriction phase in /p, t, k/ occurred. Means for the proportion and ratio measures are not displayed with error bars, since means are correlated with variability.

Fig 7. Articulatory measurements across subjects (mean and SD) for control group and ET patients with DBS-OFF and DBS-ON, shown separately for place of articulation (/pa, ta, ka/).

Fig 8. Acoustic waveform and spectrogram for /ta/ and corresponding tongue tip position (vertical and horizontal position with the same range across speakers) during the production of /t/, for one ET patient in the DBS-OFF (middle) and DBS-ON condition (right) and one age-matched control-speaker (left).

Fig 9. Acoustic waveform and spectrogram for /ta/ and corresponding tongue tip position (vertical and horizontal position with the same range across speakers) during the production of /t/, for one ET patient in the DBS-OFF (middle) and DBS-ON condition (right) and one age-matched control-speaker (left).