Both handwriting and speech are learned, highly practiced human motor skills that involve the control and coordination of complex movement sequences. From a motor control perspective, both handwriting and speech involve small, fast movements with low force levels and small movement amplitudes.

Rigidity and bradykinesia, cardinal signs of Parkinson’s disease (PD), are believed to underlie both the handwriting and the speech abnormalities observed in PD patients. Rigidity can interfere with movement because muscles are in a “state of sustained contraction”. Normal reciprocal innervation which brings about the relaxation of the antagonist muscle while the agonist muscle contracts is impaired in PD. Hence rigidity may be associated with a reduction in movement amplitude. Bradykinesia can be manifested in many ways, including delay in initiating movement and slowness in executing movement.

Levodopa is considered to be the most effective therapeutic agent for the treatment of PD, particularly in the alleviation of rigidity and bradykinesia. Levodopa, an amino acid precursor of dopamine, is converted in the brain to dopamine to replenish the depleted stores of this neurotransmitter. Specifically, there is a loss of dopamine-producing cells in the pars compacta of the substantia nigra. Levodopa levels decay over several hours so every few hours another dose of levodopa has to be taken. Consequently, we hypothesized that Parkinsonian handwriting and Parkinsonian speech would change across the levodopa cycle.

Increased rigidity is associated with reductions of movement size while increased bradykinesia is associated with increases in duration of specific movement phases. This study was designed to evaluate changes in the spatial and temporal parameters of handwriting and speech that are presumed to reflect reductions in rigidity and bradykinesia, and, therefore, a therapeutic response to levodopa.

Micrographia is the handwriting disturbance associated with Parkinson’s disease. It is characterized by an overall decrease in handwriting size, diminished legibility, and decreased movement speed. The muscles involved in handwriting are slow in executing movements and these movements are reduced in amplitude. These muscles can be conceptualized as operating in a restricted working space. Working space should increase with reduced rigidity, detectable as an increase in stroke size, while a reduction in bradykinesia should result in a decrease in movement time, detectable as a decrease in stroke duration.

The motor speech disorder characteristic of Parkinson’s disease is called hypokinetic dysarthria. Speech has a mumbling quality with syllables and words seemingly running together. It appears that hypokinetic dysarthria may result from “articulatory undershoot” concluded that micrographia resembles hypokinetic speech in that “smaller movement excursions compensate for the inability to execute high velocity strokes” (p. 1097). Kinematic and acoustic analyses of the speech of PD individuals have provided evidence of reduced and slowed articulatory movements. Kinematic studies show that lip, jaw, and velar movements of PD individuals are smaller in range and slower than the movements of healthy elderly. This suggests that the speed and range of movement of the articulators are also affected by rigidity and bradykinesia and should vary across the levodopa cycle.

Speech working space can be quantified in frequency space by plotting the first versus the second formant frequencies of specific vowels. The lowest formant (F1) varies mostly with tongue height while the second lowest formant (F2) varies mostly with tongue advancement. The first and second formants of the vowels /i/, /u/, /æ/, and // represent vowel space. Reduced vowel space has been used to express degradation of speech in various dysarthric speakers including those with amyotrophic lateral sclerosis and those with cerebellar lesions. Conversely, expanded vowel space has been associated with increased speech intelligibility. We hypothesized that in PD patients levodopa brings about a reduction in rigidity of the articulators and therefore should produce larger vowel quadrilateral areas.

Unlike vowels, diphthongs are dynamic sounds that change their articulatory shape throughout their production. Measuring the slope of the trajectory of the second formant has been used as a gross estimate of the rate of change in the articulatory pattern of the diphthong. It has been shown that these slopes are smaller for PD patients than for normal geriatric adults. When rigidity and bradykinesia are reduced, a greater slope should result as a function of the increased change in articulatory configuration per unit time.

In summary, certain spatial and temporal parameters can reflect changes in the amplitude and speed of PD movements across the levodopa cycle. Upstroke size characterizes handwriting working space while vowel quadrilateral area and diphthong slope characterize speech working space. We expect that working space is increased when levodopa is effective in reducing rigidity and bradykinesia. Upstroke duration and vowel duration reflect the temporal parameter of handwriting and speech movements, respectively, which is expected to vary with the effectiveness of levodopa.