Miriam Sach

Psycholinguistic theories propose different models of inflectional processing of regular and irregular verbs: dual mechanism models assume separate modules with lexical frequency sensitivity for irregular verbs. In contradistinction, connectionist models propose a unified process in a single module. We conducted a PET study using a 2 × 2 design with verb regularity and frequency. We found significantly shorter voice onset times for regular verbs and high frequency verbs irrespective of regularity. The PET data showed activations in inferior frontal gyrus (BA 45), nucleus lentiformis, thalamus, and superior medial cerebellum for both regular and irregular verbs but no dissociation for verb regularity. Our results support common processing components for regular and irregular verb inflection.

Amyotrophic lateral sclerosis (ALS) is a progressive neurodegenerative system disorder affecting both upper and lower motor neurons. Despite supportive electrophysiological investigations, the involvement of the upper motor neuron is often difficult to assess at an early stage of disease. Diffusion tensor MRI provides an estimate of the orientation of fibre bundles in white matter on the basis of the diffusion characteristics of water. Diffusivity is generally higher in directions along fibre tracts than perpendicular to them. This degree of directionality of diffusion can be measured as fractional anisotropy. Changes in tissue structure due to degeneration of the corticospinal fibres can lead to a modification of the degree of directionality which can be detected by diffusion tensor MRI. We investigated 15 patients with ALS, six of whom had no clinical signs of upper motor neuron involvement at the time of MRI investigation, but developed pyramidal tract symptoms later in the course of their disease. These patients met the El Escorial criteria as their disease progressed. We found a decrease in fractional anisotropy in the corticospinal tract, corpus callosum and thalamus in all 15 ALS patients, including the patients without clinical signs of upper motor neuron lesion, compared with healthy controls. Regression analysis showed a negative correlation between fractional anisotropy and central motor conduction time obtained by transcranial magnetic stimulation, allowing spatial differentiation between the degenerated corticospinal tract fibres that supply the upper and lower extremities. Thus, diffusion tensor MRI can be used to assess upper motor neuron involvement in ALS patients before clinical symptoms of corticospinal tract lesion become apparent, and it may therefore contribute to earlier diagnosis of motor neuron disease.