Involuntary attention in essential tremor: a MMN study

Background: Essential Tremor (ET) is one of the most common neurological disorders and the most common pathologic tremor in humans. Indeed, as has been observed in several other progressive movement disorders, the motor disturbances appear to be accompanied by cognitive deficits. Aims: To investigate the involuntary attention in patients with essential tremor using the Mismatch Negativity (MMN). Methods: seventeen non-demented patients with ET and seventeen age- and sex-matched healthy controls underwent an EEG recording. MMN was evoked in a classical auditory MMN paradigm: 1000 Hz standard stimuli, occasionally interrupted by 1100HZ deviant stimuli that occurred with a probability of 0.2, were administered while subjects were watching a silent movie. ISI was set at 1 s. MMN latency and amplitude were evaluated. Results: MMN latency was prolonged in patients with elderly-onset ET (>65 years) (p<0.005), while no differences emerged either in latency or in amplitude between younger-onset ET and controls. Discussion: Elderly onset ET patients displayed a difficulty in the automatic involuntary processing of the stimuli, as demonstrated by the prolonged MMN latency. MMN is generated from auditory cortices bilaterally, possibly with an additional downstream component in the frontal cortex, the latter being a selective index of attentional orienting. Cerebellum, which is thought to play a major role in the pathophysiology of ET, may interfere with cognitive functioning, due to its strong and bidirectional frontal connectivity particularly to the prefrontal cortex. The differences between elderly-onset and younger-onset ET patients indicate that ET may represent a heterogeneous family of diseases united by tremor, but heterogeneous in other characteristics, as regarding cognitive functioning and in particular the automatic pre-attentive orienting. Conclusion: Our psychophysiological data shed light on the involvement of the attentional frontal network in ET, revealing a picture of the pathological process underlying this disease that is more complex than traditionally believed.