Somatosensory Temporal Discrimination Threshold in Patients with Cognitive Disorders

Background: The temporal processing of sensory information can be evaluated by testing the somatosensory temporal discrimination threshold (STDT), which is defined as the shortest interstimulus interval needed to recognize two sequential sensory stimuli as separate in time. The STDT requires the functional integrity of the basal ganglia and of the somatosensory cortex (S1). Although there is evidence that time processing is impaired in patients with Alzheimer's disease (AD), no study has yet investigated STDT in patients with various degree of cognitive impairment. Objective: The aim of our study was to understand how cognition and attention deficits affect STDT values in patients with cognitive abnormalities. Methods: We enrolled 63 patients: 28 had mild-moderate AD, 16 had mild cognitive impairment (MCI), and the remaining 19 had subjective cognitive deficit (SCD). A group of 45 age-matched healthy subjects acted as controls. Paired tactile stimuli for STDT testing consisted of square-wave electrical pulses delivered with a constant current stimulator through surface electrodes over the distal phalanx of the index finger. Results: STDT values were higher in AD and MCI patients than in SCD subjects or healthy controls. Changes in the STDT in AD and MCI were similar in both conditions and did not correlate with disease severity. Conclusions: STDT alterations in AD and MCI may reflect a dysfunction of the dopaminergic system, which signals salient events and includes the striatum and the mesocortical and mesolimbic circuits.