Cortical correlates of auditory sensory gating: a simultaneous near-infrared spectroscopy event-related potential study.

Sensory gating refers to the ability of cerebral networks to inhibit responding to irrelevant environmental stimuli, a mechanism that protects the brain from information overflow. The reduction of the P50 amplitude (an early component of the event-related potential/ERP in electrophysiological recordings) after repeated occurrence of a particular acoustic stimulus is one means to quantitatively assess gating mechanisms. Even though P50 suppression has been extensively investigated, neuroimaging studies on the cortical correlates of auditory sensory gating are so far very sparse. Near-infrared spectroscopy (NIRS) is an optical imaging technique perfectly suitable for the investigation of auditory paradigms, since it involves virtually no noise. We conducted a simultaneous NIRS-ERP measurement to assess cortical correlates of auditory sensory gating in humans. The multi-channel NIRS recording indicated a specific activation of prefrontal and temporo-parietal cortices during conditions of increased sensory gating (dual-click trials). Combining the hemodynamic data with an electrophysiological index of the "gating quality" (gating quotient Q) revealed a positive correlation between the amount of sensory gating and the strength of the hemodynamic response during dual-clicks in the left prefrontal and temporal cortices. The results are in line with previous findings and confirm a possible inhibitory influence of the prefrontal cortex on primary auditory cortices.

Non-invasive measurement of vagus activity in the brainstem - a methodological progress towards earlier diagnosis of dementias?

In Alzheimer's disease (AD), mild functional disturbances should precede gross structural damage and even more clinical symptoms, possibly by decades. Moreover, alterations in the brainstem are supposed to occur earlier as cortical affections. Based on these considerations, we developed a new method aiming at the measurement of vagal brainstem functioning by means of evoked potentials after electrical stimulation of the cutaneous representation of the vagus nerve in the external auditory channel. In the current study, a first sample of patients with Alzheimer's disease (n = 7) and mild cognitive impairment (n = 3) were investigated (6m, 4f, range from 57 to 78 y, mean age 68.6 years). Vagus somatosensory evoked potentials (VSEP) were characterized by significantly longer latencies as compared to healthy age- and gender-matched controls (p < 0.05). Future large scale studies - also including preclinical stages of AD - have to assess the value of this non-invasive, fast and cheap method in the early diagnosis of neurodegenerative disorders.