Auditory brain-stem, middle- and long-latency evoked potentials in mild cognitive impairment

The auditory P50 component to onset and offset of sound

OBJECTIVE

The auditory Event-Related Potentials (ERP) of component P50 to sound onset and offset have been reported to be similar, but their magnetic homologue has been reported absent to sound offset. We compared the spatio-temporal distribution of cortical activity during P50 to sound onset and offset, without confounds of spectral change.

METHODS

ERPs were recorded in response to onsets and offsets of silent intervals of 0.5 s (gaps) appearing randomly in otherwise continuous white noise and compared to ERPs to randomly distributed click pairs with half second separation presented in silence. Subjects were awake and distracted from the stimuli by reading a complicated text. Measures of P50 included peak latency and amplitude, as well as source current density estimates to the clicks and sound onsets and offsets.

RESULTS

P50 occurred in response to noise onsets and to clicks, while to noise offset it was absent. Latency of P50 was similar to noise onset (56 ms) and to clicks (53 ms). Sources of P50 to noise onsets and clicks included bilateral superior parietal areas. In contrast, noise offsets activated left inferior temporal and occipital areas at the time of P50. Source current density was significantly higher to noise onset than offset in the vicinity of the temporo-parietal junction.

CONCLUSIONS

P50 to sound offset is absent compared to the distinct P50 to sound onset and to clicks, at different intracranial sources. P50 to stimulus onset and to clicks appears to reflect preattentive arousal by a new sound in the scene. Sound offset does not involve a new sound and hence the absent P50.

SIGNIFICANCE

Stimulus onset activates distinct early cortical processes that are absent to offset.

Cholinesterase inhibitors affect brain potentials in amnestic mild cognitive impairment

Amnestic mild cognitive impairment (MCI) is an isolated episodic memory disorder that has a high likelihood of progressing to Alzheimer's disease. Auditory sensory cortical responses (P50, N100) have been shown to be increased in amplitude in MCI compared to older controls. We tested whether (1) cortical potentials to other sensory modalities (somatosensory and visual) were also affected in MCI and (2) cholinesterase inhibitors (ChEIs), one of the therapies used in this disorder, modulated sensory cortical potentials in MCI. Somatosensory cortical potentials to median nerve stimulation and visual cortical potentials to reversing checkerboard stimulation were recorded from 15 older controls and 15 amnestic MCI subjects (single domain). Results were analyzed as a function of diagnosis (Control, MCI) and ChEIs treatment (Treated MCI, Untreated MCI). Somatosensory and visual potentials did not differ significantly in amplitude in MCI subjects compared to controls. When ChEIs use was considered, somatosensory potentials (N20, P50) but not visual potentials (N70, P100, N150) were of larger amplitude in untreated MCI subjects compared to treated MCI subjects. Three individual MCI subjects showed increased N20 amplitude while off ChEIs compared to while on ChEIs. An enhancement of N20 somatosensory cortical activity occurs in amnestic single-domain MCI and is sensitive to modulation by ChEIs.

Sensory gating in patients with Alzheimer's disease and their biological children

Research has shown that sensory gating is largely modualted by acetylcholine. Diminished levels of acetylcholine and sensory gating deficits have been reported in research involving Alzheimer's disease (AD) patients. However, there has been little investigation into those with a family history (FH+) of AD. The rationale of this study was to determine whether sensory gating impairments could distinguish those with early AD from individuals with increased risk for the disease while replicating previous findings of gating abnormalities in AD patients. Using the paried-click paradigm, evoked potentials were recorded from 4 groups of 20 subjects per group (AD, older controls, FH+, FH-). The results showed that while the AD group demonstrated sensory gating abnormalities, the FH+ group did not when compared to their peers with no family history of the disease (FH-). These results are discussed in relation to previous findings reporting P300 abnormalities in the FH+ group.

Assessing neurocognitive dysfunction in cranial radiotherapy: can cognitive event-related potentials help?

Cognitive changes are common sequelae of cancer and cancer treatment, particularly in patients receiving cranial radiotherapy (RT). These effects are typically assessed by subjective clinical examination or using objective neuropsychological tests. Biologically based neurophysiological methods have been increasingly applied to the study of cognitive processing in neuropsychiatric and neurological disorders and as objective measures of cognitive status for patients with dementia. These methods detect the activation of neural circuits that directly mediate cognitive function in the human brain and include metabolic and electrophysiology based techniques. Neuroimaging procedures such as 18FDG PET and more recently fMRI, which detect metabolic activation associated with cognitive processing, provide excellent spatial resolution and can be directly correlated with neuroradiological findings associated with cranial RT neurotoxicity. Clinical electrophysiology procedures such as cognitive event-related potentials (ERP), which detect the neuronal electrical activity associated with cognitive processing, offer excellent temporal resolution at low cost. Cognitive ERP techniques are already being used to assess severity and progression of cognitive dysfunction in patients with vascular and degenerative dementias, but have been largely overlooked in studies of radiation-related cognitive impairments. We review these various electrophysiological methods in the context of their relevance to assessing cranial RT effects on cognitive function, and provide recommendations for a neurophysiological approach to supplement current neuropsychological tests for RT cognitive impairments. This technology is well suited for clinical assessment of neurocognitive sequelae of cancer and should provide new insights into the mechanism of RT-related cognitive dysfunction.