The middle latency auditory evoked potential may be abnormal in dementia

Association between Excessive Daytime Sleepiness and the Cholinergic Ascending Reticular System in Parkinson's Disease

BACKGROUND

Excessive daytime sleepiness (EDS) in Parkinson's disease (PD) may occur because of dysfunction on the brain areas in controlling wakefulness; however, the pathophysiology of EDS in PD has not been completely clarified. The Pb component of a middle-latency auditory evoked response (MLR) is generated from the cholinergic ascending reticular activating system (ARAS) projecting to the auditory cortex via the thalamus. We examined the association between EDS and the Pb component in patients with PD.

METHODS

Participants were 38 patients with nondemented PD and 18 age-matched controls. EDS was evaluated using the Japanese version of the Epworth Sleepiness Scale (JESS). PD patients were classified into the high sleepiness (HS) group and the low sleepiness (LS) group by the score of JESS. MLRs were recorded from the scalp with each earlobe as a reference under presentation of 1-Hz and 65- to 90-dB click sounds.

RESULTS

There was no difference in age, duration, and motor function between the HS PD and the LS PD groups. Peak latencies of Pb were not different between PD group and controls; however, Pb amplitudes were significantly increased in the HS PD group compared with the LS PD group and controls.

CONCLUSION

One of the mechanisms of EDS in PD was suggested to be dysregulation of cholinergic neurons from the ARAS projecting to cortical cholinergic neurons.

Future Perspectives on the Relevance of Auditory Markers in Prodromal Parkinson's Disease

Research on auditory processing in Parkinson's disease (PD) has recently made substantial progress. At present, evidence has been found for altered auditory processing in the clinical stage of PD. The auditory alterations in PD have been demonstrated with low-cost and non-invasive assessments that are already used in routine clinical practice. Since auditory alterations have been reported early in disease progression, it would be highly relevant to investigate whether auditory markers could be provided in the prodromal stage of PD. In addition, auditory alterations in early stage PD might be modulated by dopaminergic medication. Therefore, the aim of this review is (1) to summarize the literature on auditory processing in PD with a specific focus on the early disease stages, (2) to give future perspectives on which audiological and electrophysiological measurements could be useful in the prodromal stage of PD and (3) to assess the effect of dopaminergic medication on potential auditory markers in the prodromal stage of PD.

Central auditory processing in parkinsonian disorders: A systematic review

Altered auditory processing has been increasingly recognized as a non-motor feature in parkinsonian disorders. This systematic review provides an overview of behavioral and electrophysiological literature on central auditory processing in patients with Parkinson's disease (PD), multiple system atrophy (MSA) and progressive supranuclear palsy (PSP). A systematic database search was conducted and yielded 88 studies that met the intelligibility criteria. The collected data revealed distinct impairments in a range of central auditory processes in PD, including altered deviance detection of basic auditory features, auditory brainstem processing, auditory gating and selective auditory attention. In contrast to PD, literature on central auditory processing in atypical parkinsonian disorders was relatively scarce, but provided some evidence for impaired central auditory processing in MSA and PSP. The interpretation of these findings is discussed and suggestions for further research are offered.

Brainstem auditory evoked potentials and middle latency auditory evoked potentials in patients with impaired glucose tolerance

AIMS

The aim was to investigate the effects of impaired glucose tolerance (IGT) on the central nervous system via brainstem auditory evoked potentials (BAEPs) and middle latency auditory evoked potentials (MLAEPs), and on the peripheral nervous system via nerve conduction studies (NCS).

METHODS

Thirty patients with IGT and 20 control subjects underwent NCS, BAEPs and MLAEPs.

RESULTS

Tibial distal motor latencies were significantly prolonged in the IGT group; no differences in other parameters, including BAEPs and MLAEPs, were observed between the IGT and control subjects.

CONCLUSION

Brainstem involvement may not be seen in IGT patients as in DM. The was no obvious electrophysiological finding indicating peripheral nervous system disfunction in our patients.

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.

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.

Noradrenergic antagonism of the P13 and N40 components of the rat auditory evoked potential

RATIONALE

Two rat auditory evoked potential (AEP) components P13 and N40 are suggested as analogues to the human P50, which has abnormal suppression properties in schizophrenia. However, P50 likely reflects neural activity from several different brain areas. Studies examining each of these components in the rat model have proposed circuitry that involves alpha2 norepinephrine (NE) receptors, and different disruption effects are predicted depending on whether effects are presynaptic or postsynaptic.

OBJECTIVES

The aim of this paper is to test differential effects of NE antagonism on disruption of normal P13 and N40 expression.

MATERIALS AND METHODS

AEPs were recorded simultaneously in alert, freely moving rats using the alpha2 antagonist yohimbine. Amplitudes of P13 and N40 elicited by 500-ms interstimulus interval click pairs were measured after administration of a placebo and three doses of the yohimbine.

RESULTS

A high dose of yohimbine yielded smaller P13 amplitudes to both clicks, consistent with presynaptic action. However, a moderate yohimbine dose yielded increased P13 amplitudes to both clicks. For N40, a moderate dose of yohimbine yielded increased amplitudes to the second stimulus, and a high dose restored normal suppression, which is consistent with previously reported findings.

CONCLUSIONS

This study demonstrated that noradrenergic activity differentially affects P13 and N40 components. As P13 and N40 are each models of human P50, these findings highlight the complex circuitry that likely underlies P50. An appreciation for these complexities is critical for understanding the mechanisms of the P50 suppression deficit in schizophrenia, which may be influenced by both trait and state factors.

Enhanced magnetic auditory steady-state response in early Alzheimer's disease

OBJECTIVE

Previous studies have reported abnormalities in both spontaneous and evoked electromagnetic brain activity in Alzheimer's disease (AD). We studied the auditory steady-state response (SSR) which represents the net effect of entrained background activity and superimposed cortical evoked responses, in AD patients and healthy controls.

METHODS

Whole-head magnetoencephalography (MEG) was used to measure SSR to monaural 40-Hz stimulation in AD patients and age-matched controls. Equivalent current dipoles (ECD) of the SSR were modeled in each hemisphere, and source amplitudes were compared between the two groups using time-varying ECD models.

RESULTS

Our results indicate that the SSR is significantly increased in AD patients with mild to moderate cognitive deterioration in comparison with healthy elderly subjects.

CONCLUSIONS

Goal-directed functioning requires optimization of inhibitory and excitatory inputs in the cortex, allowing the adaptation of responsiveness to repetitive stimulation with low relevance. The present results suggest that this balance is impaired in AD, manifesting itself in decreased inhibition in cortical auditory processing and impaired adaptation of the stimulus-locked activity, probably due to abnormalities in cholinergic modulation.

SIGNIFICANCE

MEG appears to be a sensitive tool to detect abnormalities of auditory processing already in early stages of AD.

Cholinergic modulation of preattentive auditory processing in aging

Auditory event-related potential (ERP) components P50 and N100 are thought to index preattentive auditory processing underlying stimulus detection, whereas a subsequent component termed mismatch negativity (MMN) has been proposed to reflect comparison of incoming stimuli to a short-lived sensory memory trace of preceding sounds. Existing evidence suggests impairment of preattentive auditory processing in aging, which appears to be accompanied by decline of cholinergic activity. Previous studies indicate that scopolamine, which is a centrally acting muscarinic receptor antagonist, modulates preattentive auditory processing in young subjects. It has remained elusive, however, to which extent scopolamine affects preattentive auditory processing in aged subjects. We measured auditory responses simultaneously with electroencephalogram (EEG) and magnetoencephalogram (MEG) from nine non-demented elderly subjects after intravenous injection of scopolamine or glycopyrrolate, the latter being a peripherally acting cholinergic antagonist, using a double blind protocol. Scopolamine significantly delayed electric P50, both electric and magnetic N100 responses, whereas subsequent MMN and P200 responses were not altered by scopolamine. Our results indicate that the cholinergic system modulates auditory processing underlying stimulus detection in aging. In addition, auditory evoked responses appear to have different age-related sensitivity to cholinergic modulation. The combined MEG/EEG measurements using particularly auditory N100 response might offer an objective tool to monitor cholinergic activity in aging and Alzheimer's disease (AD).

Reduced brain electric activities of frontal lobe in cortical cerebellar atrophy

OBJECTIVE

To assess the relationship between cerebellum and brain cortical activity without motor factors, we recorded the mid-latency auditory evoked responses (MLRs) with simultaneous recording of the electroencephalography (EEG) at rest in patients with 'pure cortical cerebellar atrophy (CCA)'.

METHODS

We studied 12 normal control subjects and non-demented 'pure CCA' patients determined by quantitative magnetic resonance imaging analysis. A comprehensive neuropsychological test battery assessed intelligence, frontal lobe function and word fluency. Spontaneous eyes-closed resting EEG and MLRs were recorded from 20 scalp electrodes and analysed with low-resolution electromagnetic tomography (LORETA) to compute the 3-dimensional intracerebral distribution of electric activity.

RESULTS

Neuropsychological tests revealed no differences between CCA and the control. Analysis of EEG and MLRs using classical methods also did not reveal any differences. LORETA analysis indicated significant decrease of alpha2 activity in the left inferior frontal gyrus in CCA. On MLRs, the most significant difference was observed at P1 component, and CCA patients showed significant decrease at the right superior frontal gyrus.

CONCLUSIONS

Our results indicated that the frontal lobe and ascending reticular activating system are inhibited in CCA patients, and suggested the involvement of the cerebellum in cortical electric activities irrespective of motor adjunct.

SIGNIFICANCE

Quantitative EEG and MLR measurements with LORETA pointed out frontal lobe hypoactivities in pure CCA patients.

Middle latency auditory-evoked potentials in myotonic dystrophy: relation to the size of the CTG trinucleotide repeat and intelligent quotient

OBJECTIVE

Major components of MLAEPs are thought to originate in the temporal lobe. Absence of the Pb potential has been demonstrated in MLAEPs in Alzheimer's disease and demented Parkinson's disease patients. To validate usefulness of middle latency auditory-evoked potentials (MLAEPs) in evaluating the central nervous system (CNS) involvement of myotonic dystrophy (MyD).

METHODS

MLAEPs were recorded in eight patients with MyD and nine normal control subjects. In the patient group, the size of the CTG triplet repeat expansion within the dystrophia myotonica protein kinase (DMPK) gene and the revised Wechsler Adult Intelligence Scale (WAIS-R) were also assessed.

RESULTS

The latency of the Nb potential showed a significant correlation with the size of the CTG repeat expansion (r=0.734, P=0.036). The Pb latency also tended to prolong according to CTG amplification (r=0.644, P=0.087). The amplitudes of Na and Pa significantly increased compared with those of normal control subjects (P=0.024 and 0.016, respectively). However, they did not correlate with IQ or CTG amplification.

CONCLUSIONS

Abnormal MLAEPs may indicate CNS involvement in MyD. Although the precise generating mechanisms of Nb are unclear, the correlation of Nb latency with CTG amplification suggests that MLAEPs can reflect the extent of genetic abnormality.

Memory-based comparison process not attenuated by haloperidol: a combined MEG and EEG study

Auditory P50 and N100 responses reflect preattentive processing, whereas subsequent mismatch negativity (MMN) response indexes memory-based comparison process. Divergent ERP responses have been found in schizophrenia and in Parkinson's disease (PD), which have abnormalities in cerebral dopamine activity. We used simultaneously magnetoencephalography and electroencephalography to investigate, whether a single dose of haloperidol, a dopamine D2-receptor antagonist, modulates preattentive auditory processing using a randomized, double-blind, placebo-controlled crossover design. Our results showed that haloperidol did not alter MMN to frequency and duration changes, whereas the magnetic MMN to frequency change was significantly accelerated. The amplitude and latency changes of the electric and magnetic P50 and N100 were insignificant. Our results indicate that memory-based sound comparison and preceding cortical processing underlying stimulus detection are not attenuated by haloperidol, whereas haloperidol appears to accelerate preattentive sound comparison.

Auditory event-related potentials during target detection are abnormal in mild cognitive impairment

OBJECTIVE

To define brain activity and behavioral changes in mild cognitive impairment (MCI), an isolated memory deficit in the elderly that is a major risk factor for Alzheimer's disease.

METHODS

Brain potentials and reaction time were examined in elderly controls (n=12) and MCI (n=15) using a target detection paradigm. Subjects listened to a sequence of tones and responded to high-pitched target tones (P=0.20) that were randomly mixed with low-pitched tones (P=0.80). Measures were a pre-stimulus readiness potential (RP), post-stimulus potentials (P50, N100, P200, N200, P300), and reaction time.

RESULTS

Accuracy was equivalent between groups, but there was a trend for longer reaction times in MCI (P=0.08). Two potentials differed between groups: (1) P50 amplitude and latency were significantly increased in MCI, and (2) P300 latency was significantly longer in MCI. Results from two MCI subjects that converted to Alzheimer's disease are also discussed.

CONCLUSIONS

Brain potentials in MCI subjects during target detection have certain features similar to healthy aging (RP, N100, P200, N200), and other features similar to Alzheimer's disease (delayed P300 latency, slower reaction time). P50 differences in MCI may reflect pathophysiological changes in the modulation of auditory cortex by association cortical regions having neuropathological changes in early Alzheimer's disease.

Middle latency auditory evoked potentials in patients with parkinsonism

The aim of this study was to assess the middle latency auditory evoked potential (MLAEP) findings in idiopathic Parkinson's disease (IPD) and in patients who are regarded as having atypical parkinsonian disorders (AP) and to determine whether MLAEPs could contribute to the differential diagnosis of IPD and AP.MLAEPs were evaluated in 19 control subjects and in a total of 35 patients with parkinsonism, of which 27 had IPD and 8 had AP. Among IPD patients, P1 was absent in 1 nondemented patient with young-onset Parkinson's disease (YOPD) (5%) and in 2 of 7 demented (28.5%) IPD patients. In the AP group, 3 of the 7 (42.8%) nondemented patients and the one patient with dementia showed the absence of P1. The absence of P1 was found to be significantly higher in AP patients than IPD patients (p=0.0335).In conclusion, MLAEPs were found to be normal in nondemented IPD patients with only a few exceptions. The absence of P1 in nondemented patients with parkinsonian symptoms may bring the diagnosis of IPD into question. The absence of P1 could be detected in AP patients at least as often as in demented IPD patients. Thus, the measurement of MLAEPs may be a clinically useful adjunct to the clinical examination of patients with parkinsonism.

Impaired preconscious auditory processing and cognitive functions in Alzheimer's disease

OBJECTIVE

To study whether preconscious auditory processing is deteriorated in patients with Alzheimer's disease (AD) having mild to moderate cognitive symptoms. To investigate whether auditory processing correlates with the impairment of the higher cortical functions.

METHODS

P50m and N100m responses elicited by a sequence of repetitive tones were recorded with a whole-head magnetometer from 22 patients with probable AD and from 18 healthy age-matched controls. In addition, an extensive neuropsychological test battery assessing main cognitive domains was administered to all subjects.

RESULTS

The patients with AD had significantly delayed N100m responses in the left hemisphere that correlated with the impairment of the language functions.

CONCLUSIONS

N100m auditory responses measured with magnetoencephalography may be useful in evaluating the severity and progression of the cortical dysfunction in dementia.

Individual differences in auditory middle latency responses in elderly adults and patients with Alzheimer's disease

Previous research has suggested that the Pb component of the middle-latency auditory evoked response (MLAER) is differentially abnormal in patients with Alzheimer's disease relative to control subjects. In the present study, this putative abnormality was examined in vertex-recorded MLAERs elicited by monaural stimulation in 14 patients with Alzheimer's disease (six females) and 22 age-matched control subjects (10 females). A sex difference in Pb elicitation was revealed in control subjects; Pb area was twice as large in females than males (P < 0.05). Pb and Pa amplitudes and latencies did not differ between male and female control subjects. Comparisons of Pb between patients and controls were conducted within each sex. There was no main effect of group on Pb area, amplitude, or latency, Pa amplitude was significantly larger in patients than control subjects; there was no group difference in Pa latency. This study did not replicate previous reports of differences in Pb between patients with Alzheimer's disease and elderly control subjects. We demonstrated that Pb elicitation may be unreliable in elderly control subjects and found evidence of a possible sex difference. The effects of inter-subject variables (e.g. age, sex) must be understood more fully before MLAERs can be exploited as meaningful markers of brain dysfunction.

A comparison of the repetitive click and conditioning-testing P50 paradigms

The auditory P50 ERP component has previously been studied either in the repetitive click or the conditioning-testing (C-T) paradigm. For 20 subjects, we compared 4 repetitive click and 4 C-T protocols in a single experimental session with identical recording techniques and with interclick intervals comparable to the C-T intervals. In the C-T protocols, a long interval between click pairs ensured full recovery of P50 to the C click. The analysis of P50 topographies provided strong evidence that the same component was measured in the two paradigms. For both paradigms, P50 amplitude was progressively suppressed as the interclick or C-T interval decreased (P < 0.0001), with parallel interval vs. P50 amplitude regression lines for the two paradigms. There was a strong trend (P = 0.08) for the repetitive click amplitudes to be smaller than T amplitudes for comparable repetitive click and C-T intervals. Equivalently, this strong trend suggests that repetitive click intervals must be longer (by about 300 ms) than the C-T interval to generate equivalent amplitude P50 responses. We conclude that the same component is measured in both paradigms, that P50 amplitude decreases with decreasing interstimulus intervals in both paradigms, and that in normals, for comparable inter-click and C-T intervals, there is greater P50 suppression in the repetitive click paradigm. Finally, we note that the comparison of paradigms within normals does not necessarily apply to clinical samples.

Multimodal evoked potentials in Alzheimer's disease and Binswanger's disease

Somatosensory evoked potentials (SEPs), brainstem auditory evoked potentials (BAEPs), visual evoked potentials (VEPs), and auditory event-related potentials (ERPs) were studied in 15 patients with Alzheimer's disease (AD), eight patients with Binswanger's disease (BD), and 15 normal subjects. Patients with BD showed significant prolongation of the interpeak latency between N13 and N20 (N13-N20) and N20-P40 of SEPs as compared with the normal controls, whereas patients with AD only demonstrated significant prolongation of N20-P40. The interpeak latency between waves I and V of BAEPs in patients with both AD and BD was significantly longer than that of controls. There were no significant differences in P100 latency of VEPs among these three groups. Both groups with dementia showed significant prolongation of N200 and P300 latencies of ERPs compared with normal controls. In addition, patients with AD showed significant prolongation of P200 latency. We conclude that these two dementing diseases have different electrophysiologic features that may be related to their underlying pathogenetic mechanisms. Furthermore, the measurement of multimodal evoked potentials may be helpful in the differential diagnosis of AD and BD.

The auditory P50 response is normal in Alzheimer's disease when measured via a paired click paradigm

Recent findings of missing or markedly attenuated P50 (or P1) auditory ERPs in Alzheimer's disease (AD) patients suggest this may be a useful diagnostic and/or prognostic marker of AD cholinergic deficits. Those studies used repetitive 1/sec clicks. Given P50's long recovery time, all but the first click in that paradigm was presented during the recovery of the P50 generation system from the response to the prior click. We studied 8 AD patients and 17 elderly controls using a paradigm incorporating 7-8 sec intervals between clicks, which allows examination of P50 generation separate from P50 recovery. With the long inter-click interval, we identified P50 responses in most AD patients and controls, and found no difference in P50 amplitude between groups. These results suggest that if there is a P50 deficit in AD patients, it is the result of the accumulative effect of repetitive stimulation, rather than a primary deficit in P50 generation.