Hearing Tests

A total of 2470 participants were recruited, including 1151 cognitively normal controls (NC), 898 patients with amnestic mild cognitive impairment (aMCI), and 421 patients with mild Alzheimer disease (AD).
We recruited the controls using cluster sampling in Jingansi Community Shanghai, China. The inclusion criteria for NC were: age between 50 and 90; no memory complaints verified by an informant; cognitively normal, based on the absence of significant impairment in cognitive functions or activities of daily living (ADL); Clinical Dementia Rating (CDR)  =  0; and Hamilton depression rating scale (HAMD) scored ≤ 12 on the 17-item scale in past 2 weeks. They had adequate visual and auditory acuity to allow cognitive testing. Participants with any significant neurologic disease and psychiatric disorders/psychotic features were excluded.
All the patients with aMCI and AD were recruited from the Memory Clinic, Huashan Hospital, from Jun 2004 to Oct 2011.They finished the laboratory tests and cranial CT/MRI scan, and had no clinically significant abnormalities in vitamin B12, folic acid, thyroid function (free triiodothyronine-FT3, free tetraiodothyronine-FT4, thyroid stimulating hormone-TSH), rapid plasma regain (RPR), or treponema pallidum particle agglutination (TPPA).
The aMCI patients were diagnosed according to the following criteria19]: (1) cognitive complaints verified by an informant; (2) cognitive impairment lasting more than 3 months; (3) Mini-mental state examination-Chinese version (C-MMSE)[25] (link) ≥ cut-off score for adjusted education; (4) Abnormal objective memory impairment documented by scoring below the age and education adjusted cutoff on an episodic memory test (Auditory Verbal Learning Test); (5) preserved basic ADL/minimal impairment in complex instrumental functions; (6) etiology unknown; (7) normal sense of hearing and sight; (8) has not met diagnostic criteria of dementia based on the National Institute of Neurological and Communicative Disorders and Stroke and the Alzheimer's Disease and Related Disorders Association (NINCDS-ADRDA).
The AD patients (n = 421) met the following criteria: (1) diagnosed as probable AD according to the NINCDS-ADRDA; (2) no obvious medical, neurological or psychiatric diseases or psychological dysfunction including anxiety and depression within the previous one month; (3) no visual or auditory deficit.

We presented pure tones as test stimuli (TS) simultaneously with band-eliminated noises (BENs) (Figures 1B, 2 and Additional file 1). The TS had a duration of 600 msec (10 msec rise and fall times), and a frequency of either 250, 350, 450, 570, 700, 840, 1000, 1170, 1370, 1600, 1850, 2150, 2500, 2900, 3400, or 4000 Hz (one critical band (CB) steps [29 ]). In 50% of trials, the TS contained a silent gap of 10 msec duration (with 10 msec rise and fall times) starting at latency 285 msec (deviant test stimulus, cf. Figure 2 and additional file 1). The TS with temporal gaps were targets for behavioral responses during the MEG measurement (reaction times and error rates) and ensured the subjects' compliance regarding the focus of attention. The sound onset asynchrony between two subsequent TS was fixed to 3000 msec.
The simultaneously presented BENs were prepared as follows: From 8000 Hz low-pass filtered white noise (sampling rate: 48000 Hz), spectral frequency bands with widths of either 1/4 critical band (1/4 CB), 1/2 critical band (1/2 CB), or 1 critical band (1 CB) centred at the frequency of the simultaneously presented TS were eliminated (Figure 2 and additional file 1). All BENs (duration 3000 msec; 10 msec rise and fall times) started 2200 msec prior to TS onset and ceased 200 msec after TS offset. All sound stimuli were prepared as sound files and presented under control of Presentation (Neurobehavioral Systems, Albany, CA, United States). 18000 Hz frequency tags (not perceivable) were attached to the onset of each TS in order to obtain precise timing. SRM-212 electrostatic earphones (Stax, Saitama, Japan) transduced air-conducted sounds which were delivered through silicon tubes (length: 60 cm; inner diameter: 5 mm) and silicon earpieces fitted to each subject's ears. The hearing threshold for the 1000 Hz TS was determined for each ear of each individual at the beginning of the MEG session. The 1000 Hz TS was presented binaurally at intensity of 35 dB above individual sensation level. The power of the other TS, which were also presented binaurally, was adjusted to the power of the 1000 Hz TS. The total power of the binaurally presented BENs was 15 dB larger than TS power, resulting in slightly higher spectrum levels for the BENs containing wider notches compared to the BEN with the narrowest notch (see Additional file 2; please note that the spectrum level difference is nearly invisible and therefore considered to be negligible).
In order to investigate the effects of stimulus sequencing during auditory focused attention, we contrasted two different conditions within subjects: 'constant sequencing' and 'random sequencing'. In the constant sequencing session, 30 TS with identical frequency (either solely 250, 350, 450, 570, 700, 840, 1000, 1170, 1370, 1600, 1850, 2150, 2500, 2900, 3400, or 4000 Hz) were successively (and pseudo-randomly) presented simultaneously with either the 1/4, 1/2, or the 1 CB BEN. In the random sequencing session, 30 TS with different frequencies were presented, pseudo-randomly chosen from the same frequencies that were used in the constant sequencing blocks (250, 350, 450, 570, 700, 840, 1000, 1170, 1370, 1600, 1850, 2150, 2500, 2900, 3400, or 4000 Hz). As in the constant sequencing condition, BENs with notches of either 1/4, 1/2, or 1 CB were presented simultaneously and pseudo-randomly (Figure 2 and additional file 1). Crucially, the overall amount of bottom-up auditory inputs was identical between constant sequencing and random sequencing conditions, while the patterning of stimuli was different. During all conditions, subjects were instructed to focus their attention on the auditory stimuli, and to press a response button as quickly as possible with their left or right index finger (randomized between subjects) whenever a TS with gap was detected. Constant sequencing and random sequencing blocks alternated, and block order was counterbalanced between subjects. In total, 160 trials (10 trials for 16 frequencies) for each BEN condition in each sequencing condition were presented and subjected to data analysis.

According to the time characteristics of vestibular symptoms onset, patients can be classified into acute, episodic, or chronic vestibular syndrome (AVS, EVS, or CVS) (21 (link)). All diagnoses were made by the senior authors (ZXW and XY) according to widely accepted diagnostic criteria for each vestibular disorder or the international classification of vestibular disorders (ICVD) criteria when available (22 (link)–26 (link)). The published diagnostic criteria consensus includes acute unilateral vestibulopathy (AUVP)/VN (26 (link)), persistent postural-perceptual dizziness (PPPD) (23 (link)), VM (25 (link)), VM of childhood (24 (link)), and MD (22 (link)). Besides, probably labyrinthine infarction was diagnosed in older patients with sudden onset of unilateral deafness and vertigo, especially when there is a history of stroke or known vascular risk factors (27 (link)). Benign recurrent vertigo (BRV) was diagnosed when patients showed spontaneous rotational vertigo or instability; symptoms that were not triggered by changes in position, lasting longer than 1 min; normal audiogram or symmetric hearing loss; no cochlear symptoms (tinnitus or stuffiness) during the attack phase; no migraine or migraine aura in the acute phase (26 (link), 28 (link)). Isolated acute unilateral utricular vestibulopathy was diagnosed in patients with acute onset of postural imbalance, which can be diagnosed by ocular VEMP (26 (link)).

All participants completed both visual and auditory tasks, in separate blocks, counterbalanced across participants. Each trial started with a 500-ms blank period (with no stimulus, besides the fixation point). Then, a visual or auditory test stimulus was presented. The duration of the test stimulus was one of five logarithmically spaced intervals of time, between 300 and 900 ms (i.e., 300, 395, 520, 684, 900 ms). Before beginning each block, participants were presented with five or more visual (or auditory) reference stimuli with duration 520 ms (the geometric mean of the test stimuli durations). They were instructed to remember the reference duration and to make subsequent judgments regarding test stimuli relative to the reference duration (the PSE values in Additional file 1: Fig. S6 confirm that participants indeed complied with this instruction). Before beginning the task, participants were allowed to continue to experience the reference stimulus repeatedly, until they were confident that they had remembered the reference duration. Once the block began, participants could no longer experience the reference stimulus.
Participants reported their choice on each trial after the stimulus had ended by pressing one of two keyboard buttons. The button-choice contingency was counterbalanced across participants: half the participants pressed the “F” button with their left index finger and the “J” button with their right index finger to indicate longer and shorter test durations, respectively, while the other half responded with the reverse mapping. Participants were instructed to make each response as quickly and accurately as possible. To ensure an equal number of consecutive stimulus pairs, while retaining a long-term pseudorandom structure, the order of stimulus presentation was determined by a pseudorandom 5⁴ de Bruijn sequence. This creates an optimally short (pseudorandom) sequence of stimuli (625 trials total) in which each contiguous subsequence of 4 stimuli (from the 5 possible stimulus intervals) occurs exactly once [68 (link)]. Accordingly, each individual stimulus occurred 125 times, each possible pair occurred 25 times, each possible triplet occurred 5 times, and each quadruplet occurred once (per block).