The phase of pre-stimulus brain oscillations correlates with cross-modal synchrony perception.

In everyday life multisensory events, such as a glass crashing on the floor, the different sensory inputs are often experienced as simultaneous, despite the sensory processing of sound and sight within the brain are temporally misaligned. This lack of cross-modal synchrony is the unavoidable consequence of different light and sound speeds, and their different neural transmission times in the corresponding sensory pathways. Hence, cross-modal synchrony must be reconstructed during perception. It has been suggested that spontaneous fluctuations in neural excitability might be involved in the temporal organisation of sensory events during perception and account for variability in behavioural performance. Here, we addressed the relationship between ongoing brain oscillations and the perception of cross-modal simultaneity. Participants performed an audio-visual simultaneity judgement task while their EEG was recorded. We focused on pre-stimulu activity, and found that the phase of neural oscillations at 13 ± 2 Hz 200 ms prior to the stimulus correlated with subjective simultaneity of otherwise identical sound-flash events. Remarkably, the correlation between EEG phase and behavioural report occurred in the absence of concomitant changes in EEG amplitude. The probability of simultaneity perception fluctuated significantly as a function of pre-stimulus phase, with the largest perceptual variation being accounted for phase angles nearly 180º apart. This pattern was strongly reliable for sound-flash pairs but not for flash-sound pairs. Overall, these findings suggest that the phase of ongoing brain activity might underlie internal states of the observer that influence cross-modal temporal organisation between the senses and, in turn, subjective synchrony.

The mismatch negativity as an index of cognitive decline for the early detection of Alzheimer's disease.

Evidence suggests that Alzheimer's disease (AD) is part of a continuum, characterized by long preclinical phases before the onset of clinical symptoms. In several cases, this continuum starts with a syndrome, defined as mild cognitive impairment (MCI), in which daily activities are preserved despite the presence of cognitive decline. The possibility of having a reliable and sensitive neurophysiological marker that can be used for early detection of AD is extremely valuable because of the incidence of this type of dementia. In this study, we aimed to investigate the reliability of auditory mismatch negativity (aMMN) as a marker of cognitive decline from normal ageing progressing from MCI to AD. We compared aMMN elicited in the frontal and temporal locations by duration deviant sounds in short (400 ms) and long (4000 ms) inter-trial intervals (ITI) in three groups. We found that at a short ITI, MCI showed only the temporal component of aMMN and AD the frontal component compared to healthy elderly who presented both. At a longer ITI, aMMN was elicited only in normal ageing subjects at the temporal locations. Our study provides empirical evidence for the possibility to adopt aMMN as an index for assessing cognitive decline in pathological ageing.