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Researchers Reveal How Spontaneous Fluctuations of Neural Brain Activity Impact Sensory Responses

A team of researchers from Russia, Germany and the USA have identified the mechanisms that form the basis for the impact of the brain’s current state on sensory processing. The results of the study were published in the paper ‘Multiple Mechanisms Link Prestimulus Neural Oscillations to Sensory Responses' in eLIFE

Unlike computers, the human brain may respond differently to the same stimulus, owing to the differences in the brain’s condition when stimuli are processed. Even in the absence of sensory input, the brain generates spontaneous activity, which can affect the perception of audial, visual and somatosensory stimuli. Electroencephalography (EEG) can measure this activity as neural oscillations of various frequency and amplitude. Reactions to stimuli are analyzed with event-related potentials (ERP) – registration of the brain’s bioelectric reactions to stimuli.

At present, the impact of spontaneous neural fluctuations on event-related potentials is the focus of active investigations. This type of research can contribute considerably to our understanding of how the brain improves its processing of information by changes in its background state. Furthermore, conditions such as schizophrenia and autism are also characterized by abnormal patterns in spontaneous brain activity, which, in turn, can influence the processing of sensory information.

The researchers relied on EEG and ERP methods to find out that strong alpha and beta waves in the brain, which are common for a person’s quiet awake state, suppress the early components of ERPs, which are registered by EEG during the initial 200 milliseconds. Therefore, this may be the mechanism to suppress visual stimuli that are irrelevant and may even hamper the execution of currently active tasks.

At the same time, as it turned out, later on (400 ms after the initial stimulus), the ERPs were increased in amplitude together with an increased power of prestimulus alpha wave. However, this rise runs counter to the theory of signal suppression, which was confirmed for earlier components.

Nevertheless, the authors were able to find an explanation for this paradox and prove that increased amplitude of late ERPs is not directly related to the impact of spontaneous activity on the brain’s reactiveness. It rather indicates the fact that neuronal oscillations have non-zero mean and thus modulation of their amplitude should contribute to the generation of ERP through baseline-shift mechanism. Consequently, in a state of high prestimulus alpha oscillations (~ 10-Hz), their stronger attenuation by visual stimuli resulted in the enhancement of late ERP component.

Analysing the impact of brain activity on sensory processing can help researchers to understand which states are most susceptible to information processing. The confirmation of the fact that early and late brain reactions are modulated by different mechanisms, allows for a more precise evaluation and interpretation of neural activity recorded with EEG.