Abstract :
[en] Introduction/Motivation:
Cortical excitability is modulated both by conscious states, sleep homeostasis and circadian rhythm, following a nonlinear dynamic across the day1,2. It is low during wakefulness and REM sleep, where subjects experience phenomenological states, and high in conditions such as NREM sleep and Disorders of Consciousness (DoC), where there is inappropriate or absent processing of external stimuli3,4. Cortical excitability increases across the day, peaking at night after habitual sleep time where behavioral impairments such as attentional lapses are more common. Previous researches demonstrated that, at night, lack of vigilance is connected with lower frontoparietal effective connectivity5. However, no study has investigated whether there is a transient increase of cortical excitability during attentional lapses per se. This would be in line with the idea that the brain is less efficient to engage in the ongoing task, acting similarly to an “unconscious state”. To test this hypothesis, we compared cortical excitability during normal awakening and attentional lapses.
Methods:
Data included in this analysis were retrospectively selected among 3 different studies including repeated assessment of cortical excitability using Transcranial Magnetic Stimulation (TMS) of the superior frontal gyrus coupled to high-density Electroencephalography (hdEEG). This region was selected because it is sensitive to sleep pressure1 and has been implicated both for motor and cognitive tasks6. To increase the likelihood of attention lapses, data was selected among nighttime sessions. Attention lapses were detected based on the performance to a continuous Compensatory Tracking Task (CTT) completed simultaneously to TMS-EEG recording. Volunteers with at least 25 lapses were included to reach a total sample of 26 healthy individuals in 2 age groups (young, N= 13, 18-30 y; old, N = 13, 50-69 y). As previously published1, cortical excitability was inferred from amplitude and slope of the first component of the TMS-evoked EEG potential (TEP; 0–30 ms post-TMS, Figure 1). Latencies of the negative and positive peaks of this evoked potential were also extracted. Statistics were run on SAS 9.4 with a Generalized Linear Mixed Model (GLMM) (for more details, description of Table 1).
Results and Discussion:
Cortical excitability showed a strong change from no-lapse to attention lapse. In particular, there was a significant increase of amplitude, with a smaller latency of the negative component and a bigger latency of the positive one, leading to an increase in slope (for more details, Table 1). These results suggest that there is a transient increase of cortical excitability during vigilance lapses pointing to an alteration of the brain function that could be similar to what is observed when sleep need is high compared to well-rested condition, or during states of altered consciousness (e.g. sleep). These results could constitute an epiphenomenon of the likelihood of transitions of conscious states or rather a sheer marker of errors. Alternatively, they could reflect a local sleep phenomenon over the target area. Future researches should validate the extent of this description and fathom its molecular mechanisms.
