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We examined the neural signature of directed exploration by contrasting MEG beta (16-30 Hz) power changes between disadvantageous and advantageous choices in the two-choice probabilistic reward task. We analyzed the choices made after the participants have learned the probabilistic contingency between choices and their outcomes, i.e., acquired the inner model of choice values. Therefore, rare disadvantageous choices might serve explorative, environment-probing purposes. The study brought two main findings. Firstly, decision making leading to disadvantageous choices took more time and evidenced greater large-scale suppression of beta oscillations than its advantageous alternative. Additional neural resources recruited during disadvantageous decisions strongly suggest their deliberately explorative nature. Secondly, an outcome of disadvantageous and advantageous choices had qualitatively different impact on feedback-related beta oscillations. After the disadvantageous choices, only losses – but not gains – were followed by late beta synchronization in frontal cortex. Our results are consistent with the role of frontal beta oscillations in the stabilization of neural representations for selected behavioral rule when explorative strategy conflicts with value-based behavior. Punishment for explorative choice being congruent with its low value in the reward history is more likely to strengthen, through punishment-related beta oscillations, the representation of exploitative choices consistent with the inner utility model.

BACKGROUNDAn altered balance of neuronal excitation and inhibition (E-I balance) might be implicated in the co-occurrence of autism and intellectual disability, but this hypothesis has never been tested. E-I balance changes can be estimated from the spectral slope of the aperiodic 1/f neural activity. Herein, we used magnetoencephalography (MEG) to test whether the 1/f slope would differentiate ASD children with and without intellectual disability.METHODSMEG was recorded at rest with eyes open/closed in 49 boys with ASD aged 6-15 years with a broad range of IQs, and in 49 age-matched typically developing (TD) boys. The cortical source activity was estimated using the LCMV beamformer approach. We then extracted the 1/f slope by fitting a linear function in to the log-log-scale power spectra in the high-frequency range.RESULTSThe grand averaged 1/f slope was steeper in the eyes closed than in the eyes open condition, but had high rank-order stability between them. In line with the previous research, the slope flattened with age. Children with ASD and below-average (CONCLUSIONSThe atypically flattened spectral slope of aperiodic activity in children with ASD and below-average IQ suggests a shift of the global E-I balance toward hyper-excitation. The spectral slope can provide an accessible non-invasive biomarker of the E-I ratio for translational research and making objective judgments about treatment effectiveness.

Visual gamma oscillations are generated through interactions of excitatory and inhibitory neurons and are strongly modulated by sensory input. A moderate increase in excitatory drive to the visual cortex via increasing contrast or motion velocity of drifting gratings results in strengthening of the gamma response (GR). However, increasing the velocity beyond some ‘transition point’ leads to the suppression of the GR. There are two theoretical models that can explain such suppression. The ‘excitatory drive’ model infers that, at high drifting rates, GR suppression is caused by excessive excitation of inhibitory neurons. Since contrast and velocity have an additive effect on excitatory drive, this model predicts that the GR ‘transition point’ for low-contrast gratings would be reached at a higher velocity, as compared to high-contrast gratings. The alternative ‘velocity tuning’ model implies that the GR is maximal when the drifting rate of the grating corresponds to the preferable velocity of the motion-sensitive V1 neurons. This model predicts that lowering contrast either will not affect the transition point or will shift it to a lower drifting rate. We tested these models with magnetoencephalography-based recordings of the GR during presentation of low (50%) and high (100%) contrast gratings drifting at four velocities. We found that lowering contrast led to a highly reliable shift of the GR suppression transition point to higher velocities, thus supporting the excitatory drive model. No effects of contrast or velocity were found for the alpha-beta response power. The results have important implications for the understanding of the neural mechanisms underlying gamma oscillations and the development of gamma-based biomarkers of brain disorders.

Whether the motor cortex activation accompanying concrete verbs comprehension is necessary for verbs conceptual processing is still a hotly debated topic in the literature. Answering this question, we examined to what extent the more difficult access to verb semantics requires an additional engagement of cortical motor system in verb generation task. Using power suppression of MEG beta oscillations (15-30 Hz) as an index of sensorimotor activation, we presented to our participants the noun cues which either were strongly associated with a single verb and prompted the fast and effortless verb retrieval, or were weakly associated with multiple verbs and were more difficult to respond to. A whole-brain analysis of beta suppression revealed that the only cortical regions sensitive to the difficulty of semantic access were the higher order motor areas on the medial and lateral surfaces of the frontal lobe. This differential activation of cortical motor system accompanied effortful verb retrieval and preceded the preparation of vocal response for more than 500 milliseconds. Since the mid-frontal frontal brain areas are involved in maintaining abstract representations of actions during their initiating and planning, we argue that our finding supports the view that motor associations contribute to retrieval of verb semantics.