A multidimensional model of memory complaints in older individuals and the associated hub regions.

Memory complaints are highly prevalent among middle-aged and older adults, and they are frequently reported in individuals experiencing subjective cognitive decline (SCD). SCD has received increasing attention due to its implications for the early detection of dementia. This study aims to advance our comprehension of individuals with SCD by elucidating potential cognitive/psychologic-contributing factors and characterizing cerebral hubs within the brain network. To identify these potential contributing factors, a structural equation modeling approach was employed to investigate the relationships between various factors, such as metacognitive beliefs, personality, anxiety, depression, self-esteem, and resilience, and memory complaints. Our findings revealed that self-esteem and conscientiousness significantly influenced memory complaints. At the cerebral level, analysis of delta and theta electroencephalographic frequency bands recorded during rest was conducted to identify hub regions using a local centrality metric known as betweenness centrality. Notably, our study demonstrated that certain brain regions undergo changes in their hub roles in response to the pathology of SCD. Specifically, the inferior temporal gyrus and the left orbitofrontal area transition into hubs, while the dorsolateral prefrontal cortex and the middle temporal gyrus lose their hub function in the presence of SCD. This rewiring of the neural network may be interpreted as a compensatory response employed by the brain in response to SCD, wherein functional connectivity is maintained or restored by reallocating resources to other regions.

Detecting and correcting partial errors: Evidence for efficient control without conscious access.

Appropriate reactions to erroneous actions are essential to keeping behavior adaptive. Erring, however, is not an all-or-none process: electromyographic (EMG) recordings of the responding muscles have revealed that covert incorrect response activations (termed "partial errors") occur on a proportion of overtly correct trials. The occurrence of such "partial errors" shows that incorrect response activations could be corrected online, before turning into overt errors. In the present study, we showed that, unlike overt errors, such "partial errors" are poorly consciously detected by participants, who could report only one third of their partial errors. Two parameters of the partial errors were found to predict detection: the surface of the incorrect EMG burst (larger for detected) and the correction time (between the incorrect and correct EMG onsets; longer for detected). These two parameters provided independent information. The correct(ive) responses associated with detected partial errors were larger than the "pure-correct" ones, and this increase was likely a consequence, rather than a cause, of the detection. The respective impacts of the two parameters predicting detection (incorrect surface and correction time), along with the underlying physiological processes subtending partial-error detection, are discussed.

Involvement of SMAp in the intention-related long latency stretch reflex modulation: a TMS study.

When our movement is perturbed by environmental forces, the Long Latency Stretch Reflex (LLSR), generated by a transcortical loop through the primary motor cortex (M1), is the fastest reaction adapted according to our prior intent. We investigated the involvement of the caudal part of the Supplementary Motor Area (SMAp) in this intention-related LLSR modulation. Subjects were instructed either to not react (i.e. to 'let-go') or to resist a mechanical perturbation extending the wrist and Transcranial Magnetic Stimulation (TMS) was used to transiently inactivate SMAp, either at the time of the LLSR generation (TMS was applied 50 ms before the perturbation), or at the end of the preparation period (TMS was applied 150 ms before the perturbation). The effect of SMAp transient inactivation on the LLSR modulation was compared to the effect of transient inactivation of M1 or of a Control area. Compared to the Control condition, the intention-related LLSR modulation decreased when TMS was applied either over SMAp or over M1 50 ms before perturbation occurrence, suggesting that SMAp, as M1, is involved in the LLSR modulation. Moreover, the LLSR modulation also decreased when TMS was applied over SMAp 150 ms before the perturbation, indicating that anticipatory processes taking place in SMAp participate to the LLSR modulation. In addition, TMS applied over SMAp elicited Motor-Evoked Potentials (MEPs) whose latency and shape were similar to MEPs evoked by TMS over M1, suggesting that they are due to direct corticospinal projections from SMAp. Interestingly, the SMAp MEPs amplitude was modulated depending on the subject's intention to resist or to let-go. Taken together these results strongly favor the idea that, during the expectation of a perturbation, SMAp is the seat of anticipatory processes that are specific to the subject's intent and that preset M1 in order to adapt the LLSR to this intention.

Cortical mechanisms underlying stretch reflex adaptation to intention: a combined EEG-TMS study.

During voluntary motor acts, potential perturbations due to transient external forces are counteracted very quickly by short- and long-latency stretch reflexes (SLSR and LLSR, respectively). The LLSR, presumably linked to a transcortical loop, can be modulated by the subjects' intention. Here, we used combined TMS-EEG to study cortical mechanisms involved in this intention-related modulation both before and during the reaction to a mechanical perturbation. Subjects had to prepare for a brisk wrist extension under the instruction either to 'resist' the perturbation or to 'let-go'. Following the perturbation, the early cortical evoked activity (45-75 ms) was greater in the 'let-go' condition; moreover, its amplitude was negatively correlated with the LLSR amplitude, regardless of condition. After 100 ms the pattern reversed, the late evoked activity (presumably linked to the voluntary reaction) was greater in the 'resist' condition. The early and late evoked activities also differed in their topography. Therefore, the cortical mechanisms involved in the intention-related LLSR modulation differ from those involved in the voluntary reaction. In addition, in response to a single-pulse TMS delivered during the expectation of the mechanical perturbation, the TMS-evoked N100 amplitude decreased when subjects intended to 'let-go', suggesting anticipatory decreased activity of intracortical inhibitory sensorimotor networks. Taken together, these results support the idea that anticipatory processes preset the sensorimotor cortex so as to adapt its early reaction to the perturbation relative to the subjects' intention.

Prior intention can locally tune inhibitory processes in the primary motor cortex: direct evidence from combined TMS-EEG.

Human subjects are able to prepare cognitively to resist an involuntary movement evoked by a suprathreshold transcranial magnetic stimulation (TMS) applied over the primary motor cortex (M1) by anticipatory selective modulation of corticospinal excitability. Uncovering how the sensorimotor cortical network is involved in this process could reveal directly how a prior intention can tune the intrinsic dynamics of M1 before any peripheral intervention. Here, we used combined TMS-EEG to study the cortical integrative processes that are engaged both in the preparation to react to TMS (Resist vs. Assist) and in the subsequent response to it. During the preparatory period, the contingent negative variation (CNV) amplitude was found to be smaller over central electrodes (FC1, C1, Cz) when preparing to resist compared with preparing to assist the evoked movement whereas alpha-oscillation power was similar in the two conditions. Following TMS, the amplitude of the TMS evoked-N100 component was higher in the Resist than in the Assist condition for some central electrodes (FCz, C1, Cz, CP1, CP3). Moreover, for six out of eight subjects, a single-trial-based analysis revealed a negative correlation between CNV amplitude and N100 amplitude. In conclusion, prior intention can tune the excitability of M1. When subjects prepare to resist a TMS-evoked movement, the anticipatory processes cause a decreased cortical excitability by locally increasing the inhibitory processes.