Resting state EEG phase connectivity within source-localized frontal regions in a well-powered sample of FXS and matched controls revealed in two key findings. First, individuals with FXS broadly demonstrated increased gamma and reduced alpha phase connectivity across frontal regions, both within and across hemispheres, compared to TDC. Second, significant associations between EF and frontal connectivity emerged in FXS, such that increased error rates were positively associated with gamma connectivity and inversely associated with alpha connectivity. Notably, these findings remained robust after accounting for general cognitive functioning. These latter findings document an important and not previously reported link between deficits in EF and the alterations in the coherence of specific frequencies of neural oscillations within the frontal cortices of individuals with FXS. Together, our study reveals a potential underlying neurophysiological basis for EF impairment in FXS that may represent a promising target for future intervention studies.
Phase Connectivity
Phase connectivity, when applied spatially, assesses the precise alignment neural oscillations at a specific frequency between brain regions. Phase connectivity, or coherence, between different brain regions is well known to support cognitive functions (41, 43, 47). Herein, we implemented source localization to examine point-to-point connectivity within frontal regions to clarify the source of previously reported alterations and their functional significance (14, 16, 22). Previous investigations into other neurodevelopmental and neurological disorders have linked alterations in resting frontal phase connectivity with impaired cognitive function, including EF (48–50).
Our primary results are consistent with previous electrode-level phase connectivity findings and further localize a subset of regions with gamma band hyper-connectivity and alpha band hypo-connectivity within frontal cortex in FXS (14, 16, 22). Specifically, gamma hyper-connectivity and alpha hypo-connectivity may reflect poor top-down regulation of local frontal circuits leading to hyperexcitability of local circuit function and subsequently to cognitive and behavioral dysfunction (51, 52). The observed alpha hypo-connectivity may represent deficient longer-range inhibitory mechanisms which down-regulate background neural excitability (40). From the perspective of an excitatory-inhibitory imbalance (E:I) model of neurodevelopmental disorders, these findings are consistent with a hyper-excitable phenotype that has been shown in FXS across in-vivo slice physiology and mouse model studies (17, 19, 21, 53).
More recently, a growing body of literature has raised the importance of increased variability in neural signals linked to enhanced cognition, basically because systems need to be tuned on-line to optimize them for behavioral demands (54–56). Our observations, when taken together with other EEG findings in FXS (e.g., decreased peak alpha frequency, decreased neural synchronization to the auditory chirp, reduction in global alpha power with concomitant increases in regional gamma power) suggest a diminished capacity or increased constraints on the expression of neural variability in the FXS cortex (15, 57–59). From a molecular standpoint, loss of FMRP results in a reduction of synaptic plasticity, defects in stimulus-induced synaptic protein synthesis, synaptic overgrowth, and changes in dendritic spine morphology in the FMR1-/- KO mouse and neurons derived from FXS patients (60–64). Such changes also would be predicted to dampen neural variability at the molecular and cellular levels (58).
Evolving Model of EF Physiology in FXS
The association between EF task performance and frontal lobe phase connectivity in the present study can be used to advance our understanding of higher-order cognitive processes in FXS and, for the first time, establish a neurophysiological model of impaired EF in this patient population. Importantly, the correlation and regression findings remained robust, even after correcting for general intellectual functioning. This suggests that frontal gamma hyper-connectivity and alpha hypo-connectivity may be specifically related to EF deficits rather than intellectual or general cognitive capacity more broadly. Alpha and gamma phase connectivity predicted increased error rates during the distractibility task as well as increased error rates and reduced reaction time during the cognitive flexibility task. Although faster reaction times are often thought to be better, this is not necessarily the case in the context of EF when slower reaction times can benefit participants in terms of the speed/accuracy tradeoff (46, 65). We speculate that functional consequences of connectivity abnormalities may include poorer local regulation of frontal activity and deficient inhibitory mechanisms, thus leading to difficulty in cognitive flexibility, attention shifting, ignoring distractions, and an increase in impulsivity.
Differences in Clinical Correlations between Males and Females with FXS
Contrary to our expectations, males and females with FXS had similar frontal gamma hyper-connectivity. As full-mutation males with FXS have significantly less expression of FMRP (66) and a higher burden of clinical symptoms (4), we had predicted increased gamma band phase connectivity in males than females. Previous studies have replicated the finding of increased resting local gamma power in males with FXS (15, 57, 58, 67). Phase-based measures, such as dWPLI, can show increases in phase synchronized neural oscillations across regions rather than just a parallel increase in the power at a given frequency band. Thus, our finding indicate high frequency activity in local circuits may be restricted to males with FXS, whereas high frequency activity in the mutual synchronized driving of excitability across widely distributed brain regions may be more broadly present across individuals with FXS.
Our findings indicated that associations of EF with alpha connectivity were primarily driven by females with FXS. Preserved regulatory capacity of long-distance inputs in the alpha band frequency, evident in variable degrees in female FXS participants, was related to more intact EF task performance. As full mutation females with FXS are obligate mosaics (one X chromosome still produces FMRP) it is not surprising that intermediate results (between full mutation males and controls) have been reported in resting state and event related EEG studies in FXS (15, 59, 68). Thus, greater FMRP expression in females may help to mobilize alpha band connectivity to support EF in a compensatory fashion. Our findings are consistent with the canonical role of alpha oscillations in attention and cognition, such that enhanced alpha frontal connectivity may facilitate shifting attention and cognitive resources to support behavioral flexibility and inhibit distraction from sensory information that is not relevant to contextual demands (40). Previous fMRI studies have implicated compensatory mechanisms of increased activation in prefrontal regions to support inhibitory control and prevent distractor interference in FXS (13, 69). Complementing the general activation finding from fMRI work, our EEG study highlights the breadth and spatial distance of altered coherence of neural oscillation across regions that occurs in specific frequency bands that have their own functional significance.
Evidence of Atypical Lateralization
Lateralized substrates for distinct cognitive functions within frontal cortex have been consistently observed in typically-developing individuals. For example, right inferior and superior frontal gyri have been implicated in inhibitory control, specifically proactive control related to reaction time slowing (70, 71). Atypical brain lateralization of cognitive functions has been observed in other neurodevelopmental disorders, including ASD (72, 73). Our findings add to these previous studies by documenting atypical lateralization in individuals with FXS. For example, we found increased gamma band, but reduced alpha band, in right prefrontal regions in individuals with FXS compared to TDC. This finding suggests EF skills lateralized within these regions would be affected, which our partial correlation and regression findings support. Specifically, we observed reduced alpha connectivity within these atypically lateralized regions predicted impaired performance during cognitive flexibility and distractibility tasks. Yet, consistent with our compensatory hypothesis, our findings suggest that preserved lateralized right frontal alpha connectivity may facilitate inhibition of previously learned behavior (flexibility) and irrelevant sensory stimuli (distractibility) in individuals with FXS, especially among females.
Notably, among males with FXS, we found gamma hyper-connectivity in left pars opercularis and pars triangularis, areas within the inferior frontal gyrus critical for speech and language. This suggests increased phase synchronized frontal gamma activity also may contribute to language impairments and delays in FXS, which are nearly universal among male patients (74, 75). Our previous work showed increased frontal gamma power prior to the onset of speech production in individuals with FXS compared to controls, and greater gamma increases were associated with more unintelligible speech in FXS (76). Taken together, these findings implicate increased local and synchronized high frequency frontal activity may have widespread disruptive role in FXS that is not necessarily specific to EF or speech production. Future studies are needed to determine the extent to which high frequency activity within frontal cortices more broadly affects learning and development in FXS.
Limitations
Frontal connectivity findings are limited to brain activity at rest and thus should not be equated with task-based connectivity findings. Similarly, resting connectivity findings may not generalize to neural activity during real world function. Thus, future work is needed to study dynamic changes in neural oscillation during EF task performance. Findings further are limited to short-range frontal connectivity and do not consider longer-range connectivity relevant to EF (e.g., fronto-parietal connections). Still, findings remain the first of their kind in FXS and represent a critical step to better understanding neurophysiological mechanisms underlying impaired EF in FXS. It also is important to note that only certain aspects of EF were measured using KiTAP, indicating the need to replicate findings in a broader battery of neuropsychological tests (e.g., NIH Cognitive Toolbox). We did not have an IQ- and age-matched control group. However, given that our analyses controlled for IQ, we believe our findings are specific to specific relations of EF skills to frontal lobe EEG features independent of the level of intellectual ability. Last, we note that FMRP expression is not dichotomous based on sex as presently described. Future work examining frontal connectivity in relation to a continuous measure of FMRP (66) is needed to better understand the role of FMRP in EF impairments in FXS.