Earlier animal work had already hinted that overexposure to digital-style stimulation very early in life, before weaning, could produce hyperactivity, social deficits and disrupted neuroplasticity. What remained unclear was whether the childhood period, after pups are weaned but before adolescence, carries a similar risk. This matters because human screen exposure frequently intensifies during exactly this stage, when children begin choosing their own content and spending longer stretches with tablets, televisions and gaming devices. The Iranian team, led by Amirreza Hosseinzadeh and corresponding author Monireh Mansouri, together with Hamidreza Pouretemad and Mozhan Parsa, designed their experiment to test that specific post-weaning window under tightly controlled laboratory conditions.

The experimental protocol was deliberately simple in concept but rigorous in execution. Male rats, beginning at postnatal day 22, the point of weaning, were placed in front of a display presenting colored lights paired with cartoon sounds for six hours every single day, continuing until postnatal day 52, which corresponds roughly to adolescence in the rat. This sustained regimen of excessive audiovisual stimulation, abbreviated EAVS by the authors, was intended to model, in a controlled fashion, the kind of high-intensity, fast-paced sensory input that digital media delivers to developing children. The animals were then put through a battery of standard behavioral assays designed to probe the domains most commonly affected in autism spectrum disorder: social interaction, repetitive behavior, and activity levels.

The behavioral results were unambiguous. Rats that had experienced the daily audiovisual overstimulation showed markedly increased repetitive behaviors, one of the diagnostic hallmarks of autism spectrum conditions. They also demonstrated impaired social interaction, engaging less and differently with conspecifics than their normally reared peers. In addition, the exposed animals were hyperactive, moving with an intensity and persistence not seen in controls. Together, the three behavioral changes map onto the classic triad that researchers look for when building animal models of autism-like phenotypes, and their co-occurrence after a purely environmental manipulation is the most provocative aspect of the study.

Behavior alone, however, does not explain mechanism, so the team turned to quantitative neuroanatomy. Using three-dimensional stereological measurement, a technique that allows unbiased estimation of structure volumes and cell numbers from systematically sampled tissue sections, the researchers examined the amygdala, the almond-shaped complex in the medial temporal lobe that assigns emotional significance to sensory input and is heavily implicated in social behavior. What they found was significant enlargement of the amygdala overall, along with an increased number of neurons in two of its key subregions: the basolateral nucleus, which integrates sensory and cortical information feeding into the amygdala, and the central nucleus, which serves as the main output station driving emotional and autonomic responses.

This pattern of structural plasticity is technically significant because the amygdala occupies an unusual position in autism research. Human neuroimaging studies of autism spectrum disorder have variously reported amygdala enlargement in young children with the condition, and altered connectivity between the amygdala and social brain networks is a recurring finding. The fact that a purely environmental input, six hours per day of colored lights and cartoon sounds, was sufficient to drive both volume increases and neuron number increases in the basolateral and central subregions suggests that developmental sensory overstimulation can push the amygdala along a growth trajectory that mirrors, at least superficially, the structural differences observed in clinical populations. The authors are careful, however, to frame this as an autism-like phenotype rather than a model of autism itself.

That caution is important and the researchers state it explicitly. Rodents do not voluntarily choose their media, so the model cannot capture the motivational and behavioral complexity of human device use, where children actively seek out screens and their exposure patterns vary enormously. What the model does offer is experimental control: a defined stimulus, a defined duration, a defined developmental window, and a homogenous genetic background. Within those constraints, the study isolates the neurobiological consequences of sensory overstimulation per se, free from the confounds of family environment, socioeconomic status, or pre-existing developmental differences that complicate every human screen-time study. It is precisely this control that human correlational research can never achieve.

The timing of the manipulation also carries an implicit message about brain development. Postnatal day 22 to 52 in the rat encompasses a period of intense experience-dependent plasticity, when circuits in the limbic system and cortex are being refined by environmental input. The amygdala, in particular, continues to mature through this window, and the increased neuron counts observed in the basolateral and central nuclei indicate that the overstimulation did not merely alter existing neurons but was associated with changes in neuronal population size, whether through altered neurogenesis, altered survival, or shifts in the timing of developmental cell loss. Untangling which of those mechanisms is responsible will be a natural target for follow-up work, as will determining whether the effects persist into adulthood or can be reversed by returning the animals to normal environments.

For the broader public conversation about children and screens, the study does not say that tablets cause autism. Autism spectrum disorder is a strongly heritable, multifactorial condition, and no single environmental exposure can be called its cause. What the research does support is a narrower and still consequential claim: that the developing brain, through at least the childhood years, is structurally and behaviorally responsive to the intensity and character of its sensory environment, and that sustained, excessive exposure to fast, colorful, noisy audiovisual input can bias that development toward patterns of behavior and brain structure that resemble aspects of autism. Previous work had established this vulnerability for the pre-weaning period; this study extends it into the post-weaning phase, closing a gap that parents and pediatricians had every reason to wonder about.

The study was funded by the Iran Cognitive Sciences and Technologies Council and approved by the Ethics Committee of Shahid Beheshti University, conducted in accordance with the National Institutes of Health Guide for the Care and Use of Laboratory Animals. Its publication as an open-access article ensures that the full methods, stereological protocols and raw behavioral data are available for scrutiny and replication. As debate over childhood screen exposure continues to intensify worldwide, work of this kind provides the kind of mechanistic, experimentally controlled evidence that observational studies alone cannot supply, and it will likely fuel further research into how the timing, duration and content of sensory stimulation interact to shape the social and emotional brain during development.

Subject of Research: Effects of post-weaning excessive audiovisual stimulation on autism-like behavior and amygdala structure in rats

Article Title: Post-weaning audiovisual overstimulation induces autism-like phenotypes and amygdala structural plasticity in rats

Article References: Hosseinzadeh, A., Mansouri, M., Pouretemad, H., & Parsa, M. (2026). Post-weaning audiovisual overstimulation induces autism-like phenotypes and amygdala structural plasticity in rats. BMC Neuroscience. https://doi.org/10.1186/s12868-026-01041-2

Image Credits: AI Generated

DOI: 10.1186/s12868-026-01041-2

Keywords: autism spectrum disorder, audiovisual overstimulation, amygdala, brain development, rats, screen exposure, behavioral neuroscience, stereology, social interaction, repetitive behavior, hyperactivity, childhood

The study, led by Walter E. Kaufmann of the Department of Human Genetics at Emory University School of Medicine and Paul S. Horn of the Division of Neurology at Cincinnati Children’s Hospital Medical Center, together with the FORWARD Consortium, drew on the pediatric FORWARD database, a multi-site registry and longitudinal clinical database established to study fragile X syndrome across specialty clinics in the United States. The analytical sample comprised 1,072 males and 338 females, an unusually large cohort for a rare genetic disorder, allowing the researchers to examine patterns of co-occurrence with a statistical power that smaller, single-center studies could not achieve.

The team focused on the eight most common behavioral comorbidities identified by clinicians in these specialty fragile X clinics: attention problems, anxiety, hyperactivity, autism spectrum disorder, mood disturbances, irritability and aggression, self-injurious behavior, and sleep problems. Importantly, the researchers treated these as clinician-identified behavioral concerns rather than formal diagnoses, with one exception. Autism spectrum disorder was the only comorbidity defined using explicit DSM-5 criteria, reflecting the particular diagnostic challenges that autism presents in fragile X syndrome, where the social and repetitive features of the syndrome itself can blur the boundary between behavioral phenotype and co-occurring condition.

Across the cohort, attention problems and anxiety emerged as the two most frequent behavioral comorbidities, and, notably, also as the mildest in terms of functional impact. By contrast, the cluster of disruptive behaviors captured under the umbrella of irritability, aggression, agitation, and self-injurious behavior, while less universally present, carried substantially greater weight in terms of impairment. Roughly half of the children affected by any given behavioral comorbidity showed functional impairment from that condition, a proportion that underscores how often these difficulties cross the threshold from trait to clinically meaningful problem in fragile X syndrome.

The most striking finding concerned co-occurrence. Among children with fragile X syndrome, 69 percent were reported to have impairing behavioral comorbidities occurring together, and 39 percent of those children presented with more than two comorbidities simultaneously. Children with multiple co-occurring conditions showed the greatest overall impairment, a dose-response-like pattern suggesting that the cumulative behavioral load, rather than any single diagnosis, is what most powerfully determines how much a child’s daily functioning is compromised. For families and clinicians, this finding reframes fragile X syndrome not as a condition with one dominant behavioral complication but as one in which multiple behavioral challenges frequently compound one another.

To dissect these patterns, the researchers deployed an unusually rich statistical toolkit. Chi-square analyses quantified how frequently pairs of comorbidities appeared together. Polychoric and polyserial correlations, methods designed to estimate the association between ordinal and continuous variables that underlie observed categorical ratings, allowed the team to measure the strength of association between comorbidities while correcting for the fact that co-occurrence is partly driven by simple frequency. Mann-Whitney tests compared behavioral scale scores between groups. This distinction between frequency-driven co-occurrence and genuine statistical association proved crucial: the researchers found that while which comorbidities co-occur is influenced by how common each one is, the strength of the association between any pair was relatively independent of frequency, implying that some pairings reflect shared underlying mechanisms rather than mere arithmetic.

Two of those strong associations stood out because they replicate well-documented patterns in the general, non-fragile X population. The first was the pairing of attention problems with hyperactivity, a combination familiar from attention deficit-hyperactivity disorder in typical development. The second was the coupling of anxiety with mood disturbances, echoing the extensive literature on comorbidity between anxiety and mood disorders in the general psychiatric population. That these canonical pairings also hold in fragile X syndrome suggests that at least some of the architecture of behavioral comorbidity in this genetic condition mirrors the structure seen in the broader population, even though the underlying biology is profoundly different.

The team also examined how standardized behavioral scale scores related to the clinician-identified comorbidities. Scale scores broadly aligned with other severity parameters, lending convergent validity to the clinician-based classification. However, correlations between comorbidities and specific scales tracked the relevance of the behaviors each scale evaluates, rather than behaving as simple proxies for severity. In other words, a given scale correlated most strongly with the comorbidities whose symptom content it directly captures, a finding that helps clinicians interpret behavioral checklist data in fragile X syndrome with appropriate nuance.

Perhaps the most unexpected result of the entire analysis was the discovery that all eight behavioral comorbidities were strongly correlated with the Sensory problems scale. Sensory difficulties, including hyperarousal to sound, touch, and visual stimuli, have been increasingly recognized as central features of fragile X syndrome, with prior work from the FORWARD registry documenting their prevalence and their links to hyperarousal. But the uniformity of the association across such clinically distinct comorbidities, from attention deficits to aggression to sleep disturbance, was not anticipated. The finding raises the possibility that sensory dysregulation functions as a transdiagnostic factor in fragile X syndrome, a common physiological substrate that feeds into or amplifies virtually every major behavioral problem the condition produces. If confirmed by future longitudinal work, this would carry direct therapeutic implications: interventions targeting sensory hypersensitivity and hyperarousal might yield benefits across a wide swath of behavioral comorbidities simultaneously, rather than requiring separate treatments for each condition.

The authors caution that their data are cross-sectional and clinic-based, drawn from children evaluated at specialty fragile X clinics and therefore not necessarily representative of the full population spectrum of the disorder, which includes individuals with milder presentations never referred to specialty care. Even so, the scale of the FORWARD database and the consistency of the findings give the profiles considerable weight. The research team suggests that the reported profiles of impairing behavioral comorbidities could assist clinicians in the early identification of behavioral symptoms in children with fragile X syndrome and in refining the management of those symptoms, and, they add, perhaps in other neurodevelopmental disorders as well, where multi-comorbidity burdens are similarly common and similarly under-characterized. The study was supported by cooperative agreements and a contract funded by the Centers for Disease Control and Prevention, and de-identified data from the FORWARD project are available under the project’s data sharing plan and CDC policies, opening the door for further analyses of how these behavioral trajectories unfold over time.

Subject of Research: Behavioral comorbidities and their co-occurrence patterns in children with fragile X syndrome

Article Title: Profile of Behavioral Comorbidities in Children With Fragile X Syndrome

Article References: Kaufmann, W. E., Horn, P. S., FORWARD Consortium, Berry-Kravis, E., Velinov, M., Talboy, A. L., Sherman, S. L., Kaufmann, W. E., Schuster, M., Tartaglia, N., Filipink, R. A., Budimirovic, D. B., Barbouth, D., Lightbody, A., Reiss, A., Delahunty, C. M., Hagerman, R. J., Hessl, D., Erickson, C. A., … Morris, S. M. (2026). Profile of Behavioral Comorbidities in Children With Fragile X Syndrome. Journal of Autism and Developmental Disorders. https://doi.org/10.1007/s10803-026-07525-8

Image Credits: AI Generated

DOI: 10.1007/s10803-026-07525-8

Keywords: fragile X syndrome, behavioral comorbidities, anxiety, attention problems, autism spectrum disorder, sensory problems, FORWARD registry, intellectual disability, hyperactivity, self-injurious behavior, neurodevelopmental disorders, FMR1

The findings arrive amid a widening scientific debate over fluoride, a substance added to community water supplies at 0.7 milligrams per litre to prevent tooth decay, yet one that occurs naturally in groundwater at levels that can vary dramatically with local geology. In rural New Hampshire, where many families rely on private, unregulated wells drilled into fluoride-bearing rock, concentrations can be far higher than in fluoridated municipal systems; roughly thirteen percent of wells in regions underlain by felsic igneous rock exceed the U.S. Environmental Protection Agency’s secondary maximum contaminant level of 2.0 milligrams per litre. Against this backdrop, the research team set out to test whether prenatal fluoride exposure is linked not to intelligence, which has dominated the literature, but to the social-emotional and behavioural dimensions of child development that have received far less scrutiny.

Methodologically, the study distinguishes itself through its choice of exposure biomarker. Most previous prospective studies relied on maternal urinary fluoride, which reflects intake over only about six hours—a snapshot that can fluctuate with tea drinking, meal composition, and water consumption on a given day. Toenails, by contrast, grow slowly and integrate fluoride exposure over approximately three to six months, capturing cumulative intake from all sources: drinking water, beverages prepared with fluoridated water, seafood, tea, and the inadvertent swallowing of fluoridated dental products. Because fluoride readily crosses the placenta and penetrates the blood-brain barrier, the developing fetal brain is directly exposed, making the timing and magnitude of maternal exposure biologically consequential.

The researchers analyzed toenail clippings collected from women at roughly 28 weeks of gestation and again about five weeks after delivery, yielding estimates of fluoride exposure during early pregnancy and mid-to-late pregnancy respectively. Samples were analyzed at the Indiana University School of Dentistry using a hexamethyldisiloxane microdiffusion procedure coupled with fluoride ion-selective electrode detection, and specimens weighing less than 2.5 milligrams were excluded after validation work showed that smaller samples produce unreliable results. Household tap water collected at enrollment was analyzed with a fluoride ion-selective electrode as well. The final analytic samples comprised 358 mother-child pairs with toenail fluoride data and 381 pairs with water fluoride data, drawn from a cohort recruited between 2009 and 2013 from prenatal clinics serving women using private water systems.

Children’s behaviour was assessed at age five with the Behavior Assessment System for Children, Second Edition, Parent Rating Scale for preschoolers, a validated instrument that generates age-standardized T-scores across clinical scales such as hyperactivity, aggression, attention problems, anxiety, depression, and withdrawal, along with composite indices of externalizing problems, internalizing problems, and overall behavioural symptoms. Scores of 60 or above fall into the at-risk range. In this sample, most children scored near the normative mean, with between six and twenty-eight percent in the at-risk range depending on the scale. The analysis was preregistered, and the modeling strategy used generalized estimating equations to estimate the odds of at-risk outcomes per one microgram per gram increase in toenail fluoride, with interaction terms testing for differences by child sex and exposure window.

The headline result was statistically robust: each one microgram per gram increase in toenail fluoride reflecting early pregnancy was associated with thirty-nine percent higher odds of scoring in the at-risk range for externalizing problems, with a confidence interval running from fifteen to sixty-seven percent above baseline. The late-pregnancy estimate was similar in magnitude, at thirty percent higher odds, though its confidence interval narrowly included the null. On the individual clinical scales, early-pregnancy fluoride was linked to twenty percent higher odds of at-risk hyperactivity, and the late-pregnancy estimate of twenty-four percent approached significance. The pattern suggests that no single trimester stands out as uniquely vulnerable; rather, fluoride exposure across pregnancy appears relevant to externalizing behaviour.

The sex-specific findings added a further layer of complexity. Among girls, each one microgram per gram increase in early-pregnancy toenail fluoride was associated with thirty-seven percent higher odds of at-risk depressive symptoms, while no association appeared in boys—a statistically significant interaction. Suggestive but imprecise signals also emerged for anxiety and withdrawal in girls in relation to late-pregnancy exposure. This pattern is intriguing because prior fluoride studies, particularly those examining intelligence outcomes, have often reported stronger effects in boys. The authors note that internalizing and externalizing problems may share transdiagnostic roots, including emotion dysregulation, and that sex-specific biological responses to prenatal exposures could shape which behavioural domain is affected.

Plausible biological mechanisms are beginning to take shape in the literature. Fluoride has been implicated in disruption of the thyroid axis, a concern in pregnancy because the fetus depends entirely on maternal thyroid hormones during the first trimester, and even mild maternal thyroid dysfunction has been linked to both internalizing and externalizing problems in children. Fluoride exposure has also been shown to induce oxidative stress in animal and human studies, a pathway independently associated with behavioural problems in offspring. Low iodine status, which compromises thyroid hormone synthesis, may exacerbate fluoride’s effects, and emerging evidence suggests thyroid regulation and oxidative stress responses differ by fetal sex. None of these mechanisms is confirmed as the driver of the observed associations, but together they offer a coherent framework for future investigation.

Why did water fluoride fail to predict outcomes while toenail fluoride succeeded? The authors argue that toenails capture total intake from all dietary and product sources, whereas a single tap-water measurement reflects only one exposure pathway and ignores individual differences in consumption volume. This mismatch can produce exposure misclassification that biases associations toward the null. Median water fluoride in the sample was about 0.2 milligrams per litre—comparable to other communities with naturally occurring fluoride—but values ranged from 0.02 to 13.8 milligrams per litre, and correlations between water and toenail measures, while positive, were moderate. The authors also caution that some participants may have reduced tap-water consumption after learning their wells exceeded arsenic guidelines during the study.

The study’s limitations temper its implications. The cohort was predominantly white, college-educated, married, and relatively affluent, and relied on unregulated private wells, so the findings cannot be assumed to extend to populations drinking optimally fluoridated municipal water. Postnatal fluoride exposure was not measured, toenail growth rates vary between individuals, and only a small fraction of children scored in the at-risk range, limiting statistical power. Yet extensive sensitivity analyses—adjusting for co-occurring metals, imputing missing data, removing influential observations, and modeling outcomes continuously—left the primary conclusions intact. The authors call for longitudinal follow-up, validation of toenail fluoride reference ranges in pregnant populations, and exposome-wide approaches that jointly model fluoride alongside the many other prenatal environmental exposures that shape the developing brain.

Subject of Research: The association between prenatal fluoride exposure measured in maternal toenails and social-emotional and behavioural outcomes in preschool children.

Article Title: Long-term exposure to prenatal fluoride and social-emotional and behavioural outcomes in children in the New Hampshire Birth Cohort Study

Article References: Goodman, C. V., Karagas, M. R., Peacock, J. L., Korrick, S., Flora, D., Lanphear, B., Martinez-Mier, E. A., Tamayo-Cabeza, G., Lippert, F., & Till, C. (2026). Long-term exposure to prenatal fluoride and social-emotional and behavioural outcomes in children in the New Hampshire Birth Cohort Study. Environmental Advances, 25, Article 100753. https://doi.org/10.1016/j.envadv.2026.100753

Image Credits: AI Generated

DOI: 10.1016/j.envadv.2026.100753

Keywords: fluoride, prenatal exposure, child behaviour, hyperactivity, New Hampshire Birth Cohort Study, toenail biomarker, drinking water, neurodevelopment, thyroid, externalizing problems, internalizing problems, private wells