Adverse caregiving in infancy blunts neural processing of the mother

The study investigates how adverse caregiving during infancy affects neurobehavioral processing of the attachment figure and the quality of attachment behavior. Building on the Strange Situation Procedure (SSP) that classifies attachment quality via separation and reunion with the caregiver, the authors ask whether early adversity causally disrupts attachment-related behaviors and neural processing, which aspects of maternal behavior drive these changes, and what neurohormonal mechanisms mediate them. They leverage a translational approach: human SSP data from high- vs low-risk infants serve as a benchmark, and a rodent SSP (rSSP) allows invasive neural recordings and causal hormonal manipulations. The central hypothesis is that adversity elevates infant stress hormones, which in turn blunt cortical oscillatory responses to maternal cues, yielding atypical reunion behaviors indicative of disorganized attachment. The work focuses on cortical oscillations (theta, beta, gamma) and cross-frequency coupling, known to be developmentally important and stress-sensitive, especially in the context of social and maternal cues.

Prior literature shows that attachment quality, revealed by SSP reunions, predicts later socio-emotional outcomes including disorganized attachment linked to psychopathology. Human studies are largely correlational due to ethical and methodological constraints. Cortical oscillations are key in brain development and are modulated by stress hormones (corticosterone/cortisol) and social stimuli, with maternal cues being particularly salient. Existing human work indicates maternal presence can enhance child theta/gamma activity, but conflictual dyads show reduced gamma modulation. Animal work documents that maternal care regulates infant brain state and that early stress/maltreatment impacts later neurobehavioral outcomes, with corticosterone and noradrenergic mechanisms implicated. Cross-frequency theta-gamma coupling is sensitive to development and pathology. This background motivates causal testing in rodents to bridge human observations with mechanistic insight.

Design: Three experiments in rats using the scarcity-adversity low-bedding model to induce maternal maltreatment, combined with a human SSP dataset for translational benchmarking.

• Human SSP: N=21 children (12.1–28.3 months, M=19.9, SD=5.6) and their mothers categorized into high-risk (≥6 risk factors) and low-risk (≤5). Standard SSP conducted; attachment behaviors during reunions coded, parental sensitivity assessed.
• Rodent rearing and rSSP (Experiment 1): Litters split PN8–PN12 to control housing vs scarcity-adversity (low bedding; increases maternal rough handling while maintaining nurturing behaviors). At PN13–14, pups underwent rSSP comprised of seven epochs paralleling human SSP, using the biological mother and an anesthetized “stranger” dam (novel learned maternal odor via diet manipulation). Only the final reunion epoch analyzed. Approximately equal male/female distribution. Random assignment to rearing condition.
• In-nest neurobehavioral assessment (Experiment 2): Freely behaving pups recorded telemetrically during 1 h bouts of control or adversity conditions twice daily PN10–14. Maternal behaviors scored (nonnutritive nursing, nutritive nursing/milk ejection, grooming, rough handling, mother in/out of nest) and aligned to neural data.
• Causality via corticosterone blockade (Experiment 3): Metyrapone (MET; 50 mg/kg, i.p.) vs saline administered 90 min before daily adversity/control bouts for 5 days (EXP 3A) or acutely 90 min before rSSP (EXP 3B) to reduce corticosterone synthesis. Neural recording and analysis: Pups implanted 1–2 days pretest with stainless steel neocortical electrodes connected to a subcutaneous telemetric pack (DSI ETA-F10). Signals filtered 0.5–200 Hz, digitized at 2 kHz. FFT power computed in delta (0–5 Hz), theta (5–15 Hz), beta (15–35 Hz), gamma (35–100 Hz). Cross-frequency coupling assessed by filtering theta (1.5–12 Hz) and gamma (35–80 Hz), detecting cycles/troughs, estimating phase-locking and vector length. Behavioral epochs selected artifact-free; some evoked analyses normalized to each pup’s nonnutritive nursing baseline. Statistics: Two-sided t tests, one-/two-way ANOVA with post hoc Bonferroni, chi-square tests, and circular statistics for coupling; significance at p<0.05. Validation of adversity model: Maternal behaviors quantified; low bedding increased rough handling frequency without reducing grooming or nursing. Pup plasma corticosterone (CORT) measured after 5 days to confirm elevation under adversity. Ethics: Human IRB approval with informed consent; rodent IACUC approval and NIH guidelines.

• Human SSP benchmark: High risk for maltreatment associated with more disorganized attachment behaviors [χ2(1,21)=14.32, p=0.0002; relative risk=5.5] and fewer organized/secure behaviors [χ2(1,21)=3.834, p=0.05; relative risk=2.867]. Higher parental sensitivity correlated with fewer disorganized behaviors (r=−0.44, p=0.047) and was lower in high-risk vs low-risk parents (r=−0.42, p=0.008).
• Adversity model validation (Table 1): Rough handling increased in adversity vs control (0.68±0.28% vs 3.00±0.91%, t(4)=2.98, p=0.027), while nurturing behaviors (nursing, grooming) were similar across groups. Pup plasma CORT elevated under adversity (74.19±19.76 vs 214.15±50.31 ng/dL; p<0.05).
• Rodent rSSP behavior (PN13–14): Control-reared pups displayed typical reunion behaviors (approach, nursing, sleeping at ventrum) more often than adversity-reared pups [χ2(1,16)=2.618, p=0.05; relative risk=1.5]. Adversity-reared pups exhibited more atypical reunion behaviors (e.g., sleeping alone or behind mother) [χ2(1,16)=4.267, p=0.039; relative risk=0.2]. Nursing in-nest outside rSSP did not differ, indicating the test uncovered group differences.
• rSSP cortical LFP during reunion: Significant interaction of rearing×frequency band [F(3,48)=5.415, p=0.003]. Control pups showed decreased gamma power during reunion vs alone epoch (epoch 5→7), whereas adversity pups did not; instead, adversity pups showed decreased theta. Post hoc: theta ratio control 1.135±0.088 vs adversity 0.892±0.036 (t(48)=2.424, p=0.019); gamma ratio control 0.827±0.038 vs adversity 1.053±0.064 (t(48)=2.251, p=0.029). Theta–gamma coupling observed more reliably in adversity-reared pups [χ2(1,14)=5.6, p=0.018; relative risk=8], with mean vector length doubling (0.019±0.002 vs 0.043±0.011; t(12)=2.137, p=0.025).
• In-nest LFP during specific maternal behaviors (Experiment 2): No group differences when mother was out of nest or during nonnutritive nursing. Rough handling produced minimal LFP differences, limited to delta band (main effect of rearing F(1,44)=4.303, p=0.044; delta ratio control 1.317±0.32 vs adversity 0.673±0.115, t(44)=2.51, p=0.016). Nurturing inputs robustly modulated LFP in controls but were blunted in adversity pups: • Milk ejection in controls increased beta (t(6)=2.631, p=0.039) and gamma (t(6)=3.159, p=0.019) power vs baseline; grooming increased gamma (t(6)=4.314, p=0.005). • Adversity significantly reduced these modulations (milk ejection: main effect of rearing F(1,44)=9.102, p=0.004; gamma ratio control 2.551±0.284 vs adversity 1.4±0.269, t(44)=2.47, p=0.017. Grooming: rearing×frequency interaction F(3,44)=3.029, p=0.039; main effects of frequency F(3,44)=5.451, p=0.003 and rearing F(1,44)=5.205, p=0.027; gamma ratio control 2.523±0.353 vs adversity 1.298±0.083, t(44)=3.651, p<0.001). • Theta–gamma coupling occurred more reliably during adversity bouts [χ2(1,13)=3.899, p=0.024; relative risk=2.94].
• Causality via corticosterone blockade (Experiment 3): • MET during daily adversity bouts (EXP 3A) rescued attachment behavior during rSSP (rearing×drug interaction F(1,20)=5.19, p=0.034; main effects: rearing F=8.061, p=0.01; drug F=5.48, p=0.029). Saline adversity pups showed more atypical behaviors than saline controls (1211±310.7 s vs 305.667±151.656 s, t(20)=3.266, p=0.004). MET-treated adversity pups did not differ from saline controls (208±121.341 s; t(20)=0.352, p=0.728). • Acute MET before rSSP (EXP 3B) similarly rescued behavior (rearing×drug F(1,18)=11.03, p=0.004; main effects drug F=8.103, p=0.011; rearing F=5.722, p=0.028). Saline adversity pups 206.667±43.778 s vs control saline 22.167±22.167 s atypical behaviors (t(18)=4.237, p=0.005); adversity MET vs control saline t(18)=0.31, p=0.76. • MET normalized reunion-induced beta/gamma suppression (e.g., beta: control saline 0.659±0.059 vs adversity saline 1.059±0.101, p=0.019; control saline vs adversity MET 0.705±0.106, p=0.648; gamma: control saline 0.655±0.034 vs adversity saline 1.061±0.057, p=0.035; control saline vs adversity MET 0.769±0.065, p=0.588; main effect of frequency F(3,72)=5.191, p=0.003). MET also reduced aberrant theta–gamma coupling in adversity pups [χ2(1,11)=4.412, p=0.018; relative risk=0.24].

Across species, brief separations followed by reunion reveal attachment quality; here, adversity-exposed infants (humans at high maltreatment risk and rats reared under low bedding) showed atypical reunion behaviors. In rats, adversity disrupted the maternal regulation of cortical oscillations: control pups exhibited expected reductions in high-frequency power during reunion and robust high-frequency enhancements to nurturing inputs (milk ejection, grooming), whereas adversity pups showed blunted responses to nurturing cues and minimal differences during rough handling. Adversity also increased the reliability of theta–gamma coupling during both in-nest adversity bouts and rSSP reunion, indicating altered network coordination in response to maternal cues. Elevated pup corticosterone during adversity was causally linked to these neurobehavioral changes: blocking corticosterone synthesis with metyrapone during adversity or acutely before rSSP restored typical attachment behaviors, normalized beta/gamma power dynamics during reunion, and reduced aberrant cross-frequency coupling. These findings support a model in which stress physiology during adverse caregiving degrades cortical processing of maternal signals, weakening the mother’s moment-to-moment regulation of infant brain state and behavior. Given the central role of oscillations in development, such alterations may contribute to later socio-emotional pathology and provide early biomarkers of risk.

The study establishes a translational bridge showing that early adverse caregiving induces atypical attachment behaviors and stress-dependent blunting of cortical processing of maternal cues in infancy. In rodents, adversity specifically dampened high-frequency LFP responses to nurturing maternal inputs and increased theta–gamma coupling during maternal interactions; reducing corticosterone rescued both behavior and neural metrics. These results suggest a stress-induced alteration of the infant’s processing of the attachment figure that may underlie risk for later psychopathology. Future work could delineate the precise neural circuitry and neuromodulatory mechanisms (e.g., noradrenergic–HPA interactions) mediating maternal cue processing under stress, assess the persistence of these oscillatory alterations longitudinally, and test targeted early interventions to normalize infant brain responses to caregivers.

• Translation from rodents to humans has inherent constraints; authors acknowledge limitations of using an animal model to understand human behavior.
• In rSSP, dams and “stranger” mothers were anesthetized, which may differ from naturalistic interactions.
• Human sample size was modest and groups were defined by cumulative risk rather than confirmed maltreatment; findings are correlational in humans.
• LFP delta power can be influenced by movement artifacts; although artifact-free epochs were selected, residual effects cannot be entirely excluded.
• Timing differences: rodent assessments captured immediate effects during/after adversity, whereas human EEG/behavior literature often assesses later outcomes, complicating direct comparisons of cross-frequency coupling patterns.