Emerging neurodevelopmental research indicates that screen exposure during the critical first two years of life correlates with measurable alterations in neural architecture and cognitive trajectories. Contemporary pediatric neuroscience has identified this developmental window as uniquely vulnerable to environmental perturbations, with early digital media exposure representing a novel ecological variable whose long-term neurological consequences demand rigorous empirical investigation and evidence-based clinical guidance for caregivers navigating modern parenting challenges.
What Does Current Neuroscience Reveal About Brain Development in the First Two Years?
The human brain undergoes extraordinary structural and functional transformation during the initial 24 months of postnatal development, a period characterized by unprecedented neuroplasticity and environmental sensitivity. At birth, the infant brain contains approximately 86 billion neurons—nearly the adult complement—yet weighs only 25% of its eventual mature mass. This apparent paradox resolves when examining the developmental processes that dominate early childhood: dendritic arborization, synaptogenesis, myelination, and activity-dependent synaptic pruning.
Synaptogenesis, the formation of synaptic connections between neurons, proceeds at extraordinary rates during infancy. Peak synaptic density in the visual cortex occurs around 8 months of age, reaching approximately 150% of adult levels before subsequent pruning refines neural circuits based on experiential input. The prefrontal cortex, governing executive functions, attention regulation, and impulse control, exhibits a more protracted developmental trajectory, with synaptogenesis continuing throughout early childhood and pruning extending into adolescence and early adulthood.
Myelination—the process whereby oligodendrocytes ensheath axons with lipid-rich myelin sheaths—dramatically increases neural transmission velocity and metabolic efficiency. This process follows a posterior-to-anterior and inferior-to-superior gradient, with sensory and motor systems myelinating before association cortices. The corpus callosum, facilitating interhemispheric communication, undergoes substantial myelination during the first two years, enabling increasingly sophisticated bilateral coordination and information integration.
Critical periods represent developmental windows during which specific neural systems exhibit heightened plasticity and environmental sensitivity. The primary visual cortex demonstrates a well-characterized critical period during which binocular visual experience shapes ocular dominance column formation. Deprivation or abnormal input during these windows—as demonstrated through classical studies of monocular deprivation in animal models—produces permanent alterations in neural circuitry that resist subsequent remediation. While the concept of critical periods initially emerged from sensory system research, contemporary neuroscience recognizes similar sensitive periods for language acquisition, social cognition, and executive function development.
The neurobiological substrates supporting early learning depend critically on interactive, contingent social experiences. Infant brains evolved within environments characterized by face-to-face interaction, physical manipulation of three-dimensional objects, and temporally structured caregiver responsiveness. These experiences activate distributed neural networks spanning sensory cortices, motor systems, limbic structures mediating emotional processing, and prefrontal regions supporting attention and executive control. The temporal contingency of social interaction—the immediate responsiveness of caregivers to infant behaviors—appears particularly crucial for developing neural circuits underlying social cognition and emotional regulation.
How Do Screens Affect Developing Neural Architecture?
Screen exposure during early development introduces a constellation of environmental variables that differ fundamentally from the evolutionary contexts within which human neural development systems were selected. Understanding the neurobiological mechanisms through which digital media may influence brain development requires examining multiple levels of analysis: sensory processing, attention systems, language acquisition, social cognitive development, and sleep architecture.
Visual processing systems evolved to extract information from three-dimensional environments with specific spatiotemporal characteristics. Screen-based stimuli present two-dimensional representations with abnormal luminance characteristics, refresh rates, and color spectra. The rapid scene changes characteristic of much digital content—often exceeding one cut per second in children’s programming—may exceed the temporal resolution capacities of immature visual processing systems. Neuroimaging studies in older children demonstrate that rapid-paced media activate different neural networks compared to slower-paced content, with implications for attention regulation and information processing efficiency.
The attention systems undergo critical development during infancy and toddlerhood, transitioning from exogenously driven orienting responses to endogenously controlled sustained attention. Screens present extraordinarily salient stimuli—characterized by movement, color variation, and auditory synchronization—that powerfully capture exogenous attention through bottom-up mechanisms. Chronic exposure to such stimuli during periods of attention system development may bias neural circuits toward reactive, stimulus-driven attention modes at the expense of developing volitional attention control. Longitudinal studies examining associations between early screen exposure and subsequent attention regulation consistently demonstrate correlations between higher infant media consumption and elevated attention-deficit symptoms during preschool and school-age periods, though establishing definitive causality remains methodologically challenging.
Language acquisition depends fundamentally on temporally contingent social interaction. The phonological, semantic, and pragmatic dimensions of language emerge through millions of interactive exchanges wherein caregivers provide linguistic input calibrated to the child’s developmental level, with immediate feedback reinforcing communicative attempts. Screens, even those presenting ostensibly educational content, lack genuine temporal contingency—the media content cannot adjust its complexity, pacing, or focus based on the infant’s moment-to-moment comprehension state or attentional engagement.
The “video deficit effect,” documented across numerous experimental paradigms, demonstrates that infants and toddlers learn substantially less effectively from screen-based presentations compared to equivalent information delivered through live social interaction. This effect persists even when controlling for content quality and presentation format. Neurobiological explanations invoke the absence of social-cognitive engagement during screen viewing: joint attention, eye contact, prosodic variation, and gestural communication—all critical scaffolds for early learning—remain absent or impoverished in screen-mediated contexts.
Social brain networks, encompassing the superior temporal sulcus, temporoparietal junction, medial prefrontal cortex, and amygdala, undergo substantial development during early childhood. These circuits support face processing, biological motion perception, mental state attribution, and emotional understanding. Screen time necessarily displaces time available for face-to-face social interaction, potentially creating a form of relative social deprivation during sensitive periods for social brain development. Preliminary neuroimaging evidence suggests that higher screen exposure during early childhood correlates with altered activation patterns in social cognitive networks, though longitudinal research with adequate control for confounding variables remains limited.
Sleep architecture demonstrates particular vulnerability to screen exposure through multiple mechanisms. The short-wavelength blue light emitted by electronic displays suppresses melatonin secretion through direct activation of intrinsically photosensitive retinal ganglion cells projecting to the suprachiasmatic nucleus. Even brief evening screen exposure can delay circadian phase and reduce total sleep duration. Given that sleep serves critical functions in memory consolidation, synaptic homeostasis, and neurodevelopmental processes, chronic sleep disruption during early childhood may compromise multiple aspects of brain development. Screen content itself may increase physiological arousal, further interfering with sleep onset and quality.
What Does the Empirical Evidence Demonstrate About Developmental Outcomes?
The epidemiological literature examining associations between early childhood screen exposure and developmental outcomes has expanded substantially over the past decade, though methodological heterogeneity and correlational study designs necessitate cautious interpretation. Synthesizing findings across multiple research domains reveals consistent patterns warranting clinical and public health attention.
Longitudinal cohort studies demonstrate dose-response relationships between infant screen time and subsequent language development trajectories. Research utilizing the Canadian Healthy Infant Longitudinal Development Study followed approximately 2,400 children, assessing screen exposure at multiple timepoints alongside standardized developmental assessments. Results indicated that each additional hour of daily screen time at 24 months predicted lower scores on developmental screening tests at 36 months, with language domains showing particular vulnerability. Effect sizes, while modest at the individual level, achieve clinical significance at the population level given the high prevalence of substantial screen exposure among contemporary infants.
Executive function development, assessed through behavioral tasks measuring inhibitory control, working memory, and cognitive flexibility, shows inverse associations with early screen exposure across multiple investigations. The Growing Up in Singapore Towards Healthy Outcomes (GUSTO) study examined nearly 600 children, finding that television viewing before age 2 predicted poorer executive function performance at age 9, even after controlling for maternal education, household income, and other potential confounders. These associations persisted across multiple executive function domains, suggesting broad impacts on prefrontal cortex development rather than selective effects on specific cognitive processes.
Attention regulation difficulties represent one of the most consistently reported correlations with early screen exposure. Meta-analytic syntheses incorporating data from dozens of studies across multiple countries indicate small but reliable associations between infant media consumption and subsequent attention problems, with estimated effect sizes in the range of r = 0.10-0.15. While these correlations do not definitively establish causation—alternative explanations including reverse causality and unmeasured confounding remain plausible—the consistency across diverse samples and methodological approaches strengthens causal inference.
Neuroimaging investigations provide direct evidence of structural and functional brain differences associated with screen exposure patterns. Research published in JAMA Pediatrics examined 47 children aged 3-5 years, correlating parent-reported screen time with diffusion tensor imaging measures of white matter integrity. Higher screen use associated with lower microstructural organization in white matter tracts supporting language and executive function, including the inferior fronto-occipital fasciculus and superior longitudinal fasciculus. These structural differences mediated associations between screen time and lower expressive language scores, suggesting that white matter development represents one neurobiological pathway linking screen exposure to cognitive outcomes.
Functional MRI studies demonstrate altered neural activation patterns during cognitive tasks in children with higher versus lower screen exposure histories. Tasks requiring sustained attention, inhibitory control, or language processing reveal different recruitment patterns of prefrontal, temporal, and parietal regions as a function of early media exposure. While cross-sectional neuroimaging cannot definitively establish whether these differences represent consequences of screen exposure, pre-existing individual differences, or complex bidirectional relationships, they provide compelling evidence that screen habits associate with measurable brain functional differences.
Sleep outcomes show particularly robust associations with screen exposure. Systematic reviews incorporating dozens of studies consistently document that screen access in bedrooms, evening screen use, and total daily screen time all predict shorter sleep duration, later sleep onset, and increased sleep disturbances. Given sleep’s fundamental importance for neurodevelopment, these sleep disruptions may mediate some proportion of screens’ broader developmental impacts.
What Mechanisms Might Explain Lasting Neurological Changes?
Understanding the neurobiological pathways through which early screen exposure might produce enduring neural alterations requires integrating evidence across multiple levels of analysis, from molecular neuroscience through systems-level circuit development to behavioral phenotypes. Several interconnected mechanisms likely operate synergistically rather than independently.
Experience-dependent plasticity represents the fundamental principle whereby neural activity patterns shape circuit refinement during development. Hebbian learning—”neurons that fire together, wire together”—operates through molecular cascades initiated by coincident pre- and post-synaptic activity, leading to long-term potentiation, spine enlargement, and synaptic strengthening. Conversely, inactive synapses undergo depression and eventual elimination. During early development, these activity-dependent processes sculpt neural circuits to match environmental statistics. Screen exposure patterns that differ systematically from the social-physical environments for which infant brains evolved may drive circuit development along atypical trajectories.
The displacement hypothesis proposes that screen time’s developmental impacts result primarily from opportunity costs—time spent viewing screens necessarily displaces time available for other activities more conducive to optimal development. Infants and toddlers learn most effectively through active exploration of three-dimensional environments and contingent social interaction. Each hour of screen exposure potentially displaces an hour of these developmentally optimal experiences. Given the finite duration of critical and sensitive periods, displacement during these windows may produce irreversible effects even if subsequent experiences are enriched.
Attention system biasing suggests that chronic exposure to rapid-paced, highly salient screen stimuli during periods of attention network development may bias neural circuits toward bottom-up, reactive attention modes at the expense of top-down, endogenously controlled attention. The neural competition between stimulus-driven attention systems (involving superior colliculus, pulvinar, temporoparietal junction) and goal-directed attention networks (involving dorsolateral prefrontal cortex, frontal eye fields, intraparietal sulcus) may be resolved in favor of reactive systems when developing brains receive disproportionate input from highly salient screen stimuli. This biasing could manifest as persistent difficulties with sustained attention, increased distractibility, and reduced capacity for self-directed cognitive control.
Social deprivation mechanisms invoke the possibility that screen time during sensitive periods for social brain development may constitute a form of relative social deprivation. The social brain hypothesis proposes that human cognitive evolution was driven substantially by the computational demands of navigating complex social environments. If screen exposure systematically reduces the quantity or quality of social interaction during periods when social cognitive circuits exhibit heightened plasticity, resulting neural development may deviate from species-typical trajectories. This framework predicts specific impacts on face processing, biological motion perception, mental state attribution, and emotional recognition—predictions partially supported by emerging evidence.
Sleep disruption pathways operate through both direct and indirect mechanisms. Direct effects include circadian phase delay from evening light exposure and heightened physiological arousal from stimulating content. Indirect effects include displacement of sleep by extended viewing and parent-child conflicts around screen limits creating bedtime stress. Chronic sleep restriction during early childhood compromises multiple neurodevelopmental processes: growth hormone secretion occurs predominantly during deep sleep; memory consolidation depends on sleep-dependent replay; and synaptic homeostasis theory proposes that sleep enables synaptic downscaling essential for maintaining neural circuit efficiency. Persistent sleep disruption during sensitive developmental periods may therefore impair multiple aspects of brain development.
Stress pathway activation represents an under-explored mechanism. Some research suggests that inappropriate media content—including programs too complex, fast-paced, or emotionally intense for young children’s regulatory capacities—may activate stress response systems. Chronic activation of the hypothalamic-pituitary-adrenal axis during early development alters stress system calibration, potentially producing lasting changes in stress reactivity and vulnerability to anxiety and mood disorders. While evidence for this mechanism remains preliminary, it warrants investigation given known impacts of early adversity on neurodevelopment.
How Can Parents Navigate Screen Use Guidelines Effectively?
Translating neuroscientific evidence and clinical recommendations into practical parenting strategies requires acknowledging the complex realities of contemporary family life while maintaining commitment to developmentally appropriate practices. Effective implementation depends on understanding rationales behind recommendations, identifying flexible strategies aligned with family values and circumstances, and maintaining realistic expectations about perfection versus progress.
The American Academy of Pediatrics recommends avoiding digital media use (except video chatting) in children younger than 18-24 months, then limiting screen time to high-quality programming watched together with caregivers for children 2-5 years. These guidelines reflect evidence synthesis regarding developmental risks, though they acknowledge that zero screen exposure may be impractical for many families. Understanding the neurobiological basis for these recommendations—particularly the importance of interactive, contingent learning experiences during this developmental period—helps parents make informed decisions when navigating inevitable exceptions and challenges.
Video chatting with distant relatives represents a qualitatively different experience from passive screen viewing. Unlike recorded media, video chat provides genuine temporal contingency—the remote caregiver responds to the child’s vocalizations, gestures, and emotional expressions in real time. Research indicates that toddlers can learn effectively from video chat when the remote partner engages actively, using joint attention strategies and providing contingent responses. While video chat lacks the multisensory richness of in-person interaction, it offers social connection that passive viewing cannot provide, justifying the AAP’s exception for this screen use category.
Content quality varies enormously across available media. Programming specifically designed for infant and toddler audiences using developmental science principles—slow pacing, repetition, direct address, pausing for response opportunities—produces better learning outcomes than programs created for older audiences or adults. Educational content research demonstrates that even young children can acquire vocabulary from well-designed programs when co-viewing caregivers actively scaffold understanding through labeling, repetition, and extension activities. However, “educational” marketing claims warrant skepticism; independent evaluation of developmental appropriateness provides more reliable quality indicators.
Co-viewing transforms passive consumption into an interactive learning opportunity. When caregivers watch programs with children, labeling characters and actions, asking questions, making connections to the child’s experiences, and extending learning beyond screen time, they substantially enhance learning outcomes. This active mediation essentially creates temporal contingency that the screen content itself lacks, partially compensating for video deficit effects. However, co-viewing requires substantial caregiver attention and effort, potentially limiting practical feasibility during times when parents utilize screens precisely to enable other necessary activities.
Environmental modifications prove more effective than reliance on moment-to-moment willpower. Removing televisions from bedrooms eliminates sleep disruption risks and reduces unsupervised viewing. Creating screen-free zones (bedrooms, dining areas) and screen-free times (meals, the hour before bedtime) establishes predictable routines that reduce conflicts. Keeping devices out of sight when not in use reduces children’s requests and parents’ temptation to use screens as default entertainers. These structural approaches leverage situational factors rather than depending solely on self-control.
Alternative engagement strategies address the underlying needs that screens often fulfill. When parents rely on screens primarily for occupying children during necessary activities—preparing meals, handling work calls, managing siblings—identifying alternative engagement options increases feasibility of screen reduction. Age-appropriate independent play materials, audiobooks, sensory play opportunities, and safe enclosed spaces where children can explore with minimal supervision provide alternatives that support development more effectively than screens. Normalizing that children will sometimes experience boredom—itself a developmental opportunity for cultivating imagination and self-directed play—reduces pressure for constant entertainment provision.
Self-compassion and realistic goal-setting prevent the all-or-nothing thinking that undermines sustained behavior change. Parents facing recommendations that may conflict with current practices often experience guilt, defensiveness, or overwhelm. Recognizing that incremental improvements benefit children’s development—even if falling short of ideal recommendations—maintains motivation. Each substitution of interactive play for screen time, each instance of co-viewing instead of solo viewing, each evening without screen exposure represents progress supporting healthier development.
What About Educational Apps and Interactive Media?
The proliferation of applications and interactive media marketed as educational for infants and toddlers raises complex questions about whether interactivity fundamentally alters developmental considerations around screen exposure. Critical evaluation requires distinguishing genuine interactivity—where the device responds contingently to the child’s input in pedagogically meaningful ways—from superficial interactivity providing mere entertainment value through touch responses.
Touchscreen tablets introduce motor interaction absent from passive television viewing. Toddlers can manipulate on-screen objects through direct touch, potentially engaging sensorimotor systems more fully than passive viewing. However, this two-dimensional interaction fundamentally differs from three-dimensional object manipulation that engages proprioceptive feedback, haptic exploration, and spatial reasoning about real-world physics. Research comparing learning from touchscreen activities versus analogous hands-on activities consistently demonstrates superior learning outcomes for physical manipulation, particularly for spatial and scientific concepts requiring understanding of causation, three-dimensional relationships, and physical properties.
Transfer of learning from screen-based contexts to real-world applications presents substantial challenges for young children. Studies examining whether toddlers can apply skills learned through apps to analogous real-world situations find inconsistent transfer, with success depending heavily on task demands, individual differences, and the degree of similarity between training and transfer contexts. This transfer limitation reflects young children’s immature capacity for symbolic representation and abstract mapping—the same cognitive constraints underlying the video deficit effect. Skills practiced exclusively in screen contexts may remain somewhat encapsulated, failing to generalize to the real-world situations where their application matters most.
The quality of purportedly educational apps varies enormously, with most lacking empirical validation of learning outcomes. Content analyses of apps marketed for early learning reveal that few incorporate evidence-based design principles from developmental psychology and education research. Common deficiencies include excessive distracting features, insufficient scaffolding, inappropriate difficulty progression, and prioritization of engagement (screen time maximization) over pedagogical effectiveness. The absence of robust certification standards means that parental selection requires substantial effort to identify genuinely educational content among thousands of options varying widely in quality.
“Educational” screen time may inadvertently displace more effective learning activities. When parents substitute app-based learning for activities like shared book reading, object manipulation, outdoor exploration, or pretend play, children may experience net learning disadvantages even if the app itself provides some educational value. Shared book reading, for instance, offers vocabulary exposure, narrative structure, print concept learning, and social interaction simultaneously—a combination difficult for even high-quality apps to replicate. Before age 2, when interactive capabilities remain limited and symbolic representation is emerging, the opportunity cost of screen-based learning appears particularly unfavorable.
Interactive video chat represents the screen-based activity with strongest evidence for supporting learning in very young children. Unlike apps or programming, video chat provides genuine contingent social interaction—the defining feature of optimal early learning contexts. Research demonstrates that toddlers learn words, solve problems, and engage socially through video chat when remote partners use effective pedagogical strategies. However, even video chat appears less effective than in-person interaction for the youngest infants, suggesting that developmental thresholds determine when children can effectively utilize screen-mediated communication.
What Do Long-Term Studies Reveal About Outcomes?
Assessing the long-term developmental and neurological consequences of early screen exposure requires longitudinal research tracking children from infancy through school age and beyond. While this literature remains limited due to the methodological demands of lengthy follow-up and the relatively recent emergence of substantial infant screen exposure as a widespread phenomenon, available evidence provides preliminary insights into potential enduring effects.
The Adolescent Brain Cognitive Development Study represents the largest long-term brain development investigation in the United States, following nearly 12,000 children from ages 9-10 through early adulthood. While participants were recruited after infancy, retrospective reports of early childhood screen exposure combined with prospective tracking enable examination of associations between early media habits and later brain structure and function. Preliminary analyses indicate that children with higher early screen exposure histories demonstrate subtle but measurable differences in cortical thickness patterns, particularly in regions supporting language and cognitive control. However, isolating early exposure effects from ongoing screen habits and numerous potential confounding variables remains analytically challenging.
Educational achievement outcomes show modest but consistent associations with early screen exposure patterns. Studies following children from toddlerhood through elementary school find that higher infant and toddler screen time predicts lower academic readiness at school entry and reduced academic achievement in early grades, even after adjusting for socioeconomic status, parental education, and other demographic factors. Effect sizes typically classify as small by conventional standards, yet given the population-level prevalence of screen exposure, even small effects aggregate to substantial public health significance.
Socioemotional development trajectories demonstrate associations with early media exposure across multiple dimensions. Longitudinal research indicates that higher infant screen time predicts increased behavioral problems, reduced prosocial behavior, and greater difficulty with emotion regulation during preschool and early school years. The mechanisms likely involve both direct effects—such as displacement of social interaction during sensitive periods—and indirect pathways including sleep disruption and reduced parent-child interaction quality. Importantly, the strength of these associations varies substantially based on content type, co-viewing practices, and overall family context, underscoring that screen exposure operates within complex ecological systems rather than as an isolated risk factor.
Attention and executive function outcomes show perhaps the most robust long-term associations. Multiple longitudinal investigations consistently demonstrate that early screen exposure predicts increased attention problems, reduced sustained attention capacity, and poorer executive function performance years later. While debate continues regarding whether these associations reflect causal effects of screens, reverse causality (children with pre-existing regulatory difficulties receiving more screen exposure), or correlated risk factors, the consistency across studies and the presence of dose-response relationships strengthen arguments for at least some causal contribution.
Physical health outcomes extend beyond purely neurological considerations but warrant mention given interconnections between physical and brain health. Longitudinal research links early screen exposure to increased obesity risk, reduced physical activity levels, and poorer sleep quality persisting through childhood. These physical health impacts may secondarily affect brain development through multiple pathways: obesity associates with systemic inflammation affecting brain health; physical inactivity reduces production of neurotrophic factors supporting neuroplasticity; and chronic sleep restriction impairs multiple neurodevelopmental processes. The developmental trajectories of physical and brain health appear substantially intertwined.
Resilience and protective factors substantially moderate associations between early screen exposure and developmental outcomes. Children in environments characterized by responsive caregiving, cognitive stimulation, emotional support, and material resources demonstrate smaller negative associations with screen exposure than children in less supportive contexts. This pattern suggests that screen exposure may interact with other environmental factors, with effects amplified in contexts where screens displace limited parent-child interaction opportunities or where families face multiple accumulated risks. Conversely, high-quality caregiving may buffer against screen exposure’s potential harms, perhaps through compensatory mechanisms supporting development despite suboptimal screen habits.
Conclusion
The neuroscientific evidence examining early screen exposure’s impacts on brain development reveals a complex picture requiring nuanced interpretation and thoughtful application to clinical and parenting guidance. While definitive proof of lasting brain damage from screen exposure before age 2 exceeds current empirical evidence’s reach—given methodological limitations of existing research—the preponderance of available data indicates concerning associations between early media exposure and multiple developmental domains. The neurobiological plausibility of these associations, grounded in understanding of critical periods, experience-dependent plasticity, and the importance of contingent social interaction for optimal development, strengthens causal interpretations.
Parents navigating screen use decisions benefit from understanding both the empirical evidence and the underlying developmental principles. The recommendation to minimize screen exposure before age 2 reflects recognition that this developmental period exhibits unique vulnerability to environmental influences, that screen time necessarily displaces more developmentally optimal activities, and that the purported benefits of infant media exposure lack robust empirical support. However, realistic implementation requires acknowledging modern family realities, focusing on progress rather than perfection, and recognizing that screens represent one factor among many influencing developmental trajectories.
The path forward involves continued rigorous research clarifying causal relationships and identifying mechanisms, development of genuinely evidence-based media content when screen use occurs, and clinical guidance supporting families in making informed decisions aligned with their values and circumstances. Your commitment to understanding these issues and implementing developmentally supportive practices—even imperfectly—supports your child’s optimal brain development during these foundational years. Approach screen use decisions with both scientific knowledge and self-compassion, recognizing that informed, intentional parenting practices accumulated across countless daily decisions ultimately shape developmental outcomes more powerfully than any single factor.
Important Disclaimer: This article is for informational purposes only and should not replace professional advice. For health-related topics, consult healthcare providers. For financial or legal matters, seek qualified professional guidance. For safety procedures, verify current best practices and local regulations. Individual results may vary, and personal circumstances should always be considered when implementing any suggestions.