Screen time can damage the brains of under sixes leaving them struggling in language and literacy, according to new research.
The study showed tablets, smartphones and TV rewires their brains, affecting speech, thinking and reading.
These skills include recognising objects and executive function - the process involving mental control and self-regulation.
Lead author Dr John Hutton said: "Screen-based media use is prevalent and increasing in home, childcare and school settings at ever younger ages.
"The findings highlight the need to understand effects of screen time on the brain - particularly during stages of dynamic brain development in early childhood - so providers, policymakers and parents can set healthy limits."
His team used a scanning technique called diffusion tensor imaging (DTI) to assess the condition of the brain's white matter in 47 boys and girls aged three to five.
The participants, who were all healthy, first completed standard cognitive tests. Their screen use was measured through a 15-point questionnaire filled in by their parents.
Those who spent the most time on electronic devices or watching TV were much more likely to have lower expressive language.
They were also liable to struggle to rapidly name images of objects - indicating slow brain processing speed - and were prone to poor literacy.
Dr Hutton said 28 of the children, or six in ten, had their own smartphone or tablet and 19 (41%) a television or portable device in their bedroom.
He said: "To our knowledge, this study is the first to describe structural neurobiological links to screen-based media use in preschool-aged children."
Scientists believe screen time fails to stimulate the brain in the same way as reading books and can reduce sleep - which is essential for a child's development.
Dr Hutton, director of the Reading & Literacy Discovery Centre at Cincinnati Children's Hospital, said: "This study raises questions as to whether at least some aspects of screen-based media use in early childhood may provide sub-optimal stimulation during this rapid, formative state of brain development."
The study found pre-school children hooked on computer games and other screen activities had less white matter that enables communication between neurons.
This impairs brain organisation and the formation of myelin - a protective sheath around nerves that allows their impulses to move quickly.
In particular, the harm was identified in tracts involving language executive function and other literacy skills, reports JAMA Pediatrics.
The results remained after taking into account other factors such as age, gender and household income.
In particular, the harm was identified in tracts involving language executive function and other literacy skills.
Dr Hutton said: "While we can't yet determine whether screen time causes these structural changes or implies long-term neuro-developmental risks, these findings warrant further study to understand what they mean and how to set appropriate limits on technology use."
The American Academy of Pediatrics (AAP) recommends screen use for two to five year-olds be limited to one hour per day of high-quality programs.
Parents should also watch with them to understand the information - and apply it to the world around them.
Dr Hutton said: "In a single generation, through what has been described as a vast 'uncontrolled experiment,' the landscape of childhood has been digitised, affecting how children play, learn and form relationships.
"In addition to traditional programming, rapidly emerging technologies, particularly portable electronic devices, provide unprecedented access to a wide range of media.
"Use begins in infancy and increases with age, and it was recently estimated at more than two hours per day in children younger than nine years, aside from use during childcare and school."
The AAP also advises designated shared times when screens are banned - such as during dinner or in the car - as well as media-free locations like bedrooms.
And parents ought to deny any screen media to those younger than 18 months other than video-chatting.
Parents of children aged 18 to 24 months who want to introduce digital media should choose high-quality programming and view it with them to explain it.
Dr Hutton said: "This study found an association between increased screen-based media use, compared with the AAP guidelines, and lower micro-structural integrity of brain white matter tracts supporting language and emergent literacy skills in pre-kindergarten children."
He added: "Given that screen-based media use is ubiquitous and increasing in children in home, childcare, and school settings, these findings suggest the need for further study to identify the implications for the developing brain, particularly during stages of dynamic brain growth in early childhood."
Last year another US study found children who spent the most time on screens had around a five per cent lower cognitive function than other eight to 11-year-olds.
British experts are sceptical about the dangers of screen-time in children and have criticised the quality of scientific evidence.
More than half of three to four-year-olds in the UK use the internet every week - and one in five have their own tablet.