Too Much of a Good Thing
There is a good reason for limiting screen time no matter how great the programs you're showing your child are. It's something experts describe as "the displacement effect." By this rationale, television may not be harmful in and of itself, but there is a danger that too much television watching could displace other activities that would be more beneficial to the child.
High-quality TV programs can certainly aid learning, but they can never replace real interactions between parents and children.
"Children learn better from [someone] speaking to them in person," notes Christakis, commenting on a 2005 study in which children were randomized to either hear a Mandarin speaker live, or to see the same speaker on screen saying exactly the same thing. His findings suggest that children's language learning is vastly enhanced when it is a live person doing the talking.
Several studies have linked total television viewing in early childhood to attention problems in later life. According to a 2004 study led by Christakis, "The more TV children watched before the age of 3, the more likely they were to have attention problems at age 7. Specifically, for each hour of TV the children watched on average before the age of 3, their chances of having attentional problems at age 7 were increased by about 10 percent." (It should be noted that all types of content were included in this study.)
Supervision
In the beginning, your child should never watch TV unsupervised. Your baby needs you -or another loving adult - to help her make sense of what is on the screen. Repeat after the words your baby hears, ask questions, and talk about what she is seeing.
It is natural for parents to make use of the window of time TV programs afford to do chores or get ready to go out. If you do this, keep it to one show per day unsupervised, and make sure it is one your child is already familiar with and enjoys. For new programs, make a point of being there to judge your child's reaction, and to help explain what's going on.