Multi-stage process, from the elementary treatment of luminous points by the retina to the interpretation of images in higher concepts by consciousness. The complete answer to your question is quite lengthy if one has to consider the effects at each of these levels. Choose two:
Is there physical and neural fatigue caused by prolonged processing of repetitive signals such as a screen? At first no more than for any other activity. Whatever you look at, there are always images to process. Visual fatigue and neurons of vision comes from keeping your eyes open for too long, without a period of sleep, more than what is being watched. Of course the criteria of brightness, wavelength, and frequency of image changes can occur but are relatively incidental to the sleep time to be respected.
Is there an influence of the content of the images, a programming of the mind by the transferred concepts? Undoubtedly. We are our mental representations. Of course, all do not move around freely in consciousness. Existing concepts combine to reject the inconsistent. However, if the transferred content is presented skilfully, it fits seamlessly into the conceptual assembly that shapes your personality, in the same way that a smart talker within a group will greatly influence his behavior.
However, the most important and least known effect of repetitive mental activity is elsewhere. It is neglected because there is still no official theory of the general functioning of the brain. To understand it, you must give credence to the following axiom: the more a mental task is organized, the less it can be exercised independently of the others. For example, the neurological regulation of visceral functions is very low in the organization hierarchy. It is running almost automatically even with a brain in coma. While a highly organized task such as solving a thorny problem mobilizes all conscious attention, other mental tasks are put on stand-by.
But we must not believe that unused neural networks simply remain in the shadows, ready to serve the identical to the next solicitation. Unusual connections are fading away. This is the way the brain manages to reconfigure itself. A conceptual element proves to be wrong or of little use? The links are reformed. It is forgotten. The more the mental representations are sophisticated, elevated in the conceptual hierarchy, the more they disintegrate quickly in case of underemployment. You have certainly noticed it: for example, you assimilate complicated knowledge by having worked on it for an exam. But then it is rarely used. A few months or years later, it became very foggy in your memory. You would have a hard time explaining it as clearly as when you were learning it.
Consequence: The prolonged exercise of a mental activity, such as watching television or working on a computer screen, is at the expense of other activities of the same level. These tend to lose acuity, responsiveness, and of course complementary developments.
This does not make television or computer work harmful in itself. Question of proportion in all activities, and presence of attractive alternatives: it is the lack of rivals that makes an occupation addictive.
What is the neurological reason? It requires a small revision of our free will. High-level tasks are acting representations, that is, they call themselves. They are our tastes, our desires, our decisions. When one is used very frequently, its celebrity makes it come back to the stage, especially when it recruits a lot of hopes, rewards, identity reinforcement. Glued to a video game or a TV series? Nothing suprising. You are, temporarily, the game or the series. At least its mental representation. This popularity changes you. It is only by constructing even higher hierarchical representations, which are the image of the self in its environment, that one can « become aware of » it.