I tend to agree with the observation, but I'm hesitant to accept the conclusion that appears to be that there is something inherent in silent movies that requires more attention than sound movies.When there's dialogue, especially a conversation between two characters, we often stop watching the actors and just listen to them. Sometimes, we look at their surroundings, especially if it's an open air scene filmed in a recognizable setting (try watching "Vertigo" or "Breakfast at Tiffany's" without being distracted occasionally by the setting), but often we look away from the screen completely, because we know we won't miss anything.
On the other hand, with a silent looking away even briefly can cause us to miss an important gesture or look, so we avoid allowing ourselves to be distracted.
We (including me) tend to fool ourselves into believing that just because there is audible dialogue, listening to it is sufficient..."Oh, I know what happened. I can hear it and that is telling me." No it's not. Mise-en-scene and gestures (aka acting) is important in sound films. That they're not, or that they're somehow diminished vis-a-vis silent film appears to be the conclusion when, instead, we should fault our own viewing habits. Given the rapture that many of us have expressed about, oh, how Ford characters enter a frame or how Alton can show inner turmoil by shutting off three-quarters of the light a scene might typically have, tells me that it's the quality that we attribute to a movie -- not the absence or use of sound -- that is determinative. Then there's...
The revolution in home viewing has probably contributed mightily to this phenomenon. If I walk out of the room during a Silent, I have nothing. If I walk out of the room during a Talkie, I may think I'm missing nothing, but I'm actually missing just as much. At a theater, I would never walk out (unless I have foolishly consumed to much liquid and the director has not adhered to Hitchcock's dictum regarding the endurance of the human bladder). Yes, it's that damnable "Pause" button.
The fault, dear Brutus, is not in our stars, but in ourselves.
Query: Does everyone behaviorally treat sub-titled movies as if they were Silent (oh, there's sound, but it's meaningless to me) or as the Sound movies that they are?