This study, by Soeda, DiZio & Lackner (2002), found that - without including the visual effects of gazing out a spacecraft's window - Coriolis forces rising from rotation caused subjects to soon lose some control while doing certain tasks, including just standing and moving about.
The results of one of their tests found that
The experimenter counted the number of times the subject had to use the railings in order to avoid losing balance. Each contact involved a momentary push against the rail with the back of the hand. Subjects had to use the rails
more frequently during rotation than pre- or post-rotation, and more frequently when their eyes were closed than open. Pre-rotation there were 0.25 touches per trial in the dark and none with vision; per-rotation there were 5.04 touches per trial in the dark and 1.16 when the subject’s eyes were open; post-rotation there were 0.125 touches per trial in both visual conditions.
In all the tests, closing eyes did not help subjects retain control; in fact, it lessened their ability to control themselves. However, the latter two of the authors had found (in previous experiments) that adaptation was possible:
However, when permitted to make repeated movements, subjects quickly become more accurate and are able to perform near normally within 10–15 reaches
(Lackner and DiZio 1994). Such adaptation means that the subjects’ nervous systems have planned anticipatory muscle innervations which cancel the consequences of the Coriolis forces. Aftereffects are present post-rotation with
movement paths being mirror image to the initial ones during rotation.
A NASA page on the researchers' other work noted that
DiZio and Lackner have found that people can adapt to rotational speeds as fast as a carnival-ride-like 25 rpm. That's about as fast as people turn their bodies during day-to-day life.
This would seem to be a decent estimate of an upper limit if only Coriolis forces are taken into account.
In this study (and related ones), it seems like simply the rotation - not the visual perception - is the cause of problems. That said, microgravity environments on their own can cause problems (see Oman et al.). This also states
Windows continue to be a matter of disagreement. In addition to concerns about structural integrity, environmental control (including radiation shielding), and cost, rotation introduces the issue of dizziness. Payne [20] and others have suggested that, "to avoid disorientation," windows should not be provided in rotating environments. But depriving the inhabitants of an outside view would do nothing to alleviate the vestibular effects of rotation. On the contrary, it may promote the mismatch between visual and vestibular perception that leads to motion sickness [15]. Windows might provide an obvious, natural aid to orientation, in addition to the abstract, formalist cues discussed previously.
