The accommodative response (AR) of the eye while reading a text with BoW or WoB contrast polarity was obtained objectively with an adaptive optics system that corrected all aberrations but subject's own. 
adaptive optics system
Two different letter sizes (visual acuity conditions), shown on a microdisplay, were tested.  Contrast polarity does not seem to influence the accommodative response when reading text from an electronic microdisplay.
microdisplay