An infrared optometer was used to investigate the relation between cognitive induced shifts in tonic accommodation (TA) and ciliary muscle innervation. Twenty emmetropic men volunteers took part in a double-masked protocol which involved high (a reverse counting task) and low levels of mental load and blocking of sympathetic innervation of the ciliary muscle with the nonselective β receptor antagonist timolol maleate (0.5%). The high level of cognitive demand induced shifts in TA of up to +1 D. There was no clear reason why large differences in intersubject susceptibility to these shifts occurred. The effect of β-antagonism on these shifts was insignificant for subjects having initial TA values <1.2 D. However, sympathetic antagonism induced significant increases in cognitive shifts for the remaining subjects. The inference is that cognitive induced shifts in TA are predominantly parasympathetic mediated although a sympathetic attenuation may occur at higher levels of TA.