Aspects of tonic accommodation in emmetropia and late-onset myopia.

An objective infrared optometer was used to investigate the influence of cognitive demand on tonic accommodation (TA) for two groups of young emmetropes (N = 15) and late-onset myopes (N = 15). TA was measured under a passive condition and while subjects carried out a reverse counting task. The TA of myopes under the passive condition (mean = 0.81 D, SD = 0.46 D) was shown to have lower values than the TA of emmetropes (mean = 1.14 D, SD = 0.46 D). For the myopic group the counting task induced a positive shift in TA (mean = +0.35 D, SD = 0.31 D) that was significantly higher than that for the emmetropic group (mean = +0.07 D, SD = 0.27 D). The implications for theories of refractive error development are discussed.