Changing size (looming) as a stimulus to accommodation and vergence.

Changing size (looming) produces changes in accommodation and vergence. Dynamic responses of vergence and accommodation to sinusoidal looming of a Maltese cross were recorded with an SRI dual-Purkinje-image eyetracker and optometer. The ratio of these two motor responses was compared with the response accommodative convergence/accommodation (AC/A) ratio and convergence accommodation/convergence (CA/A) ratio determined from sinusoidal variations of blur and disparity respectively. The response to changing size was found to be more similar to the AC/A ratio than the CA/C ratio. In addition, when a changing disparity or changing blur stimulus was combined with a changing size stimulus, the response phase lags of accommodation and vergence were decreased. In addition, the CA/C ratio was increased when changing size was added to changing disparity, but the AC/A ratio was unaltered when changing size was added to changing blur. These results indicate that changing size is stimulating accommodation directly and vergence secondarily through an AC/A crosslink.