Stabilization of gaze: a relationship between ciliary muscle contraction and trapezius muscle activity.

In an experimental study four levels of oculomotor load were induced binocularly. Trapezius muscle activity was measured with bipolar surface electromyography and normalized to a submaximal contraction. Twenty-eight subjects with a mean age of 29 (range 19-42, std 8) viewed a high-contrast fixation target for four 5-min periods through: (i) -3.5 dioptre (D) lenses; (ii) 0 D lenses; (iii) individually adjusted prism D lenses (1-2 D base out); and (iv) +3.5 D lenses. The target was placed close to the individual's age-appropriate near point of accommodation in conditions (i-iii) and at 3m in condition (iv). Each subject's ability to compensate for the added blur was extracted via infrared photorefraction measurements. A bitwise linear regression model was fitted on group level with eye-lens refraction on the x-axis and normalized trapezius muscle EMG (%RVE) on the y-axis. The model had a constant level of trapezius muscle activity--where subjects had not compensated for the incurred defocus by a change in eye-lens accommodation--and a slope, where the subjects had compensated. The slope coefficient was significantly positive in the -D (i) and the +D blur conditions (iv). During no blur (ii) and prism blur (iii) there were no signs of relationships. Nor was there any sign of relationship between the convergence response and trapezius muscle EMG in any of the experimental conditions. The results appear directly attributable to an engagement of the eye-lens accommodative system and most likely reflect sensorimotor processing along its reflex arc for the purpose of achieving stabilization of gaze.