Adaptive plasticity of head movement propensity

Individual humans exhibit differing propensity to move the head in association with saccadic shifts in gaze. We assessed whether this tendency can be modified in normal subjects by either reducing neck mobility with a cervical collar or restricting the field of view using aperture spectacles. We quantified head movement propensity in terms of the range of orbital eccentricity within which the eyes are customarily maintained (customary ocular motor range), and the range of final eye-in-head eccentricity for which a planned saccade is likely to be executed without a concomitant head movement (eye-only range). Three subjects wore rigid collars during waking hours for periods of up to 9 days. We measured customary and eye-only ranges with the collar removed, at various times during the adaptation and recovery periods. Collar adaptation reduced head movement propensity in all three subjects, increasing the average customary ocular range from 27.6 +/- 8.9 degrees (mean +/- SD) to 66.1 +/- 4.5 degrees and the eye-only range from 24.6 +/- 17.0 degrees to 67.6 +/- 7.4 degrees. In two subjects the modifications persisted for weeks following final collar removal. In parallel with the reduction in head movement propensity, all subjects improved in their ability to maintain eccentric gaze, suggesting that neck restriction led to effects at the level of the brainstem. Three subjects were adapted to spectacles, masked to restrict the field of view to approximately 20 degrees. The aperture spectacles were worn for periods of up to 9 days. When tested without the apertures, one subject exhibited a definite increase in head movement propensity; in the other two, the data were equivocal, indicating either a small increase in head movement propensity or no effect. Averaged across subjects, customary ocular motor range decreased from 35.1 +/- 12.8 degrees to 25.4 +/- 13.4 degrees and eye-only range decreased from 35.1 +/- 7.5 degrees to 23.0 +/- 4.0 degrees. The marked difference in the magnitudes of collar- and spectacle-induced changes suggests that the responses to the two restrictive appliances are mediated by different mechanisms. Collar adaptation may involve parametric modulation of circuits mediating reflex recruitment of head movements, while aperture adaptation may primarily reflect substitution of an alternative mode of head control triggered by the presence of the restricted field of view, with only minor parametric modulation of the underlying head recruitment circuit. The enduring effects of restricting neck mobility upon head movement tendencies may relate to the common clinical association between neck injury and persistent dysequilibrium.