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AM F Wong, JA Sharpe, D Tweed; Adaptive Neural Mechanism For Listing's Law Revealed In Patients With Fourth Nerve Palsy . Invest. Ophthalmol. Vis. Sci. 2002;43(13):2659.
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Purpose: During fixation and saccades, human eye movements obey Listing's law, which specifies the eye's torsional angle as a function of its horizontal and vertical position. Torsion of the eye is in part controlled by the fourth nerve. This study investigates whether the brain adapts to defective torsional control after fourth nerve palsy. Methods: Thirteen patients with fourth nerve palsy (11 chronic, 2 acute), and ten normal subjects were studied using scleral search coils. With head immobile, subjects made saccades to a target that moved between straight ahead and 8 eccentric positions. At each target position, fixation was maintained for 3 seconds before the next saccade. From the eye position data, we computed a plane of best fit, called Listing's plane. Violations of Listing's law were quantified by computing the 'thickness' of this plane, defined as the standard deviation of the distances to the plane from the data points. Results: Patients with chronic fourth nerve palsy obeyed Listing's law in both the paretic and non-paretic eyes during fixation and saccades; however, Listing's planes of both eyes had abnormal orientations, being rotated temporally. In contrast, the paretic eye of patients with acute fourth nerve palsy violated Listing's law during saccades. During downward saccades, transient torsional deviations moved the paretic eye out of Listing's plane. Torsional drifts returned the eye to Listing's plane during subsequent fixation. Conclusion: During saccades, acute fourth nerve palsies violate Listing's law, whereas chronic palsies obey it, indicating that neural adaptation can restore Listing's law by adjusting the innervations to the remaining extraocular muscles, even when one eye muscle remains paretic. Although Listing's law is obeyed in chronic palsy during fixation and saccades, Listing's plane is rotated temporally, as a manifestation of excyclotorsion during downgaze and incyclotorsion during upgaze. In acute palsy, rapid torsional deviations and slow torsional drifts occur during and immediately after downward saccades. These saccadic intrusions are attributed to pulse-step mismatch, as a result of lesions in the trochlear nerve, which lead to an imbalance of phasic and tonic signals reaching the muscles.
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