Unilateral ablation of the frontal eye field of the rat affects the beating field of ocular nystagmus

Abstract

Spontaneous saccadic orientation and compensatory eye movements in response to optokinetic and vestibular velocity steps were studied in head-restrained, pigmented rats before and 1–2 weeks after unilateral ablation of the frontal eye field (FEF). One group of rats (n=5) received a deep lesion and another group of rats (n=4) received a superficial lesion of the left FEF. Postoperative response parameters such as the duration of slow buildup of eye velocity, the steady state velocity gain, the duration of optokinetic afternystagmus and of per- and postrotatory vestibular nystagmus were similar in the two groups of rats and did not differ from preoperative values measured in the same individuals. Superimposed upon these velocity components of nystagmus was a transient orienting response that expressed itself by a shift of the beating field of nystagmus in quick phase direction (gaze shift). The amplitudes of this gaze shift in quick phase direction were asymmetric in rats with a deep FEF lesion. Gaze shift amplitudes toward the side of the lesion were significantly enhanced and gaze shift amplitudes toward the intact side were significantly reduced. Similar asymmetries were observed in the distribution of spontaneous orienting movements of these rats in the light. Spontaneous saccadic eye movements of the same animals in darkness, however, were symmetric in amplitude to either side. These deficits suggest a partial sensory hemineglect after a deep unilateral lesion of the FEF and an involvement of this structure in the selective attention for targets in visual space. Thus the FEF orients the gaze at rest by means of saccades toward points of interest and during simulated circular locomotion by means of a shift of the beating field of nystagmus toward the visual sector that will be approached next.