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Saccadic Eye Movement Conjugation in Children
F. FIORAVANTI,*t P. INCHINGOLO,* S. PENSIERO,$ M. SPANIO§ Received 6 January 1995; in revisedform 10 May 1995
In recent years the conjugacyof the saccadic eye movements has been studied extensively in adult humans, while little investigation has been carried out in children. We studied the characteristics of binocular saccades in school-age children, finding significant differences with adults, particularly in their dynamics and binocular coordination. The largest deviations were found in the youngest children. An incompleteoptimization of the saccadic waveform and a poor disconjugate compensation of the mechanical asymmetries of the plants are hypotesized to explain the results.
Saccades Children Main sequence Post-saccadic drift
INTRODUCTION Saccadic eye movements are generated by a combination of phasic and tonic activity in the motoneurons. It is generally assumed that the pulse components are produced by a burst ofneuronal firing with the goal of compensating the viscous forces produced by the muscles, while the step components are tonic changes in the discharge rate required to counteract the elastic forces produced by the orbital tissues and the muscles that hold the eye in the final position (Robinson, 1975). In normal adult humans, the matching of the pulse and step components is responsible for theaccuracy of the dynamics of the movement. In case of mismatching, a post-saccadic drift motion of the eye towards the final position is produced. If the pulse is too small, the eye will drift onwards; if too big, the drift will be in a backward direction. Even though the pulse-step of innervation is normally assumed to be sent equally to both eyes, according to Hering's law (Hering, 1868), thepresence of asymmetrical post-saccadic drift in the two eyes has frequently been reported by many researchers (Collewijn, Erkelens & Steinman, 1988; Lemij & Collewijn, 1991a,b, 1992; Kapoula, Optican & Robinson, 1989). Moreover, the disconjugate drift compensation behaviour of the system has been tested in some studies, particularly in presence of strong asymmetrical weakening of the muscles(Inchingolo, Optican, Fitzgibbon & Goldberg, 1991), but
also with pure visual stimulation (Kapoula, Hain, Zee & Robinson, 1994; Kapoula, Optican & Robinson, 1990; Kapoula, Eggert & Bucci, 1994). Both the origin of the disconjugacy of the post-saccadic drift (different mechanical properties of the two plants, different pulse-step matching in the two eyes etc.) and the disconjugate adaptive properties ofthe system have mainly been studied in adult humans and primates, while little investigation has been done in children until now (Accardo, Pensiero, Da Pozzo & Perissutti, 1992; Aslin & Ciuffreda, 1983; Dannermiller, Banks, Stephens & Hartmann, 1983; Hainline, Turkel, Abramov, Lemerise & Herris, 1984). In the present study we have investigated the main characteristics of binocular saccades inschool-age children (5-13 yr), in order to point out the enhancement, if any, of the saccadic binocular co-ordination and conjugacy during growth.
Binocular eye movements were recorded in 12 children aged between 5 and 13 yr (mean age, 9.25 yr) and four adults (mean age, 31.25 yr), as reported in Table 1. All subjects were normal, and none of them had a history of ocular oroculomotor pathology. No refractive anomalies were present and visual acuity was 10/10 in each eye for all the subjects. Both children and adult subjects were naive as to the purpose of the experiment. All subjects had good binocular vision, as ascertained by the Bagolini *Dipartimento di Elettrotecnia,Elettronicaed Informatica,University striated glasses test for binocularity and random-dot test of...