Perceptual andMotor Skills, 1990, 71, 1259-1264. O Perceptual and Motor Skills 1990

VERGENCE MOVEMENTS I N COMITANT STRABISMUS ' CARL0 CIPOLLI

ROBERTO BOLZANI

RUGGERO CORAZZA

Institute of Human Physiology Uniuenity of Modena

Institute of Ophthalmology University of Modena

Institute of Human Physiology Uniuersify of Modena

GUALTIERO GUALDI

MANFRED1 ORCIUOLO

EMILIO CAMPOS

Institute of Ophthalmology Uniuersify of Modena

Biette Foundation Roma

Institute of Ophthalmology Uniuersit~of Modena

Summary.-Vergence movements induced by base-out prisms were recorded, with an infrared eye tracker, for 6 patients with comitant esotropia (i.e., convergent strabismus) and anomalous retinal correspondence and for 6 normal subjects. Vergence movements of strabismic subjects were much slower and showed characteristics different from those of controls. It may be argued that vergence movements are induced by disparity and represent the motor fusion component left over in strabismus. In fact, in this study accommodative as well as proximal vergence had been ruled out. Therefore, the movements recorded can be considered the objective representation of the well-known phenomenon of prism compensation or adaptation, found in many strabismic patients.

Cornitant strabismus, i.e., the pathological condition in which the angle of deviation of one eye does not vary significantly in different gaze positions, deserves the attention of perception researchers. I n fact, some degree of binocular cooperation (even if anomalous) is maintained in the space perception of patients with small-angle strabismus, i.e., with a deviation not e x c e e l n g 7 O (Campos, 1986). This type of binocularity is sustained by a rearrangement of the spatial value of the retinal elements of the deviated eye, called anomalous retinal correspondence. This anomalous correspondence allows a binocular single perception, in spite of the angle of strabismus. In normal subjects, binocular single vision is sustained, from the sensorial point of view, by normal retinal correspondence and, from the sensorimotor point of view, by normal fusional movements. The latter are effective in maintaining binocular single perception when images are projected onto disparate retinal elements. I n strabismus, anomalous retinal correspondence is the sensorial aspect of binocular single vision. The sensorimotor component involves the effectiveness of vergence movements, the interpretation of w h c h in strabismus has been a matter of debate since Bielschowsky's description (1900). The aim of vergence movements is to maintain an angle of deviation in which an anomalous binocular cooperation is possible (Bagolini, 1976; Campos & Zanasi, 1978). There is consistent evidence from studies based on the observation of ' a s research was supported by an M.P.I. grant awarded to C. Cipolli. Requests for re rints should be sent to C. Cipolli, Institute of Human Physiology, University of Modena, Via t a m p i 287, 41100 Modena, Italy.

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the compensation of prismatic application that disjunctive movements in comitant strabismus are slow (they cannot be observed by the naked eye, but they have been inferred from the variation occurring after a while in the angle of deviation) and imprecise [the compensation of the prismatic application may be incomplete; see Bagolini (1976); Bagolini and Campos (1982)l. However, to some extent they are similar to those induced by disparity in normal observers: disjunctive movements in all directions are common in strabismic persons as are normal disparity vergence movements (Campos & Zanasi, 1978). Moreover, they can be stimulated not only by the peripheral field of vision, but also by the central one (Campos, Bolzani, & Cipolli, 1988). To gain further insight on the pattern of vergence movements in strabismus, the infrared recording technique seems to be a useful tool, even if early attempts to use i t did not show disparity-induced vergence movements in strabismus (Kenyon, Ciuffreda, & Stark, 1980; Stark, Ciuffreda, & Kenyon, 1984). Assuming that the failure was due to too short a recording time (a few seconds), we decided to record vergence movements for a longer interval (3 min.) and to compare the pattern with that resulting for t h s lund of eye movement in normal observers. To enhance the reliability of the movement recording, we eliminated all accommodative demands.

Subjects Six patients (ages 18 to 30 yr.) with cornitant esotropia and anomalous retinal correspondence, tested with Bagolini's striated glasses, took part in this study. They were selected according to three criteria: (a) the range of the angle of deviation had to be 4 to 10 prism diopters, both to facilitate the analysis of the compensation of prisms and to avoid the artificial induction of suppression and/or diplopia by anteposing high-power prisms. (b) The exact prismatic correction of the angle of deviation had to be compensated in less than 2 min., to allow recording of possible vergence movements. (c) Deep amblyopia was absent. Pertinent data on the six strabismic subjects can be found in Table 1. I n addition, six normal subjects (ages 22 to 35 yr.), i.e., observers without optical correction, participated as controls. Having been enrolled, respectively, for the masking task of cooperating in the calibration of a recently developed eye-tracker and for periodic eye examinations, both controls and patients were unaware of the experiment's rationale. Apparatus Eye movements were recorded through an eye-tracker with infrared emitting diodes and two photodetectors for each eye (designed by Collins at the Srnith-Kettlewell Institute of Visual Sciences, San Francisco), mounted

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TABLE 1

CHARACTERISTICS OF SIX STRABISMIC PATIENTS Patient

Age (yc)

Visual Acuity Right Left Eye Eye

Onset of Strabismus

Angle of Deviation (in Prism Diopters)

1 2

18 20

616 616

616 6/12

< 6 mo. of age