During fixation the eye executes a characteristic pattern of movements consisting of drift, tremor, and microsaccades (Alpern, 1969). The function of the movements in the visual process is not fully understood and one difficulty in making progress in this area is the problem of recording fixation eye movements with sufficient sensitivity. The apparatus described in this note represents a development of existing techniques and allows microsaccades (i.e. rapid jump-like movements) to be recorded with considerable accuracy and the other components to be estimated. This locates the iris-sclera boundary by means of the differential reflectivity of the two regions. The essential components of a piece of equipment to achieve this are a light source to illuminate a region covering the iris-sclera boundary and a detector to measure the amount of light reflected. As the eye moves across the illuminated region the proportion covered by the relatively darker iris changes and correspondingly the amount of light reflected alters. A number of variations on this basic technique have been reported (Smith and Warter, 1960; Rashbass, 1960; Wheeless, Boynton and Cohen, 1960), which all claim sensitivities of around 10 min of arc visual angle. This just fails to be adequate to record microsaccades, since these have a median amplitude of 4 min of arc (Ditchburn and Foley-Fisher, 1967) and a range of 2-30 min arc. Consequently almost all useful recording of fixation eye movements has been carried out with contact lens techniques (Alpern, 1969). The present apparatus uses the photoelectric principle but introduces a development which achieves greater sensitivity and also greater convenience. The basic component is a piece of light guide known as a “random Y-guide” (Rank Taylor Hobson Fibre Optics). In this, a fibre optic “trunk” bundle bifurcates into two “branch” bundles in such a way that fibres from any part of the cross-section of the trunk are equally likely to emerge into either of the branches. The Y-guide can then be used as a reflectance measuring device by bringing the end of the trunk fibre into proximity with the surface whose reflectance is to be measured. Illumination is provided through one of the arms and the intensity of reflected light emerging from the other is measured with a detector. The Y-guide used has a trunk with circular cross-section of diameter I mm, and contains about 300 fibres. The chief advantage conferred by the Y-guide system is that light source and detector are both effectively placed closer to the eye resulting in a much smaller illuminated area sensed by the detector. The complete apparatus set up to measure horizontal movements is shown in Fig. I. The subject wears an optician’s spectacle trial frame (Curry and Paxton Croydonian). This is modified by an outward projection attached to the lens holder to support the trunk of the light guide. The trunk is mounted on a tube which runs in a closely fitting slideway on this projection, allowing movement towards or away from the subject. This movement is controlled manually by means of a rack and pinion drive (A in Fig. I). The trial frame allows movement of the lens holder in a horizontal direction (control B) and a limited amount of movement in a vertical direction is possible by means of control C which rotates the lens The method is based on the “photoelectric” technique.
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