For the purposes of this communication it is necessary to define binocular fixation. So far as I know, this function has never been accurately defined. In fact, defining it is not generally recognized as constituting any problem. The following definitions seem adequate for the purposes at hand and will, I believe, prove of general usefulness. Binocular fixation of two punctate retinal images is the act of maintaining these images on corresponding retinal points by means of motor response to these images. Corresponding retinal points I have defined elsewhere.1 Binocular fixation of two contours, one on each retina, is the act of maintaining at least two geometric points, one in each contour, on corresponding retinal points by means of motor response to these contours. Punctate retinal images or contours may by chance fall on corresponding retinal points. Hence to prove the existence of the required motor response to these images
VERHOEFF FH. HYPERPHORIA TESTS BASED ON A NEW PRINCIPLE. Arch Ophthalmol. 1939;22(5):743–760. doi:10.1001/archopht.1939.00860110029003
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