Involuntary eye movements during fixation

Eye movements form one of the basic physical data which must be considered in a theory of the process by which visual information is transmitted to the brain. The behaviour of the eyes during fixation, i.e. when the subject is attempting to maintain his gaze on a stationary point, has a special interest both as an example of the dynamic equilibrium of a physiological system and in relation to theories of the retinal basis of vision. Some of the evidence in favour of the Polychromatic Theory (Hartridge, 1947), for example, involves the assumption that the visual image during fixation on a small target is effectively stationary on the retina. On the other hand, certain 'dynamic' theories of vision (Marshall & Talbot, 1942; Jones & Higgins, 1947) postulate a continuous scanning of the visual image by the retina. The results of the three most recent investigations give the following results: Ratliff & Riggs (1950) find (1) an irregular movement of high frequency (30-70 c/s) and small extent (median value about 20" arc); (2) rapid fficks each of a few minutes of arc lasting about 0x02 sec and occurring at irregular intervals of the order of 1 sec; (3) slow irregular movements. Similar types of movement had previously been found by Adler & Fliegelman (1934). Lord & Wright (1948) and Lord (1951) find movements of type (2). They do not report movements of types (1) and (3). Hartridge & Thomson (1948) do not find any of the above types of movement and believe the eye is effectively stationary during short periods of fixation. Further details are given in Table 1 (p. 13). Apart from the differences between these results, which refer to the behaviour of the eyeball as a whole, the problem of deducing the movement of the image relative to the retina during fixation is further complicated if movements of the eye are accompanied by movements of the crystalline lens within the eyeball. Such movements, which have been suggested by Park & Park (1940), would shift the position of the visual axis relative to the eyeball. If there are variations of this type,