Eye movements

Our eyes move about three or four times a second, and these eye movements generally produce substantial effects on the retinal image. In spite of that, we normally perceive the environment as stable and unmoving. There are several ways in which our visual systems could achieve this stability. One possibility is that the visual system monitors actual changes in the extra-ocular muscles controlling eye movements, and then uses that information to interpret changes in the retinal image. However, it would be important for information about eye-muscle movements to be used before the retinal image changed (or at the same time), because otherwise the altered retinal image might be misinterpreted.

The second possibility was favoured by Helmholtz (1866). He proposed an outflow theory, in which image movement is interpreted by using information about intended movement sent to the eye muscles. The fact that the visual world appears to move when the side of the eyeball is pressed supports this theory. There is movement within the retinal image unaccompanied by commands to the eye muscles, and so it is perceived as genuine. Sekuler and Blake (1994, p. 267) spelled out some of the details: "Perceived direction [of an object] develops from a comparison between two quantities, the command signals to the extraocular [outside the eye] muscles and the accompanying retinal image motion. To derive perceived direction, simply subtract the retinal image motion from the command signal." As predicted, when the eyeball is pressed in one direction, the visual environment seems to move in the opposite direction.

One way of testing outflow theory is to study the effects of immobilising the eyes by means of a paralysing drug. According to the theory, the visual world should appear to move in the opposite direction when participants given such a drug try unsuccessfully to produce eye movements. This prediction has been supported (e.g., Matin et al., 1982). However, Stevens et al. (1976) obtained a slightly different effect. Their participant (John Stevens) reported that attempted eye movements following muscle paralysis produced a kind of relocation of the visual world but without movement.

Evidence for Helmholtz's theory was reported by Duhamel, Colby, and Goldberg (1992). They found that the parietal cortex in the monkey brain is of major importance to an understanding of how the visual system handles eye movements. Duhamel et al. (1992, p. 91) concluded as follows: "At the time a saccade [rapid, jerky eye movement] is planned, the parietal representation of the visual world undergoes a shift analogous to the shift of the image on the retina." Thus, visual processing in the parietal cortex anticipates the next eye movement in the period between its planning and execution.

In spite of the successes of outflow theory, it cannot be the whole story. As Tresilian (1994b, p. 336) remarked, outflow theory "predicts that if the eyes are stationary in the head, as the head rotates, the resulting image motion will be interpreted as motion of the environment, yet everyone knows that this does not happen." What probably happens is that we do not rely exclusively on information about intended eye movements in order to perceive a stable environment. Movement of the entire retinal image is probably attributed to movement of the head or eye, whereas movement of part of the retinal image is interpreted as movement of an external object. In addition, information about head movements is used in the same way as eye-movement information to permit us to see the environment as stable. The parietal lobe seems to be the site at which information about eye movements and head movements is integrated (see Andersen et al., 1997, for a review).

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