Each of 12 subjects set a binocularly viewed target to apparent eye level; the target was projected on the rear wall of an open box, the floor of which was horizontal or pitched up and down at angles of 7.5 degrees and 15 degrees. Settings of the target were systematically biased by 60% of the pitch angle when the interior of the box was illuminated, but by only 5% when the interior of the box was darkened. Within-subjects variability of the settings was less under illuminated viewing conditions than in the dark, but was independent of box pitch angle. In a second experiment, 11 subjects were tested with an illuminated pitched box, yielding biases of 53% and 49% for binocular and monocular viewing conditions, respectively. The results are discussed in terms of individual and interactive effects of optical, gravitational, and extraretinal eye-position information in determining judgements of eye level.

The apparent suppression of the target in metacontrast is often accompanied by "split" apparent motion. In Experiments 1, 4, 5, and 6 "neighboring stimuli" (similar to and flanking the mask stimuli) were added to the display, and subjects rated both metacontrast and split motion. Under some conditions, both split motion and metacontrast were completely eliminated (Experiment 1), supporting the assumption that apparent motion is necessary for metacontrast. However, under other conditions, neighboring stimuli caused a much stronger depression of metacontrast than of split motion (Experiments 4 and 5), sometimes even enhancing the latter (Experiment 6), supporting the assumption that the mechanisms underlying the two phenomena are essentially independent. Further, peripheral presentation and close spacing of target and mask with no neighboring stimuli (Experiments 2 and 3) gave strong metacontrast while completely eliminating split motion, showing clearly that apparent motion is not necessary for metacontrast. Results are interpreted in terms of a "fusion" process underlying metacontrast and a "direction-sensitive unit" underlying apparent motion. Interactions between these two processes that might account for the common co-occurrence of motion and metacontrast are proposed.

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In normal illumination, retrograde motion of the background (the Filehne illusion) can be seen during ocular pursuit, in contrast to stability seen during a saccade. In the present experiment, two stimuli were presented sequentially: (I) at disparate physical locations such that the pursuit movement of the eye caused them to excite the same retinal location, (II) at the same physical location, with p,ursuit movement causing disparate retinal excitations, and (III) with stationary fixation but with disparate physical locations such that the retinal excitation was identical to that of Condition II. Optimal movement was never reported for Condition I but was reported with essentially equal frequency in Conditions II and III. These results indicate a failure of compensation for pursuit movement, as does the F ilehne illusion. The nature of the pursuit extraretinal signal was discussed, and it was argued that a distinctly different extraretinal signal is necessary for perceived stability during the saccade.Interest in the subject of motion perception has recently been concentrated on the coding of image motion with respect to the retina; physiological mechanisms which seem capable of this coding have been discovered in the mammalian visual system (Hubel & Wiesel, 1962;Barlow & Hill, 1963). It is well known, however, that motion can be experienced without such image motion. If, for example, a smoothly moving luminous object is accurately pursued by the eye in a dark room, the motion of this object will be seen even though image motion with respect to the retina may be virtually eliminated. l The generally accepted explanation of this perceived motion involves some process which "takes into account" the motion of the eye itself.The existence of a "taking-into-account" process was proposed over 100 years ago by Helmholtz (1866), and its hypothetical functions have recently been clearly stated by Gregory (1958Gregory ( ,1966. Gregory postulates two distinct systems for the perception of motion. One system is responsible for detecting image movement with respect to the retina and is presumably mediated by low-level detectors such as those found by Barlow and Hubel and Wiesel; Gregory calls it the "image/retina system." The other system is responsible for detecting motion of an object which is followed by the eye. It works by taking into account movements of the eye; Gregory calls it the "eye/head system." The nature of this taking-in to-account process is examined in this paper.Traditionally, several other motion phenomena associated with eye movements have been explained by means of some form of taking eye movements into account. In addition to (I) the perceived motion of a pursued object, these are (2) the perceived stability of a scene even though voluntary saccadic eye motions cause its image to move over the retina, (3) the apparent motion of a stationary scene when the eye is moved "passively," Le., by finger pressure, (4) the apparent motion of a stationary scene when a person with a paralyzed eye attempts, but fa...

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