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Two experiments are reported which indicate that neural and photochemical satiation processes are inadequate to explain the fading of the stabilized retinal image. When eye movements are attempted over the stimulus, fading occurs more rapidly, indicating an information processing component in the disappearance of the image.

Motion thresholds were determined in the fovea and in the horizontal peripheral meridian under conditions of practice, correction of refractive error, and feedback. While foveal determinations did not change significantly, peripheral motion thresholds were lowered for all conditions. Both practice and practice with feedback produced significantly better motion discrimination, these effects showing a longevity of several months. However, the largest improvement resulted from correction of peripheral refractive error, which also reduced individual differences. This suggests that peripheral sensitivity to movement is ordinarily limited primarily by dioptric rather than neural factors. Implications for perceptual learning and peripheral vision research are discussed.

It is shown that there may be major problems in the use of mtf's in assessing visual systems because of the coherence of coupling between the visual aid and the refraction optics of the eye. A simple computer program has been set up to help study the errors in predictions of retinal image quality made from mtf measurements. This can be used to produce complex amplitude spread functions cheaply for each element of a system and for cascaded systems. Mtf's derived from multiplying the individual mtf's of a visual aid and of the eye's refraction optics are compared with those derived from complex amplitude wavefront analysis

The relationship between perceived size and distance and oculomotor adjustments were assessed in two experiments. In both experiments, Ss were required to make scalar linear size, angular size, and distance judgments of stimuli subtending a constant retinal image size at different levels of convergence. The results of the first experiment indicate that the perceived linear size, angular size, and distance of the stimulus decreased with increased convergence, the decrease in perceived linear size being greater than that of perceived angular size. While again showing a decrease in perceived linear and angular size, the results of the second experiment also show that there was a smaller decrease in perceived distance with increased convergence when Ss continued to view the stimulus as convergence was changed than when they did not view the stimulus as convergence was changed. The implications these results have for size and distance perception are discussed.

In certain commonly used experimental situations in visual perception, the proximal stimulus of a perceived object (i.e. its retinal image) is changed and the resulting phenomenal character is foun...

There is an invariant relationship between the size of an object, its distance from the retina, and the size of the image that object forms on the retina. The relationship between size and distance is called visual angle. Visual angle, and thus retinal image size, can be varied by changing either the size of the object or its distance from the retina. The effect of such variations on size judgements and distance judgements under two different viewing conditions can be examined. The effect of variation in distance on the judged size of an object of constant physical size (and thus a changing retinal image size) has most often been tested. Under normal viewing conditions size constancy is reported (2); the size judgements approximate the tapemeasure size of the object irrespective of distance. Boring (I) thus speaks of an invariance between physical size and judged size despite changes in retinal image size. A relation can be expressed, R = f(s =s)

An examination of the implications of the theory of geometric illusions proposed by Gregory (1963) leads to the prediction that the illusion error should be determined by the retinal image projected by the figure rather than by its perceived characteristics where these differ. This prediction is tested in two experiments, using a version of the Zöllner illusion. In the first experiment, distance is used to manipulate the retinal image characteristics and, in the second, slant. In both cases, error is found to follow changes in the retinal image rather than being held relatively constant by the perceived size and shape. It is suggested that the effect of slant and distance on illusions, as reported by others, can be attributed to the change in retinal image characteristics involved in such operations.

Visual thresholds were measured during the 200 msec interval spanning small, spontaneous saccadic eye movements. Brightness thresholds measured foveally with a stabilized retinal image showed a maximum increase of 0.5 log units, starting 60 msec before the flick and lasting 75 msec afterwards. Another experiment measured the subject's ability to detect 15 min arc target motions. Ninety-three per cent of these step movements were visible normally, but none were perceived when they coincided with the eye movements. This effect spans the saccade by 100 msec, and is not the result of the brightness threshold change.

The fusion faculty is little needed or developed in most lower animals. In the carnivora and those that dwell in trees accurate judgment of distance is important, the eyes turn forward so that the fields of vision overlap and binocular fusion develops, with the necessary mechanism for securing it, including the fibers connecting the eye and various visual centers; and these in turn with the motor apparatus. Points not on the horopter impressing points of the retina not identical give the sense of depth. Differences of light and color falling on corresponding points of the two eyes lead to rivalry of the two retinal fields, and specially interpretation of impressions. Suppression of one retinal image is learned when it serves to help the individual's visual needs. Read before the Chicago Ophthalmological Society, October 19, 1925. (See p. 291)

Judgments of relative slant were elicited by a paired-comparison method from 24 S"s in each of three experiments. The stimuli were computergenerated slides representing regular dot patterns (Exp. I), regular line patterns (Exp. II), or random dot patterns (Exp. Ill) rotated about a horizontal axis. The ratio of horizontal to vertical separations (form ratio) and perspective were independently varied in Exp. I and II. Perspective clearly dominated slant judgments when in conflict with form ratio as an indicator of degree of slant. Perspective alone was varied in Exp. Ill and was found to be less effective in determining slant judgments for random dot patterns. The equivalence of perspective and optical theta as explanations of slant perception is discussed. The retinal image of a slanted surface pro- The form ratio, however, is independent of vides at least two potential sources of veridical slant information. The first is the proximal stimulus form, relative to an assumed original form. For a surface rotated about a horizontal axis, form is defined as the ratio of a vertical distance between identifiable features of the surface to a horizontal distance between identifiable features. The vertical distance is measured from the axis of rotation along a line perpendicular to that axis to a horizontal contour line or other identifiable surface feature. The horizontal distance is measured at that vertical distance along a line parallel to the axis of rotation. Then viewing distance when its components are computed in the manner specified. The second source of slant information may be expressed in several ways. Flock's (1962) optical theta is one such expression

Criteria for defining the efficiency of an apparatus for stabilizing the retinal image are formulated. A distinction is made between geometrical stabilization and stabilization of illumination. A new technique is described which employs a telescopic normal incidence system. This makes it possible to obtain geometrical compensation both for rotations and for translations of the eye. It also gives good illumination stabilization. The degree of compensation achieved may be evaluated by precise physical measurements. About 99.7 per cent of natural eye rotations in horizontal and vertical planes is compensated and the effect of translations is negligible. The apparatus is designed to permit easy interchange of normal and stabilized viewing conditions.

A recent investigation (McDermott, 1965) revealed individual cues to distance had little effect on the perception of size. The present investigation paired each distance cue with linear perspective to determine the effect on size of combined distance cues. Separate groups of Ss were run in each of six combined cue conditions at three distances. For the most part, size matches were made in terms of retinal image size. It was hypothesized that relative or familiar size may be more relevant to size perception than distance cues.

Perceptual adaptation to prismatically created tilt of the retinal image was measured after three conditions of exposure: active, passive and no movement of the observer. No difference in magnitude of effect was found between the two movement conditions. The effects of no movement were considerably less than those attained with movement, thus indicating the effectiveness of movement-produced information for adaptation. An explanation is suggested for the no movement effect in terms of a Wertheimer righting-of-the-field.

Pritchard's contact lens method of fixing a stimulus object with respect to the retina allows considerable latitude in the variation of stimuli and is the technique adopted in the present series of studies (Pritchard, 1961) .' The method employs a tight-fitting contact lens with a stalk to which is attached a collimator assembly on which different stimulus targets are mounted. The assembly consists of an aluminum tube, a magnifying lens, stimulus target, and light source, mounted by a ball and socket joint to the stalk carried on a scleral contact lens that is cight fitting enough to be responsive to small eye movements. Stimulus targets can be slipped in and out of a slit in the assembly while the lens and the aaached assembly are worn by S. In the present experiments, stimulus targets were made by photographing India ink drawings using microfile film. The targets placed in the assembly were on 5-mm. circular cutouts of the negative, were 1 mm. in maximum dimension, and were seen as if located at infinity, making possible clear viewing of the target with a relaxed eye. All the targets appeared within a central 2" visual field and were seen as illuminated white contours on a black background. Vision was monocular, with the other eye occluded. Ss lay on a bed in a darkened, sound-proofed room. Testing sessions lasted about 45 min. Approximately 3 wk. of adaptation to the contact lens and assembly preceded experimentation with the two Ss whose data are reported. The method of report required Ss to press a key in keeping with appearance of phenomena specified by E, e.g., "any fragmentation at all," "complete disappearance," etc. The response phenomena selected for quantification were tot& disappearance of a figure, and appearance and disappearance of its parts. Measurements were made in 1- or 2-min.

The influence of luminance on horizontal pursuit movements has been investigated. Almost identical effects were found for static and for dynamic visual acuity up to 80°/sec. In both cases acuity reaches a maximum around 103 cd/m2. It is known that dynamic acuity decreases with increasing angular velocity. For velocities below 80°/sec the authors do not believe that this is because of the decreasing contrast due to relative movement of the retinal image.

The modulation transfer function (MTF) of the frog’s eye was used to calculate retinal intensity distributions of several test patterns. The state of refraction was varied between ––14 D and + 8 D, and the influence of defocussing was studied. Conclusions: (1) The optical resolving power of the eye depends on the type of the stimulus. (2) The criterion that is used (maximum amplitude or half-width) results in different values of resolving power for points and lines. (3) The retinal image of the double line and the double point has a threshold characteristic. Below a certain threshold there is no modulation of the image, and the separation of the maxima of the image is zero. (4) Defocussing in the positive direction (i.e., when the refractive power of the eye and the auxiliary lens is too high) degrades the image more than an equal negative refractive error. (5) The double line is a more suitable stimulus for accommodation than the double point.

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Subjective reports of the spontaneous fluctuations of perception of a stabilized retinal image were recorded simultaneously with the subjects' EEG. Periods of fade-out of the image showed a high correlation with the occurrence of EEG alpha trains, whereas periods of visibility were correlated with low voltage fast activity. The consideration of motor reaction times and alpha blocking times or latencies to visual stimulation indicates that central processes control the time at which fluctuations of perception of a stabilized retinal image take place. These are not retinal events reflected in the higher levels at later times.

Shape constancy responses, obtained by asking Ss to match the shape of an inclined circular test object with one of a series of comparison ellipses, were obtained for adult groups of educated Senas, uneducated Senas, and educated Europeans, as well as for uneducated Manglanja children between the ages of 5 and 15 years and Manglanja adults. The uneducated African Ss tended to respond to the true shape of the objects regardless of age while the European Ss tended to respond to the retinal image shape; the educated Africans gave responses which were intermediate between those of the uneducated Africans and the educated Europeans. The differences between African Ss and European Ss on shape matching responses were believed to be due to cultural influences, but it could not be determined by the present shape matching method whether the response differences were due to differential responding to the instructions by the cultural groups or to differences in perception.

When an observer is confronted with a stimulus pattern that in some aspect does not change over time, perception of that aspect of the pattern does change. This paper documents several different types of change, all of which progress linearly with the square root of the observing time. Examples are drawn from studies of figurai after-effects, motion after-effects, vigilance, motion neutralization, visibility of the stabilized retinal image, effects of contours on visibility and fluctuations in the perceptual organization of ambiguous figures.

Two important theories of upright vision hold that the inversion of the retinal image is necessary for the perception of things as upright. According to the first, which we may call the projection theory, objects are projected back into space in the directions in which the rays of light fall upon the retina. And the crossing of these lines of direction within the eye requires that if the object is to be projected right side up the retinal image must be inverted. The second theory, which may be termed the eye-movement theory, holds that the movements of the eye and our perception of the direction of such movements are the means by which we judge of the spatial relation of objects in the visual field. Upper and lower, according to this theory, mean positions which require an upward or downward movement of the eye to bring them into clear vision. But an upward movement of the eye brings into clear vision only what lies below the fovea on the retina. So that here too the perception of objects as upright requires that their retinal images be inverted. The purpose of the experiments, of which only the preliminary ones are here reported, was to throw some light, if possible, on the correctness of this assumption. Is the inverted image a necessary condition of our seeing things in an upright position? The method of approaching the problem was to substitute an upright retinal image for the normal inverted one and watch the result. This was done by binding on the eyes a simple optical con