Search

Summary.-Discrimination performance of two classes of patterns (histoforms and polygons) was determined as a function of the size of the forms at three levels of complexity (4, 6, and 8 columns or angles). The visual angle subtense of the stimulus forms was varied over a range of sizes from near threshold (1.88 min.) to well above it (16.56 min.). The important finding is that below a visual angle of 8 min. discrimination of the form is essentially identical in terms of both accuracy and speed of identification. At angular sizes above 8 rnin. consistent differences in the rate of processing the two classes of forms emerged. The results were interpreted as indicating that identification performance depends on "emergent features" of the forms as they increase in size. The concept of "emergence" has often been used to account for various perceptual phenomena. Emergence is said to occur when changes in perceptual performance appear as constituent parts are added together to form wholes. The process may be due to the configuration of the whole but it is not easily assigned to its component parts. The present study was undertaken to determine if accuracy and speed of identification of complex visual forms depend upon some "emergent properties," i.e., perception of the forms as wholes. In order to provide changes in forms that would permit the emergence of different properties, histoforms and polygons were increased from sizes so small that identification performance was no better than chance (threshold) to relatively large sizes. One of the prime considerations of the study was how small the forms should be in order to assure that they were near or below threshold. The literature concerning visual acuity functions provides little information relating to the visual angle subtense required of complex forms for accurate identification. Most of the research conducted on these acuity functions has employed Landolt sings and parallel bar gratings. Previous studies of visual acuity indicate that the critical detail of a form which differentiates it from other forms must have a visual subtense of at least 1 min. under ideal conditions. This is a general statement, however, and its application is made difficult by the lack of technique for quantifying the critical detail of a form which can be used to differentiate it from a class of other forms. Clinical measures of visual acuity, e.g., the Snellen acuity chart, indicate that with normal vision an individual is just able to recognize letters of the alphabet which subtend an over-all visual angle of 5 min. However, letters of the alphabet are over-learned, and for the most part it is not difficult to dis'This research was su ported in part by the Army Research Office under Army THEMIS Contract DA H~19-&-~-0009, "Studies of Performance Assessment and Enhancement." The authors wish to thank G. Abel, J. D. Repko, and K. E. Rothrock for technical assistance.

The hypothesis was tested that zebra danios (Brachydanio rerio) react to an approaching predator when the rate of change of the angle subtended by the predator at the prey's eye (dα/dt) exceeds some threshold level. Danios were presented with real predators (largemouth bass, Micropterus salmoides) and artificial predators of two types: a model predator, whose size and velocity could be varied, and a film of an approaching object. The reactive distance for flight (RD) could be predicted from the relationship: RD=[(VS/k)−(S2/4)]1/2 where V is the predator approach velocity (cm/s), S is the predator front diameter (cm), and k is the threshold rate of change of visual angle (radians/s). The best overall estimate of k was 0·43 rad/s. The reactive field was shown to be circular, demonstrating that the mechanism is not solely a binocular one. Escape velocity, though higher in response to real than to artificial predators, was not correlated with reactive distance.

Two experiments tested the hypothesis that the paradoxical relative distance judgment associated with the size-distance paradox is due to the visual system’s assuming equal linear size and perceiving a smaller angular size for the closer stimulus equal in visual angle. In Experiment I, two different sized coins were presented successively, and 16 Ss were asked to give ordinal judgments of apparent distance and apparent size. When the two coins depicted the same figures, the closer stimulus was judged to be farther and smaller, more frequently, than when two coins depicted different figures. In Experiment II, 48 Ss were asked to give ratio judgments of apparent distance, apparent linear size, and apparent angular size for two stimuli which were presented successively. When the stimuli were of equal shape, the mean ratios of the far stimulus to the near stimulus were smaller for the apparent distance but larger for the apparent linear size and angular size than when the stimuli were of different shape. The obtained distance judgments were consistent with the hypothesis but the obtained judgments of linear size and angular size were not.

Two experiments were conducted to investigate the unsettled question of whether the visual search process is selfterminating or exhaustive. In the experiments three letters were placed on an imaginary circle round the fixation point. Two different letters were used, one of which was defined as the ‘signal’. S s had to respond ‘yes’ when one or more signals were in the display, ‘no’ otherwise. In both experiments the number of signals in the display was varied from 0 to 3. Decreasing latencies with increasing number of signals were observed in both experiments, indicating a selfterminating visual search. In experiment 1 a significant increase in latencies with increasing visual angle was found, in experiment 2 an increase in latency resulting from neighbouring contours. Both factors probably contribute to the slope of the function relating positive responses to display size and to the slope of the function relating negative responses to display size when stimuli are presented in linear arrays as in the experiment by Atkinson et al. (1969). As a result the slopes become more equal, falsely suggesting an exhaustive visual search process.

The caridean shrimp Hippolyte californiensis Holmes locates its host plant, the eel grass Zostera marina, visually, discriminating it on the basis of its form. Using models of the plant, the form-specific sign stimuli optimally releasing the host-selection response were investigated. Shrimps preferentially approached dark forms presented against a lighter background. The more a form contrasted with the background, the higher was its stimulatory value. Shrimps showed differential responses to solid forms of varying shape, preferring a stripe-like rectangle in a vertical orientation. Rectangular forms 50° (visual angle) or more in height and between 12° and 20° in width were maximally stimulating. Patterns of several vertical stripes had a greater releasing value for the approach response than a solid rectangle form of optimal dimensions. The greater the spatial frequency of a striped pattern, the reater was its attractiveness. Shrimps discriminated patterns of stripes whose elements were as small as 2° wide, giving an estimate of their visual acuity. Along with the relative contrast, the most important stimulus parameters of striped patterns are the continuous, straight form of the edges and their vertical orientation. It is postulated that the scototactic response of H. californiensis to relatively large, dark, solid forms is involved in the distant detection of clumps of eel grass. At closer range, the eel grass plants are then discriminated on the basis of the stripelike pattern of the leaves.

The latency of the crayfish,Procambarus clarki, visually evoked defense reflex varies inversely with the velocity of an approaching object (Figs. 3, 4, 5). Several lines of evidence demonstrate that the latency variations may be attributed to the time required for the target visual angle to expand by a criterion number of degrees (Table 1). The probability that a stimulus will elicit the defense reflex, increases monotonically with the velocity of target approach. Single unit analysis of optic nerve interneurons indicate that the sustaining unit response latency was ≧ reflex latency (Fig. 8). Dimming units were only weakly responsive to approaching objects. Furthermore, the dimming units exhibited very little differential responsiveness over 76% of the behaviorally relevant range of stimulus velocity (Fig. 10). Motion detectors exhibited strong responses to approaching targets (Fig. 11) and both the mean discharge rate and the number of brief interspike intervals/stimulus increased linearly with stimulus velocity (Fig. 14, 15). It is proposed that a central neuronal threshold for eliciting the defense reflex is a criterion number of motion detector spikes or brief interspike intervals.

When a stick is used to extend the boundaries of the world perceived by touch, haptic parallels to visual angle and size constancy appear. In a pilot study, Ss using a stick to feel an out-of-reach gap were more likely to underestimate its size than were Ss feeling a nearby gap with the index finger, but size constancy was relatively high with the stick. The main study, comparing judgments of gaps at intermediate and far positions, confirmed these findings. Errors were greater for the more distant gap, but constancy was high in both positions. Correlations between far and intermediate judgments indicated that Ss were consistent in the type of error made. Similar findings emerged from a partial replication with blind and partially sighted Ss.

Effects of visual angle and convergence upon the perceived sizes and perceived distances of a familiar object (playing card) and a nonrepresentational object (blank white card) were investigated by means of a projector stereoscope with polarizing filters. The results obtained with six Ss indicated that size estimates increased nearly proportionally as the visual angle increased and decreased nearly linearly as the convergence increased. Distance estimates decreased nearly linearly as either the visual angle or the convergence increased. The ratio of the size estimate to the distance estimate for a given visual angle was almost constant irrespective of convergence. In this sense, the size-distance invariance hypothesis held. No clear effect of familiarity was found. Partial correlations were used to discriminate direct and indirect causal relationships between the stimulus variables and perceptual estimates. Both perceived size and perceived distance were found to be determined directly by the two stimulus variables, but to be mutually related only indirectly.

Ss made judgments of whether or not they were being looked in the eye as an E fixated several points on and off their faces.Distance between sender and receiver, sex, and whether the sender made a sequence of fixation sprior to the terminal fixation were variables of interest. The sender-receiver distance produced less effect than predicted from the hypothesis that receiver judgments were determined solely by the discrepancy between the sender’s iris/sclera configuration (ISC) during true eye contact and the current fixation point. There was no stable differencein accuracy as a function of whether the sender’s terminal fixation was preceded by other fixations or not. These findings cast doubt on the belief expressed in recent studies that the ISC can be used to accurately discriminate eye gazes from other gazes. The results also suggest that fixations on some portions of the face yield a judgment of eye contact which might be predicted on the basis of social experience.

Using the method of constant stimuli, horizontal black and white line gratings were presented to a bottlenose dolphin in a successive discrimination task. Over a constant viewing distance of 2.8 m, a minimal visual angle of 18 min of arc was obtained. This value is poorer than that reported for pinnipeds, but comparable to functions reported for the antelope, elephant, and clawless otter.

Naive zebra danios (Brachydanio rerio) flee from an approaching predator when the angle subtended at the danio's eye by the predator changes at a rate greater than some characteristic threshold value. The hypothesis was tested that this value declines, and hence reactive distance increases, as a function of the number of past experiences with the predator. Artificial predators of two types, a model and a film of an approaching object, were presented to the danios daily and changes in behaviour recorded. The rate of change of visual angle at the time of response to the predator (dα/dt threshold) declined asymptotically with increasing experience. Consequently, flight distance increased asymptotically. The change was not due to growth, maturation, or increased familiarity with the testing situation. The danios apparently associated the visual characteristics of the predator with its subsequent high rate of angular growth and began to respond to these visual characteristics prior to dα/dt exceeding the threshold. The angle of escape was not affected by experience. Escape velocity increased with experience of the film predator, but not with experience of the model predator. No decrement of the learned change in reactive distance was evident after 10 days without experience with the film predator.

The purpose of this study was to investigate the relationship between the visual after-image and corresponding brain waves (EEG) unde more specific stimulus conditions thar Jasper & Cruikshank (1936, 1937) and Motokawa & Mita (1941). The stimulus conditiom used in the present experiment were : 15 to 200 lux in intensity, 3 to 60 sec. in duration and 1×1 sq. cm. to 60×60 sq. cm. in size. Analysis of EEG corresponding to after-images was made in two ways : (1) in terms of the amplitude of glpha waves, and (2) by the use of a band-filtered method developed by Komizo & Yamaoka (1954).Procedure and SubjectsThe subject was asked to be seated facing a stimulus figure in the dark room. EEG's were recorded of the subject while he was closing his eyes. As soon as the light stimulus was exposed he was instructed to open his eyes and when the stimulus was extinguished after a certain duration of time, he was required to close the eyes. When an after-image was perceived, he had to press on an electric key.When the stimulus intensity was varied, a constant duration of 30 sec. and a constant size of 3×3 sq. cm. were used. The stinlulus duration was changed with the same area and intensity (3×3 sq. cm. and 50 lux). When the stimulus size was variable, a constant intensity of 50 lux and a constant duration of 30 sec. were utilized. An EEG recorder with 6 channels, manufactured by Sanei Co. was used for experimentation. Four experienced subjects were observed.Results and Discussions1. Appearance of after-images agreed with the decrease in alpha wave amplitude and its disappearance with the increase in the amplitude. This was most conspicuous at the beginning of the whole phase, but was obscured at the end where alpha waves reached a certain level of amplitude.2. After-images were seen longer and more frequently when the stimulus intensity was 200 lux. With the optimal intensity for the subject, after-images became clearer in contour and their intensity increased. The corresponding EEG showed regular fluctuations and the amplitude of alpha waves indicated regular increases and decreases.3. When the duration of a light stimulus was varied, corresponding changes were shown in after-images and EEG. The longer the stimulus duration was, longer was its after-image prolonged and the EEG more regular in fluctuations.4. When the stimulus size was of a optimal visual angle (7.2°to 13.9°) its atter-imags was seen longer and the EEG more regular in fluctuations.5. A band-filtered analysis of EEG showed that wave components of 20, 10, 5 and 2.5 c/s were involved in the EEG of after-images. Components of 10 c/s changed with components of 5 c/s while those of 20 c/s varied with those of 2.5 c/s. They behaved antagonistically.6. A slight difference was suggested between the EEG of a light stimulus and that of its after-image. In the former dominance of 20 c/s components was shown for a certain length of time, while in the latter 10 c/s and 5 c/s components were more conspicuous and 20 c/s components were inhibited, being shifted to a lower frequency of 18 c/s.

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)

The visual resolution of a single opaque line against an evenly illuminated background has been studied over a large range of background brightness. It was found that the visual angle occupied by the thickness of the line when it is just resolved varies from about 10 minutes at the lowest illuminations to 0.5 second at the highest illuminations, a range of 1200 to 1. The relation between background brightness and just resolvable visual angle shows two sections similar to those found in other visual functions; the data at low light intensities represent rod vision while those at the higher intensities represent cone vision. With violet light instead of white the two sections become even more clearly defined and separated. The retinal image produced by the finest perceptible line at the highest brightness is not a sharp narrow shadow, but a thin broad shadow whose density distribution is described in terms of diffraction optics. The line of foveal cones occupying the center of this shadow suffers a decrease in the light intensity by very nearly 1 per cent in comparison either with the general retinal illumination or with that on the row of cones to either side of the central row. Since this percentage difference is near the limit of intensity discrimination by the retina, its retinal recognition is probably the limiting factor in the visual resolution of the line. The resolution of a line at any light intensity may also be limited by the just recognizable intensity difference, because this percentage difference varies with the prevailing light intensity. As evidence for this it is found that the just resolvable visual angle varies with the light intensity in the same way that the power of intensity discrimination of the eye varies with light intensity. It is possible that visual resolution of test objects like hooks and broken circles is determined by the recognition of intensity differences in their diffracted images, since the way in which their resolution varies with the light intensity is similar to the relation between intensity discrimination and light intensity.

Recognition thresholds for lunar crater size were determined, analytically, for various look angles and magnifications, at an orbital altitude of 80 nautical miles. Elliptical image measurements for various sized craters were combined with some previous threshold recognition data for the ellipse (Casperson, 1950). Elliptical image measurements consisted of the visual angle of the major axis, and elliptical form (the ratio of minor axis to major axis). A computer program was generated from which the visual angle and form measurements of anticipated elliptical crater images were computed for various combinations of crater size, look angle, and magnification. Casperson's data was then re-worked to obtain the visual angle and form measurements associated with his recognition threshold data for the ellipse. By graphically combining the visual angle and form data from both computations, 50% and 75% threshold recognition curves were generated, relating crater size, magnification and look angle. Implications of these data are discussed.

Definitions and statements about visual size-distance perception are presented which emphasize neglected distinctions, first between physical, phenomenal, and cognitive variables, and secondly between an object's phenomenal visual size, s, and its phenomenal visual angle, v. It is reiterated that size-constancy pertains to s not v, and that the perceptual correlate of the size, R′, of the retinal image is v, not s. It is argued that R′ and v are not immutably related, and proposed that micropsia is a reduction of v but not necessarily of s. It is suggested that accommodation-convergence micropsia stems from normal perceptual compensation for the difference in location between the eye's entrance pupil and the visual egocenter.

The electrical threshold of the eye was measured by the alternating current of 20 c. p. s. with flicker sensation caused by it as an index under various adaptation states. 1. The threshold-time curve during complete dark adaptation following light adaptation of 10 minutes to illumination of 9, 800 lux shows the following course: At first the threshold remained low for about 8 minutes, and then increased rapidly to attain a constant level in about 15 minutes. Thus, the curve showed an inflection at about 8 minutes. 2. The curve suffered from a modification when the retina was constantly exposed to a small point of light of 40' in visual angle following the same light adaptation as above. The inflection at the middle stage became more obscure and the final level lower as local illumination was moved farther towards the periphery of the retina. 3. The effect of similar local illumination was investigated with colored lights. When blue, green and red lights were used for local illumination, it was found that the red light showed an effect similar to white light confined to the fovea, but that the green and blue light acted as white light illuminating the periphery of the retina. 4. Next the effect of exposure to broad illumination was investigated upon the curve following the same light adaptation as above. The curves by illumination of 0.1 lux, 1 lux and 10 lux were low and almost steady, but those by 100 and 1, 000 lux were generally much higher. From these experiments it is likely that the threshold as determined by the A. C. of 20 c. p. s. is lowered when visual cells, especially rods are excited to some extent.

Spatial summation and inhibition were studied with two methods, Motokawa's ζ-method and the binocular brightness match method. 1. The electrical excitability of the eye was measured after exposure to white five spots together and to the central spot of them alone. The excitability was greater for the five spots together (summation), when the distance between the centrally fixated central spot and other four spots was a few degrees in visual angle, but smaller (inhibition) when the distance was 8°-11°. 2. Distribution of summation and inhibition in the retina was mapped by exploring each region with 2 spots 3° apart from one another, and it was found that the central part of about 12° in diameter was a sum-mation area, which was surrounded by an inhibitory zone of 5° in width. 3. The rod process which was determined by the ζ-method, showed spatial summation, but no inhibition at all. 4. The effect of a glare patch G upon the brightness of a test patch T presented to one eye was measured in comparison with a control patch viewed by the other eye, and it was found that the inhibitory effect of G upon T was greatest when G was presented to the inhibitory zone of the retina determined by the ζ-method. 5. Under experimental conditions advantageous for scotopic vision no inhibition could be found by the brightness match method in accordance with the result obtained by the ζ-method.

This study was conducted to determine the speed and accuracy of form recognition as a function of the size of target forms for various amounts of detail resolution. The stimulus forms were generated by filling in, on a statistical basis, some of the cells of a 90,000-cell matrix. The subjects were shown a “briefing target” and instructed to locate that target on a display containing numerous other forms. The significant finding is that both search time and errors remain invariant until the visual angle subtense of the targets falls below 12 min; at values below 12 min performance deteriorates. This effect is independent of the range of resolutions investigated. The implications of these findings to equipment design are discussed.

Su?nnzary.-The effects of the constant errors, anticipation and habituation, on measurement of visual fields war, investigated in 79 Ss. Visual fields were measured using 8 arc positions on a radius perimeter. On all arc positions, especially in rhe inferior arc positions, the peripheral threshold means, based on inward movement of the stimulus object, were larger (indicative of a larger field) than those based on outward movements. Peripheral threshold measurement, as conducted in this study, was seen to possess high reliability. In clinical perimetry, approximate values are only spoken of with respect to the size of the normal visual field. This is because there are so many factors which influence measurement of visual fields, e.g., lack of uniformicy regarding apparatus, methodology, illumination, isopters, and facial contours, to mention a few. However, it has been suggested that, for the values given for the nor7nal visual isopters, ". . . there is surprisingly close agreement among investigators when one considers the difficulty involved in establishing such standards for a subjective examination" (Harrington, 1956, p. 100). Textbooks of modern clinical quantitative perimetry suggest, as a routine methodological procedure, that each isopter (an isopter is the contin~ious line which represents the peripheral extent of the visual field for a particular visual angle) be plotted in the same fashion. The stimulus object is moved from outside the visual field toward the area of fixation. S (or patient) is instructed to signal as soon as the stimulus comes into view (cf. Harrington, 1956; Hughes, 1954). This type of systematic manipulation of stimuli is quite similar to one of the traditional procedures used in psychophysics and is referred to by various names: method of least noticeable stimuli, method of limits, method of minimal changes, and method of serial exploration. Such procedures involve presentation of stimuli involving ascending and descending series; each series is carried far enough to locate the transition point or threshold from one to another response category. With such serial exploration procedures, errors of anticipation and habituation are expected to be confounded into the calculation of threshold values. It is expected that the alternate ascending and descending series will average out these constant errors when they are present. The purpose of the present investigation was to examine the possible influ- ~p

In determining the smallest amount of energy necessary for vision fluctuating results are found: a flash originating from a constant light source is sometimes seen and sometimes not. Out of the work of van der Velden it proved that the fluctuating phenomenon mentioned is due to the statistical behaviour of the light quanta: a pure quantum physical aspect. From his investigation on the seeing of flashes he concluded that 7° nasal from the fovea for rod vision of the dark adapted eye independent of the observer two quanta must be absorbed in the visual purple within about 0,02 seconds and within an area of the retina corresponding to a visual angle of about 10′ in order to perceive light. As it appeared to be possible to distinguish between the physical and physiological aspect of vision, it can be expected that by a consistent extension of such experiments other fundamental properties of the behaviour of the visual sense organ can be found. To this purpose are the experiments described in this paper performed. In Chapter I the two quanta theory, its fundamental experiments and our experimental confirmation for rod vision are discussed. Chapter II gives an exposition of threshold experiments for rod vision in which a larger number of combinations of visual angle d and flash time t are studied. It proved that deviations from the two quanta theory occur when t and d are large. These deviations can be explained by the fact that some time after the beginning of the flash the chance for a light perception is impeded. This state has developed after a time T (ca 0,1 sec.) and extends over an area O (about 20′). Chapter III deals with the investigation of cone vision. With the aid of the results of experiments on flashes on various places of the retina with monochromatic and heterochromatic light it is concluded that every receptor of the eye reacts at the absorption of a quantum with a nerve impulse to its nerve connection and a light impression is caused when a second quantum is absorbed in a receptor within a certain distance D of the first receptor and within a time τ after the absorption of the first quantum. The time τ is in our experiments independent of the place of the retina and of the kinds of excitated receptors (0,04 seconds), so that it is probable that the interaction between the two impulses in the nerve connections of the receptors always occur in a nerve element of the same kind. The distance D is in the fovea almost independent of the wavelength, so that this distance is almost equal for the several cone systems (2–4 minutes), 7° nasal from the fovea, D is for the cone system sensitive in the extreme red about 4′ and for the rod system 10′. For the development of the light impression it is indifferent whether the two quanta are absorbed in receptors of the same kind or not, so that a completely mutual dependence between the rod and cone systems in the periphery and between the cone systems in the fovea exist. In Chapter IV some general proporties of the nerve system are discussed. The general principles of the histology of the nervous system and the nervous transmission are reviewed. In according with the mutual dependence found by us between the several cone systems in the fovea we concluded that there exist mutual connections between the cones, somewhere in the visual pathways. Out of the work of Sherrington on the excitation of simple reflex arcs and of Eccles on the potential disturbances of nerve-elements it proved that the results of investigations from two fields of science, nerve-physiology and physiological optics are similar; the visual sense organ and simple reflex arcs are excitated by two impulses. In Chapter V experiments on the spectral sensitivity curve for rod and cone vision are presented. The shape of our foveal curves obtained with the aid of absolute threshold measurements differ from the curves usually found with luminosity experiments. In ours a few rather small peaks occur due to the intrinsic sensitivity curves of the seperate receptor systems. It seems out of the shape of the peaks that the half-width values of these intrinsic sensitivity curves are rather smaller and agree with these values for the “modulators” of Granit, contrary to the fundamental response curves found by other authors with luminosity experiments, which are all wider. No shift in the scale of Wave-lengths of the peaks for a deuteranomalous observer occurs, so that anomaly in trichomatic vision might be of nervous origin and has probably no photochemical foundation. The deviations from the “additional law” for small visual angles suggested by Bouma must be due to the chromatic aberration of the eye. Fundamental deviations from the additional law are impossible at the threshold of vision. In Chapter VI the behaviour of the brightness impression is studied with the aid of experiments on the comparison of the impression of flashes of different time and size. For peripheral vision it is found that for a certain brightness the average number of effective absorptions within 0,11 seconds within an area with 25′ diameter for rod vision and 4′ for cone vision must be constant. It is concluded that the impulses within the area and time mentioned co-operate to create the brightness impression. The brightness impression will generally increase when the average number of absorptions increases. For “red” cone vision in the area O 7° nasal from the fovea only one “red” cone is situated, so that in the extreme red all quanta necessary for a certain brightness impression must be absorbed within one cone. For rod vision the area O contains about 400 rods. The area O has the nature of a recipient unit, the time T of an averaging time. Chapter VII deals with the visual acuity. As far as the visual angle of the details of the test figure are large compared with the diameter of the recipient unit the behaviour of the visual acuity is similar to the behaviour of the threshold values and agrees with the two quanta explanation: at the threshold of recognition the intensity is proportional to visual acuity. When the details of the test figure are of the same order of magnitude as the diameter of the recipient unit deviations from the two quanta behaviour for simple threshold experiments occur due to the fact that within such an unit separate light sensations cannot be observed. In Chapter VIII some general aspects of vision are discussed in relation to the simultaneous and successive comparison of sensations. It is deduced that for the perception of differences in sensations the first and last averaging time T of the observation and moreover for the visual sense the border of the test spot are of special importance. The consequences of these facts are confronted with the deviations mentioned in chapter II, problems of the visual acuity and the influence of the retina grey. It is possible that the “psychological” considerations of this chapter are a paraphrase of the transmission of impulses by the nerve system and it is suggested that the on-effect, off-effect and on-off effect of nerve transmission has a close bearing on the problems of vision of this chapter.

According to a model for motion and form perception proposed by Johansson (1964). every two-dimensional change in a changing proximal stimulation is projected out as a motion in depth The model assumes that the amount of perceivedrelative motion (the fraction between the perceived amount of motion of the object and the perceived initial distance to the object) is determined only by the amount ofrelative change (the fraction between the absolute amount of change and the initial size). The aim of the present study was to test this hypothesis by studying the effect of some other variables on perceived relative motion in depth. As stimuli, continuously shrinking and growing squares were used. No effects were found when varying the absolute amount of change. Neither did the rate of change influence the perceived relative motion in any important way. The only variable that gave rise to strong and systematic effects on perceived relative motion was the initial distance to the perceived object. The greater the initial distance, the less relative motion was perceived.

This article gives a consecutive description of the typical scotoma of glaucoma from its beginning through the various stages of its development until central vision is lost. At the same time, it shows the limitations of the perimeter and emphasizes the importance of using test objects with small visual angle against a flat surface, if these defects are to be found at a time when treatment is effective. Much of the literature of this subject comes from pretonometer days and was written from the standpoint of differential diagnosis, while today we know that field studies are the only reliable indicators of gain or loss.

Vision is a light and form sense. From the ability of the eye to discriminate detail of form is evolved the function of resolving power. The problem in medicolegal cases is the determination of the percentage loss of functional values. To determine these percentage functional disabilities, the relation between visual acuities and the associated functional losses must be established on a mathematical basis. The term “visual acuity” does not include the concept of function. Acuity notations are not fractions of vision nor are they fractional equivalents of resolving power. Comparing the successive gradations of acuity notations in the geometrical progression of Green and assuming that acuity notations express fractional ability of the form sense, it is shown that the better the acuity the poorer is functional ability—when vision is good it has relatively poor resolving power. This disproves the assumption. A method for computing the functional and proportionate value for acuity notations has been established. By this method each equal increase in the visual angle reduces visual resolving power (efficiency) by the same percentage. By application of this method to the medicolegal problem of evaluating visual disabilities, for identical degrees of damage to function identical percentage losses are established.

Using a stimulus subtending a visual angle of 0.17 degree, the reduction in the field as compared with that obtained for a one-degree stimulus was greater for the presbyopic group of subjects and less (in the order named) for the myopic, hyperopic, and emmetropic groups. The shape of the field obtained for the 0.17-degree stimulus differed from that obtained with the larger stimulus, the reduction being greater on the temporal side. A greater range of scatter was obtained in the results with the 0.17-degree stimulus, and this fact may render of little or no value the use of the smaller stimulus for refinement of diagnosis based on size of field.

The color sensitivity to four spectral colors, red, green, yellow and blue around the spot of Mariotte was investigated under dark adaptation of the eye. 1. It was found that there is a color-blind zone around the blind spot, and that the zone is wider in the following order: Red>green>yellow>blue. 2. The zone is not strictly concentric with the blind spot, but wider towards the periphery than towards the fovea. 3. The extent of the zone increases by dark adaptation, especially in the vertical meridian and the upper and the lower parts of the zone rupture at a certain stage of dark adaptation. This phenomenon is especially remarkable for red and green lights. 4. Reduction of the visual angle of the test patch has an effect similar to dark adaptation on the form and size of the color-blind zone. 5. The zones for red and green lights are more sensitive to changes of intensity of test light than those for yellow and blue liglts. 6. In short, the color sense around the blind spot depends on many factors such as the intensity, the wave-length, the size of the test stimulus, the state of dark adaptation, etc.

The influence of lateral inhibition upon stabilized images was studied by presenting to three subjects concentric triangles and systematically varying the visual angle subtended, the interval between lines and the thickness of the line. When the images were stabilized on the retina subjects reported the disappearance of the image in various ways. In particular, the number of times fusion of lines before disappearance was reported was found to be inversely related to the size of the interval and directly related to the thickness of the lines. These findings were consistent with quantitative estimates of lateral inhibition.

Measurements of apparent size were obtained by distance adjustment of a peripherally viewed stimulus to produce a match to a foveally viewed standard. As eccentricity increased, the peripheral stimulus was adjusted at distances of progressively greater visual angle, indicating that a continuous diminution in apparent size occurs with increased eccentricity. This effect was found to be stable for several conditions of illumination and for changes in the light adaptive state of S. Apparent size diminution and apparent distance increase were also found for familiar objects viewed in an open field.

The perception of discrete movement it related to visual angle, the angle subtended at the spectator by the displacement of the screen image between two successive frames. This paper determines the perceptual threshold of discrete movement in terms of visual angle for a limited set of conditions.

THIS STUDY EXAMINED THE EFFECTS OF CAFFEINE ON A PROTRACTED VISUAL MONITORING TASK ANALOGOUS TO AN ASPECT OF AUTOMOBILE NIGHT DRIVING. THE TASK INVOLVED MONITORING TWO READ LIGHTS THAT MOVED APART AT RANDOM INTERVALS. THIS DISPLAY SIMULATED THE RATE-OF-CLOSURE CUE OF CHANGE IN VISUAL ANGLE FOR A VEHICLE FOLLOWING 60 YARDS BEHIND ANOTHER VEHICLE. TWENTY MEASURES OF RESPONSE LATENCY WERE PROVIDED FOR EACH S IN EACH OF FOUR HOURS OF CONTINUOUS TESTING. FIVE GROUPS (N=20 IN EACH) WERE GIVEN THE FOLLOWING TREATMENTS: PLACEBO, 200 MG. OF CAFFEINE AT THE BEGINNING AT THE SECOND HOUR; 200 MG. AT THE THIRD HOUR; 400 MG. AT THE SECOND, AND 400 MG. AT THE THIRD. THE RESULTS INDICATED THAT CAFFEINE SIFNIFICANTLY INHIBITED RESPONSE BLOCKING (ATTENTION LAPSES). THE EFFECTS WERE APPARENT WITHIN ONE HOUR FOLLOWING ADMINISTRATION AND PERSISTED OVER THE REMAINING HOURS. NO DIFFERENCE WAS FOUND BETWEEN DOSAGE LEVELS OR IN MAGNITUDE OF EFFECT AT THE DIFFERENT ADMINISTRATION TIMES. /AUTHOR/

An analysis was conducted to determine some of the visual requirements for landing a shuttle spacecraft on the surface of the moon and subsequently achieving rendezvous with the command module in orbit. The major questions investigated were: (1) Is the astronaut capable of perceiving moon landmarks while in orbit in order to detect and identify a desired landing area? (2) Can the astronaut perceive the command module from the moon during all phases of its orbit from lunar horizon to entering the moon's shadow? To evaluate these questions, the properties of the moon landmarks were examined in terms of size (visual angle subtended), contrast, and velocity of the moving imagery. In addition, stellar magnitude and luminance data were calculated for the command module and the earth as seen from the moon for different phase angles and albedos. Finally, the effects of glare on the discrimination of the command module were discussed.

When a multiletter display is preceded by a bar designating one of the letters for report, reaction time (RT) to voice the indicated letter decreases. Previous research had indicated that the efficiency of this selective mechanism decreases as the number of display elements increases. Two experiments were conducted to determine whether the effect of display size could be eliminated when the indicator precedes the display at long intervals. Results indicated that the display size effect was maintained. The results could not be attributed to eye movements, but were interpreted in terms of a central encoding mechanism that is limited in its precision of localization and exclusion. At the phenomenal level, we can look at an object and see it as an entire object without concentrating on specific details. For example, we may look at the clock on the wall, even noticing what time it is, but not paying "attention" to specific characteristics of the numerals. Or we can phenomenally direct our attention to one of the numerals on the clock such as the 3 and in the process seem to exclude the details of the other numerals. This experience can occur even though our distance from the clock is such that the whole clock face is falling within the fovea and assuring adequate resolution of detail for all parts of the clock. It is as though we had a limited capacity for processing information per unit of time, and we can select which information and in what order it is going to be processed and encoded. We have been employing an experimental analog of this phenomenal experience. Circular displays of a small subset of letters are presented to Ss with a black bar indicator designating one of the letters in the display. The display as a whole is foveal. sub tending approximately 2 deg of visual angle on the diameter. The bar indicator is quite conspicuous, both in terms of its size and shape, but primarily in terms of its location as an extension of an imaginary radius from the center of the display extending beyond the circumference of the imaginary circle along which the letters are arrayed. Ss are instructed to name the designated or target letter as quickly as possible. By manipulating the display variables, we have hoped to delineate some of the characteristics by which selective attention or encoding occurs.

The ability to detect small excursions of apparent movement of a point light source was investigated. Apparent movement was achieved by alternately presenting a point source in two different planes. The presentations, each lasting about 500 milliseconds, had an overlap of approximately 8 milliseconds. Using 7 subjects, the limen for apparent motion was a stimulus separation of 43.9 mm, which is a visual angle of 1′ 21″. Possible application of the effect in a highly precise visual guidance system is discussed in light of the results.

When a perceived frontal size is matched to a perceived egocentric distance with only primary cues available, the corresponding physical size-distance ratio is about 0.5. Perceived visual angle exceeds physical angle by much less. It follows that under these conditions the visual space is not Euclidean. This phenomenon is demonstrated in two experiments. It may be the basic phenomenon responsible for the non-Euclidean character of visual space. It is, however, inconsistent with the assumption of homogeneity of the Luneburg-Blank theory. When additional cues to distance are introduced, the physical ratio is set much closer to the perceived ratio with no corresponding change in the perceived magnitude of visual angles. The results are interpreted as disconfirming the hypothesis that perceived size is inferred from visual angle and perceived distance. Rather they suggest two independent modes of spatial information processing.

1. A glass microelectrode was inserted into the unit responding to movements of a spot in the lobula or, on rare occasions, in the medulla of the optic lobe of fleshflies (Boettcherisca peregrina). 2. By the sequential presentation of several or two stationary spots placed along the preferred-null axis of the directionally selective unit and by the turning on and off of a stationary spot (test stimulus) placed along the preferred-null axis at various distances from another continuously lighted, fixed spot (conditioned stimulus), it is demonstrated that spatial, excitatory effects were produced on the preferred direction side of the conditioned spot, and simultaneously, spatial, inhibitory effects were seen on the side of the null direction. Therefore, it is considered that movement perception is analyzed on the basis of the stationary and elemental light sensation and is completed by both the mechanism of excitation in the preferred direction and inhibition in the null direction. 3. The field with excitatory effect is relatively wider (20 ° or more in visual angle) than that with inhibitory one (8 °–20 °, rarely more than 20 °). The inhibitory effect decreases gradually with increasing distance from the conditioned spot stimulus, while the excitatory effect increases with each 5 ° of separation in visual angle from the conditioned spot. Therefore, it is assumed that the neural mechanism for excitatory effects is different from that for inhibitory effects. 4. Movements of a bright slit on a dark background and a dark slit on a light background were given using various slit lengths. The discharge rate in the directionally selective unit shows an increase, a decrease or an increase after a decrease by increasing the length of the slit. These three kinds of responses are explained by the width of the interactivity between the excitatory and inhibitory fields to spot stimulation. 5. The neural organization of excitatory and inhibitory field effects underlying the directional selectivity in movement perception is discussed from the morphological view point.

The glare effect of monochromatic light was measured as a function of wavelength, the variation of the visual threshold serving as criterion. Test light (35′ visual angle) and glare light (3° 19′ visual angle) were presented to the eye in light flashes of 0·06 resp. 0·1 sec duration at different retinal places. The measurements, carried out by four observers are made up of three groups: in the first group the test light was white, in the second one of the same colour as the glare light. In both cases glare lights of the same subjective brightness were used, in the third group glare lights of the same radiance. The difference of the logarithms of radiance for the visual thresholds with and without glare were plotted as a function of the wavelength and the obtained results were discussed.

Thirty-six volunteer subjects, screened for normal visual acuity, viewed televised displays during a one-hour testing session. There were 232 displays; each consisted of four characters (letters and numbers) of a given size and contrast condition. The study design made it possible to compare visibility across: (1) six viewing distances, (2) three viewing angles, (3) three figure-background contrasts, (4) four character sizes and (5) three time-segments within the testing session. Results indicate that no visual fatigue occurred, that black-on-white and white-on-black contrasts produced about equal visibility, and that characters subtending 10 min of vertical visual angle could be perceived with almost complete accuracy.

A four alternative Choice Reaction Task, oil alternatives equally likely, was used to study the effect of size and contrast ratio of the visual stimulus on response time. A full factorial experiment was designed in which three levels of both the variables were tested. Sizes subtending visual angle of 1 min, 3 min, and 5 mill and the contrast ratios of 0-94, 0-75, and 0-34 were used. Seventeen subjects wore tested for this experiment and each subject responded 250 times under each experimental condition. The analysis has indicated that the contrast ratios tested did not significantly (a = 0'05) effect the response time.