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The perception of a pair of contours in a retinal image cannot be understood simply by adding up the perceptions of the individual contours, especially when they form a perpendicular junction, or are parallel to one another. It is the relationship among the contours that determines what is perceived. Note that it is hard to actually compare the perception of such configurations quantitatively. We managed to do this by testing the perception of such configurations in three psychophysical experiments in which the perception was characterized by measuring the orientation threshold of a single contour. This threshold was estimated by using a modified Method of Constant Stimuli based on the assumption that contours forming a configuration are perceived individually, and that they are integrated linearly. This assumption made the quantitative comparison of the perceived configurations possible. We found that changes of the estimated threshold depended on the type of the configuration, specifically thresholds estimated from a perpendicular junction were substantially lower than thresholds estimated from a single contour or from a non-perpendicular junction. The lowest thresholds were observed when the threshold was estimated from a pair of parallel contours. These results suggest that the visual system is sensitive to perpendicular junctions and parallel contours in a retinal image.

Detecting motion changes is a fundamental prerequisite for solving tasks in sports and in everyday life. It is known that peripheral vision is used to detect these changes and that saccades impair detection performance. However, comparatively little is known about the role of smooth-pursuit eye-movements (SPEMs) during these tasks. Therefore, we compared peripheral motion-change detection during SPEM and fixation at eccentricities up to 18°, simulating the perceptual demands of real-life situations. Based on expert gaze behavior in sports, we predicted that motion detection should be better during fixation than SPEM. In a series of three experiments, we consistently found that detection rates and response times were impaired during SPEM compared to fixation, particularly at 18° eccentricity. With an invisible pursuit object and targets moving ahead rather than behind the pursued object, performance differences in response times declined, whereas differences in detection rates interestingly remained unmoved. We argue that retinal image motion and attentional demands are reasons for SPEM impairments.

Two experiments evaluated the effect of retinal image size on the proto-object model of visual clutter perception. Experiment 1 had 20 participants order 90 small images of random-category real-world scenes from least to most cluttered. Aggregating these individual rankings into a single median clutter ranking and comparing it to a previously reported clutter ranking of larger versions of the identical scenes yielded a Spearman’s ρ = .953 (p

It is currently debated whether the perception of gloss is linked to the statistical parameters of the retinal image. In particular, it has been suggested that gloss is highly correlated with the skewness of the luminance histogram. However, other psychophysical work with artificial stimuli has shown that skewness alone is not enough to induce the perception of gloss. Here, we analyzed many images of natural surfaces to search for potential statistical correlates of perceived gloss. We found that skewness indeed correlates with gloss when using rendered stimuli, but that the standard deviation, a measure of contrast, correlates better with perceived gloss when using photographs of natural surfaces. We verified the important role of contrast by manipulating skewness and contrast within images. Changing the contrast in images significantly modulates perceived gloss, but manipulating the skewness of the luminance histogram had only a small effect.

School aged children with progressive myopia show large accommodative lags to blur only cue which is suggestive of a large depth of focus (DOF). While DOF measures are lacking in this age group, their blur detection and discrimination capacities appear to be similar to their non-myopic peers. Accordingly, the current study quantified DOF and blur detection ability in progressive myopic children showing large accommodative lags compared to their non-myopic peers and adults. Blur sensitivity measures were taken from 12 children (8–13 years, 6 myopes and 6 emmetropes) and 6 adults (20–35 years). DOF was quantified using step changes in the lens induced defocus while the subjects viewed a high contrast target through a Badal lens at either 2 or 4D demand. Blur detection thresholds (BDT) were tested using a similar high contrast target in a 2-alternate forced-choice paradigm (2AFC) at both the demands. In addition to the large accommodative lags, micro fluctuations and DOF were significantly larger in myopic children compared to the other groups. However, BDTs were similar across the three groups. When limited to blur cues, the findings of a large DOF coupled with large response lags suggests that myopes are less sensitive to retinal defocus. However, in agreement to a previous study, refractive error had no influence on their BDTs suggesting that the reduced sensitivity to the defocus in a myopic eye appears to be compensated by some form of an adjustment in the higher visual processes to preserve the subjective percept even with a poor retinal image quality.

Our visual system can restore information missing within the portion of the retinal image corresponding to the blind spot where the optic nerve exits the eye. Previous studies of the properties of filled-in surfaces at the blind spot have found similarities and dissimilarities between filled-in and real surfaces and have therefore not provided a consistent view of the characteristics of the filled-in surface. First, we investigated whether filling-in utilizes a contour integration mechanism. Gratings with collinear lines filled in the blind spot more effectively than those both with orthogonal lines and without any line, suggesting that collinear facilitation underlies the filling-in of the blind spot. Second, the dynamics of binocular rivalry was examined by comparing the dominance duration distributions of filled-in and real surfaces. The results indicated that the strength of the filled-in surface was attenuated compared to that of the real surface during rivalry. Lastly, we tested whether travelling waves of dominance in rivalry could occur at the blind spot. The travelling waves could propagate through a hole only at the blind spot, suggesting that the filled-in surface helps perceptual waves to travel across the blind spot. These results suggest that the filled-in surface shares a common mechanism via a horizontal connection but that it has weak strength to suppress the opposite eye during binocular viewing.

To investigate the effect of head roll tilt on the binocular coordination of ocular counterroll in non-human primates, we measured binocular ocular counterroll in two rhesus monkeys fixating a straight ahead target, while adopting different head roll tilt positions. We used two infrared cameras to take snapshots of the left and the right eye in order to measure the resulting ocular counterroll responses. The horizontal and vertical components of the position of one of the two eyes where measured using an implanted 2D-search coil in one monkey and video-based eye tracking in the second one. We consistently observed disconjugate ocular counterroll responses to static head roll in both monkeys. Invariably, the eye positioned further away from ground level by roll tilting the head always exhibited larger ocular counterroll than the other eye. The pattern of disconjugacy of the ocular counterroll responses exhibited by rhesus monkey parallels the one described for humans. The correspondence between the two species suggests that monkeys may serve as useful models in studies of the neuronal underpinnings of tilt-induced ocular counterroll and the perceptual compensation of uncompensated retinal image tilt.

When objects move in the environment, their retinal images can undergo drastic changes and features of different objects can be inter-mixed in the retinal image. Notwithstanding these changes and ambiguities, the visual system is capable of establishing correctly feature-object relationships as well as maintaining individual identities of objects through space and time. Recently, by using a Ternus-Pikler display, we have shown that perceived motion correspondences serve as the medium for non-retinotopic attribution of features to objects. The purpose of the work reported in this manuscript was to assess whether perceived motion correspondences provide a sufficient condition for feature attribution. Our results show that the introduction of a static "barrier" stimulus can interfere with the feature attribution process. Our results also indicate that the barrier stops feature attribution based on interferences related to the feature attribution process itself rather than on mechanisms related to perceived motion. (C) 2011 Elsevier Ltd. All rights reserved.

Refractive error, higher order aberrations (HOA), axial length (AL), anterior chamber depth (ACD) and average corneal radius of curvature were measured after cycloplegia from 166 emmetropic participants at the Sydney Myopia Study (SMS, 2004-2005, age 12.63 ± 0.48 years). Measurements were repeated approximately 5 years later at the Sydney Adolescent Vascular and Eye Study (SAVES, 2009-2010, age 17.08 ± 0.67 years). The baseline spherical equivalent (M) did not differ significantly between the participants lost to follow-up (65%) and the participants enrolled in SAVES study (p = 0.932). Refractive error and HOA were measured using a Shack-Hartmann aberrometer for a pupil diameter of 5 mm and AL, ACD and average corneal curvature measured using IOL Master at both visits. Retinal image quality in terms of Visual Strehl ratio (VSOTF) for a 5 mm pupil diameter was determined using on-axis lower and HOA. General linear model was used to determine the association of HOA and retinal image quality with change in refraction. Of the 166 emmetropes, 41 (25%) had myopic change (change in M-0.50 D) and 125 (75%) had no change in refraction (change in M between +0.49 D and -0.49 D). Change in C[4, 0] (p0.001, R² = 0.236), fourth order RMS (p = 0.003, R² = 0.097) and coma RMS (p = 0.004, R² = 0.056) from baseline were significantly correlated with change in refraction. More positive change in C[4, 0] was associated with lesser myopic change in refraction. The eyes with myopic change in refraction decreased in positive C[4, 0] (at baseline = +0.049 ± 0.05 μm, at follow-up = +0.024 ± 0.05 μm, p0.05). In comparison, eyes with no change increased in positive C[4, 0] (at baseline = +0.033 ± 0.04 μm, at follow-up = +0.047 ± 0.04 μm, p0.05). Thus in conclusion, no significant association was observed between HOA and retinal image quality at baseline and development and progression of myopia among emmetropic eyes. The change in spherical aberration (C[4, 0]) with myopic change is possibly associated with changes occurring in crystalline lens during ocular growth.

When a visual stimulus changes direction and distance simultaneously, Hering's Law argues that the resulting eye movements are the result of combined version and vergence control processes. Recently, it has been suggested that slow asymmetrical eye movements might be guided by monocular control processes wherein each eye is driven by its own retinal image. Experimental results presented here show behavioral differences between slow version and slow vergence eye movements, indicating that different control processes drive the two “pure” responses. Specifically, version tracking of constant velocity stimuli (i.e., smooth pursuit) is more precise, showing less variation in tracking velocity than movements of equal velocity produced by vergence stimuli. When the two stimuli are combined, the variability in tracking is consistent with the addition of the two components in proportion to their respective stimuli. These results provide support for Hering's Law, at least for low velocity, smooth tracking movements (i.e., slow version and slow vergence)

Position judgments, which are exquisitely precise in the fovea, are markedly degraded in the periphery. In a recent article [ Hess and Field 1993 Vision Research, 33 , 2663–2670] argue that the poor representation of positional information in peripheral vision is a consequence of uncalibrated spatial disorder of cortical connections rather than due to undersampling of the retinal image. Specifically, Hess and Field argued that if positional uncertainty is due to undersampling, then because of univariance, there should be an associated contrast uncertainty. In this report we show that the univariance model is limited in its generality since: (1) the Hess and Field data in which contrast and position discrimination are decoupled do not preclude undersampling with large univariant filters; (2) aliasing can decouple position from contrast; (3) undersampling or noise at a second stage of processing can lead to selective losses of position information without any degradation of contrast information. Copyright © 1996 Elsevier Science Ltd.

To determine whether speed discrimination improves when the retinal image is stabilized against the effects of eye movements, thresholds were measured under stabilized and normal viewing conditions. In the normal viewing conditions, eye movements were recorded and used to estimate retinal-image speeds. Stimulus reference speed for sinusoidal gratings varied from 0.5 to 8.0 deg/sec. Results showed that speed discrimination thresholds, expressed as Weber ratios, decreased with increasing stimulus speed for both the normal and stabilized viewing conditions. Stabilized viewing thresholds were higher than normal viewing thresholds only at the slowest stimulus reference speed. However, when speed discrimination thresholds were expressed as a function of the estimated retinal speed, there was no difference in thresholds for the stabilized and normal viewing conditions. A retinal-image model, whereby speed discrimination depends on retinal-image motion, explains the results.

We simulated the effects of longitudinal (axial) chromatic aberration and defocus on contrast of the long-, middle- and short-wavelength components of the retinal image to determine whether the effects of chromatic aberration are sufficient to drive accommodation. Accommodation was monitored continuously while subjects (12) viewed a 3 c/deg white sine-wave grating (0.92 contrast) in a Badal stimulus system. The contrasts (amplitudes) of the red, green and blue components of the white grating changed independently to simulate a grating oscillating from 1 D behind the retina to 1 D in front of the retina at 0.2 Hz. Subjects responded strongly to the chromatic simulation but poorly to a luminance control. The results support the hypothesis that focus is specified by the contrast of spectral-wavebands of the retinal image, and that conventional color mechanisms, monitoring chromatic contrast at luminance borders (1–8 c/deg), mediate the signals that specify dioptric vergence.

The addition of a uniform increment of luminance (L) to a faded retinally-stabilized target results in the subjective reappearance of the image with contrast opposite to that of the target. This phenomenon, called apparent phase reversal (APR), reveals a nonlinear gain mechanism in the adaptation process. The magnitude of the threshold increment to elicit APR (Lapr) is a measure of the state of stabilized adaptation. In the experiments reported here, Lapr was studied as a function of background luminance (Lo) and contrast (m) of the adapting stimulus. It was found that Lapr increases with increasing Lo, but does not depend on m. The data are analyzed within the context of a previously proposed model of stabilized image fading consisting of a multiplicative inverse gain followed by a subtractive process. It was found that the addition of a contrast processing stage was required to account for the relationship between Lapr and m.

We have investigated the spatiotemporal transfer function of human "reflex" accommodation. An accommodative mechanism that is sensitive to an intermediate temporal rate of retinal image contrast change is proposed as the basis of the fine focus control hypothesis. To test the proposed mechanism accommodative responses were monitored by a dynamic infrared optometer while the subject focused on sinusoidal gratings (0.98-10.5 c/deg) which were moving sinusoidally at temporal frequencies in the range of 0.05-0.80 Hz over a 0.50 or 2.00 D peak-to-peak amplitude. The accommodative responses were best at 3 and 5 c/deg at both amplitudes of target motion. This result does not support the proposed mechanism or the fine focus control hypothesis for "reflex" accommodation. Fitting the data with first-order response functions showed little evidence of prediction. In addition, a second experiment found that the profile of the accommodative gain function is not altered by instruction at spatial frequencies above 5 c/deg in this type of dynamic accommodation experiment. The use of sinusoidally moving accommodative blur targets, particularly with careful instruction, seems to discourage voluntary accommodation in investigations of "reflex" control mechanisms of accommodation.

During a pursuit eye movement made across a stationary stimulus, that stimulus is often perceived as moving slightly in the direction opposite to the eyes (Filehne illusion). The illusion is generally thought to increase in strength when the stimulus is made visible only briefly. In two experiments the illusion was indeed observed with young subjects. However, with older subjects brief stimulus presentations yielded a strong inverted Filehne illusion (the stimulus appeared to move in the same direction as the eyes). This age dependency of the Filehne illusion is caused by an increase of only the threshold for stimulus motion in the direction opposite to the eyes. No such effect happens with the threshold for stimulus motion in the same direction as the eyes. These findings can be explained if we assume that with increasing age it takes more time to properly register retinal image velocity within the perceptual system.

In human far peripheral vision, drifting stimuli of particular periodicities appear to move in the opposite direction from their true direction of motion. This “reverse motion illustion” is a consequence of spatial undersampling of the retinal image. The illusion occurs for spatial frequencies an order of magnitude lower than that expected on the basis of anatomical measurements of human photoreccptor density. We conclude that for naturally imaged stimuli the site of undersampling in far peripheral vision must be post-receptoral.

The relative visual positions of briefly flashed stimuli are systematically modified in the presence of motion signals. We have recently shown that the perceived position of a spatially extended flash stimulus is anisotropically shifted toward a single convergent point back along the trajectory of a moving object without a significant change in the perceived shape of the flash [Watanabe, K., & Yokoi, K. (2006). Object-based anisotropies in the flash-lag effect. Psychological Science, 17 , 728–735]. In the previous experiment, the moving stimulus moved in both retinotopic and environmental coordinates. In the present study, we examined whether the anisotropic mislocalization depends on retinotopic or object motion signals. When the retinal image of a moving stimulus was rendered stationary by smooth pursuit, the anisotropic pattern of mislocalization was not observed. In contrast, when the retinal image of a stationary stimulus was moved by eye movements, anisotropic mislocalization was observed, with the magnitude of the mislocalization comparable to that in the previous study. In both cases, there was little indication of shape distortion of the flash stimulus. These results demonstrate a clear case of object-based mislocalization by retinotopic motion signals; retinotopic—not object—motion signals distort the perceived positions of visual objects after the shape representations are established.

The extent of perceived motion smear was compared for targets that underwent similar velocities of retinal image motion during the vestibulo-ocular reflex (VOR) in the dark, the visually enhanced VOR (VVOR), VOR suppression, and fixation. Compared to the extent of perceived motion smear during fixation, observers reported significantly less smear when the target moved either in the same direction or against the direction of the head movement during the VVOR and VOR. We also confirmed a previous finding that perceived smear is attenuated asymmetrically during VOR suppression, with attenuation occurring primarily for targets that move against the direction of the observer’s head motion. The results support the hypothesis that the visual system employs extra-retinal signals that accompany eye and head movements to reduce the perception of motion smear for targets that move physically in the opposite direction of eye and/or head movements. 2006 Elsevier Ltd. All rights reserved.

A primary task of the visual system is to extract the direction and speed of animate objects from the retinal image. We examined global speed processing by determining how local speeds are integrated and whether integration occurs across all speeds or within fixed speed ranges. The first experiment addressed how local motion signals are combined to determine the speed of an object in motion. Observers judged the speed of a moving cloud of dots that took a random walk in direction while the dots inside the cloud moved somewhat independently of the cloud itself. The apparent speed of the cloud of dots is found to change in proportion with the dot speed and is well predicted by calculating the average speed resulting from nearest neighbour matches across stimulus frames. The second experiment addressed whether local speeds are combined across all speeds or within fixed speed ranges for the detection of global motion. Global dot motion (GDM) stimuli that moved in a radial or rotational directions moving at a low speed of 1.2 degrees /s or a high speed of 9.6 degrees /s were used to measure the thresholds for detecting structured motion as a function of the speed of noise dots (0 degrees /s-10.8 degrees /s) added to the stimulus. With low-speed targets, only additional noise dots moving at low speeds interfered with signal detection. High-speed targets were only interfered with by dots moving at high speeds. This finding established the existence of at least two independent speed tuned systems in the range of speeds tested. Experiment 3 investigated how speed signals are combined within a system to determine the global speed. Using sectored radial GDM stimuli the perceived speed of the fastest dots was measured as a function of whether the speed of the dots in alternate sectors either activated the high or low-speed systems. Averaging only occurred when dots were all within the sensitivity range of the high-speed system, however, if alternate sectors activated separate speed systems, averaging did not occur. Thus local speeds are averaged, independent of direction, to derive a global speed estimate, but averaging only occurs within, and not across, speed tuned mechanisms.

This study determined how contour interaction (the degradation of visual acuity by the presence of nearby contours) is affected by the incessant retinal image motion that occurs in observers with congenital nystagmus (CN). Visual acuity was measured for single, high-contrast, black Landolt Cs, presented without and with flanking bars (contour-to-C separation = 1, 2, 5 or 10 multiples of the gap width of the C). Stimuli were presented against either a white or a black surround. For comparison, acuity was also determined in normal observes, with and without motion of the stimulus to simulate the retinal image motion in jerk CN. The results show that the peak magnitude of contour interaction (the maximal degradation in acuity attributable to contour interaction) is significantly larger in the observers with CN than in normals. When acuity targets are presented against a black surround, contour interaction also occurs over a wider spatial extent in the observers with CN. Imposed image motion increases the extent of contour interaction in normal observers, but not sufficiently to account fully for the results of the observers with CN. We suggest that the additional contour interaction found in observers with CN may be attributable to the presence of amblyopia. For a small contour-to-C separation, contour interaction is significantly greater when stimuli are presented against a black rather than a white surround. Consequently, single-letter acuity may be appreciably underestimated clinically when an adjustable window is used to isolate letters on a projected acuity chart.

A photic stimulus flashed just before a saccade in the dark tends to be mislocalized in the direction of the saccade. This mislocalization is not only perceptual; it is also expressed by errors of ocular targeting. A particular situation arises if the point of light is flashed twice at the same place, the second time, just before a saccade. The point of light may appear at two different places even though neither the site of its retinal image nor the direction of gaze change between the flashes. Experiments were run on five human subjects, head fixed in the dark, with flashes repeated at the site of the saccade goal or at the initial point of fixation. In both cases, the test stimulus was mislocalized. However, its apparent displacement never produced the perception of a streak. Streak were reported only when there was an actual stimulus movement on the retina (e.g. by flashing the stimulus during the saccade). Mislocalization did not occur if the two flashes were not separated by a dark interval. This implies that, as long as a steady stimulus remains continually visible, there is no updating of the internal representation of eye position assumed to be used for stimulus localization.

Smooth pursuit initiation to step-ramp stimuli was investigated in normal subjects, young and elderly. Older subjects had significant reductions in initial pursuit acceleration before saccades, and in post-saccadic and peak pursuit velocities. Aging impairs the open-loop performance of the pursuit system, possibly by decreasing sensitivity to retinal image motion or by limiting the conversion of visual motion signals into commands for smooth pursuit. Lower open-loop pursuit gain degrades steady-state, closed-loop smooth pursuit in senescence. Our elderly subjects also made less accurate saccades to moving targets, implying defective use of visual motion information by the saccadic system.

Myopia, as characterized by a large refractive error (e.g. −10.7 ± 0.4 D), was induced in post-hatch chicks by a 14 day application of a goggle that was designed to blur the retinal image. In comparison to untreated eyes, the treated eye showed significant changes in wet eye weight and both axial and equatorial lengths. However, the lenses of myopic and non-myopic eyes were not significantly different in focal characteristics, light transmittance or total soluble protein content. Thus the lens neither contributes to, nor compensates for the large refractive error observed in experimentally induced myopia.

We had the rare opportunity to examine a patient with a unilateral complete ophthalmoplegia in an eye with normal visual acuity. We recorded the movements of the covered, sound eye of the patient during full-field optokinetic stimulation of his seeing, immobile eye (open loop condition). The open loop gain for low velocity stimuli (less than 1 deg/sec) was high (greater than 50) but progressively fell with faster stimuli. This finding is inconsistent with the representation of the optokinetic system as a linear velocity servo. In a second experiment, the patient foveally fixated, with the immobile eye, a head-fixed target during sinusoidal, en block rotation. The vestibular movements of the sound eye were suppressed, even though no retinal image motion occurred in the seeing eye. This suggests that visual attention to a stable retinal image can be used to suppress the vestibulo-ocular reflex. Our results were from one patient only and any generalizations must be made cautiously. Nevertheless, our findings are comparable with previous "open loop" studies in normal human beings and patients.

The influence of smooth pursuit eye movements on the visibility of stationary and moving objects was investigated by measuring eye movements and contrast thresholds for seeing bright and dark bars of a 7 cycle (5.14c/deg) sine grating in three different viewing conditions: (1) Subjects maintained fixation, without saccades, on a stationary 2-point target while the grating oscillated at a constant speed across the fovea (2) Subjects smoothly pursued the 2-point target superimposed on the oscillating grating and moving with it (3) Subjects smoothly pursued the 2-point target back and forth across the stationary grating. It was found that: (1) Inaccuracies of smooth pursuit eye movements produce substantial amounts of retinal image motion for target velocities greater than 1†/sec (2) Contrast thresholds during smooth pursuit were equal to contrast thresholds during maintained fixation when equal amounts of retinal image motion were present (3) Self-imposed and externally-imposed retinal image motions have equal effects on contrast detection (4) Improvements in contrast sensitivity were accompanied by improvements in smooth pursuit accuracy. These results imply that: (1) Visual sensitivity is not reduced by smooth pursuit eye movements. (2) The visibility of moving objects is primarily limited by velocity errors of smooth pursuit eye movements for the range of target velocities studied (49–420′/sec).

The illuminance distribution in the retinal image of a luminous line was measured using a fiber optic probe inserted into the otherwise intact eye of the anesthetized cat. This method allowed the line-spread function of the eye to be determined over a very large range of distances from the center of the image. While the line-spread function fell to half-height within a few minutes of arc for all pupil sizes (1.25′ for the optimum pupil diameter of 1 mm), the tail of the function declined inversely as the three-halves power of the distance and had not reached zero at 20°.

Using the counterphase-modulated-tritan-metamers method of Wisowaty and Boynton [Vision Res.20, 895–909 (1980)], we measured the modulation sensitivity function of the blue mechanism on dark backgrounds and on uniform yellow backgrounds that were produced either by illuminating the retina with yellow light or by filling-in of a stabilized retinal image. When measured on yellow backgrounds, temporal modulation sensitivity was markedly reduced whether the background was produced by retinal illumination or by filling-in. This sensitivity reduction has been attributed by other researchers to changes in the transmission characteristics of the blue/yellow pathway caused by yellow adaptation. Our results obtained with backgrounds that appeared yellow due to filling-in indicate that the site of adaptation must be extraretinal as there was no yellow light reaching the retina in the vicinity of the test probe.

It has long been recognised that chromatic aberration can introduce luminance artifacts into nominally isoluminant colour stimuli. In this study the effects of chromatic aberration (along with those of defocus and stimulus spatial frequency) on the chromaticity of the retinal image are considered. Such optical effects have important methodological and functional implications for visual physiology. The "Silent Substitution" principle is a fundamental feature of modern colorimetry, being employed in both psychophysical and electrophysiological approaches to the visual system. The theoretical colour spaces introduced by MacLeod and Boynton (1979) and Derrington et al. (1984) are also ultimately based on this principle. All such applications of the silent substitution principle are sensitive to the optical effects of chromatic aberration, defocus, spatial frequency and stimulus chromaticity. The spatial acuity of the mechanisms of colour vision are appreciably lower than those of the luminance system (Mullen, 1985). In addition chromatic aberration has been shown to be a cue to ocular accommodation (Fincham, 1951). The analysis presented in this study suggests a possible explanation for these findings in terms of the ecological and computational constraints placed on the visual system by chromatic aberration.

A prior study showed that after horizontal saccades the abducting eye has little post-saccadic drift (about 0.5 deg/sec) while the abducting eye has considerable onward drift (about 1.7 deg/sec). To investigate this further, five subjects patched one eye for three days. This reduced the drift after adducting saccades in the viewing eye to the level of that after abducting saccades. The changes were a combination of conjugate and disconjugate alterations. Decreases in drift in the viewing eye did not cause increases in drift in the covered eye. These changes appear functional in that retinal image slip is decreased in the viewing eye but why this goal is not attained when both eyes habitually view is not understood. Also, post-saccadic drift could depend on which eye was used to view the target.

The amplitudes and implicit times of the P-1 component of monocular transient pattern VERs elicited by reversing checkerboard targets were measured in 6 conditions: (1) Ocular accommodation (Ao) elicited by minus power ophthalmic lenses; (2) Ao stimulated by minus lenses with the natural pupil of the test eye dilated and replaced by an artificial pupil; (3) accommodative demand and pupil size fixed with retinal image size (Ir) changed by afocal minifying lenses, (4) accommodative demand and Ir fixed but pupil size changed; (5) Ao elicited by minus lenses for three sizes of reversing checkerboards, and (6) Ao elicited by minus lenses for a constant checkerboard size with three check sizes. The observed decrease in VER amplitude with Ao could not be attributed entirely to the effects of pupil size or Ir. The rate at which the amplitude of P-1 decreased with Ao was not affected by checkerboard check size, but was closely correlated with checkerboard field size. The implicit time of P-1 first decreased slightly below baseline values at low and moderate levels of Ao and then tended to increase significantly at higher levels. The increase in implicit time at high levels of Ao could be explained adequately by the effects of pupil size alone on the VER. However, where Ao occurred without pupillary miosis, the increase in implicit time could only be explained by focusing error.

Binocular eye position was monitored by the photoelectric technique during accommodative vergence. Contrary to previous reports indicating that accommodative vergence was a uniocular phenomenon, without exception, binocular accommodative vergence movements were recorded. The total vergence amplitude in the viewing eye was reduced, on the average, by approximately 88% with respect to the vergence movement measured in the covered eye. Some saccadic eye movements that occurred during vergence movements were likewise reduced in amplitude in the viewing eye by up to 20%. Smooth eye movements were utilized to counteract the vergence movement in the viewing eye. This smooth movement alone, or in conjunction with a late saccade, returned the eye to the target and helped to maintain the retinal image of the target coincident with the foveal center for the duration of the accommodative vergence movement. Thus, there appears to be a fixation-holding mechanism which produced a general attenuation of both vergence and some saccadic movements in the viewing eye. Although this control strategy produced violations of Hering's law with respect to the magnitude of the movements in the eyes but not with respect to the direction of the movement, it was implemented in the interest of retaining the target within the sensitive foveal region.

The latency of the first major positive component (P1) of the transient visually evoked potential (VEP) was measured using a checkerboard pattern stimulus under varying conditions of retinal blur. One emmetropic and four ametropic subjects were tested with spherical lenses of varying dioptric power. The results showed that P1 latency was shortest when visual acuity was optimal (i.e. when the pattern was least blurred) and that defocusing the retinal image had a greater effect on VEP latency for small checks (intermediate spatial frequencies) than for large checks (low spatial frequencies). When emmetropic subjects were tested with cylindrical lenses of varying dioptric power placed at each of four orientations (180°, 45°, 90° and 135°), P1 latency was delayed when the lenses were oriented such that both fundamental Fourier components of the checkerboard were blurred but was unaffected as long as at least one of the two oblique fundamentals remained in focus.

Vestibular, optokinetic, and spontaneous eye and head movements have been examined in the hooded rat. Eye movement range was 18–20°, and frequency of ocular saccades was 5–20/min; there was a weak linkage of eye and head movements and a weak vestibulocollic reflex. Response to optokinetic stimulation with unity gain (eye velocity matches drum velocity) was seen only at velocities below 1 deg/sec: maximal eye velocity evoked by drum velocities of over 20 deg/sec never exceeded 4–6 deg/sec. These motor responses were not altered by head movements: thus gaze velocity is not improved by optocollic (head movement) responses, and such optocollic activity occurs only when substantial retinal image motion is present.

Lid closure initiated early in life produces axial myopia in a variety of species. However, it is currently not known what aspects of the anomalous visual experience associated with lid closure disrupt the emmetropization process and cause abnormal ocular axial elongation. This study was designed to determine if a degradation in the quality of the spatial characteristics of the retinal image was sufficient to produce an experimental myopia. Optical rearing procedures were employed to defocus one eye of developing kittens, and retinoscopic and ultrasonic procedures were used to evaluate the effects of chronic optical defocus on the kitten's refractive status. Different defocusing lens designs and rearing protocols were included to evaluate variables that may have confounded previous investigations. The major finding is that early chronic optical defocus results in axial myopia. The prevalence and magnitude of the induced refractive errors were dependent on the magnitude of optical defocus, but they were not affected by the sign or form of the defocusing lens. The results demonstrated that the potential for a clear retinal image is important for regulating normal ocular growth and maintaining a near emmetropic refractive status.

The steady-state, monocular, accommodation response to sinusoidal grating targets (spatial frequencies 0.67, 2.0, 6.0 and 18 c/deg) at effective distances between 0.2 and 1.0m was measured with a laser optometer, as a function of the grating modulation. In general, only a rather weak dependence of the response on modulation was observed. The results are discussed in terms of the theoretical modulation of the retinal image as a function of defocus. It is argued that the accommodative system usually works to produce a contrast in the grating image which is well above the detection threshold. Considerable inter-subject differences in response were observed, suggesting that a variety of strategies may be employed by different subjects when engaged upon the same voluntary accommodation task.

Two cylindrical lenses of equal power, one positive and one negative, were used to correct astigmatism of oblique incidence in a wide-angle optical model of the human eye. Tangential and sagittal vergences were calculated at different visual angles from the posterior pole to the peripheral retina. The positive cylinder was placed with its axis parallel to the tangential meridian while the negative cylinder was placed with its axis parallel to the sagittal meridian. Improvement in the quality of the retinal image at different angles of incidence was obtained by varying the angle between the axes of the correcting cylinders. Corresponding gains in resolution have been calculated and confirmed by peripheral fundus photography.

The fusional vergence system is under the control of a fast neural integrator which aligns the eyes and a slow neural integrator which maintains binocular alignment. These controllers are distinguished by their decay time constants and their stimuli. Previous studies indicate that the fast fusional vergence controller responds to retinal image disparity and the slow fusional vergence controller responds to the output of the fast neural integrator. Slow fusional vergence as evidenced by adaptation of the phoria is unaffected by accommodative vergence when disparity vergence is open loop. Under closed loop conditions both accommodation and disparity induced vergence influence slow fusional vergence. These results indicate that the slow vergence controller is located before the site of interaction between convergence and accommodation. Fixation disparity is described as a steady-state error of the neural integrator controlling fast fusional vergence and its amplitude is shown to be inversely related to adaptation of the phoria to prism.

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 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 percentage of time (V) for which a sinusoidally flickered stabilised retinal image is visible, was measured for frequencies in the range 0–25 c/s, and in relation to several parameters. It was found that V reaches a maximum at a certain frequency, fp, and decreases fairly rapidly for frequencies either side. For a 1.5° bipartite field, located on the fovea, flicker has an overall positive effect on V, and fp is around 5 c/s for all colours except blue, which gives anomalous results. The value of fp decreases with decreasing luminance, with increasing field size, or as a small field is moved to the parafovea. Variations of contrast, or blurring the field, have no effect on fp although V is affected. If square-wave or highly modulated flicker is used, non-linear effects become pronounced and fp depends on colour. The results are discussed in relation to the increase in size of retinal receptive fields with distance from the fovea, and in relation to the “pseudo-resonance” ideas of de Lange and Kelly.

The behaviour of accommodation and convergence of the eyes in total darkness and in very low illumination has been studied by an objective method. Also the ability of subjects to accommodate and converge to the position of their own finger held up in darkness has been recorded. Simultaneous records show that both accommodation and convergence are active in darkness, but fluctuate without relation to one another. Results also show that the brain is unable to use the information from proprioceptor nerves of the hand and arm to direct the eyes and control accommodation in darkness. The synergic relation between the innervations to accommodation and convergence still holds near the threshold, although the accommodation activity of night myopia is present, but when there is no retinal image the two mechanisms act independently.

Several research-workers in psychophysical visual experiments have noticed that a large stimulus at threshold level is often seen in its entirety. If we consider this observation in connexion with current theories on the visual threshold mechanism it can only be explained by supposing the existence of a mechanism that facilitates subliminal activity whenever a supraliminal effect is present in the retinal image. Experiments were undertaken to study the spatial and temporal aspects ot this facilitation mechanism. The distance and the time within which it is active exceed the distance and time within addition of subliminal effects occurs, to a degree that depends upon the experimental conditions which determine the density of retinal activity elicited by the stimulus.

From numerical values previously chosen for all the elements of the eye and combined in a way to introduce determined variations and anomalies, we study a certain number of eyes, with paraxial approximation, by automatical matricial calculation. After an account of the method of calculation used, some results are given, concerning: 1. A quantitative description of the accommodation, variation of the optical system, according to the deformations that interfere in focusing: power, principal points, nodal points, shape factor, and fluctuations of the retinal image in a given interval of accommodation, and for a determined way, in the diagram of the possible states of accommodation. 2. The participation of the divers components to the adjustment of the eye, during the emmetropisation phase, the variations of the shape factor of the eye of a child in the course of its evolution and their repercussion on the size of the retinal image—the precision to find for each element, in the measurement on the living eye. 3. The power, the refraction and the retinal image, of some eyes obtained by combining elements which the values stay in the dispersion given by statistical studies: divers emmetropic eyes, combination ametropic eyes, pathological ametropic eyes, aphakic eyes coming from these divers eyes, and corrected with eye glasses or contact lenses. Power, refraction, shape factor, and retinal image are calculated each time, in making interfering all the elements.

Microelectrode recordings were made from retinal ganglion cells in cats, and the receptive field organization of these cells was investigated with particular reference to the effect of defocusing the retinal image. Cells within 5° of the area centralis were critically affected by defocusing the stimulus spot, while peripheral cells continued to respond to defocused stimuli. These differences have been related to differences in the receptive field organization of central and peripheral ganglion cells in respect of the spatial distribution of excitation and inhibition, and the “sustained” and “transient” response types. These results form the basis of a hypothesis about the causation of various forms of amblyopia. For instance, it is suggested that amblyopia which follows long-standing squint of early onset may be a functional degeneration resulting from the habitual exposure of the fovea of the squinting eye to defocused images of low constrast and detail, which are shown here to be inadequate stimuli for central retinal ganglion cells.