Intra- and inter-hemispheric processing during binocular rivalry in mild glaucoma.

Glaucoma is considered a progressive optic neuropathy because of the damage and death of the retinal ganglion cells. It is also a neurodegenerative disease because it affects neural structures in the visual system and beyond, including the corpus callosum-the largest white matter structure involved in inter-hemispheric transfer of information. In this study we probed the dysfunction of the inter-hemispheric processing in patients with mild glaucoma using the phenomenon of binocular rivalry. Patients with mild glaucoma and no measurable visual field defects and age-matched controls underwent a thorough visual assessment. Then they participated in a series of psychophysical tests designed to examine the binocular rivalry derived from intra- and inter-hemispheric processing. Static horizontal and vertical sinewave gratings were presented dichoptically using a double-mirror stereoscope in 3 locations: centrally, to probe inter-hemispheric processing, and peripherally to the left or to the right, to probe intra-hemispheric processing. Although the two groups were matched in functional measures, rivalry rate of the glaucoma group was significantly lower than that of the control group for the central location, but not for the peripheral location. These results were driven mainly by the patients with normal tension glaucoma whose average rivalry rate for the central location (from which information reaches the two hemispheres) was almost half (46% lower) that of the controls. These results indicate a dysfunction in inter-hemispheric transfer in mild glaucoma that can be detected behaviourally before any changes in standard functional measures.

Dominance wave propagation during binocular rivalry in mild glaucoma.

Glaucoma is both a progressive optic neuropathy and a neurodegenerative disease affecting structures in the primary visual pathway. Other vision-associated areas may also be affected, including the corpus callosum which is involved in inter-hemispheric transfer. This study evaluated dominance wave propagation during binocular rivalry to probe the efficacy of the inter-hemispheric transfer in 20 patients with mild open angle glaucoma and 25 age-matched controls. The two groups were matched for functional measures such as stereo-acuity, binocular visual acuity, and visual field mean deviation. Monocular functional and structural measures were equivalent for the left and right eye of each participant. Using Wilson et al.'s travelling wave paradigm [Nature, 412 (2001) 907-910], intra- and inter-hemispheric failure rates of traveling wave transmission and the travelling wave propagation times were recorded for the two groups. For the control group, the wave propagation failure rate was significantly greater for the inter- than for the intra-hemispheric condition, but for the glaucoma group, the failure rates were equally high for the two conditions. The wave propagation time was significantly longer for the inter- than for the intra-hemispheric condition for the control group, while the opposite was true for the glaucoma group. These results reveal changes in the wave dynamics of rivalry dominance in patients with mild glaucoma who otherwise have normal performance on standard functional measures.

Effect of Visual Feedback on the Eye Position Stability of Patients with AMD.

The sources of the reduced fixation stability exhibited by patients with central vision loss in the light are relatively well understood, but we have no information on how they control eye position in complete darkness, in the absence of visual error signals. We therefore explored the effect of visual feedback on eye position stability by testing patients with age-related macular degeneration (AMD) and controls with normal vision in the light and in complete darkness. Nine patients (ages 67 to 92 years) and 16 controls (ages 16 to 74 years) were tested binocularly in the light and in complete darkness while remembering the location of a now invisible target. Binocular eye position was recorded with a video-based eye tracker. Results show that eye position stability both in the light and in the dark is worse for patients than for controls and that, for the two groups, eye position stability in the dark is, on average, 5.9 times worse than in the light. Large instability of fixation in patients with AMD was found even in absolute darkness when the scotoma cannot impair vision. These data reflect permanent changes in the oculomotor reference of patients with AMD.

Smooth pursuit of amodally completed images.

In order to evaluate the effect of the parafoveal area of the retina on smooth pursuit, we compared the horizontal smooth pursuit of visible and amodally completed stimuli in people with central vision loss and controls. In the amodally completed stimuli, a black mask covered the bottom vertex of a moving diamond which is the feature whose movement participants had to track. Both the visible and the amodally completed stimuli moved along regular and irregular sinusoidal motion paths. Our results show that people with central vision loss are able to track a perceptually completed moving stimulus albeit with lower gain, larger tracking errors, and more saccadic eye movements than people with normal vision. Just as the controls, however, people with central vision do better in the no mask condition showing that visual feedback from eccentric vision can improve performance.

Vection Responses in Patients With Early Glaucoma.

PURPOSE: Our lab has previously shown that patients with early glaucoma have longer vection latencies than controls. We attempted to explain this finding using a combined index of structure and function (CSFI), as proposed by Medeiros and colleagues. The CSFI estimates the proportion of retinal ganglion cell loss. METHODS: Roll and circular vection were evoked using a back-projected screen (experiment 1) and the Oculus Rift system (experiment 2). Vection latency and duration were measured using a button response box. In experiment 1, tilt angles were measured with a tilt sensor, whereas subjective tilt was determined using a joystick attached to a protractor. In experiment 2, subjective vection strength was rated on a 1 to 10 scale. These measurements were compared with the CSFI, which utilizes visual field and optical coherence tomography data. RESULTS: For experiment 1 we tested 22 patients (mean age, 70.3±6 y) with glaucoma and 18 controls (mean age, 54.6±9 y); and for experiment 2 we tested 24 patients (mean age, 71.1 ±5 y) and 23 controls (mean age 61.4±10 y), but not all patients experienced vection. In both experiments, vection latency was significantly longer for patients than for controls (smallest P=0.02). The CSFI was not related to vection latency, duration, or objective and subjective measures of vection strength (smallest P=0.06) in either experiment. CONCLUSIONS: Two experiments have replicated the finding that vection responses are longer in patients with glaucoma than in controls; however, the CSFI is not related to vection responses.

Fixation parameter test-retest repeatability of the worse eye in central field loss.

OBJECTIVE: Patients with bilateral central field loss develop peripheral retinal loci (PRLs) in the functional eccentric retina. PRL characteristics and visual performance in the better-seeing eye (BE) of these patients have previously been reported. In this study, we determined the test-retest repeatability of fixation parameters, including fixation stability, PRL eccentricity, and PRL span in the worse-seeing eye (WE). DESIGN: Retrospective consecutive case series. PARTICIPANTS: Thirty-six patients with bilateral central field loss referred from the Toronto Western Hospital Retina Clinic, who had completed 2 consecutive fixation examinations on the same day. METHODS: Fixation stability was recorded using the Nidek MP-1 microperimeter (Nidek Technologies Srl., Padova, Italy). For each fixation recording, the following parameters were retrieved: (i) 68.2% bivariate contour ellipse area (BCEA), (ii) PRL span (major and minor axes of the BCEA), (iii) PRL meridian (polar angle), and (iv) PRL eccentricity. Test-retest repeatability for each parameter was assessed using Bland-Altman plots to determine 95% limits of agreement. RESULTS: The mean difference between the fixation trial pairs and the 95% limits of agreement for fixation stability, PRL major axis, PRL minor axis, PRL meridian, and PRL eccentricity were 0.06 ± 0.47 log deg2, 0.05° ± 1.42°, 0.07° ± 0.63°, -0.44° ± 66.0°, and -0.23° ±1.56°, respectively. CONCLUSIONS: The fixation parameters in the WE showed robust repeatability, comparable to that of the BE as determined from previous studies. The WE's fixation repeatability should be considered in the interpretation of fixation outcome measures subsequent to treatment interventions.

Image Stabilization in Central Vision Loss: The Horizontal Vestibulo-Ocular Reflex.

For patients with central vision loss and controls with normal vision, we examined the horizontal vestibulo-ocular reflex (VOR) in complete darkness and in the light when enhanced by vision (VVOR). We expected that the visual-vestibular interaction during VVOR would produce an asymmetry in the gain due to the location of the preferred retinal locus (PRL) of the patients. In the dark, we hypothesized that the VOR would not be affected by the loss of central vision. Nine patients (ages 67 to 92 years) and 17 controls (ages 16 to 81 years) were tested in 10-s active VVOR and VOR procedures at a constant frequency of 0.5 Hz while their eyes and head movements were recorded with a video-based binocular eye tracker. We computed the gain by analyzing the eye and head peak velocities produced during the intervals between saccades. In the light and in darkness, a significant proportion of patients showed larger leftward than rightward peak velocities, consistent with a PRL to the left of the scotoma. No asymmetries were found for the controls. These data support the notion that, after central vision loss, the preferred retinal locus (PRL) in eccentric vision becomes the centre of visual direction, even in the dark.

Changes in Fixation Stability with Time during Binocular and Monocular Viewing in Maculopathy.

The purpose of this study was to examine changes in fixation stability over time during binocular and monocular viewing in patients with age-related macular degeneration (AMD). Seventeen patients with AMD and 17 controls were enrolled. Using an EyeLink eyetracker (SR Research Ltd., Mississauga, Ontario, Canada), fixation stability was recorded binocularly and monocularly with each eye for a duration of 15 s while the fellow eye was covered. Fixation stability was analyzed over 3 s intervals for each condition using a 68% bivariate contour ellipse area. Fixation stability did not change with time during binocular viewing for both groups, both monocular conditions for the control group, and monocular viewing with the better eye for the AMD group. However, during monocular viewing with the worse eye, the test of within-subject contrasts showed linear improvement in fixation stability with time (p = 0.016). In conclusion, in patients with AMD, monocular fixational control with the worse eye is poor, but improves with time.

The reading accessibility index and quality of reading grid of patients with central vision loss.

PURPOSE: In this study we evaluated the reading accessibility index (ACC) and a quality of reading grid as assessment tools for reading and as outcome measures for reading rehabilitation of patients with central vision loss. METHODS: Reading performances on the MNRead chart (www.precision-vision.com) were reviewed from our research database. Participants were 24 controls with normal vision [mean age: 34 (SD, 14) years] and 61 patients with bilateral central vision loss [mean age: 81 (SD, 9) years] among which a subgroup of 18 patients [mean age, 76 (SD, 13) years] had undergone perceptual learning training for reading rehabilitation. The outcome measures were maximum reading speed, reading acuity, critical print size, ACC, and the reading quality. A reading quality grid that classified reading speed as spot, slow, functional, or fluent and print size as small, regular, medium, or large was used. All reading speed values were normalised (i.e., divided by 200, the average reading speed in young adults with normal vision measured with the MNRead). RESULTS: The ACC was associated perfectly with the maximum reading speed in the control group (r22  = 0.99, P < 0.001) and strongly with all parameters of reading in the patient group (smallest r value: r59  = -0.66, P < 0.001). For patients with central vision loss, reading was functional for large print, but slow for medium print and spot for regular print. For some patients with the same ACC values, the quality of reading grid revealed important performance differences. For the subgroup (n = 18) of patients who were trained, the ACC revealed a greater effect of training than the other three parameters of reading, and although there were statistically significant improvements across all print size categories, a qualitative improvement in reading was noticed only for the medium print sizes. CONCLUSIONS: The ACC is a good measure of reading performance in patients with central vision loss. Examining reading quality for different print size categories can provide a more detailed picture of reading impairment and should be considered as an outcome for rehabilitation in addition to the ACC.

Mechanisms of Image Stabilization in Central Vision Loss: Smooth Pursuit.

SIGNIFICANCE: In addition to understanding the adaptive mechanisms of eccentric viewing during smooth pursuit, the ocular motor adaptations of patients with central vision loss give us a better understanding of the basic mechanisms of smooth pursuit in the healthy visual system. PURPOSE: For patients with age-related macular degeneration and controls with normal vision, we examined the closed-loop gain of horizontal and vertical smooth pursuit eye movements as a function of stimulus speed and direction. We hypothesized that pursuit gain functions would be affected by stimulus speed and the location of the preferred retinal locus (PRL) in relation to the scotoma as determined by a fixation stability task using a microperimeter. Specifically, that a PRL on the left of the scotoma in the visual field would decrease the rightward gain relative to the leftward gain and a PRL below the scotoma in the visual field would decrease the upward gain. METHODS: Ten patients and 15 controls were tested in a step-ramp procedure with direction (left/right for horizontal motion; up/down for vertical motion), speed (5, 10, 15, 20, and 30 deg/s), and five replication conditions randomized and blocked by orientation (horizontal vs. vertical). RESULTS: Horizontal pursuit had a higher gain than vertical pursuit. The two eyes of the patients moved conjugately with similar smooth pursuit gains. For horizontal pursuit, all patients, regardless of PRL location, showed significantly better pursuit of leftward motion. For vertical pursuit, downward pursuit had a higher gain than upward pursuit for most patients. CONCLUSIONS: PRL location was not predictive of the directional preponderance of pursuit performance. These results imply that patients may not use the PRL that was initially found during a static fixation task; they may adapt to the task by using a PRL that appears more suitable.

Fixation Stability Recording: How Long for Eyes with Central Vision Loss?

PURPOSE: Fixation examination with the MP-1 microperimeter (Nidek Technologies Srl., Vigonza, PD, Italy) determines the preferred retinal loci and fixation stability in patients with central vision loss. It is typically done for periods of 15 to 30 s as per the manual's recommendations, which are arbitrary. In this study, we examined (1) whether fixation stability depends on the duration of recording and (2) whether fixation stability changes over time. METHODS: Raw eye-position data from 76 patients with bilateral central vision loss (mean age = 80 ± 9.6 yrs) who had fixation examination recordings with the MP-1of at least 15 s were used. Bivariate contour ellipse areas (BCEAs) were calculated and compared for intervals of 0 to 5 s, 0 to 10 s, and 0 to 15 s and for three consecutive 5-s intervals (0-5 s, 5-10 s, and 10-15 s). Ellipse's centroid location, axes extent, and tilt angle were also evaluated for each of these intervals. RESULTS: BCEA worsened significantly with increasing the time of fixation recording (P < .001). Compared to the BCEA during the first 5 s of examination recording, median BCEA increased by a factor of 1.4 for the first 10 s and of 1.6 for the first 15 s of recording. However, the bivariate ellipses for the three consecutive 5-s intervals were the same in terms of area, centroid location, and axes extent, but differed significantly in tilt angle (P = .005). Fixation stability (BCEAs) results were also confirmed with an additional analysis performed on shorter sampling intervals. CONCLUSIONS: Fixation stability deteriorates with increasing duration of the fixation recording, but when fixation is evaluated in shorter consecutive 5-s intervals, only a difference in ellipse's tilt angle is found. These results suggest that the current recommendations for fixation stability recording with the MP-1 can be shortened to a less demanding duration.

Aperture extent and stimulus speed affect the perception of visual acceleration.

Humans are generally poor at detecting the presence of visual acceleration, but it is unclear whether the extent of a field of moving objects through an aperture affects this ability. Hypothetically, the farther a stimulus can accelerate uninterrupted by an aperture's physical constraints, the easier it should be to discern its motion profile. We varied the horizontal extent of the aperture through which continuously accelerating or decelerating random dot arrays were presented at different average speeds, and measured acceleration and deceleration detection thresholds. We also hypothesized that manipulating aperture extent at different speeds would change how observers visually pursue acceleration, which we tested in a control experiment. Results showed that, while there was no difference between the acceleration and deceleration conditions, detection was better in the larger than small aperture conditions. Regardless of aperture size, smaller acceleration and deceleration rates (relative to average speed) were needed to detect changing speed in faster than slower speed ranges. Similarly, observers tracked the stimuli to a greater extent in the larger than small apertures, and smooth pursuit was overall poorer at faster than slower speeds. Notably, the effect of speed on pursuit was greater for the larger than small aperture conditions, suggesting that the small aperture restricted pursuit. Furthermore, there was little difference in psychophysical and eye movement data between the medium and large aperture conditions within each speed range, indicating that it is easier to detect an accelerating profile when the aperture is large enough to encourage a minimum level of pursuit.

Fixation Stability and Viewing Distance in Patients with AMD.

PURPOSE: People with normal vision perform activities of daily living binocularly, while changing viewing distance frequently and effortlessly. Typically, in patients with age-related macular degeneration (AMD), fixation stability is recorded with monocular instruments at a fixed viewing distance (i.e. optical infinity) to determine the location and precision of the preferred retinal loci (PRLs)-the part of the functional retina that fulfills the role of a pseudo-fovea. Fixation stability recorded with these instruments has been related to performance on visual tasks at shorter viewing distances, although it is not known how viewing distance affects the precision of ocular motor control in these patients. This study examined whether viewing distance affects fixation stability during binocular and monocular viewing. METHODS: Thirty patients with bilateral AMD, 10 older controls, and 10 younger controls participated. Each patient's better eye (BE) and worse eye (WE) were identified based on their visual acuity. Fixation stability was recorded with a binocular eye-tracker at three viewing distances (40 cm, 1 m, 6 m) in binocular and monocular (with BE and with WE) viewing conditions. Fixation stability was evaluated with a bivariate contour ellipse area. RESULTS: For the AMD group, there was no effect of viewing distance on fixation stability, regardless of viewing condition (i.e. binocular, monocular with the BE or with the WE). The same pattern of results was found for the two control groups. CONCLUSIONS: Viewing distance does not affect fixation stability in patients with AMD. Fixation stability data recorded with an instrument at a fixed viewing distance can be related to performance on visual tasks at other viewing distances.

Characteristics of the preferred retinal loci of better and worse seeing eyes of patients with a central scotoma.

OBJECTIVE: Patients with central vision loss develop preferred retinal loci (PRLs) in the eccentric retina. The characteristics of the PRLs for the better eye (BE) are well studied, but not those of the worse eye (WE). We examined the distribution of monocular PRLs in the visual field (VF), as well as visual acuity, fixation stability, and PRL eccentricity for the BE and WE of patients with central vision loss. DESIGN: Retrospective consecutive case series. PARTICIPANTS: A total of 87 BE and 50 WE of patients with central vision loss. METHODS: Visual acuity, fixation stability, PRL location, and PRL eccentricity measures were retrieved from our database. PRL location was categorized into 5 VF segments: central, superior, inferior, left, and right. RESULTS: For BE, PRL frequency distribution was significantly different for the 5 VF segments, χ2(4) = 19.9, p = 0.001. Most PRLs occurred in inferior (31%) and left (31%) VF segments. Visual acuity, fixation stability, and PRL eccentricity depended on the VF segment. Visual acuity correlated with fixation stability and PRL eccentricity. For WE, PRL frequency distribution was not different for the 5 VF segments and visual acuity was not dependent on the VF segment. No relationships between visual acuity and PRL eccentricity or fixation stability were found. CONCLUSIONS: Different patterns of PRL characteristics were found for BEs and for WEs. These findings are important to consider when factors such as PRL eccentricity, visual acuity, fixation stability, and PRL location are used as outcome measures after treatment or rehabilitation and when monitoring disease progression.

Effects of Vertical Direction and Aperture Size on the Perception of Visual Acceleration.

It is not well understood whether the distance over which moving stimuli are visible affects our sensitivity to the presence of acceleration or our ability to track such stimuli. It is also uncertain whether our experience with gravity creates anisotropies in how we detect vertical acceleration and deceleration. To address these questions, we varied the vertical extent of the aperture through which we presented vertically accelerating and decelerating random dot arrays. We hypothesized that observers would better detect and pursue accelerating and decelerating stimuli that extend over larger than smaller distances. In Experiment 1, we tested the effects of vertical direction and aperture size on acceleration and deceleration detection accuracy. Results indicated that detection is better for downward motion and for large apertures, but there is no difference between vertical acceleration and deceleration detection. A control experiment revealed that our manipulation of vertical aperture size affects the ability to track vertical motion. Smooth pursuit is better (i.e., with higher peak velocities) for large apertures than for small apertures. Our findings suggest that the ability to detect vertical acceleration and deceleration varies as a function of the direction and vertical extent over which an observer can track the moving stimulus.

Identifying Absolute Preferred Retinal Locations during Binocular Viewing.

PURPOSE: We present a new method for identifying the absolute location (i.e., relative to the optic disc) of the preferred retinal location (PRL) simultaneously for the two eyes of patients with central vision loss. For this, we used a binocular eye-tracking system that determines the pupillary axes of both eyes without a user calibration routine. METHODS: During monocular viewing, we measured the pupillary axis and the angle between it and the visual axis (angle Kappa) for 10 eyes with normal vision. We also determined their fovea location relative to the middle of the optic disc with the MP-1 microperimeter. Then, we created a transformation between the eye-tracking and microperimeter measurements. We used this transformation to predict the absolute location of the monocular and binocular PRLs of nine patients with central vision loss. The accuracy of the monocular prediction was evaluated with the microperimeter. The binocular PRLs were checked for retinal correspondence and functionality by placing them on fundus photographs. RESULTS: The transformation yielded an average error for the monocular measures of 0.2 (95% confidence interval, 1.0 to -0.6 degrees) horizontally and 0.5 (95% confidence interval, 1.1 to -0.1 degrees) vertically. The predicted binocular measures showed that the PRLs were generally in corresponding locations in the two eyes. One patient whose PRLs were not in corresponding positions complained about diplopia. For all patients, at least one PRL fell onto functional retina during binocular viewing. CONCLUSIONS: This study shows that measurements of the location of the binocular PRLs relative to the pupillary axes can be transformed into absolute locations.

Repeatability of Nidek MP-1 Fixation Measurements in Patients With Bilateral Central Field Loss.

PURPOSE: Visual performance in patients with bilateral central field loss is related to fixation stability. This study evaluated the repeatability of visual-fixation parameters in patients with bilateral central field loss, measured with the MP-1 microperimeter for fixation durations on the order of 15 to 30 seconds. METHODS: Bivariate contour ellipse area (BCEA), and the eccentricity and meridian of the preferred retinal locus (PRL) were determined in 56 eyes of 56 patients, sampled at two investigational sites. Repeatability and agreement were assessed by estimating 95% limits of agreement for each parameter from two fixation examinations conducted on the same day. RESULTS: The 95% confidence intervals (CI) for log BCEA and for PRL eccentricity and meridian were ±0.67 log deg2, ±2.0°, and ±65.9°, respectively. Each CI decreased substantially if a small number of outlying data points were excluded. For all parameters, the mean difference between the two fixation examinations was close to zero. CONCLUSIONS: For most patients with bilateral central field loss, the repeatability of estimated BCEA and PRL eccentricity and meridian is good. When repeated estimates of fixation parameters do not agree, the absence of a well-developed PRL or the use of multiple PRLs may be suspected.

Unaffected smooth pursuit but impaired motion perception in monocularly enucleated observers.

The objective of this paper was to study the characteristics of closed-loop smooth pursuit eye movements of 15 unilaterally eye enucleated individuals and 18 age-matched controls and to compare them to their performance in two tests of motion perception: relative motion and motion coherence. The relative motion test used a brief (150 ms) small stimulus with a continuously present fixation target to preclude pursuit eye movements. The duration of the motion coherence trials was 1s, which allowed a brief pursuit of the stimuli. Smooth pursuit data were obtained with a step-ramp procedure. Controls were tested both monocularly and binocularly. The data showed worse performance by the enucleated observers in the relative motion task but no statistically significant differences in motion coherence between the two groups. On the other hand, the smooth pursuit gain of the enucleated participants was as good as that of controls for whom we found no binocular advantage. The data show that enucleated observers do not exhibit deficits in the afferent or sensory pathways or in the efferent or motor pathways of the steady-state smooth pursuit system even though their visual processing of motion is impaired.

Vection in patients with glaucoma.

PURPOSE: Large moving scenes can induce a sensation of self-motion in stationary observers. This illusion is called "vection." Glaucoma progressively affects the functioning of peripheral vision, which plays an important role in inducing vection. It is still not known whether vection can be induced in these patients and, if it can, whether the interaction between visual and vestibular inputs is solved appropriately. The aim of this study was to investigate vection responses in patients with mild to moderate open-angle glaucoma. METHODS: Fifteen patients with mild to moderate glaucoma and 15 age-matched controls were exposed to a random-dot pattern at a short viewing distance and in a dark room. The pattern was projected on a large screen and rotated clockwise with an angular speed of 45 degrees per second to induce a sensation of self-rotation. Vection latency, vection duration, and objective and subjective measures of tilt were obtained in three viewing conditions (binocular, and monocular with each eye). Each condition lasted 2 minutes. RESULTS: Patients with glaucoma had longer vection latencies (p = 0.005) than, but the same vection duration as, age-matched controls. Viewing condition did not affect vection responses for either group. The control group estimated the tilt angle as being significantly larger than the actual maximum tilt angle measured with the tilt sensor (p = 0.038). There was no relationship between vection measures and visual field sensitivity for the glaucoma group. CONCLUSIONS: These findings suggest that, despite an altered visual input that delays vection, the neural responses involved in canceling the illusion of self-motion remain intact in patients with mild peripheral visual field loss.

Psychophysical point: a disc tends to become a point when Weber's law fails.

In three experiments, we showed that the visual system treats a dot somewhat like a geometrical point, which has a location but no area. We represented a "point" or "dot" with a small disc (diameter of 0.08º of visual angle), and a "disc" with a larger disc (diameter of 1.5º). The Weber fraction of the dot was larger than that of the disc. In Experiment 1, the relative retinal image size cues for depth for the dot and the disc were placed in conflict with the motion parallax cue. We found that the dot indicated the positions defined by the motion parallax cue better than the disc did. In Experiment 2, we placed a constant retinal image size in conflict with convergence eye movements. We found that a binocularly fused dot appeared to move in depth with convergence eye movement, whereas a fused disc appeared to move less. In Experiment 3, we examined the apparent sizes of the afterimages of a dot and a disc and found that Emmert's law failed for the dot afterimage; the apparent size of the dot afterimage changed very little for different distances-as though it had no area-whereas the apparent size of the disc afterimage changed by an extent predicted by Emmert's law. The differences in the dot and disc conditions could not be explained by the differences in the Weber fractions alone.