Interocular Phase Disparity Tuning of Binocular Contrast Summation Depends on Carrier Spatial Frequency and Orientation.

SIGNIFICANCE: Binocular summation is a sensitive metric of binocular integration. As such, characterization of the mechanisms underlying binocular summation is a key step in translating and applying this knowledge to abnormal binocular systems afflicted with strabismus and amblyopia. PURPOSE: Computational models of binocular summation have advocated the operation of mechanisms sensitive to the interocular phase disparity of first-order carrier gratings. This study investigated if such generalization depended on carrier spatial frequency and orientation. METHODS: Monocular and binocular contrast detection thresholds were measured in nine observers with normal binocular vision. Stimuli comprised Gabor targets presented with one of three spatial frequencies (1, 3, and 9 cycles per degree [cpd]), two orientations (horizontal and vertical), and five interocular phase disparities (0, ± π /2, ± π radians). Horizontal and vertical fixation disparities were measured for each binocular threshold condition. Binocular summation ratios were computed by dividing the mean monocular detection threshold by the respective binocular detection threshold. RESULTS: Binocular summation ratio varied significantly with interocular phase disparity for the 1- and 3-cpd horizontal and vertical gratings. Phase dependency was reduced with the 9-cpd horizontal grating and absent for the 9-cpd vertical grating, even though binocular summation ratio exceeded predictions of probability summation. Computational modeling that incorporated the variability of fixation disparity into a vector summation model predicted a reduction in peak binocular summation ratio with increasing carrier spatial frequency but did not account for the reduction of phase sensitivity noted with the 9-cpd stimulus. CONCLUSIONS: Binocular summation magnitude is less dependent on interocular phase disparities for carrier spatial frequencies that exceed 3 cpd, especially with vertical gratings. Although vergence variability due to fixation disparities contributes to the overall reduction in binocular summation magnitude with increasing carrier spatial frequency, it does not provide a complete account for the lack of interocular phase disparity tuning noted with high grating spatial frequencies.

The effect of letter string length and report condition on letter recognition accuracy.

PURPOSE: Letter sequence recognition accuracy has been postulated to be limited primarily by low-level visual factors. The influence of high level factors such as visual memory (load and decay) has been largely overlooked. This study provides insight into the role of these factors by investigating the interaction between letter sequence recognition accuracy, letter string length and report condition. METHODS: Letter sequence recognition accuracy for trigrams and pentagrams were measured in 10 adult subjects for two report conditions. In the complete report condition subjects reported all 3 or all 5 letters comprising trigrams and pentagrams, respectively. In the partial report condition, subjects reported only a single letter in the trigram or pentagram. Letters were presented for 100ms and rendered in high contrast, using black lowercase Courier font that subtended 0.4° at the fixation distance of 0.57m. RESULTS: Letter sequence recognition accuracy was consistently higher for trigrams compared to pentagrams especially for letter positions away from fixation. While partial report increased recognition accuracy in both string length conditions, the effect was larger for pentagrams, and most evident for the final letter positions within trigrams and pentagrams. The effect of partial report on recognition accuracy for the final letter positions increased as eccentricity increased away from fixation, and was independent of the inner/outer position of a letter. CONCLUSIONS: Higher-level visual memory functions (memory load and decay) play a role in letter sequence recognition accuracy. There is also suggestion of additional delays imposed on memory encoding by crowded letter elements.

Binocular capture: the role of non-linear position mechanisms.

When monocular Vernier targets are presented with binocular disparate elements, an increase in vertical separation elevates alignment thresholds and also shifts its perceived visual direction towards the visual direction of the binocular disparate surround. This observation has been termed binocular capture. There is increasing evidence that this shift in the visual direction of the monocular target may be related to the type of position encoding mechanism involved in processing the relative position signal. This study investigated the interaction between capture magnitude and vertical separation for stimulus conditions that favored the recruitment of linear or non-linear position encoding mechanisms. Relative alignment thresholds and bias were measured for a pair of vertically separated (8', 30', 60', 120') monocular Gabor gratings (1, 2, 4 and 8 cpd). Grating stimuli were constructed to constrain relative alignment judgments to the carrier grating (CO) or to the envelope (EO). Relative alignment thresholds and bias were also measured for a pair of vertically separated monocular Gabor gratings comprising a 1 cpd vertical square wave grating (SQ) or a 1 cpd missing fundamental grating (MF). Capture magnitudes were significantly larger across vertical separation and varied proportionally with relative alignment threshold for the EO and MF conditions. This was not evident with the CO and SQ conditions. The stark difference in capture magnitudes between the stimuli conditions suggest that the increase in capture magnitude observed with increasing vertical separation is intimately related to the transition from a "capture-immune" first-order spatial filter mechanism to a "capture-vulnerable" non-linear/feature-based position encoding mechanism.

Binocular capture: the effects of spatial frequency and contrast polarity of the monocular target.

The influence of binocular disparate targets on the perceived visual direction of adjacent monocular targets has been termed "binocular capture". The magnitude of capture increases significantly with increasing vertical separation between monocular targets. This study sets out to elucidate the interaction between spatial frequency content, contrast polarity and vertical separation between monocular targets to establish the roles of the monocular target's positional uncertainty and the underlying position-encoding mechanism in the production of binocular capture. Relative alignment thresholds and bias were measured separately for a pair of vertically separated monocular Vernier spatial frequency ribbons and a pair of monocular Gaussian bars presented across a random dot stereogram. Ribbon pairs comprised carrier frequencies that were either matched (8 cpd or 1 cpd) or mismatched (top ribbon 1 cpd, bottom ribbon 8 cpd, and vice versa). The Gaussian bars were presented with either matched contrast (bright/bright) or opposite polarity (bright/dark) contrast. Capture magnitudes increased significantly with vertical separation for all ribbon conditions and for both contrast polarity conditions. In these conditions, capture magnitude co-varied with relative alignment threshold. The matched 1 cpd ribbons showed a significant effect of separation and relative alignment threshold on capture magnitude for low contrast stimuli but not for high contrast stimuli. The results are consistent with the hypothesis that perceived visual direction of a monocular target becomes increasingly dependent on the surround visual direction when its relative position signals are poor. Furthermore, its vulnerability to capture by the surround visual direction seems to be dependent on the underlying position encoding mechanism employed to compute relative position information of the monocular target.

Spatial scaling of the binocular capture effect.

PURPOSE: Binocular "capture" occurs when the perceived visual direction of a monocular stimulus is displaced in the direction of the cyclopean visual direction of nearby binocular targets. This effect increases with the vertical separation of broadband monocular stimuli. The present study investigated whether the "capture" effect exhibits a systematic relationship with the spatial frequency composition of monocular lines and vertical separation. METHODS: Subjects judged the horizontal misalignment of 66 arc min vertical spatial frequency ribbons that were temporally interleaved with a random dot depth edge (3.2 degrees) for 108 ms. Spatial frequency ribbons were constructed from horizontal cosine gratings windowed by a 4 arc min vertical Gaussian envelope. The bottom half of the depth edge was presented with zero relative disparity, whereas the top half was presented with 10 arc min of crossed or uncrossed relative disparity. Four vertical separations (8, 16, 30, and 60 arc min) and three ribbon spatial frequencies (1, 4, and 8 cpd) were tested. The horizontal ribbon offset corresponding to 50% performance was calculated for each combination of depth condition, ribbon spatial frequency, and vertical separation. RESULTS: The magnitude of the "capture" effect was consistently larger for higher spatial frequency ribbons and decreased with decreasing vertical separation. When vertical separation was expressed as multiples of spatial periods of the respective ribbon spatial frequency, the magnitude of effect was significantly larger for separations greater than about one spatial period. CONCLUSIONS: The systematic scaling of the "capture" effect with spatial frequency and vertical separation is strongly suggestive of the operation of multiple spatial scale mechanisms; similar to those advocated for the processing of relative positional acuity with increasing vertical separation of monocular targets.

The utility of clinical electrophysiology in a case of nonorganic vision loss.

BACKGROUND: Nonorganic vision loss (NOVL) usually refers to reports of acuity reduction, field constrictions, or both without any associated organic pathology. Regardless of the underlying cause, the primary concern of the eye care practitioner is to demonstrate visual potential better than suggested by the patient's subjective reports. CASE REPORT: This article presents a case report of a 43-year-old woman with NOVL. The report shows the utility of visual electrodiagnostics in providing an objective assessment of the functional integrity of retinal and afferent visual pathway integrity and highlights how careful case history, subjective testing, observation of visual behavior, and objective testing can be used to establish a diagnosis of NOVL. CONCLUSIONS: The clever use of subjective testing and the careful selection and interpretation of objective tests such as visual electrodiagnostics can be exceptionally useful in making a diagnosis of NOVL.

Psychophysical measurements of referenced and unreferenced motion processing using high-resolution retinal imaging.

BACKGROUND: Motion detection thresholds with a stationary frame of reference are significantly lower than unreferenced motion thresholds. To account for this, previous studies have postulated the existence of compensatory mechanisms, driven by the presence of a surround, that cancel the effects of eye movements. In the present study we used an adaptive optics scanning laser ophthalmoscope (AOSLO) to investigate the effects of retinal jitter due to fixation eye movements on referenced and unreferenced motion thresholds. METHODS: The stimuli were produced by modulation of the AOSLO imaging beam, so that the absolute retinal position of targets was recorded. In Experiment 1 subjects made up/down motion judgments of a dark horizontal bar presented against a stationary 1-degree bright background. In Experiment 2 unreferenced motion thresholds were measured with isolated bright horizontal bars in otherwise complete darkness. In both experiments, AOSLO images for each trial were analyzed offline to extract retinal jitter and the retinal position of targets. RESULTS: For referenced motion, the results were consistent with complete compensation for eye movements by the visual system. In the unreferenced motion case eye movements adversely affected motion judgments, although there was evidence of partial compensation for such eye movements. CONCLUSIONS: Compensatory processes completely cancel the effect of fixation jitter for referenced motion but such compensation is partial for unreferenced motion.

Refractive error and visual impairment in African children in South Africa.

PURPOSE: To assess the prevalence of refractive error and visual impairment in school-aged African children in South Africa. METHODS: Random selection of geographically defined clusters was used to identify a sample of children 5 to 15 years of age in the Durban area. From January to August 2002, children in 35 clusters were enumerated through a door-to-door survey and examined in temporary facilities. The examination included visual acuity measurements, ocular motility evaluation, retinoscopy and autorefraction under cycloplegia, and examination of the anterior segment, media, and fundus. In nine clusters, children with reduced vision and a sample of those with normal vision underwent independent replicate examinations for quality assurance. RESULTS: A total of 5599 children living in 2712 households were enumerated, and 4890 (87.3%) were examined. The prevalence of uncorrected, presenting, and best-corrected visual acuity of 20/40 or worse in the better eye was 1.4%, 1.2%, and 0.32%, respectively. Refractive error was the cause in 63.6% of the 191 eyes with reduced vision, amblyopia in 7.3%, retinal disorders in 9.9%, corneal opacity in 3.7%, other causes in 3.1%, and unexplained causes in the remaining 12.0%. Exterior and anterior segment abnormalities were observed in 528 (10.8%) children, mainly corneal and conjunctival. Myopia (at least -0.50 D) in one or both eyes was present in 2.9% of children when measured with retinoscopy and in 4.0% measured with autorefraction. Beginning with an upward trend at age 14, myopia prevalence with autorefraction reached 9.6% at age 15. Myopia was also associated with increased parental education. Hyperopia (+2.00 D or more) in at least one eye was present in 1.8% of children when measured with retinoscopy and in 2.6% measured with autorefraction, with no significant predictors of hyperopia risk. CONCLUSIONS: The prevalence of reduced vision is low in school-age African children, most of it because of uncorrected refractive error. The high prevalence of corneal and other anterior segment abnormalities is a reflection of the inadequacy of primary eye care services in this area.