Binocular fusion, suppression and diplopia for blurred edges

Abstract

Purpose(1) To devise a model-based method for estimating the probabilities of binocular fusion, interocular suppression and diplopia from psychophysical judgements, (2) To map out the way fusion, suppression and diplopia vary with binocular disparity and blur of single edges shown to each eye, (3) To compare the binocular interactions found for edges of the same vs opposite contrast polarity.MethodsTest images were single, horizontal, Gaussian-blurred edges, with blur B = 1–32 min arc, and vertical disparity 0–8.B, shown for 200 ms. In the main experiment, observers reported whether they saw one central edge, one offset edge, or two edges. We argue that the relation between these three response categories and the three perceptual states (fusion, suppression, diplopia) is indirect and likely to be distorted by positional noise and criterion effects, and so we developed a descriptive, probabilistic model to estimate both the perceptual states and the noise/criterion parameters from the data.Results(1) Using simulated data, we validated the model-based method by showing that it recovered fairly accurately the disparity ranges for fusion and suppression, (2) The disparity range for fusion (Panum's limit) increased greatly with blur, in line with previous studies. The disparity range for suppression was similar to the fusion limit at large blurs, but two or three times the fusion limit at small blurs. This meant that diplopia was much more prevalent at larger blurs, (3) Diplopia was much more frequent when the two edges had opposite contrast polarity. A formal comparison of models indicated that fusion occurs for same, but not opposite, polarities. Probability of suppression was greater for unequal contrasts, and it was always the lower-contrast edge that was suppressed.ConclusionsOur model-based data analysis offers a useful tool for probing binocular fusion and suppression psychophysically. The disparity range for fusion increased with edge blur but fell short of complete scale-invariance. The disparity range for suppression also increased with blur but was not close to scale-invariance. Single vision occurs through fusion, but also beyond the fusion range, through suppression. Thus suppression can serve as a mechanism for extending single vision to larger disparities, but mainly for sharper edges where the fusion range is small (5–10 min arc). For large blurs the fusion range is so much larger that no such extension may be needed.

Ocular dominance plasticity: inhibitory interactions and contrast equivalence

“…Fusion is possible only over a limited range of disparities. Our model presumes, similarly to the one of Georgeson & Wallis15 that the highest probability of fusion occurs at zero disparity ( pFuse 0 ) and it falls with increasing disparity as a generalized Gaussian function:…”

“…To quantify the relationship between fusion, suppression, and diplopia, we employed a modified version of the model presented in Georgeson and Wallis15 (Fig. 3).…”

“…In this study, we adopted an elegant task recently introduced by Georgeson and Vallis15. Using a simple visual stimulus, blurred horizontal edge with different levels of blur/spatial scale (1, 2, 4, 8, 16, and 32 arcmin), vertical disparity (0, 1, 2, 3, 4, 5, 6, 7, and 8-times blur width), and interocular contrast offset (0, 6, and 12 dB) allowed them to evaluate the inter-relation between fusion, suppression, and diplopia in normal and abnormal visual conditions using one unified model.…”

“…Using a simple visual stimulus, blurred horizontal edge with different levels of blur/spatial scale (1, 2, 4, 8, 16, and 32 arcmin), vertical disparity (0, 1, 2, 3, 4, 5, 6, 7, and 8-times blur width), and interocular contrast offset (0, 6, and 12 dB) allowed them to evaluate the inter-relation between fusion, suppression, and diplopia in normal and abnormal visual conditions using one unified model. Among other, Georgeson and Wallis15 have shown that in normal binocular vision fusion dominates at small vertical disparities and diplopia (which they explained as a lack of both fusion and interocular suppression) at large vertical disparities corresponding to the stimulus scale. Suppression of one eye by the other, as a mechanisms maintaining a single vision outside of fusional disparities was less prevalent, and peaked at intermediate disparities.…”

“…We reason that if they occur in a scale-independent but contrast-dependent fashion we can advance a unified explanation in terms of a change in contrast gain arising from imbalanced inhibitory contralateral signals at a relatively early stage of visual processing prior to binocular combination. To investigate the relationship between patching and interocular contrast imbalance and their effect on binocular balance, we used a modified approach of Georgeson and Vallis15. The adaptation of their task was that we introduced a small tilt to the blurred edges with an opposite direction in each eye that allowed for a dissociation between fusion and suppression at small disparities.…”