Monocular and Binocular Temporal Visual Perception of Infantile Nystagmus

What Factors Affect Binocular Summation?

Binocular vision may serve as a good model for research on awareness. Binocular summation (BS) can be defined as the superiority of binocular over monocular visual performance. Early studies of BS found an improvement of a factor of about 1.4 (empirically), leading to models suggesting a quadratic summation of the two monocular inputs (√2). Neural interaction modulates a target's visibility within the same eye or between eyes (facilitation or suppression). Recent results indicated that at a closely flanked stimulus, BS is characterized by instability; it relies on the specific order in which the stimulus condition is displayed. Otherwise, BS is stable. These results were revealed in experiments where the tested eye was open, whereas the other eye was occluded (mono-optic glasses, blocked presentation); thus, the participants were aware of the tested eye. Therefore, in this study, we repeated the same experiments but utilized stereoscopic glasses (intermixed at random presentation) to control the monocular and binocular vision, thus potentially eliminating awareness of the tested condition. The stimuli consisted of a central vertically oriented Gabor target and high-contrast Gabor flankers positioned in two configurations (orthogonal or collinear) with target-flanker separations of either two or three wavelengths (λ), presented at four different presentation times (40, 80, 120, and 200 ms). The results indicate that when utilizing stereoscopic glasses and mixing the testing conditions, the BS is normal, raising the possibility that awareness may be involved.

Dynamics of the perceptive field size in human adults

The receptive field (RF) is the fundamental processing unit of human vision; both masking and crowding depend on its size. The RF has a psychophysical corresponding term, the perceptive field (PF); whereas the RF is measured physiologically, the PF is measured psychophysically (a perceptual response). We investigated how spatial (lateral interactions), temporal (the stimulus presentation time), and the procedure affect the PF size for both monocular and binocular viewing. The stimuli consisted of a central vertically oriented Gabor target and high-contrast Gabor flankers positioned in two configurations (orthogonal or collinear) with target-flanker separations of either 2 or 3 wavelengths (λ). We used two main methods to control the monocular and binocular vision: mono-optic glasses vs. stereo glasses. The presentation order was either mixed or non-mixed for the presentation time and the eye condition. We estimated the PF size for both monocular and binocular viewing at 4 different presentation times (40, 80,120, and 200 ms) with different orders of presentation in each experiment (mono-optic glasses vs. stereo glasses, utilizing the lateral masking paradigm). In each experiment we explored one variable: how changing one parameter would affect the PF size in both monocular and binocular viewing (the temporal duration, the testing order of conditions, and the spatial distance) while keeping the others constant. We found that both the monocular and binocular PF size were dynamic and were significantly affected by the presentation order, leading to reduced lateral suppression under the collinear 2λ condition. Hence, both the monocular and binocular PF size depended on the sequence of the stimulus presentation time and the testing order of the conditions. Furthermore, we found that the binocular PF size was significantly larger than the monocular PF size.

Crowding under scotopic and photopic vision in albino and normal-sighted participants

Crowding is a phenomenon in which the ability to recognize an object in a clutter deteriorates. It is, therefore, a fundamental aspect of object recognition and crucial in deciphering resolution. For visually impaired individuals, deficiency in crowding has a tremendous effect on vision and may reflect and predict the amount of deterioration in vision. It is well established that albinos suffer much more from crowding than normally sighted individuals under daylight luminance conditions. However, to our knowledge, this study is the first to investigate crowding in albino participants under low light conditions. In this study, we explored the crowding effect in a group of albino participants (n = 9) and a control group of normally sighted participants (n = 9). Crowding was conducted under daylight (photopic vision) and low light (scotopic vision). We measured the visual acuity threshold under crowding in three-letter spacing (0.5, 1, and 1.5) and compared it to a single target. Results indicate that albino participants experienced stronger crowding than the control under the photopic condition, while crowding under the scotopic condition was apparent in the albino but abolished for the control group. These findings highlight the importance of considering luminance when discussing the visually impaired population in general. In particular, it suggests that crowding in albinism is based on a peripheral-like mechanism and may indicate a cessation in visual development.

An interaction effect of external noise and exposure duration on the spatial contrast sensitivity function

It has been demonstrated that contrast sensitivity is sensitive to stimulus exposure duration. Here, we investigated how the duration effect on contrast sensitivity was modulated by the spatial frequency and intensity of external noise. Through a contrast detection task, the contrast sensitivity function under 10 spatial frequencies, three external noise, and two exposure duration conditions was measured. The temporal integration effect was defined by the difference in contrast sensitivity or the area under the log contrast sensitivity function between short and long exposure durations. We found that (1) the temporal integration effect was less pronounced in the zero-noise condition than in the low- or high-noise condition; (2) in the zero-noise condition, a stronger temporal integration effect was observed at high spatial frequencies; (3) in the high-noise condition, a stronger temporal integration effect was observed at low spatial frequencies; (4) the spatial-frequency-dependent transient or sustained mechanism is also sensitive to external noise level; and (5) perceptual template model analysis revealed that both decreased additive internal noise and an improved perceptual template accounted for the temporal integration effect, and these two factors were tuned to spatial frequency.

Binocular fusion disorders impair basic visual processing

In an era of increasing screen consumption, the requirement for binocular vision is demanding, leading to the emergence of syndromes such as the computer vision syndrome (CVS) or visual discomfort reported by virtual reality (VR) users. Heterophoria (phoria) is a latent eye misalignment (with a prevalence up to 35%) that appears in conditions that disrupt binocular vision and may affect the quality of binocular fusion. Collinear facilitation (CF), the mechanism for grouping contour elements, is a process that reveals lateral interactions by improving the visibility of a target by flankers placed collinearly. An abnormal pattern of CF has been observed in strabismic amblyopia. We hypothesize that phoria may affect CF in the horizontal meridian (HM) due to latent eye misalignment and its impact on binocular fusion. Fully corrected participants (phoria group and controls) completed a standard CF experiment for horizontal and vertical meridians during binocular and monocular viewing. Phoric observers exhibited (1) an asymmetry and an abnormal pattern of CF only for the HM, during both monocular and binocular viewing, (2) poor binocular summation between the monocular inputs, and (3) no binocular advantage of the CF. Phoria affects the CF in a way that is reminiscent of meridional amblyopia without being attributed to abnormal refraction. The abnormal pattern of CF in monocular viewing suggests that phoria could be a binocular developmental disorder that affects monocular spatial interactions. We suggest that the results could contribute to explain the visual discomfort experienced with VR users or symptoms when presenting CVS.

Binocular summation is affected by crowding and tagging

In perceptual crowding, a letter easily recognized on its own, becomes unrecognizable if it is surrounded by other letters, an effect that confers a limit on the visual processing. Models assume that crowding is a hallmark of the periphery but that it is almost absent in the fovea. However, recently it was shown that crowding occurs in the fovea of people with an abnormal development of functional vision (amblyopia), when the stimulus is presented for a very short time. When targets and flankers are dissimilar, the crowding is reduced (tagging). Since a combination of binocular inputs increases the processing load, we investigated whether color tagging the target reduces crowding in the fovea of subjects with normal vision and determined how crowding is combined with binocular vision. The crowding effect at the fovea was significantly reduced by tagging with a color target. Interestingly, whereas binocular summation for a single letter was expected to be about 40%, it was significantly reduced and almost absent under crowding conditions. Our results are consistent with the notion that the crowding effect produces a high processing load on visual processing, which interferes with other processes such as binocular summation. We assume that the tagging effect in our experiment improved the subject's abilities (sensitivity and RT) by creating a "segmentation", i.e., a visual simulated separation between the target letter and the background. Interestingly, tagging the target with a distinct color can eliminate or reduce the crowding effect and consequently, binocular summation recovers.

Perceptual learning improves visual functions in patients with albinistic bilateral amblyopia: A pilot study

BACKGROUND

Several visual functions are impaired in patients with oculocutaneous albinism (OCA) associated to albinistic bilateral amblyopia (ABA).

OBJECTIVE

In this study, we aimed at exploring whether perceptual learning (PL) can improve visual functions in albinism.

METHOD

Six patients and six normal sighted controls, were trained in a contrast detection task with lateral masking. Participants were asked to choose which of the two intervals contained a foveally presented low-contrast Gabor patch. Targets were presented between higher contrast collinear flankers with equal spatial frequency. When increasing target-to-flanker distance, lateral interactions effect normally switches from inhibition to facilitation, up to no effect.

RESULTS

Our findings showed that before PL, only controls showed facilitation. After PL, results suggest that facilitatory lateral interactions are found both in controls as well as in albino patients. These results suggest that PL could induce higher processing efficiency at early cortical level. Moreover, PL positive effect seems to transfer to higher-level visual functions, but results were not very consistent among tasks (visual acuity, contrast sensitivity function, hyperacuity and foveal crowding).

CONCLUSIONS

Although a small sample size was tested, our findings suggest a rehabilitative potential of PL in improving visual functions in albinism.