Spatial & Binocular Vision

Spatial Vision

Retinal image quality of the human eye is degraded by ocular aberrations. Lower-order aberrations, defocus and astigmatism, can be easily corrected using traditional methods as spectacles and contact lenses. Higher order aberrations usually have a lower impact in the visual performance in normal eyes than lower order aberrations. However, due to some ocular pathologies, as keratoconus, or after ocular surgeries, magnitude of the higher aberrations increases having a significant impact on the visual performance.


Figure 1. Simulated images of the 20/20 Snellen E letter for different real eyes' higher order aberrations (RMS=0.11, 0.30 and 0.67µm, respectively) and 6mm pupil.

Neural Adaptation

It has been shown that we are neural adapted to our own aberrations (Artal et al. 2004). Subjects preferred the blur induced by their own higher-order aberrations rather than a rotated version of the same. This might lead to improved visual performance by compensating for a blurred retinal image. However, we have shown that this mechanism produce neural insensitivity to a perfect retinal image (Sabesan and Yoon 2009). Our results show that visual performance in keratoconic eyes was worse than that for normal eyes when higher order aberrations were corrected in both groups (Figure 2). It may indicate that long-term visual experience with poor retinal image quality may restrict the visual benefit achievable immediately after correction, at least in keratoconic eyes. However, we have also proved that perceptual learning correcting higher order aberrations has the potential to enhance neural function to improve visual benefit after a customized correction in highly aberrated keratoconic eyes (Sabesan and Yoon 2013).


Figure 2. Visual acuity for normal and keratoconic eyes after correction of all aberrations (Sabesan et al. 2009)

Binocular Vision

It is known that binocular vision enhances visual perception. The quality of the binocular visual performance is typically better than the quality of the better monocular image perceived, effect known as binocular summation. Binocular monochromatic aberration correction improved visual acuity and contrast sensitivity significantly (Sabesan et al. 2012). However, binocular summation is larger in the presence of subject's native higher order aberrations and was reduced when these aberrations were corrected. Therefore, binocular summation can be considered as a compensatory mechanism exhibited by the binocular visual system to reduce the impact of optical blur and thus might undermine the binocular visual benefit compared to that measured monocularly.


Figure 3. Binocular summation for contrast sensitivity and different spatial frequencies, as a function of the optical quality. (Sabesan et al. 2012)

Moreover, we have found that the binocular blur perception is reduced when interocular blur orientations differ. It has been reported that interocular mirror symmetry in the aberrations (or optical blur) is present in the normal population (Porter et al. 2001). We are analyzing how this particular interocular distribution of the aberrations affects binocular perception. Our results suggest that there is a compensatory mechanism of the binocular visual system to cope with different interocular blur orientation, which is going to depend on the magnitude and relative orientation of blur between the eyes (Alarcon et al. 2013).

TRY YOURSELF!!


Figure 4. Place your finger between the screen and your nose right in the center of the four images. Look at the tip of your finger with both eyes open and move it closer to your nose. You must perceive a central column between the four images, which are the fused images. Which one is better: the top one (where left and right images have the same blur orientation) or the bottom one (where left and right images have different blur orientation)???