This study shows, for the first time, that binocular vision ameliorates the effects of retinal blur on spatial visual performance. This was tested using two different performance measures: suprathreshold responses of the pattern reversal VEP and high-contrast letter acuity.
It is interesting that binocular facilitation increases with the amount of induced defocus. The binocular summation ratio in VEP P100 amplitude was correlated linearly with defocus. Similarly, VEP P100 latency was prolonged under monocular compared to binocular stimulation, with the delay being more pronounced the higher the shift from optimal focus. The binocular superiority in conditions of blur was also observed in the VA task (ranging from 1.18 D in focus to 1.81 at 1.75 D defocus).
An obvious question is: “What are the neural mechanisms underlying the binocular facilitation under conditions of retinal blur?” The diminished VEP responses as a function of monocular defocus can be attributed to optical factors: increased retinal blur leads to attenuation of image contrast and an overall loss in its modulation transfer function.
6,9,11,20 It is notable that binocular facilitation, as evaluated with electrophysiological studies in humans, is more evident as stimulus contrast decreases
40,41 and that the response versus contrast slopes are steeper under binocular than under monocular observation.
41 The binocular superiority for low contrast stimuli has also been shown in Vernier acuity.
51 There is physiological evidence
52 that, under binocular stimulation, cells in the cat's striate cortex show enhanced contrast sensitivity. Moreover, low (close-to-threshold) contrast detection activates more cortical neurons showing facilitatory interactions.
52
All participants in this work were young people with normal binocular vision. Binocular summation under in-focus conditions decreases with aging, reflecting deterioration in cortical activity, and/or an increasing interocular difference in spatial performance, with the better eye dominating the overall visual performance.
28,31,53 Binocular summation is also expected to be hampered in the presence of inhibitory conditions such as amblyopia.
39,54,55
In the absence of blur, the average binocular enhancement ratio for the amplitude of the VEP P100 component was 2.1. This agrees with ratios reported in previous electrophysiological studies,
32,35 –42 which ranged from 1.3 to 2.5, varying with observer, stimulus characteristics, and recording techniques.
36 Facilitatory interaction between the signals from the two eyes is supported by electromyography
56 and single-cell electrophysiology, demonstrating that binocular interactions exist at the level of single cortical cells
57 and that a larger population of neurons contributes to contrast detection under binocular stimulation.
52 The higher binocular summation ratio found in this and earlier VEP studies, compared to the typical “neural summation” ratio of 1.4 (
) reported in human psychophysical threshold-based work,
23,27 may be a result of the different populations of neurons responsible for threshold and suprathreshold perception.
58 –60 Higher summation ratios have also been revealed in masking experiments.
30,61
In the absence of blur, the improvement in high-contrast VA with binocular observation was significantly lower than that occurring for VEPs, on average being 0.07 logMAR in focus (summation ratio 1.18). Small amounts of binocular interaction for high-contrast targets have also been reported in previous studies.
26,32,33 with binocular superiority improving over that predicted by probability summation as letter contrast decreased.
26
A limitation of this study is that only positive spherical defocus blur was used and that optical factors, such as pupil size and accommodation, were not controlled. Usually the degradation in spatial visual performance is more rapid in the positive than in the negative direction.
15,62 –64 This is attributed to positive spherical aberration, which occurs in most unaccommodated eyes,
65,66 ameliorating the effect of negative defocus on VA. Intersubject variability in spherical aberration may account for some of the intersubject variability observed in the effect of defocus, especially on VEP responses, since recordings were performed for larger pupil sizes compared to VA.
Average pupil diameter was smaller under binocular compared to monocular viewing conditions in VA and VEP recordings by 0.5 and 0.4 mm, respectively, in close agreement with a previous study.
67 It is expected that a smaller pupil with binocular vision would have little effect when in focus, but would result to a better vision for defocus-induced blur. To estimate the effect of the difference in pupil diameter between the two conditions on binocular advantage, we used data from a previous study.
68 We calculated that a 0.5 mm reduction in pupil diameter would explain only 0.02 logMAR of the 0.17 logMAR and 0.19 logMAR superiority of binocular vision at 1.00 and 2.00 D defocus, respectively.
In conclusion, this study has demonstrated that binocular observation, compared with monocular observation, ameliorates the influence of blur on visual performance as measured by VA and the visual evoked potential. The increased binocular facilitation observed with retinal blur may be due to the activation of a larger population of neurons at close-to-threshold detection under binocular stimulation. Further investigation of the relationship between binocular facilitation and blur using other measures of visual performance is needed.
Supported by an International Travel Grant for presentation at ARVO 2010.
Presented at the annual meeting of the Association for Research in Vision and Ophthalmology, Fort Lauderdale, Florida, 2010.