The physiological blind spot (BS) is the spatial projection of the optic nerve head wherein ganglion cell axons collect to form the optic nerve. The BS is 15.5° temporal to fixation, 5.5° wide by 7° high. Since the optic nerve head lacks photoreceptors, the BS is presumed to be an absolute scotoma readily mapped on tangent screen, but perceptually filled-in during normal viewing. Intrinsically photosensitive sensitive retinal ganglion cells (ipRGCs) absorb light with melanopsin mediating pupil responses, photoentrainment, cognition, alertness, and vision [1,2,3,4]. Since ipRGC axons contain melanopsin, they may respond to appropriate stimulation in the BS [4]. Our purpose was to: (1) determine whether vision can occur within the BS via ipRGCs (2) exploit this epiphenomenon to develop a rapid test of melanopsin pathway sensitivity.
30 healthy adults provided written informed consent to participate in our IRB approved protocol (mean age±SD: 27 ± 9). Visual acuity, contrast sensitivity, and Ishihara testing verified normal vision and color vision. The Blind Spot Vision Test (BSVT, Fig. 1) was displayed on a Microsoft Surface in a dark room with left eye occluded. The subject’s right eye was centred before a red X (left side of black display viewed at 50–60 cm). The blind spot target was a 1.4° x 2.5° vertical oval 15° temporal to fixation. The first stimulus was red followed by a green oval each 12.7 cd/m2. The subject moved fore/aft until the red oval disappeared followed by green oval disappearance to ensure targets were in the BS. This blue oval (41.2 cd/m2, CIE x,y = 0.206, 0.278, dominant wavelength 480 nm, melanopsin peak) [1,2,3] was presented next. In response to the blue oval, subjects reported if anything appeared and its perceived color. The method of descending/ascending limits was used to determine three blue target thresholds by changing luminance up/down in 0.05 log steps. Mean, confidence intervals (CI), and the coefficient of repeatability (COR) were calculated from pooled data.
After BS determination (red, green oval disappearance) 100% reported reappearance of a light blue haze or glow in response to the blue oval. Two subjects tested at closer and farther distances reported that the glow became a well-formed peripheral blue oval suggesting cone mediation at distances outside the BS. Mean ± 2 SD ipRGC BSVT threshold was 32.0 ± 12.6 cd/m2 (1.5 ± 0.2 log cd/m2, Fig. 2A). The COR was 0.11 log cd/m2 (95% CI for within-subject change, Fig. 2B). Red, green, and blue thresholds from three subjects to stimuli 15° nasal to fixation were > 100X lower than BSVT thresholds substantiating mediation of the BSVT by less sensitive ipRGCs [1,2,3,4].
Study limitations include possible artifacts from fixational shifts or subject reliability which could be mitigated by fixation monitoring and catch trials. The results do support visual perception within the BS with stimuli optimized for ipRGCs. Testing across a blue luminance range yielded repeatable thresholds providing a metric of ipRGC pathway sensitivity. Potential applications include detection/monitoring of ocular, systemic and/or neurologic disease, and mood, alertness, pupil, and circadian rhythm function. BVST testing may also serve as an outcome measure for gene therapy efficacy in eye disease [5].
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Research reported in this publication was supported by the National Eye Institute of the National Institutes of Health under Award Number T35EY032441. The content is solely the responsibility of the authors and does not necessarily represent the official views of the National Institutes of Health. Open access funding provided by SCELC, Statewide California Electronic Library Consortium.
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JR was responsible for conducting the search, designing/writing the protocol, data collection, statistical analyses, results interpretation, writing the manuscript, and research oversight. JL was responsible for conducting the search, designing/writing the protocol, data collection, statistical analyses, results interpretation, and writing the manuscript. MB, AV, AM, GW, DH, EP were responsible for designing the protocol, data collection, collating data, and results interpretation. Contributions and Access to Data: The Principal Investigator, Dr. Jeff Rabin, had full access to all the data and takes responsibility for the integrity of the data and the accuracy of the data analysis.
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Rabin, J., Lundby, J., Brase, M. et al. Vision within the blind spot: a new test to quantify melanopsin pathway sensitivity. Eye (2024). https://doi.org/10.1038/s41433-024-03102-4
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DOI: https://doi.org/10.1038/s41433-024-03102-4