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Frequency-Doubling Illusion Under Scotopic Illumination and in Peripheral Vision

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Frequency-Doubling Illusion Under Scotopic Illumination and in Peripheral Vision Frequency-Doubling Illusion under Scotopic Illumination and in Peripheral Vision Kunjam Vallam,1,2 Ioannis Pataridis,1 and Andrew B. Metha1 3–6 PURPOSE. The authors sought to determine whether frequency- sponse characteristics. However, the existence of such non- doubling illusion (FDI) could be perceived under scotopic linear responses in M cells is not universally agreed,7–9 and a illumination at central and peripheral retinal locations. For cortical locus has been suggested.9 comparison, perception of the FDI at the central and periph- There are several differences in the parvocellular (P) and M eral retina under photopic illumination was also evaluated. ganglion cells; it is generally agreed that M cells primarily METHODS. Five subjects matched the apparent spatial frequency convey information concerned with the perception of visual of counterphase flickering sinusoidal gratings with stationary motion and luminance information, whereas P cells primarily sinusoidal gratings presented foveally and out to 20° eccentric- convey information concerned with the perception of color ity under photopic and scotopic illumination conditions. Two and form.10,11 These two types of cells also respond differently spatial frequencies (0.25 and 0.50 cpd) were used at four to luminance patterns at varying levels of retinal illumination. temporal frequencies (2, 8, 15, and 25 Hz). Subsequent exper- Purpura12 reported that the responses of both types of cells iments explored the range of spatial and temporal frequency decrease with reduction in mean retinal illumination, but P stimulus conditions under which the scotopic FDI might be cells are affected relatively more than the M cells. As a result, observed. in the scotopic ranges of mean retinal illumination, M cells are RESULTS. Under scotopic illumination conditions, the apparent the predominant conveyor of spatial contrast information to spatial frequency of eccentrically presented 0.25- and 0.50-cpd the visual cortex.12 Hence, if it is true that, at least under flickering gratings gradually increased as a function of flicker photopic conditions, the FDI is perceived because of inputs frequency and approaches “doubling” at 15 Hz. Under pho- from the M cells, then the FDI should also be perceived under topic conditions, the apparent spatial frequency of 0.25-cpd scotopic conditions. Until now, no psychophysical data have flickering at 25 Hz was approximately doubled in all four been published to suggest that the FDI can be perceived under primary meridians at central and peripheral eccentricities. The scotopic conditions. One report13 demonstrated increases of final experiment showed that the spatiotemporal range under apparent spatial frequency with retinal illuminance levels as which the scotopic FDI could be seen was similar to the low as 8 photopic trolands. However, without the results of photopic illumination condition reported earlier. functional tests such as color vision or the demonstration of a CONCLUSIONS. Scotopic FDI is similar to photopic FDI at the central central scotoma, it is not possible to rule out the influence of and the peripheral retina. This suggests that similar mechanisms cone-based mechanisms in mediating these results. Hence, the are responsible for generating the illusion under both photopic first aim of this article is to provide further insight into this and scotopic illumination conditions. (Invest Ophthalmol Vis Sci. issue of perception of the FDI under scotopic conditions to add 2007;48:3413–3418) DOI:10.1167/iovs.06-1091 further insight into what neuronal mechanisms may be causing the illusion. patial frequency-doubling illusion (FDI) occurs when the Of potential clinical relevance is the finding that patients Scontrast of a low spatial frequency sinusoidal grating is with glaucoma experience greater loss of scotopic sensitivity counterphase modulated at high temporal frequencies. In than of photopic sensitivity.14,15 A recent study using a rat other words, its apparent spatial frequency increases.1 Earlier model of experimental glaucoma confirms that, in the early suggestions were that some form of nonlinear processing in stages of glaucoma, when no structural damage to the optic 1 our visual system is responsible for this illusion. Maddess et nerve is identifiable, some loss of scotopic functionality can 2 al. (Maddess T, et al. IOVS 1990;31:ARVO Abstract 230) attrib- occur before photopic functions are affected.16 This raises the uted the source of this nonlinearity to a specific class of interesting possibility that, if the FDI can be perceived at primate magnocellular (M) ganglion cells, M(y) cells, which scotopic levels and can be shown to arise from the same resemble cat Y cells in their nonlinear spatial summation re- mechanisms responsible for the photopic FDI and if those mechanisms are themselves selectively damaged early in glau- coma in a way that raises their contrast threshold for detection From the 1Department of Optometry and Vision Sciences, Univer- (Maddess T, et al. IOVS 1990;31:ARVO Abstract 230),2 glauco- sity of Melbourne, Carlton, Victoria, Australia. ma-related reductions in scotopic function might be more 2Present address: ARC Centre of Excellence in Vision Science, reliably sought by using stimuli that give rise to the scotopic School of Psychology, The Australian National University Canberra, FDI. ACT 0200 Australia. Supported by a Kaye Merlin Brutton Award and the David Hay Before entertaining thoughts on the development of a Memorial Fund (KV), The University of Melbourne, Australia. scotopic FDI-based clinical test for glaucoma, it is necessary to Submitted for publication September 12, 2006; revised December determine how scotopic FDI is perceived at central and pe- 14, 2006, and February 8, 2007; accepted April 27, 2007. ripheral retinal locations and how this relates to the FDI elic- Disclosure: K. Vallam, None; I. Pataridis, None; A.B. Metha, ited under photopic conditions. In the photopic case, although None early studies reported perception of the FDI only within 2° The publication costs of this article were defrayed in part by page eccentricity,17 McKendrick18 and James19 later confirmed that charge payment. This article must therefore be marked “advertise- ment” in accordance with 18 U.S.C. §1734 solely to indicate this fact. the FDI is perceived at retinal eccentricities up to 20°. The Corresponding author: Andrew B. Metha, Department of Optom- second aim of this article is thus to characterize and compare etry and Vision Sciences, The University of Melbourne, cnr Keppel and the FDI at different retinal eccentricities under photopic and Cardigan Streets, Carlton, Victoria 3053; [email protected]. scotopic conditions. Investigative Ophthalmology & Visual Science, July 2007, Vol. 48, No. 7 Copyright © Association for Research in Vision and Ophthalmology 3413 Downloaded from iovs.arvojournals.org on 09/29/2021 3414 Vallam et al. IOVS, July 2007, Vol. 48, No. 7 SUBJECTS AND METHODS ensure that cone pathways were rendered nonfunctional under the reduced illumination conditions (wearing goggles), all subjects were Subjects tested with the Ishihara and D-15 color vision tests after dark adapta- tion. None of the five subjects passed either of these color vision tests, Five subjects volunteered to participate in this study (age range, 21–26 confirming that both reduced illumination conditions can be consid- years), and all had normal corrected or uncorrected visual acuity ered scotopic. (20/20) in the tested eye. To be included in the study, subjects were required to have refractive error in the range of ϩ4 D to –4 D of sphere Stimuli and less than2Dofastigmatism, normal findings on an eye examina- tion, and normal color vision when tested on Ishihara and D-15 color Stimuli consisted of vertically oriented sinusoidal gratings presented in vision tests under room (photopic) conditions. All experiments were the center of the CRT monitor within an 8°-diameter, software-gener- performed monocularly using the eye with better best-corrected visual ated circular window. Fixation spots were used on the monitor and on acuity or the right eye if both eyes had equal acuity. Two of the five a surrounding black cardboard background. All stimuli were presented subjects (IP, CT) were aware of the purposes of the experiments, and at 80% Michelson contrast. All combinations of spatial and temporal the other three subjects (JB, TD, OC) were naive to the purposes. All frequencies were randomized in a session. Only one retinal location subjects read and signed informed consent documents before any was explored in a session. testing, and all procedures were carried out in accordance with Na- tional Health and Medical Research Council guidelines for human FDI under Reduced Illumination observers, which is based on the tenets of the Declaration of Helsinki Two spatial frequencies (0.25 and 0.50 cpd) were used at three tem- (approved by the National Vision Research Institute/Department of poral frequencies (2, 8, and 15 Hz) under 0.27 photopic troland retinal Optometry and Vision Sciences/Victorian College of Optometry Hu- illumination at 5° temporal retinal eccentricity and 1.99 photopic man Research Ethics Committee). troland retinal illumination at 15° temporal retinal eccentricity. To Subjects viewed the monitor screen at a viewing distance of 57 cm avoid any confounding effects of the central 2° rod-free area, experi- in dim ambient lighting conditions; their heads were left unrestrained. ments under scotopic conditions were not performed with central Pupils were dilated to 8 mm with tropicamide 0.5% drops. Each subject fixation. was involved in approximately 5 hours of data collection conducted in 30- to 60-minute blocks (beginning 15 to 20 minutes after instillation of FDI under Photopic Illumination mydriatic) over several weeks. At the beginning of each session, sub- For photopic experiments, only one spatial frequency (0.25 cpd) was jects were preadapted for 5 minutes for photopic experiments and at explored at 25 Hz at five retinal eccentricities (0°, 5°, 10°, 15°, and 20°) least 15 minutes for scotopic experiments. Before data collection for in all four principal retinal meridians without any ND filter.
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