Senescent changes in photopic spatial summation in the peripheral retina

Journal of Vision (2011) 11(10):15, 1–15 http://www.journalofvision.org/content/11/10/15 Senescent changes in photopic spatial summation Maka Malania Frederic Devinck Kenneth Knoblauch Peter B. Delahunt Joseph L. Hardy John S. Werner Department of Ophthalmology and Vision Science, University of California, Davis, USA Laboratoire de Psychologie Experimentale (E.A. 1285), Universite de Rennes 2, Rennes, France INSERM, U846, Stem-cell and Brain Research Institute, Department of Integrative Neurosciences, Bron, France, & Universite de Lyon, Lyon, France Posit Science Corporation, San Francisco, CA, USA Lumos Labs, Incorporated, San Francisco, CA, USA Department of Ophthalmology and Vision Science, University of California, Davis, USA Previous studies have demonstrated an inverse relation between the size of the complete spatial summation area and ganglion cell density. We hypothesized that if this relation is dynamic, the spatial summation area at 6- nasal would expand to compensate for age-related losses of retinal ganglion cells but not in the fovea where age-related loss in ganglion cell density is not significant. This hypothesis was tested by measuring contrast thresholds with a series of Gabor patches varying in size. The spatial summation area was defined by the intersection of the segments of a two-branched, piece-wise linear function fitted to the data with slopes of j0.5 and 0 on a plot of log threshold vs. log area. Results demonstrate a 31% increase in the parafoveal spatial summation area in older observers with no significant age-related change in the fovea. The average foveal data show a significant increase in thresholds with age. Contrary to the foveal data, age comparisons of the parafoveal peak contrast thresholds display no significant difference above the summation area. Nevertheless, as expected from the increase in summation area, expressing the parafoveal thresholds as contrast energy reveals a significant difference for stimuli that are smaller than the maximal summation area. Keywords: spatial summation, human aging, contrast sensitivity, ganglion cells Citation: Malania, M., Devinck, F., Knoblauch, K., Delahunt, P. B., Hardy, J. L., & Werner, J. S. (2011). Senescent changes in photopic spatial summation. Journal of Vision, 11(10):15, 1–15, http://www.journalofvision.org/content/11/10/15, doi:10.1167/11.10.15. Introduction The sampling limits of the human visual system depend upon optical and neural factors, both of which change across the life span (Werner, Schefrin, & Bradley, 2010). These senescent changes provide a probe into possible dynamic modifications that support vision in elderly observers, notwithstanding inevitable age-related losses in vision even in the absence of age-related disease. Classically, spatial integration has been quantified from measurements of thresholds for a series of small test stimuli. Over small regions, these thresholds depend on a constant amount of energy regardless of its spatial distribution. Therefore, over a restricted region of the visual field, the energy required for threshold and stimulus area is reciprocally related. The largest area over which this relation obtains is known as the complete summation area or Ricco’s (1877) area in honor of the Italian astronomer doi: 1 0. 11 67 / 11. 1 0. 15 who described this as the “Law of Compensation.” Ricco’s law is valid for relatively small stimuli, ranging from 2 arcmin in the fovea at photopic luminances to 1 deg in the periphery (Graham & Margaria, 1935; Scholtes & Bouman, 1977) depending on the stimulus conditions. For medium stimulus sizes, and when using gratings, thresh- olds typically decrease in proportion to the square root of stimulus area, following Piper’s (1903) law. Figure 1 (solid lines) illustrates a typical two-branched, piece-wise linear summation curve. The upper branch of the curve has a slope of j0.5, which is followed by a size-independent region having a slope of 0. Whether one observes Ricco’s law, Piper’s law, or both before reaching the size- independent region depends on numerous parameters (Barlow, 1958; Cohn, 1990; Sakitt, 1971). Here, we tested sensitivity using a series of Gabor patches, so the primary interest is the intersection of the descending function (slope of j0.5 for our conditions) and the size-independent area, which we define as the spatial summation area. Received September 26, 2010; published September 22, 2011 Downloaded From: http://jov.arvojournals.org/pdfaccess.ashx?url=/data/journals/jov/932789/ on 04/25/2017 ISSN 1534-7362 * ARVO