Estimating the Critical Duration for Temporal Summation of Standard Achromatic Perimetric Stimuli

Purpose: To estimate the critical duration of temporal summation for achromatic Goldmann III stimuli under the conditions of standard automated perimetry (SAP) and quantify response variability for short duration stimuli. Methods: Contrast thresholds were gathered using the method of constant stimuli for seven circular (0.48° diameter) incremental stimuli of varying duration (sum-of-frames equivalent: 8.3-198.3 msec), at an eccentricity of 8.8° along the four principal meridians of the visual field in two healthy, psychophysically experienced observers. Stimuli were presented on a high-resolution CRT display with a background luminance of 10 cd/m2. Psychometric functions were fitted using a probit model and non-parametric local linear analysis. The critical duration was estimated using iterative two-phase regression analysis, the results also being compared with values produced using previously published methods of analysis. Results: The median critical duration estimated using iterative two-phase regression analysis was 27.7 msec (IQR 22.5-29.8). A slight steepening of the psychometric function slope (lower variability) was observed for longer stimulus durations, using both probit and local-linear analysis techniques, but this was not statistically significant. Conclusions: Critical duration estimates in this study are substantially shorter than those previously reported for a Goldmann III stimulus under the conditions of SAP. Further work is required to firmly establish the relationship between measurement variability and the degree of local temporal and spatial summation

The effect of age on the area of complete spatial summation for chromatic and achromatic stimuli

Purpose. Previously, an association between the area of complete spatial summation (Ricco's area) and age under scotopic conditions had been found. The authors sought to determine whether Ricco's area is similarly associated with age under photopic achromatic and selective S-cone conditions in peripheral vision and whether any association relates to a loss of ganglion cell density as determined by measurements of peripheral grating resolution acuity. Methods. Achromatic spatial summation functions were plotted for 68 healthy subjects (aged 20–77 years) in four oblique meridians on a gray background field of 10 cd/m2. Similar functions were generated for the S-cone pathway (isolated using Stiles' two-color threshold method) for the same locations. Ricco's area was determined using two-phase regression analysis. Achromatic peripheral grating resolution acuity was measured at the same locations using high-contrast Gabor stimuli, as an estimate of localized functional ganglion cell density. Results. There was a notable decrease in overall contrast sensitivity with age for all stimulus sizes. However, there was no evidence of age-related change in Ricco's area for either achromatic (superior field, r2 = 0.05; inferior field, r2 = 0.0007; all P > 0.05) or chromatic (superior field, r2 = 0.01; inferior field, r2 = 0.006; all P > 0.05) stimuli, despite a significant decrease in peripheral grating resolution acuity with age (superior field, r2 = 0.15; inferior field, r2 = 0.17; both P < 0.05). Conclusions. An age-related decline in functional ganglion cell density is not accompanied by a significant change in Ricco's area for achromatic or chromatic stimuli

Sensitivity loss in early glaucoma can be mapped to an enlargement of the area of complete spatial summation

Purpose. The area of complete spatial summation (Ricco's area) is the largest stimulus size for which area × intensity is constant at threshold. The authors sought to investigate whether Ricco's area changes in early glaucoma to account for the decreased visual signal/noise ratio that may accompany retinal ganglion cell loss. Methods. Spatial summation functions were measured, and Ricco's area was determined at four 10° retinal locations in 24 patients with early glaucoma (total deviation at test locations, mean, −1.3 dB; range, +2 dB to −8 dB) and 26 age-similar healthy subjects under achromatic and S-cone isolation conditions. Achromatic grating resolution acuity was measured at the same locations to estimate functional ganglion cell density. Results. Ricco's area was enlarged in patients compared with controls for both achromatic (enlarged by: superior field, 0.57 log units, P < 0.01; inferior field, 0.72 log units, P < 0.01) and chromatic (enlarged by: superior field, 0.26 log units, P < 0.01; inferior field, 0.25 log units, P = 0.065) stimuli, with negligible vertical summation curve shifts along the intensity axis. Resolution acuity was significantly reduced in glaucoma patients in both hemifields (P < 0.001). There was a weak, but significant, relationship between Ricco's area and resolution acuity. Conclusions. Enlargement of Ricco's area completely compensates for reduced perimetric sensitivity in early glaucoma to maintain constant threshold at Ricco's area, suggesting an increase in signal pooling in response to ganglion cell loss. The rightward displacement of the spatial summation curve indicates that perimetric stimuli should be capable of modulating in size as well as/instead of contrast, which may boost the glaucoma signal within measurement noise

Changes in Ricco's Area with background luminance in the S-Cone Pathway

Purpose: The area of complete spatial summation (Ricco’s area) for achromatic stimuli has previously been shown to decrease with increased background luminance. A popular hypothesis is that such a phenomenon reflects increased center-surround antagonism within the receptive field of the retinal ganglion cell. We wished to investigate if similar changes in Ricco’s area occur with blue background luminance for the S-cone pathway, guided by the knowledge that the retinal ganglion cells with S-cone input do not display S-cone–mediated center-surround antagonism (S+/S-). Methods: Spatial summation functions were measured for four young healthy observers under S-cone pathway isolation by presenting blue test stimuli on a background consisting of intense fixed yellow (600 cd/m2) component in combination with a variable blue component (background range, 1.78 to 2.82 log S-Td). Ricco’s area was estimated by two-phase regression analysis. Results: All subjects demonstrated a notable decrease in Ricco’s area with increasing blue background luminance. On average, Ricco’s area decreased in size by 0.39 log units per log unit increase in blue background luminance. Conclusions: The change in Ricco’s area with the blue background component is not what one would initially expect given the known organization of S-cone–driven cells at the retinal level. Spatial reorganization by the suppressive surround of the receptive fields at a cortical level and a reduction in the contribution from S-cones with the lowest weights in the retinal receptive field periphery are among the possible mechanisms of the summation changes observed. These findings have implications for the design of clinical tests of the S-cone pathway

The effect of cataract on early stage glaucoma detection using spatial and temporal contrast sensitivity tests

Background: To investigate the effect of cataract on the ability of spatial and temporal contrast sensitivity tests used to detect early glaucoma. Methods: Twenty-seven glaucoma subjects with early cataract (mean age 60 ±10.2 years) which constituted the test group were recruited together with twenty-seven controls (cataract only) matched for age and cataract type from a primary eye care setting. Contrast sensitivity to flickering gratings at 20 Hz and stationary gratings with and without glare, were measured for 0.5, 1.5 and 3 cycles per degree (cpd) in central vision. Perimetry and structural measurements with the Heidelberg Retinal Tomograph (HRT) were also performed. Results: After considering the effect of cataract, contrast sensitivity to stationary gratings was reduced in the test group compared with controls with a statistically significant mean difference of 0.2 log units independent of spatial frequency. The flicker test showed a significant difference between test and control group at 1.5 and 3 cpd (p = 0.019 and p = 0.011 respectively). The percentage of glaucoma patients who could not see the temporal modulation was much higher compared with their cataract only counterparts. A significant correlation was found between the reduction of contrast sensitivity caused by glare and the Glaucoma Probability Score (GPS) as measured with the HRT (p<0.005). Conclusions: These findings indicate that both spatial and temporal contrast sensitivity tests are suitable for distinguishing between vision loss as a consequence of glaucoma and vision loss caused by cataract only. The correlation between glare factor and GPS suggests that there may be an increase in intraocular stray light in glaucoma

The effect of age-related lens yellowing on Farnsworth-Munsell 100 hue error score

Purpose:  To investigate the effects of real and simulated age-related changes in crystalline lens yellowing on Farnsworth–Munsell (FM) 100 hue performance. Methods:  FM 100 hue total and partial error scores (PES) were measured in a group of younger (n = 10, mean age = 22.2 ± 2.65 years) and a group of older (n = 10, mean age = 54.5 ± 2.64 years) normal observers along with psychophysical estimates of crystalline lens optical density and pupil size. Three younger observers underwent repeated FM 100 hue testing under a variety of simulated age-related lens yellowing conditions, using filters with well-defined transmittance properties which attempted to mimic the real age-related lens yellowing changes of the older group. Results:  FM 100 hue total and PES were significantly higher in the older age group compared with the younger group (p < 0.01). Lens density measures were significantly higher in the older age group compared with the young group (p < 0.01), but showed less scatter than individual FM 100 hue error scores. Simulated lens yellowing in the three younger observers, equivalent to the level of that of the older observers, did not affect any of their FM 100 hue total or PES. Conclusions:  Simulation of age-related lens yellowing in younger observers has little effect on FM 100 hue error score. A variety of other factors such as pupil size, background illuminaton level, iris colour and macular pigment density may contribute to the age-related increase in FM 100 hue scores

Sensitivity Loss in Early Glaucoma Can Be Mapped to an Enlargement of the Area of Complete Spatial Summation

PURPOSE. The area of complete spatial summation (Ricco&apos;s area) is the largest stimulus size for which area ϫ intensity is constant at threshold. The authors sought to investigate whether Ricco&apos;s area changes in early glaucoma to account for the decreased visual signal/noise ratio that may accompany retinal ganglion cell loss. METHODS. Spatial summation functions were measured, and Ricco&apos;s area was determined at four 10°retinal locations in 24 patients with early glaucoma (total deviation at test locations, mean, Ϫ1.3 dB; range, ϩ2 dB to Ϫ8 dB) and 26 age-similar healthy subjects under achromatic and S-cone isolation conditions. Achromatic grating resolution acuity was measured at the same locations to estimate functional ganglion cell density. RESULTS. Ricco&apos;s area was enlarged in patients compared with controls for both achromatic (enlarged by: superior field, 0.57 log units, P Ͻ 0.01; inferior field, 0.72 log units, P Ͻ 0.01) and chromatic (enlarged by: superior field, 0.26 log units, P Ͻ 0.01; inferior field, 0.25 log units, P ϭ 0.065) stimuli, with negligible vertical summation curve shifts along the intensity axis. Resolution acuity was significantly reduced in glaucoma patients in both hemifields (P Ͻ 0.001). There was a weak, but significant, relationship between Ricco&apos;s area and resolution acuity. CONCLUSIONS. Enlargement of Ricco&apos;s area completely compensates for reduced perimetric sensitivity in early glaucoma to maintain constant threshold at Ricco&apos;s area, suggesting an increase in signal pooling in response to ganglion cell loss. The rightward displacement of the spatial summation curve indicates that perimetric stimuli should be capable of modulating in size as well as/instead of contrast, which may boost the glaucoma signal within measurement noise. (Invest Ophthalmol Vis Sci. 2010;51:6540 -6548) DOI:10.1167/iovs.10-5718 S tandard automated perimetry (SAP) is routinely used in ophthalmic clinics across the world for the detection, evaluation, and monitoring of neural damage to the visual pathway, particularly in conditions such as glaucoma. Glaucoma is characterized by a loss of retinal ganglion cells, which can be observed directly using histologic techniques 13 It is well known that perimetric sensitivity declines with progressive ganglion cell death in glaucoma. It has also been well established that the relationship between ganglion cell density and perimetric sensitivity is not monotonic over the entire course of the disease. 24 -28 Considering Ricco&apos;s law and the classical spatial summation curve it is apparent that, in the normal eye, thresholds for conventional Goldmann III stimuli (area ϭ Ϫ0.83 log deg 2 ) in SAP are determined by complete spatial summation only for retinal eccentricities greater than approximately 15°. This is because the stimulus size is smaller than Ricco&apos;s area for those retinal regions. Within 15°, thresholds for SAP are determined by probability summation because stimuli are larger than Ricco&apos;s area. If one uses a larger stimulus for conventional perimetry (e.g., Goldman V), then thresholds will be determined by probability summation for a greater region of the retina under test. It is clear that the relative size of Ricco&apos;s area and the stimulus size used are important when determining and interpreting threshold values from SAP. The exact physiological mechanism underlying Ricco&apos;s area is still heavily debated 22,24,25,28 -31 ; however, some studies suggest that it changes with retinal eccentricity to maintain a constant number of underlying ganglion cells

Sensitivity Loss in Early Glaucoma Can Be Mapped to an Enlargement of the Area of Complete Spatial Summation

Purpose. The area of complete spatial summation (Ricco's area) is the largest stimulus size for which area × intensity is constant at threshold. The authors sought to investigate whether Ricco's area changes in early glaucoma to account for the decreased visual signal/noise ratio that may accompany retinal ganglion cell loss. Methods. Spatial summation functions were measured, and Ricco's area was determined at four 10° retinal locations in 24 patients with early glaucoma (total deviation at test locations, mean, −1.3 dB; range, +2 dB to −8 dB) and 26 age-similar healthy subjects under achromatic and S-cone isolation conditions. Achromatic grating resolution acuity was measured at the same locations to estimate functional ganglion cell density. Results. Ricco's area was enlarged in patients compared with controls for both achromatic (enlarged by: superior field, 0.57 log units, P < 0.01; inferior field, 0.72 log units, P < 0.01) and chromatic (enlarged by: superior field, 0.26 log units, P < 0.01; inferior field, 0.25 log units, P = 0.065) stimuli, with negligible vertical summation curve shifts along the intensity axis. Resolution acuity was significantly reduced in glaucoma patients in both hemifields (P < 0.001). There was a weak, but significant, relationship between Ricco's area and resolution acuity. Conclusions. Enlargement of Ricco's area completely compensates for reduced perimetric sensitivity in early glaucoma to maintain constant threshold at Ricco's area, suggesting an increase in signal pooling in response to ganglion cell loss. The rightward displacement of the spatial summation curve indicates that perimetric stimuli should be capable of modulating in size as well as/instead of contrast, which may boost the glaucoma signal within measurement noise

The Robustness of Various Forms of Perimetry to Different Levels of Induced Intraocular Stray Light

PURPOSE: To compare directly the robustness of standard automated perimetry (SAP), short-wavelength automated perimetry (SWAP), frequency-doubling perimetry (FDP), and grating-resolution perimetry (GRP) stimuli to different degrees of intraocular stray light induced by commercially available opacity-containing filters. METHODS: Five white opacity filters of increasing density were used to simulate the typical forward light scatter and stray light values associated with age-related lens opacification and significant cataract. The individually induced intraocular stray light value for each filter was quantified with a stray light meter and plotted against individual perimetric thresholds for the right eyes of three normally sighted trained observers for SAP, SWAP, FDP, and GRP. RESULTS: All tests were significantly but differently affected by increasing stray light. Overall average declines over a 1 log unit change in the stray light values were as follows: SAP, 4.85 dB; SWAP, 9.03 dB; FDP, 4.29 dB; and GRP, 1.36 dB. Standardized (z) scores were calculated after normalization to the spread of the normative data values for each instrument. These indicated that the standardized changes from baseline over the range of the five filters per log stray light unit were as follows: SAP, 2.177; SWAP, 1.96; FDP, 1.277; and GRP, 1.04. CONCLUSIONS: The increased stray light values induced by cataract-simulating filters has a significant effect on all tests. However, GRP, which is known to be limited by retinal sampling rather than contrast, remains the most robust of the tests to the effects of intraocular stray light. The degree to which the normative "sensitivity" range for different types of perimetry might incorporate a component caused by individual differences in intraocular stray light is discussed and requires further research

The appropriateness of luminance vs. energy as a descriptor of CRT stimulus output when measuring the temporal aspects of vision

The use of cathode-ray-tube monitors is widespread in the vision science community. The temporal response of these displays is, however, known to vary with the frame rate selected. No previous study has investigated if variations in frame rate impacts upon the temporal processing of spatially identical stimuli. In this study we found the upper limit of complete temporal summation for an achromatic spot stimulus to be greater at 60Hz compared with a frame rate of 160Hz when visual thresholds were classified using luminance output. No such variation was found when a novel metric quantifying total energy output was used. This finding has significant implications for the accurate quantification of visual thresholds collected using display monitors used in visual psychophysics investigations