Evidence against age-related enlargements of ganglion cell receptive field centers under scotopic conditions.

Age-related changes in the neural organization of spatial information are required to account for much of the senescent loss in human scotopic spatial vision, specifically declines in the high spatial frequency cut-off of the contrast sensitivity function and enlargements of the area over which there is complete spatial summation (Ricco's area). These results are consistent with hypothesized enlargements of ganglion cell receptive field centers during adulthood. This hypothesis was tested with 50 subjects (19-88 years) by measuring contrast thresholds for two low spatial frequency gratings (0.3 and 1.2 cycles per degree) at a series of scotopic mean illuminance levels. Contrast sensitivity increased with retinal illuminance and then reached a plateau, corresponding to the onset of Weber-like behavior. No age-related change in the light level associated with the onset of Weber-like behavior was found at either spatial frequency. This result is inconsistent with proposed age-related enlargements of ganglion cell receptive field centers under scotopic conditions.

Ricco's Areas for S- and L-Cone Mechanisms Across the Retina.

The purposes of this study were to measure areas of complete spatial summation (i.e., Ricco's area) for S- and L-cone mechanisms and to evaluate whether the sizes of Ricco's area could be explained in terms of either the densities of photoreceptors or ganglion cells. Increment thresholds were measured at the fovea and at 1.5°, 4°, 8°, and 20° in the superior retina using a temporal two-alternative forced-choice procedure. Test stimuli ranging from -0.36 to 4.61 log area (min(2)) were presented on concentric 12.3° adapting and auxiliary fields, which isolated either an S- or L-cone mechanism on the plateau of the respective threshold vs. intensity function. The data indicate that from 0-20° retinal eccentricity, the size of Ricco's area is larger for the S-cone mechanism than the L-cone mechanism, increases monotonically for the L-cone mechanism, and, for both cone mechanisms, increases between 8-20° retinal eccentricity. This latter finding suggests that ganglion cell density rather than cone density defines the size of Ricco's area in the parafoveal and peripheral retina.