AREA AND VISUAL THRESHOLD

Spatial summation in human cone mechanisms from 0 degrees to 20 degrees in the superior retina

The maximum area of complete spatial summation (i.e., Ricco's area) for human short-wavelength-sensitive-(S-) and long-wavelength-sensitive- (L-) cone mechanisms was measured psychophysically at the fovea and at 1.5 degrees , 4 degrees , 8 degrees , and 20 degrees along the vertical meridian in the superior retina. Increment thresholds were measured for three observers by a temporal two-alternative forced-choice procedure. Test stimuli ranging from -0.36 to 4.61 log area (min2) were presented on concentric 12.3 degrees adapting and auxiliary fields, which isolated either an S- or an L-cone mechanism on the plateau of its respective threshold versus intensity function. Test flash durations were 50 and 10 ms for the S- and L-cone mechanisms, respectively. The data indicate that, from 0 degrees to 20 degrees, Ricco's area increases monotonically for the L-cone mechanism, is variable for the S-cone mechanism, and is larger for the S-cone mechanism than for the L-cone mechanism for essentially all retinal locations. This pattern of results most likely reflects differences in ganglion cell density and changes in neural convergence with retinal eccentricity.

Spatial summation among coextensive and parallel line segments across wide separations (50 degrees): egocentric localization and the great circle model

The elevation at which an observer sets a target to appear at eye level (VPEL) is systematically related to the angle of pitch of the visual field and is only a little less for a visual field consisting of a single line in darkness than for a complexly structured field [Matin and Li (1994a) Vision Research, 34, 311-330]. Three experiments are described which measure the quantitative characteristics of spatial summation among individual pitched-from-vertical line segments that control the visual influence on VPEL. As the length of a one-line stimulus increased from 0 degrees to 64 degrees the slope of the VPEL-vs-pitch function (S) increased from 0 to +0.56 along a negatively accelerated exponential with a 15.1 degree space constant. The combined influence on S of two simultaneously-presented, parallel, pitched-from-vertical lines, horizontally separated by 50 degrees, is slightly greater than the combined influence of two coextensive line segments with the same total length. S saturates at a locus that lies beyond any separate neural locus for the processing of the individual line. The results are effectively treated by the Great Circle Model (GCM) which converts stimulus "nonlocality" to neural "locality" by mapping the intersections of the images of parallel line sets in a spherical approximation of the eye on to a set of neural nodes. A neurophysiological realization of GCM is compatible with mediation by the long horizontal connections afferent to layer 6 of primary visual cortex (V1). The combination of visual influences with extraretinal information is compatible with the characteristics of posterior parietal cortex downstream from V1. The increase in the effectiveness of a line with increase in length is in accord with a more general division between the utilization of long lines for egocentric orientation and short lines for figural processes; end-inhibition from elongated layer 6 cells (which process long lines) onto layer 4 cells (which process short lines) in V1 may provide a means for separating the two streams of information.

[The effect of the pupil as aperture and field stop on the various components of the human electroretinogram (author's transl)]

The electroretinogram (ERG) was recorded in 10 normal subjects under scotopic and photopic conditions with the pupil of one eye constricted and that of the other eye dilated. The human iris, being displaced from the nodal point of the eye, acts not only as an aperture, regulating the retinal illumination, but also as a field stop, limiting the visual angle, especially for large object fields, as in Ganzfeld illumination. This double effect of a constricted pupil clearly influences the Ganzfeld ERG, not only shifting the intensity response functions to higher luminances but also diminishing the maximal responses. Control experiments with smaller test fields, which are less affected by the pupillary field-stop properties, reveal no diminution of the scotopic b-wave amplitudes. Implicit time functions, being nearly independent of the number of receptors stimulated, can be matched by taking into account the pupillary diameter and calculating the actual retinal illumination (Troland). Amplitudes, being highly affected by a decrease of responding neurons due to the angle-limiting field-stop character of the pupil, cannot be matched with regard to the pupillary diameter. This effect is most noticeable in Ganzfeld illumination for the scotopic b-wave, generated mainly by neurons of the peripheral retina, and has less effect on the photopic responses that are generated more centrally.