Clemens C. Fach

The spectral sensitivity of the human short-wavelength sensitive cones derived from thresholds and color matches.

We used two methods to estimate short-wave (S) cone spectral sensitivity. Firstly, we measured S-cone thresholds centrally and peripherally in five trichromats, and in three blue-cone monochromats, who lack functioning middle-wave (M) and long-wave (L) cones. Secondly, we analyzed standard color-matching data. Both methods yielded equivalent results, on the basis of which we propose new S-cone spectral sensitivity functions. At short and middle-wavelengths, our measurements are consistent with the color matching data of Stiles and Burch (1955, Optica Acta, 2, 168-181; 1959, Optica Acta, 6, 1-26), and other psychophysically measured functions, such as pi 3 (Stiles, 1953, Coloquio sobre problemas opticos de la vision, 1, 65-103). At longer wavelengths, S-cone sensitivity has previously been over-estimated.

Assimilative hue shifts in color depend on bar width

Hue shifts were measured in isoluminant color gratings whose bar width was varied from 2′ to 20′ of visual angle. Subjects matched the hues in each grating with individual Munsell swatches. Hue shifts were largest for bar widths of 2′; however, they depended on the color combination used. Green and red shifted toward (i.e., assimilated with) whatever second grating color they were paired with. Blue, on the other hand, assimilated with red and with yellow, but remained relatively unchanged when combined with green. Yellow shifted only minimally, regardless of the second grating color. Hue shifts decreased with increasing stripe width and disappeared between 4.5′ and 7.5′. Compared with the assimilative hue shifts, color contrast effects were slight or absent. These results cannot be attributed merely to chromatic aberration, macular pigment, eye movements, or field size.

Temporal and spatial summation in the human rod visual system.

1. Absolute and increment thresholds were measured in a retinal region 12 deg temporal from the fovea with 520 nm targets of varying size and duration. Measurements were made under rod‐isolation conditions in two normal observers and in a typical, complete achromat observer who has no cone‐mediated vision. The purpose of these experiments was to determine how the temporal and spatial summation of rod‐mediated vision changes with light adaptation. 2. The absolute threshold and the rise in increment threshold with background intensity depend upon target size and duration, but the psychophysically estimated dark light of the eye (the hypothetical light assumed to be equivalent to photoreceptor noise) does not. 3. The rise in increment threshold for tiny (10 min of arc), brief (10 ms) targets approaches the de Vries‐Rose square‐root law, varying according to the quantal fluctuations of the background light. The slope of the rod increment threshold versus background intensity (TVI) curves in logarithmic co‐ordinates is about 0.56 +/‐ 0.04 (when cones are not influencing rod field adaptation). For large (6 deg) and long (200 ms) targets, a maximum slope of about 0.77 +/‐ 0.03 is attained. 4. The steeper slopes of the rod‐detected TVI curves for large, long targets implies some reduction in temporal or spatial summation. In fact, the change in summation area is much more critical: under conditions where only the rod system is active the TVI curve slope is independent of target duration, suggesting that temporal summation is practically independent of background intensity. 5. The rise in threshold also depends on the wavelength of the background field in the normal observer but not in the achromat, confirming reports that the field adaptation of the rods is not independent of the quantal absorptions in the cones. The cone influence is most conspicuous on long‐wavelength backgrounds and is found for all target sizes and durations, but is greater for large and long targets than for the other conditions.

The field adaptation of the human rod visual system.

1. Incremental thresholds were measured in a retinal region 12 deg temporal from the fovea with a target of 200 ms in duration and 6 deg in diameter superimposed on background fields of various intensities and wavelengths. Measurements were made under rod‐isolation conditions in five normal observers and in a typical, complete achromat observer who had no cone function. 2. The rise in threshold with background intensity changes with background wavelength in the normal trichromat observers. On 450, 520 and 560 nm backgrounds the average slope in logarithmic co‐ordinates (0.78 +/‐ 0.04, S.D.) is similar to that found for the achromat‐‐whose slope is independent of background wavelength (0.79 +/‐ 0.03)‐‐but on a 640 nm background it more nearly approaches Webers law (0.91 +/‐ 0.02). This indicates that the sensitivity of the rods to an incremental target is not determined by quantal absorptions in the rods alone but by quantal absorptions in both the rods and the cones. 3. Rod incremental thresholds were also measured in various colour‐blind observers lacking one or more of the cone classes: a blue‐cone monochromat, four deuteranopes and a protanope. For the blue‐cone monochromat, like the achromat, the slope of the increment threshold curve is constant with background wavelength. For the deuteranopes and the protanope, like the normal, the slope increases with wavelength. The protanope, however, shows a smaller increase in slope, consistent with the lower sensitivity of his cones to long‐wavelength light. 4. The dependence of the field adaptation of the rods on the cones was confirmed by field‐mixture experiments, in which the incremental threshold was measured against bichromatic backgrounds, and in silent substitution experiments, in which backgrounds equated for their effects on either the cones or the rods but not both were instantaneously substituted for one another.

Temporal summation in the achromat

We investigated temporal summation of the rods in a complete achromat, who lacks cone vision. Critical duration (tc) was estimated both at the achromats preferred area of fixation and at an area 12 deg laterally in the nasal visual field. Comparable tc determinations were made in a normal trichromat. At background luminances of 0.0 and 0.6 scot. td, where the rods mediate detection, the values of tc were similar for the achromat and the normal. At a luminance of 813 scot. tds, however, where the middle-wavelength sensitive cones mediate detection in the trichromat, the tc for the achromat was much longer than that for the trichromat.

Contingent aftereffects: Lateral interactions between color and motion

A randomly dotted yellow disk was rotated at a speed of 5 rpm, alternating in direction every 10 sec. Its change in direction of rotation was paired with a change in surround color, which was either red or green. After 15 min of exposure, observers reported vivid motion aftereffects contingent on the color of both the stationary disk and the surround, even though during adaptation only motion or color was associated with either alone. In further experiments, it was established that a change in color (or direction of motion) of the disk could be associated with a change in direction of motion (or color) of the surround. Such lateral effects were found even when a wide (50) annulus was introduced between the disk and the surround during adaptation and testing. Furthermore, the aftereffects generalized to the annulus, which was not associated with either color or motion during adaptation. However, when the disk alone was adapted to color and motion, no generalization to the surround was found (and vice versa), suggesting that the effects are not produced by adaptation of large receptive fields or by scatter of light within the eye. The results appear to conflict with the ideas that contingent aftereffects are confined to the adapted area of the retina and that they are built up by links between single-duty neurones, and with an extreme view of the segregation of color and motion early in human vision.

Spatial relations of flicker signals in the two rod pathways in man.

1. Flicker signals originating from the human rod photoreceptors seem to have access to two retinal pathways: one slow and sensitive, the other fast and insensitive. The phase lag between signals in the two pathways grows monotonically with frequency, reaching 180 deg near 15 Hz. 2. At 15 Hz, destructive interference between the slow and the fast signals can cause two related phenomena: (i) a suprathreshold intensity region‐‐the perceptual null‐‐within which the perception of flicker vanishes, and (ii) a double branching of the 15 Hz rod‐detected flicker threshold versus intensity (TVI) curve. 3. Here we investigate the effect of changing target size on these phenomena in normal human observers. We find that the double‐branched flicker TVI curve and the perceptual null are found for all targets larger than 2 deg in diameter. For smaller diameter targets, however, neither the lower branch of the double‐branched flicker TVI curve nor the null are found. 4. While this might suggest that the slow rod signals are selectively disadvantaged by the use of small targets, phase measurements relative to a cone standard reveal that the slow signals are always present. For targets < or = 2 deg in diameter, however, they remain below detection threshold because of destructive interference with the fast rod signals. Thus, for small targets, the perceptual null is not absent, but has merged with (and therefore obliterated) the lower branch of the double‐branched flicker TVI function. 5. This situation could arise if decreasing the target size causes a parallel reduction in the sensitivities of both pathways, rather than a selective reduction in the sensitivity of either one. Our findings are therefore consistent with a model in which the large‐scale spatial organization of the two rod pathways is roughly similar.

Temporal summation in a typical, complete achromat

Skottun (1989) raises a number of issues concerning his previous (Skottun, Nordby & Rosness, 1982) and our recent report (Sharpe, Fach & Nordby, 1988) about temporal summation in the same achromat observer (co-author K.N.). We welcome the opportunity to clarify a few points. First, at the single background intensity where the two sets of measurements can be compared (approx. 430 Scot. td), we report a much longer temporal summation for both the achromat (cu 125 msec) and the normal control observer (ca 42 msec) than Skotton et al. do (cu 15 msec for both). Since our findings accord with those of others (Krauskopf & Mollon, 1971; Uetsuki & Ikeda, 1971) while those of Skottun et al. do not, we suggested that their results might “be partly attributed to the adverse effects of using an electronic tachistoscope . . . to measure threshold”. Skottun argues that this cannot be the grounds for the discrepancy because he and his collaborators carefully calibrated the tachistoscopic test flashes. We accept his statement, but do not abandon our belief that turning fluorescent lamps on and off in commercially-available tachistoscopes is a poor means of modulating light intensity in threshold-duration experiments (to see why, cf. Mollon & Polden, 1978). Second, unlike Skottun et al., we find that the critical duration measured on a background near 430 scat. td is considerably longer for the achromat (cu 125 msec) than for the normal observer (ca 42msec), because the achromat must rely on rod vision to set threshold, while