The field adaptation of the human rod visual system.

Abstract

SUMMARY1. 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 +003) -but on a 640 nm background it more nearly approaches Weber's 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. INTRODUCTIONThe visual system adapts to variations in ambient illumination by adjusting its sensitivity. This change in sensitivity is typically monitored by measuring the change in detection threshold for a small target flash as a function of the intensity of a larger adapting background (Stiles, 1939

Gap-Junctional Coupling and Absolute Sensitivity of Photoreceptors in Macaque Retina

“…Equation 2 was used to calculate P C of the pool. For comparison with a human psychophysical two-alternative forced-choice task with a criterion of 73% correct (Sharpe, 1990), we calculated the total number of flash-evoked R* within the detection pool required to achieve a P C of 0.73.…”

“…With linear summation, the size of a single photon response relative to the pooled dark noise is expected to be 100 times smaller than the signal-to-noise ratio of a single rod. Thus it is remarkable that human observers can detect stimuli evoking only 10 photoisomerizations (Sharpe, 1990); the signal-to-noise ratio for this stimulus would be only ϳ0.5. Considering the problem of rod summation, Baylor et al (1984) proposed that the rod synapse filters out low-amplitude dark fluctuations and preferentially transmits the larger photon responses.…”

The spectral properties of the two rod pathways

“…The dependence of the field adaptation of the rods on the cones has been shown for flashed rod targets using steady and transient adapting fields (e.g. Makous & Boothe, 1974;Frumkes & Temme, 1977;Ingling, Lewis, Loose & Meyers, 1977;Makous & Peeples, 1979;Sharpe et al, , 1992. Since the dependence is only marked on long-wave adapting fields, the influence of the M-cones (which are less sensitive to long-wave lights than the L-cones) is always likely to seem less than the influence of the L-cones, even though the two could be equally effective (in quanta1 terms) in elevating rod threshold.…”

“…Flash detection measurements of spectral sensitivity demonstrate the L-cone influence more clearly (e.g. Alexander & Kelly, 1984;Alexander, Kelly & Morris, 1986;Sharpe, Fach & Stockman, 1992), pres~ably because the L-cones are more sensitive at long wavelengths, where the cone influence on rods is most easily measured. Yet, if transient cone excitation is used, a clear M-cone influence on rod flash detection can be revealed (Sharpe et al, 1992).…”

“…Alexander & Kelly, 1984;Alexander, Kelly & Morris, 1986;Sharpe, Fach & Stockman, 1992), pres~ably because the L-cones are more sensitive at long wavelengths, where the cone influence on rods is most easily measured. Yet, if transient cone excitation is used, a clear M-cone influence on rod flash detection can be revealed (Sharpe et al, 1992). In rod flicker detection experiments, Knight, Buck and Sanocki (1991) claimed to find a clear effect of M-cones on the sensitivity on rrh, but little or no effect of L-cones.…”