Psychophysical and electroretinographic observations in normal observers and achromats (who lack ... more Psychophysical and electroretinographic observations in normal observers and achromats (who lack cone vision) suggest that rod signals have access to two retinal pathways: one, slow and sensitive; the other, fast and insensitive. The two are revealed by double-branched rod-detected flicker threshold vs intensity (ftvi) curves, and by a suprathreshold intensity region (below cone detection)—the perceptual null—within which the sensation of 15-Hz flicker is canceled. The cause of the null is believed to be destructive interference between signals conveyed by the two pathways. Here we report that the break in the 15-Hz ftvi curve and the perceptual null is not found, in either the normal or achromat observer when the flickering target is ≤2° in diameter. Phase measurements (relative to a cone standard) indicate that the faster rod pathway mediates flicker detection for targets of ≤2°, not because the signals from the slow pathway are severely diminished but because destructive interfer...
Hue shifts were measured in isoluminant color gratings whose bar width was varied from 2' to 20' ... more Hue shifts were measured in isoluminant color gratings whose bar width was varied from 2' to 20' ofvisual 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.
1. Absolute and increment thresholds were measured in a retinal region 12 deg temporal from the f... more 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) curve...
1. Incremental thresholds were measured in a retinal region 12 deg temporal from the fovea with a... more 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 coordinates (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. MS 9236 L. T. SHARPE, C. C. FACH AND A. STOCKMAN incremental threshold method, as well as several modifications of it, to determine the extent to which the rod visual system adapts independently of the cone systems. In making our measurements, we used not only normal observers, but also a typical, complete achromat, a blue-cone monochromat, a protanope and four deuteranopes. Rod adaptation can be assumed to be more or less independent of the cones on dim adapting backgrounds that elevate rod threshold but have little or no direct effect on the cones (e.g. Flamant & Stiles, 1948). As the intensity of the 'rod' adapting background is increased however, it eventually begins to desensitize cones as well as rods. When this happens, rod adaptive independence can no longer be safely assumed (see Makous & Boothe, 1974; Makous & Peeples, 1979). The sensitivity difference between rods and cones decreases with increasing wavelength (Wald, 1945), so that it is 'rod' adapting backgrounds of long wavelength that most severely desensitize the cones. For this reason, long-wavelength backgrounds are typically used to characterize rod adaptation at high scotopic intensities (e.g. Aguilar & Stiles, 1954), where the rod threshold response would be obscured by cones if shorter wavelength backgrounds were used. However, because they strongly adapt the cones, long-wavelength backgrounds are precisely the ones on which failures of rod adaptive independence are most likely to be found.
The incremental threshold of the isolated rod visual system is believed, under certain conditions... more The incremental threshold of the isolated rod visual system is believed, under certain conditions, to obey Weber's law (that is, to increase in direct proportion to the intensity of the background). This relation was tested at several background wavelengths, over an intensity range for which the target was seen only by the rods. Although the slope on long-wavelength background approximates unity (that is, Weber's law on log-log coordinates), it averages less than 0.8 on short- and middle-wavelength backgrounds. This is the same value as that found for the thresholds of a typical, complete achromat--who lacks cone vision--regardless of background wavelength. These results force the conclusion that Weber's law for incremental threshold detection is achieved not by the rods alone but only by the rods acting together with the cones.
A randomly dotted yellow disk was rotated at a speed of 5 rpm, alternating in direction every 10 ... more 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.
1. Absolute and increment thresholds were measured in a retinal region 12 deg temporal from the f... more 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 coordinates 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.
We investigated temporal summation of the rods in a complete achromat, who lacks cone vision. Cri... more We investigated temporal summation of the rods in a complete achromat, who lacks cone vision. Critical duration (I,) was estimated both at the achromat's preferred area of fixation and at an area 12 deg laterally in the nasal visual field. Comparable & dete~nations were made in a normal trichromat. At background luminances of 0.0 and 0.6 scat. td, where the rods mediate detection, tire values of r, were similar for the achromat and the normal. At a luminance of 813 scat. tds, however, where the middle-wavelength sensitive cones mediate detection in the trichromat, the t, for the achromat was much longer than that for the trichromat.
Psychophysical and electroretinographic observations in normal and achromat observers suggest tha... more Psychophysical and electroretinographic observations in normal and achromat observers suggest that rod flicker signals have access to at least two retinal pathways: one (pi 0), slow and sensitive, predominating at scotopic luminance levels; the other (pi'0), fast and insensitive, predominating at mesopic ones. We have measured steady-state flicker detection sensitivities on background fields ranging from 430 to 640 nm in normal observers. Our results suggest that cone signals can reduce the sensitivity of pi'0, but have comparatively little effect on pi 0. The pi'0 field sensitivities derived from these measurements have been fitted with linear combinations of the scotopic luminosity function, V' lambda, the M-cone spectral sensitivity function, M lambda, and the L-cone function, L lambda. These fits demonstrate a clear cone influence on pi'0, but they cannot tell us unequivocally whether the influence is from the M-cones, from the L-cones or from both. According...
Skottun (1989) raises a number of issues concerning his previous (Skottun, Nordby & Rosness, 1982... more 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 flash...
1. Absolute and increment thresholds were measured in a retinal region 12 deg temporal from the f... more 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 coordinates 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.
We used two methods to estimate short-wave (S) cone spectral sensitivity. Firstly, we measured S-... more 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.
Psychophysical and electroretinographic observations in normal observers and achromats (who lack ... more Psychophysical and electroretinographic observations in normal observers and achromats (who lack cone vision) suggest that rod signals have access to two retinal pathways: one, slow and sensitive; the other, fast and insensitive. The two are revealed by double-branched rod-detected flicker threshold vs intensity (ftvi) curves, and by a suprathreshold intensity region (below cone detection)—the perceptual null—within which the sensation of 15-Hz flicker is canceled. The cause of the null is believed to be destructive interference between signals conveyed by the two pathways. Here we report that the break in the 15-Hz ftvi curve and the perceptual null is not found, in either the normal or achromat observer when the flickering target is ≤2° in diameter. Phase measurements (relative to a cone standard) indicate that the faster rod pathway mediates flicker detection for targets of ≤2°, not because the signals from the slow pathway are severely diminished but because destructive interfer...
Hue shifts were measured in isoluminant color gratings whose bar width was varied from 2' to 20' ... more Hue shifts were measured in isoluminant color gratings whose bar width was varied from 2' to 20' ofvisual 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.
1. Absolute and increment thresholds were measured in a retinal region 12 deg temporal from the f... more 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) curve...
1. Incremental thresholds were measured in a retinal region 12 deg temporal from the fovea with a... more 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 coordinates (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. MS 9236 L. T. SHARPE, C. C. FACH AND A. STOCKMAN incremental threshold method, as well as several modifications of it, to determine the extent to which the rod visual system adapts independently of the cone systems. In making our measurements, we used not only normal observers, but also a typical, complete achromat, a blue-cone monochromat, a protanope and four deuteranopes. Rod adaptation can be assumed to be more or less independent of the cones on dim adapting backgrounds that elevate rod threshold but have little or no direct effect on the cones (e.g. Flamant & Stiles, 1948). As the intensity of the 'rod' adapting background is increased however, it eventually begins to desensitize cones as well as rods. When this happens, rod adaptive independence can no longer be safely assumed (see Makous & Boothe, 1974; Makous & Peeples, 1979). The sensitivity difference between rods and cones decreases with increasing wavelength (Wald, 1945), so that it is 'rod' adapting backgrounds of long wavelength that most severely desensitize the cones. For this reason, long-wavelength backgrounds are typically used to characterize rod adaptation at high scotopic intensities (e.g. Aguilar & Stiles, 1954), where the rod threshold response would be obscured by cones if shorter wavelength backgrounds were used. However, because they strongly adapt the cones, long-wavelength backgrounds are precisely the ones on which failures of rod adaptive independence are most likely to be found.
The incremental threshold of the isolated rod visual system is believed, under certain conditions... more The incremental threshold of the isolated rod visual system is believed, under certain conditions, to obey Weber's law (that is, to increase in direct proportion to the intensity of the background). This relation was tested at several background wavelengths, over an intensity range for which the target was seen only by the rods. Although the slope on long-wavelength background approximates unity (that is, Weber's law on log-log coordinates), it averages less than 0.8 on short- and middle-wavelength backgrounds. This is the same value as that found for the thresholds of a typical, complete achromat--who lacks cone vision--regardless of background wavelength. These results force the conclusion that Weber's law for incremental threshold detection is achieved not by the rods alone but only by the rods acting together with the cones.
A randomly dotted yellow disk was rotated at a speed of 5 rpm, alternating in direction every 10 ... more 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.
1. Absolute and increment thresholds were measured in a retinal region 12 deg temporal from the f... more 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 coordinates 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.
We investigated temporal summation of the rods in a complete achromat, who lacks cone vision. Cri... more We investigated temporal summation of the rods in a complete achromat, who lacks cone vision. Critical duration (I,) was estimated both at the achromat's preferred area of fixation and at an area 12 deg laterally in the nasal visual field. Comparable & dete~nations were made in a normal trichromat. At background luminances of 0.0 and 0.6 scat. td, where the rods mediate detection, tire values of r, were similar for the achromat and the normal. At a luminance of 813 scat. tds, however, where the middle-wavelength sensitive cones mediate detection in the trichromat, the t, for the achromat was much longer than that for the trichromat.
Psychophysical and electroretinographic observations in normal and achromat observers suggest tha... more Psychophysical and electroretinographic observations in normal and achromat observers suggest that rod flicker signals have access to at least two retinal pathways: one (pi 0), slow and sensitive, predominating at scotopic luminance levels; the other (pi'0), fast and insensitive, predominating at mesopic ones. We have measured steady-state flicker detection sensitivities on background fields ranging from 430 to 640 nm in normal observers. Our results suggest that cone signals can reduce the sensitivity of pi'0, but have comparatively little effect on pi 0. The pi'0 field sensitivities derived from these measurements have been fitted with linear combinations of the scotopic luminosity function, V' lambda, the M-cone spectral sensitivity function, M lambda, and the L-cone function, L lambda. These fits demonstrate a clear cone influence on pi'0, but they cannot tell us unequivocally whether the influence is from the M-cones, from the L-cones or from both. According...
Skottun (1989) raises a number of issues concerning his previous (Skottun, Nordby & Rosness, 1982... more 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 flash...
1. Absolute and increment thresholds were measured in a retinal region 12 deg temporal from the f... more 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 coordinates 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.
We used two methods to estimate short-wave (S) cone spectral sensitivity. Firstly, we measured S-... more 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.
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Papers by Clemens Fach