Increment-threshold Spectral Sensitivity Research Articles

Angular dependence of polarisation contrast sensitivity in octopus

Cephalopod photoreceptors are polarisation-sensitive, giving them an ability to discriminate between lights of different angle and degree of polarisation. While colour vision is achieved by comparison of signals of photoreceptors tuned to different parts of light spectra, polarisation vision is achieved by comparison of signals of photoreceptors tuned to different orientations of e-vector. Therefore, from a theoretical point of view, polarisation vision is similar to colour vision. In particular, detection of polarised light against an unpolarised background is analogous to detection of chromatic light against grey. The dependence of polarisation contrast sensitivity on the angle of polarisation can be theoretically predicted using a receptor noise limited model in much the same way as it has been done for predicting the shape of the increment threshold spectral sensitivity in animals with colour vision. Here we report angular dependence of polarisation contrast sensitivity in octopus (O. tetricus, Gould 1852) and compare the theoretical predictions of polarisation contrast with the experimental results. Polarisation gratings were generated using LCD screens with removed polarisers and the orientation of polarisation was changed by rotating the screen. Reaction to the stimulus was recorded using a fixation reflex. We show that, in agreement with the theoretical predictions, the maximum contrast sensitivity is achieved at horizontal and vertical orientations of polarisation. Our results demonstrate that the dependence of polarisation contrast sensitivity on the angle of polarisation can be analysed in the same way as the dependence of colour thresholds on wavelength of monochromatic light added to a grey background.

Increment-threshold spectral sensitivity during saccadic eye movements in uniform visual field

We measured increment-threshold spectral sensitivity functions during saccades, immediately after saccades and during fixation. A uniform white background field which covered observer’s whole visual field prevented any retinal masking effects. Visual sensitivity was degraded during saccades or after saccades. The reduction in sensitivity depended upon the wavelength of the test stimulus. The spectral sensitivity function during fixation produced a broad smooth curve in the middle and long wavelength region, while saccades caused a prominent dip around 570 nm. This finding indicates that saccadic suppression, which cannot be attributed to retinal masking, has more effect on the achromatic channel than the chromatic channel. The role of central and retinal processes dealing with the perceptual clearness and stableness across saccades will be discussed.

L and M cone input into spectral sensitivity functions: a reanalysis.

In order to better understand the nature of long-wavelength (L) and middle-wavelength (M) cone input into spectral sensitivity functions and determine the reliability with which it is possible to predict L:M cone inputs, we developed analytical methods to determine confidence intervals for L:M cone input for spectral sensitivity functions or data transformed to cone-contrast space. Spectral sensitivity functions measured by direct heterochromatic brightness matches are dominated by the L/M opponent channel over most of the spectral range. For detection of large/ long test stimuli, spectral sensitivity functions show a characteristic "notch" at the adapting wavelength, with the L/M opponent channel dominating most of the spectral range. Flicker increment threshold (FIT) spectral sensitivity functions display many of the characteristics of the luminance flicker mechanism described by Stromeyer et al. (1987). [Vision Research, 27, 1113-1137]. Previous modelling of FIT spectral sensitivity functions proposed a 2:1 L:M cone input for most of testing conditions. We show that FIT spectral sensitivity functions are dominated by L cones but show L cone suppression under bright red adapting fields. For the fitted spectral sensitivity functions or simulated data sets, we found small confidence intervals for L:M cone input into the L/M opponent channel and conclude that it is possible to reliably predict L:M cone input ratios. However, for similar data sets of additive spectral sensitivity functions, we found large confidence intervals for L:M cone input ratios and conclude that it is not possible reliably predict L:M cone input into the L/M non-opponent channel using available spectral sensitivity functions.

Increment threshold and purity discrimination spectral sensitivities of X-chromosome-linked color-defective observers

The goal of the study was to evaluate spectral opponency in nine X-chromosome-linked color-defective observers. The tasks included increment threshold spectral sensitivity on an achromatic background, heterochromatic flicker photometry, and colorimetric purity discrimination. With a task of heterochromatic flicker photometry, the anomalous trichromatic observers showed spectral sensitivity of the corresponding dichromat. The increment threshold spectral sensitivity and colorimetric purity discrimination data were analyzed using the concept of standard cone photopigment spectral sensitivities for normal and defective vision, and a model that postulates one cone-additive and two cone-antagonistic systems. The model incorporated a shift of the peak spectral sensitivity of the long-wavelength-sensitive (LWS) pigment (for protan observers) or of the middle-wavelength-sensitive (MWS) pigment (for deutan observers). Two dichromats and two anomalous trichromats did not show clear evidence of LWS vs MWS cone antagonism. Five anomalous trichromats showed such cone antagonism. Molecular genetic analysis of the opsin genes is presented for eight of the observers.

Saccadic suppression of achromatic and chromatic responses measured by increment-threshold spectral sensitivity.

We measured spectral-sensitivity functions during saccadic eye movement by the increment-threshold method to test whether saccades selectively suppressed achromatic or chromatic responses. A circular monochromatic test stimulus of 12-deg diameter was presented for 10 ms on a 62 deg x 43 deg white background, and observations were made under three conditions: during fixation, during 6-deg saccades, and immediately after saccades. In two additional conditions the test stimulus was made to move during fixation and during 6-deg saccades at the same speed and in the same direction as the saccades. The during-fixation spectral-sensitivity function was found to resemble the relative luminous efficiency V (lambda) function in shape except for the case of short wavelengths, whereas the during-saccade spectral-sensitivity function showed lower sensitivity for all wavelengths and had three prominent peaks at approximately 440, 530, and 600 nm. These characteristics did not depend on whether the stimulus was stationary or moving. These results indicated that saccadic suppression was greater for achromatic than for chromatic response. A possible suppression mechanism was discussed involving the magno and parvo pathways.

Modelling sensitivity losses in ocular disorders: colour vision anomalies following intense blue-light exposure in monkeys.

The effects of prolonged exposure to intense, short-wavelength light were studied in monkeys through the measurement of increment-threshold spectral sensitivity (ITSS) and threshold-versus-intensity (TVI) functions using a behavioural method. The long-term effect of intense blue-light exposure was to induce a short-wavelength (SW) sensitivity loss which did not depend on the intensity or chromatic composition of the adapting field. The TVI curves for short wavelength stimuli revealed an increase in test threshold without changes in field sensitivity. Since this SW sensitivity loss may generalize to characteristic colour vision defects found in many outer retinal diseases, models of acquired alterations of colour vision mechanisms are considered. These models describe probable changes in ITSS functions and TVI curves in diseases affecting the inner or outer retina as well as changes in dark adaptation.

Colour vision anomalies following experimental glaucoma in monkeys

Spectral sensitivity defects, associated with chronic elevated intraocular pressure (IOP) produced by Argon laser trabeculoplasty, were studied in monkeys. Increment-threshold spectral sensitivity (ITSS) and threshold versus intensity (TVI) functions were measured using a behavioural model. Elevated IOP resulted in short wavelength (SW) sensitivity losses characteristic of many ocular diseases. The amount of SW sensitivity loss for ITSS functions depended upon the intensity level and chromatic composition of the background field. The optimum condition identifying the greatest SW sensitivity reduction was a yellow background of moderate intensity (100-1000 Td). In the early stages of experimental glaucoma, the cone mechanisms and the rod mechanism typically showed decreased test and field sensitivities. The SW cone pathway has slightly greater threshold elevation (approximately 0.3 log unit) compared to the rod and cone pathways. On the other hand, in the advanced stages of experimental glaucoma, the largest sensitivity losses were in the longer-wavelength, red-green opponent mechanisms, with the rod and SW cone pathways showing smaller losses. The similarities of the colour vision anomalies in this animal model with those of patients with glaucoma, provides support for its use as an experimental model for human glaucoma.

Spectral sensitivity of monocularly deprived cats.

The reports of rod-dominated psychophysical spectral sensitivity from the deprived eye of monocularly lid-sutured (MD) monkeys are intriguing but difficult to reconcile with the absence of any reported deprivation effects in retina. As most studies of MD retina have been from cat, we have examined psychophysically the increment threshold spectral sensitivity of MD cats using both reaction time and simultaneous two-choice behavioral procedures. Although the deprived eyes exhibited an absolute increment threshold sensitivity deficit, both rod and cone spectral sensitivity functions were obtained on large white backgrounds. This normal transition from rod to cone vision, as background luminance increased, was also found in threshold vs. intensity functions. Using their deprived eye, some cats exhibited a rod spectral sensitivity function when a smaller, normally photopic, background was used providing some support for a hypothesis that the rod-dominated spectral sensitivity observed in monkey may represent detection of scattered stimulus light. Alternatively monocular deprivation may reveal a rod-dominated mechanism which exists in monkey but not in cat.

Monochromatism determined at a long-wavelength/middle-wavelength cone-antagonistic locus

The foveal increment threshold spectral sensitivity function for a 500 msec raised cosine stimulus without spatial edges exhibits a sharp drop or “notch” in sensitivity that coincides with the wavelength of a long-wavelength adapting field. An appropriate name for this phenomenon is the “Sloan notch”, after Louise Sloan, who first observed a notch in a foveal threshold spectrum. We have examined suprathreshold discriminability on both sides of the Sloan notch produced by a 6700 td, 578 nm adapting field. In a temporal two-alternative forced-choice paradigm, a suprathreshold 650 nm low-frequency “standard” stimulus was paired with low-frequency “test” stimuli, of wavelength between 600 and 670 nm and varied intensity; the observer's task was to identify the interval containing the standard. Discriminability of the test and standard typically dropped to chance for some particular test intensity, producing “indiscriminability action spectra”, up to 0.7 log units above threshold. Truncated spectra (between about 530 and 560 nm) were also obtained from observers on the middle wavelength side of the Sloan notch, for a 550 nm standard. The indiscriminability action spectra of each observer were identical, up to scaling, with the observer's threshold action spectrum. Analysis of the action spectra shows that the indiscriminate stimuli are rendered equivalent at the input to a neural pathway where L- and M-cone signals converge with opposite sign. We also investigated discriminability in the spectral region containing and immediately surrounding the Sloan notch. Suprathreshold stimuli in the spectral region near the notch produce percepts that are always discriminable from 650 and 550 nm standards (and from one another), and thus we conclude that in this spectral region, perception is mediated in part by a pathway distinct from that which signals the standards. The action spectrum of this latter pathway was estimated with a variant of the discrimination procedure, and found similar to V λ, over the spectral region 575–610 nm.

Effects of interocular suppression on spectral sensitivity.

A stable form of interocular suppression, referred to as "permanent suppression," can be produced by presenting a contoured field to one eye and a spatially homogenous field to the fellow eye, a viewing condition analogous to the classic "hole-in-the-hand" illusion. We examined the relation between permanent suppression and binocular rivalry suppression by comparing the changes in the increment-threshold spectral sensitivity function produced by these two forms of suppression. Permanent suppression produced a reduction in spectral sensitivity; however, in contrast to binocular rivalry suppression, the sensitivity alterations associated with permanent suppression were independent of the test-probe wavelength. The different patterns of sensitivity loss observed during binocular rivalry and permanent suppression indicate that different neural mechanisms mediate these two forms of interocular suppression.

Wavelength discrimination in blindsight.

In the circumscribed, long-standing, clinically absolute visual field defects of three patients with vascular lesions that involved the optic radiation and visual cortex, forced-choice discrimination between coloured stimuli was tested. Paired stimuli were matched for luminous efficiency on the basis of previous measurements of increment-threshold spectral sensitivity made in the same patients and at the same retinal positions. To different extents all patients could discriminate between narrowband wavelength stimuli. The results imply that despite the effects of retrograde degeneration on thalamic and retinal colour-processing channels, neurons which process wavelength information are still functional, although the information they transmit is not consciously perceived.

Effects of chromatic adaptation on opponent interactions in monkey increment-threshold spectral-sensitivity functions

The effects of chromatic adaptation on the opponent interactions of cone mechanisms were investigated by using increment-threshold spectral-sensitivity (ITSS) functions and threshold-versus-radiance (TVR) curves in rhesus monkey subjects. The TVR curves showed shape- and field-sensitivity invariance for both 580- and 500-nm adapting backgrounds and indicated that three cone mechanisms were mediating detection over moderate adapting-field intensity levels. Differential adaptation between the long-wavelength-sensitive (L) and the middle-wavelength-sensitive (M) opponent (L - M) and nonopponent (L + M) channels and the short-wavelength-sensitive (S) channel caused changes in the shape of the ITSS function as the adapting-field intensity was increased without changes in the level of cone interaction. Chromatic adaptation also resulted in significant changes in the shape of the ITSS functions, but it still exhibited characteristic L-M opponent interactions. Converting ITSS data to cone-contrast coordinates for R-G adapting fields indicated that the relative contribution of the L and M cones at the second site was approximately equal (detection contour slope approximately 1). Consequently, most of the changes in the shape of ITSS functions under chromatic adaptation are explained by the von Kries adaptation principle. ITSS functions on a green background also exhibited opponent interactions between S cones and longer-wavelength cones. The cone-contrast coordinates, when expressed for S cones, showed that the inhibitory interactions occur because the S-cone signal subtracts from both M and L cones.

Effects of luminance level on the saturation function: Sensitivities based on saturation discrimination

AbstractThe saturation of a monochromatic light λ is defined as the reciprocal of the just noticeable colorimetric purity ΔP(λ) from a white, which is a conventional definition having been used previously often. The present study measures saturation functions of monochromatic lights at various luminance levels ranging from 10 to 1000 Td. The main result of this article is that the saturation function systematically changes in shape with the luminance level; as the luminance increases, the values of 1/ΔP(λ) relatively increase for shorter wavelengths. The chromatic sensitivity function C(λ) is defined by the reciprocal of the radiance of a monochromatic light λ added to the white field in order to detect a difference in chromaticity from the equal‐luminance reference white. The C(λ) functions at various luminance levels are shown to have three peaks at the short‐, middle‐, and long‐wavelength regions. These peaks are similar to those of the increment threshold functions, and the saturation function can be approximated by the ratio of the increment threshold spectral sensitivity to the relative luminous efficiency, except for the 570 nm data.

Red-green interactions in the spectral sensitivity of primates as derived from ERG and behavioral data

Different techniques were used to manipulate the inhibitory interaction between the red and green photoreceptors (R and G cones) of rhesus and human primates. The response techniques that were used were the corneal electroretinogram (ERG) and psychophysical increment-threshold spectral sensitivity functions. Red-green opponency, as measured by the depth of the notch at 580 nm, is removed by intravitreal injection of bicuculline but not by strychnine. Therefore, red-green opponency is mediated by GABA and not glycine. The depth of the notch is dependent upon stimulus size. Between 30' and 15' test light diameters, this sign of red-green opponency disappears. Psychophysical increment thresholds are shown to produce the notch while decrements do not and intravitreal APB is shown to reduce the notch, evidence that red-green opponency is carried by the "ON" and not the "OFF" bipolar pathways of the retina. Red and green annuli are shown to selectively reduce red and green inhibition, as though there were selective reduction of the surround response in center-surround organized red-green receptive fields.

Behavioral and electrophysiological sensitivity to temporally modulated visual stimuli in the ground squirrel

Behavioral and electrophysiological methods were used to measure sensitivity to flickering lights in a dichromatic species, the California ground squirrel (Spermophilus beecheyi). Discrimination tests were used to determine spectral sensitivity at stimulus frequencies from 5-50 Hz and increment threshold spectral sensitivity. The contributions of retinal mechanisms to these capacities were assessed by recording the responses of optic nerve fibers to temporally modulated monochromatic lights. In the ground squirrel, as in the human, the shape of the spectral-sensitivity function depends on the temporal frequency of the stimulus. Results from single-unit recording show that all of the classes of optic nerve fibers in the ground squirrel are highly phase-locked to the stimulus for modulation rates as high as 50 Hz. Neither the responses of photoreceptors nor any class of optic nerve fiber can singly account for the behavioral results. The electrophysiological results are also counter to models which propose that temporally dependent changes in the spectral sensitivity of spectrally opponent fibers account for the behavior. The temporal resolution of the optic nerve fibers exceeds that of the behaving animal suggesting that retinal mechanisms do not limit behavioral temporal resolution.

INCREMENT-THRESHOLD SPECTRAL SENSITIVITY IN BLINDSIGHT: EVIDENCE FOR COLOUR OPPONENCY

In the circumscribed visual field defects of 3 patients, increment-threshold spectral sensitivity was measured with a guessing paradigm. Nine 116', 200 ms narrowband stimuli with maximum transmission between 450 and 660 nm were presented on a white background of photopic or scotopic luminance. Sensitivity measured in the blind field was compared with that at matched positions in the patients' normal hemifield, and with that at corresponding positions in 2 control subjects. Results show that spectral sensitivity in the blind field, albeit reduced by up to 1 log unit, shows normal dependence on adaptation level, reflecting rod activity under scotopic, and cone activity under photopic conditions. Characteristic discontinuities in the spectral sensitivity curve seen under light adaptation are evidence for colour-opponent processes, presumably involving primate beta retinal ganglion cells.

Induced color blindness in goldfish: A behavioral and electrophysiological study

To answer the question whether, like man, ethambutol treated fish would become color-blind, wavelength discrimination was measured behaviorally in goldfish, preceding, during and after ethambutol treatment. The results are that of the three high discrimination abilities at around 400, 500 and 600 nm, ethambutol affected the latter one. Red-green discrimination is lost reversibly leaving the discriminations in the blue-green and violet range unaffected. This red-green discrimination deficiency cannot be accounted for by a loss of long wavelength cones since the ERG and luminosity functions remain unaffected. Intracellular horizontal cell recordings in goldfish show that ethambutol hyperpolarizes all three types of cone driven horizontal cells and changes their color coding such that their spectral characteristics become cone-like as is the case in dark adapted retina. So, the initial effect induced by ethambutol seems to be an adaptation deficiency in color vision related tasks. Human wavelength discrimination and increment threshold spectral sensitivity functions obtained at low luminance levels are compared to behavioral functions in ethambutol treated goldfish. The high similarity between the ethambutol effects in man and goldfish, and the effects observed in the horizontal cell responses in goldfish are highly indicative that horizontal cells play a key role in color vision. So far their function has been puzzling.

Behavioral studies of the sensitive periods of development of visual functions in monkeys

The age-dependent effects of monocular form deprivation on psychophysically determined visual functions were investigated in rhesus monkeys. Monocular form deprivation was initiated at various ages from 1 to 25 months and maintained for 18 months. The effects of form deprivation varied across the visual functions evaluated. Absolute scotopic sensitivity was depressed by form deprivation initiated only at 1 or 2 months of age. Photopic, increment-threshold spectral sensitivity functions showed alterations in sensitivity levels for monkeys treated at 5 months of age or earlier, with no effect thereafter. Monocular form deprivation at 1 to 5 months of age resulted in profound deficits in spatial modulation sensitivity. The effects on spatial vision decreased systematically as the age of onset was delayed from age 6 to 18 months. Finally, binocular summation measures revealed an absence of binocular vision even for the monkey form-deprived at 25 months of age, i.e. the experimental treatment series failed to define the upper limit of the age-range for the effects of monocular form deprivation on binocular summation mechanisms. Comparisons of the age-dependent effects of monocular form deprivation across the various functions demonstrated that the sensitive period, i.e. the period of life during which a visual function may be altered by monocular form deprivation, was different for each of the psychophysical measures of visual function.

Spectral sensitivity and adaptation characteristics of cone mechanisms under white-light adaptation

Increment-threshold spectral-sensitivity (ITSS) functions and threshold-versus-intensity (tvi) curves were measured under white-light adaptation in rhesus monkeys. The tvi curves showed shape and test wavelength invariance, implying that three cone mechanisms were mediating detection. In general, the results were in agreement with the differential adaptation hypothesis proposed by Stiles that predicted spectral shape invariance of the cone mechanisms but overall changes in the shape of the spectral-sensitivity function with increases in the intensity of the adapting field. The principal changes occurring in the ITSS function as the level of adaptation increased involved a smaller loss in sensitivity of the short-wavelength and the long-wavelength peaks compared with the corresponding loss in sensitivity of the middle-wavelength peak. A three-channel model with an opponent L-M mechanism and a nonopponent L-M mechanism (both with S-cone input) and an independent S-cone mechanism described the ITSS data as well as other increment-threshold and suprathreshold data. The model values for the ITSS functions, along with parameters derived from the transformation of these data to cone-contrast coordinates, permitted the factoring out of first-site adaptation, second-site adaptation, and the relative strengths of contribution of the L and M cones within the opponent and nonopponent L-M channels.

Color vision in the dog

The color vision of three domestic dogs was examined in a series of behavioral discrimination experiments. Measurements of increment-threshold spectral sensitivity functions and direct tests of color matching indicate that the dog retina contains two classes of cone photopigment. These two pigments are computed to have spectral peaks of about 429 nm and 555 nm. The results of the color vision tests are all consistent with the conclusion that dogs have dichromatic color vision.