Spectral sensitivity of the peripheral retina to large and small stimuli

Increment-threshold spectral sensitivity functions were taken foveally and at four other retinal locations with different sized test stimuli. For the fovea and parafovea at 4°, a 1°, 250 msec stimulus of variable wavelength, flashed on a 1000 td white background yielded spectral sensitivity curves with three distinct maxima located near 455, 530–540 and 600 nm. As eccentricity increased so did the minimum spot size needed to produce spectral curves with three peaks. In particular, the 2°10′ spot at 20°, 4°10′ at 30° and 5.5° at 45° gave this result. Smaller stimuli in the periphery yielded curves with a minor peak at 455 nm, present under some conditions, and a major broad peak at 560 nm that resembled the CIE photopic luminosity function. Two different systems subserve detection : the three-peaked curves indicate mediation by the opponent-color system while the other type of function can be attributed to the non-opponent system.     

Psychophysical experiments on spatial summation at threshold level of the human peripheral retina

Experiments on spatial summation were done on the temporal retina between 4° and 40° eccentricity. The stimuli were circular discs between 7′ and 280′ of arc dia and 0.01 sec exposure at backgrounds from 0 to 10^?8 W/deg². The wavelength of the light was 510 nm for both background and test stimulus. The diameter of “Ricco's region” increases between 4° and 15° eccentricity and has an almost constant value of 100′ between 15° and 40°. For the smallest stimuli, deviations from Ricco's law were found. It appeared in increased thresholds between 15° and 40° eccentricity. We called this phenomenon the non-summation effect. The dependence of retinal sensitivity on eccentricity differs for different sizes of test stimulus and for absolute, scotopic and photopic background intensities. At photopic conditions increment thresholds are governed by Weber's law, the Weber fraction: increment intensity/background intensity (Δ/LL) being dependent on retinal location and test stimulus size.For scotopic and mesopic adaptation levels at 7° eccentricity and small stimulus sizes the De Vries-Rose law:Δ/LL^0.50= constant holds. The transition from these power functions into Weber's lawΔ/LL= c occurs all over the retina within the same narrow range of background intensities. Only the small stimuli that are subject to the non-summation effect also show a different behaviour in this respect.     

Increment spectral sensitivities for spatial periodic grating patterns: evidence for variable tuning of the chromatic system

Increment spectral sensitivities were measured for spatial periodic grating patterns. First, the increment threshold was determined as a function of wavelength, for various spatial frequencies and white-background intensities. Second, the additivity of test mixture was tested. Thirdly, the threshold vs intensity curves were determined for various spatial frequencies of test monochromatic stimuli. Finally, the increment threshold was determined as a function of spatial frequency. The following results were obtained: the background intensity and the spatial frequency affected the increment spectral sensitivity functions in different manners; the result of test mixture showed subadditivity for high background intensity and approximately linear additivity for low background intensity; the resultant t.v.i. curves converged toward Weber's law at high background intensity; and the sensitivity vs spatial frequency curve showed a loss of the sensitivity at low spatial frequency for high background intensity. These results called in question the existing hypothesis of separate chromatic and achromatic systems whose spectral tunings are invariant with a spatial parameter, but favored the hypothesis of variable tuning of the chromatic system.     

Colour coding of pattern responses in man investigated by evoked potential feedback and direct plot techniques

Visual sensitivity can be directly measured by arranging that evoked potential (EP) amplitude controls stimulus intensity (method of evoked potential feedback). A second technique is to directly plot graphs of EP amplitude vs the required stimulus parameter: this method gives more precise estimates of theshapes of EP plots than the conventional procedure of measuring individual points. The practical details, advantages and drawbacks of these two rapid techniques are discussed.Pattern EPs are mediated via colour channels, whereas EPs to rapidly flickering lights are mediated by pooled colour signals. The red pattern channel's peak sensitivity falls at a wavelength no shorter than 580–610 nm compared with the 555 nm peak for flicker sensitivity.An objective analogue of the psychophysical increment threshold procedure is made possible by means of EP feedback. Stimulus pattern intensity is approximately proportional to adapting intensity down to a pattern intensity of 5–10 td. below which EP amplitude attenuates rapidly.     

Absolute visual sensitivity of the goldfish

Absolute visual threshold was measured in dark-adapted goldfish, using classical conditioning of heart and respiration rates. For threshold detection of a 132° diameter stimulus near the peak of the rod pigment's sensitivity, only 1 quantum was incident for every 2000–4000 rods, and 400–830 photoisomerizations were needed for vision. Spectral sensitivity was also determined at absolute threshold, sing respiration rate conditioning. The shape of the function matches the electrophysiologically determined spectral sensitivity of goldfish retinal ganglion cells, but both curves are relatively more sensitive in the long wavelengths than would be predicted on the basis of the rod pigment alone. A dim 703 nm background depressed sensitivity to a 636 nm test light more than to a 532 nm test, proving that another class of photoreceptors is active at absolute threshold. If the other receptors were the red cones, as seems most likely, then only 1 quantum was incident per 220 red cones at their absolute threshold, and photoisomerizations occurred in each of 164 red cones at threshold.     

The equivalent background and retinal eccentricity.

The validity of the equivalent background concept was tested for 8 retinal loci ranging from 3° to 70° on the nasal half of the horizontal meridian. Dark adaptation and increment threshold curves were obtained for two different test field conditions: 2°175msec and 0.5°40msec, both monochromatic at 550 nm. Results show that the equivalent background follows its own time course at each location. In photopic vision, the equivalent background for the inner periphery is lower than for the outer periphery. In scotopic vision, this relationship reverses. The findings suggest that the equivalent background concept may obtain only for loci having comparable spatial and temporal properties.     

The effects of temperature on the psychophysical and electroretinographic spectral sensitivity of the chromatically-adapted goldfish

Psychophysical and electroretinographic spectral sensitivity curves were determined for goldfish acclimated to 15°C or 25°C and chromatically-adapted to Wratten 26 or Wratten 79. At 25°C long-wavelength sensitivity was lower with Wratten 26 adaptation, whereas midspectral sensitivity was lower with Wratten 79 adaptation, for both measures. At 15°C the psychophysical threshold was elevated throughout the spectrum and the selective effect of chromatic adaptation was exaggerated. The spectral sensitivity of theb-wave, however, was relatively uninfluenced by acclimation to 15°C, in both the chromatically-adapted and darkadapted state. Neither temperature nor chromatic adaptation affected thea-wave.     

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.     

Detection of quantum flux modulation by single photopigments in human observers.

The action spectra of single cone photopigments of trichromatic human observers were measured by a modification of the exchange threshold technique. The quantum flux of two superimposed 2° fields, 540 and 640 nm. was adjusted to the inverse ratio of the sensitivity of either chlorolabe or erythrolabe at these wavelengths, so that one of the photopigments made a constant quantum catch. The 540 and 640 nm lights were flickered sinusoidally in counterphase above the flicker fusion frequency of the blue cones so that when the quantum fluxes were equated for chlorolabe. only erythrolabe could detect the alternation, and vice versa. This flickering field was superimposed upon a 12° background which was variable in both quantum flux and wavelength. At any wavelength between 410 and 690 nm the log of the reciprocal of the number of quanta required to extinguish the flicker of the540/640 field described the log sensitivity at that wavelength of the only photopigment capable of detecting the flicker. The log sensitivity curves thus obtained were consistent with other measures of the cone photopigments of dichromatic and trichromatic human observers.     

Effects of stimulus size on spectral sensitivity in a fish (Scardinius erythrophthalmus), measured with a classical conditioning paradigm

The photopic sensitivity of rudd (Scardinius erythrophthalmus) to various monochromatic stimuli was measured, using a classical conditioning paradigm in which the unconditioned stimulus was shock, and the conditioned stimulus an increase in swimming activity. The stimuli were presented above the animals in all cases.An action spectrum obtained with moving bar stimuli of approximately 4.75° width showed a maximum near 630 nm. Chromatic adaptation showed that more than one spectral class of receptor contributed to these results. With diffuse light stimuli the absolute sensitivity was greatly decreased, and the form of the action spectrum was also different, showing in particular a marked region of insensitivity near 600 nm. Tests with moving and stationary 610nm bars of various sizes showed that the sensitivity was greatest when the width of the stimulus was between 3° and 5°, falling off for both bigger and smaller stimuli, and was unaffected by movement.The data can be largely interpreted in terms of known properties of fish retinal ganglion cells. It is assumed that the diffuse stimuli invade the inhibitory surrounds of the ganglion cells, reducing sensitivity and allowing chromatic interactions to take place.Behavioural determinations of the spectral sensitivity of the rudd to monochromatic stimuli superimposed on tungsten light backgrounds have now been carried out in four different ways, and the resulting spectral sensitivity curve is different in each case. Possible causes and consequences of these differences are considered.     

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 indiscriminable 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 lambda over the spectral region 575-610 nm.     

Isolation of opponent-colour mechanisms at increment threshold

An experimental examination was made of some paradigms designed to isolate the opponent-colour system at increment threshold. The effectiveness of a uniform white conditioning field spatially coincident with a 1.05-deg uniform test field was assessed by measuring intensity thresholds for simple detection and for colour discrimination. Values were obtained both by a method of adjustment and by a two-interval forced-choice procedure. For sufficiently high luminances of the conditioning field (3000 td or greater) little or no difference was found between simple-detection and colour-discrimination thresholds over the critical test-flash spectral range 520-620 nm, implying that the paradigm produced almost complete isolation of the opponent-colour system at increment threshold. A control experiment in which thresholds were obtained for a conditioning field larger than the test field gave less satisfactory isolation; near 580 nm the luminance system was found to be at least 0.3 log unit more sensitive than the opponent-colour system. A comparison was also made of the spatially coincident field paradigm with a paradigm in which a modified test stimulus of low temporal and spatial frequency content was presented on a large conditioning field. Test spectral sensitivity curves for simple detection obtained by a method of adjustment showed little difference in effectiveness in opponent-colour isolation.     

Spatial summation of blue-on-yellow light increments and decrements in human vision

In the primate retina, blue-OFF cells are less numerous than blue-ON cells but no psychophysical equivalent of this asymmetry has been found so far. The hypothesis put forward in the present study is that the ON-OFF asymmetry should manifest itself in the size and effectiveness of spatial summation of S-cone signals of opposite polarity. To test this hypothesis upon selective stimulation of the S-cones in man, a 3 cd/m(2) blue light was superimposed on a 300 cd/m(2) yellow background and the test stimulus consisted in a luminance increment or decrement of the blue light from its steady level over a circular area of variable size. The test stimuli were presented at 12.5 degrees retinal eccentricity. Within the test-stimulus spectral band, sensitivity was that of Stiles' pi(1) mechanism. Increasing stimulus area reduced more the decrement threshold than the increment threshold, and Ricco's area was larger for luminance decrements (0.8-2 degrees ) than for increments (0.6-0.9 degrees ). Experiments with red-on-red stimuli confirmed that the large summation area and stimulus-polarity-dependent spatial summation are specific for the isolated S-cone signals. The sign-dependency of spatial summation is probably a psychophysical correlate of the asymmetry of the ON- and OFF- visual pathways receiving S-cone input.     

Studies on light-sensitive units in the deep mesencephalon of blinded frogs

Unit responses susceptible to light stimulation of a small area on the frog's head were recorded in the deep encephalon of blinded specimens of Rana esculenta. The responses consisted of a spike discharge upon illumination. Using a threshold criterion the dark adaptation curves showed two parts, separated by a kink, the final dark threshold being complete after 20–30 min in darkness. Using a threshold criterion the spectral sensitivity curves under dark adapted conditions were broad with a peak at 548 nm. The dark adapted intensity threshold for a stimulus of 548 nm ranged between 0.15–1.4 μW/cm².     

Detection of long-duration,long-wavelength incremental flashes by a chromatically coded pathway

This paper reports tests of the hypothesis that a 200 msec 667 nm, foveal test flash, presented upon a steady, 8° background, is detected by a pathway whose sensitivity is monotonically related to a linear functional of photoreceptor quantum catch. The measurement conditions used in this paper are the same as those used by Stiles in his π-mechanism analysis. The hypothesis is tested by measuring increment-threshold curves for various background wavelengths and for various mixtures of two backgrounds.The results clearly reject the hypothesis. We propose two models that are consistent with the measurements reported here as well as measurements—using a 10 msec test flash—reported previously. We then discuss related empirical and theoretical work.     

Isolation of short-wavelength-sensitive cone photoreceptors in 4-6-week-old human infants

Spectral sensitivity was measured for nine infants, 4-6 weeks of age, and three adults under conditions of chromatic adaptation chosen to reveal the presence of short-wavelength-sensitive cones. Monochromatic test stimuli (400-550 nm) were presented at 2 Hz superimposed on a broadband, yellow background. Following 4 min of adaptation to the background, test stimuli were presented while recording the steady-state, visually-evoked cortical potential (VECP). Response averages were obtained for several radiance levels at each test wavelength, and the amplitude of the fundamental frequency was extracted from the digitized response with a fast-Fourier transform. These data were used to construct response vs intensity functions for each wavelength. A fixed criterion response was chosen from the latter family of functions to generate individual spectral sensitivity curves. These VECP spectral sensitivity functions matched the psychophysically-determined functions of adults, measured by the method of adjustment and with the same stimulus configuration. Peak sensitivity for infants and adults under these conditions occurred at about 440 nm, and the main lobe of the curve (400-500 nm) was well fitted by the Vos-Walraven short-wavelength cone fundamental. The only major difference between the infant and adult data was in the relative sensitivity of the secondary mode of the curves (above 500 nm). These results demonstrate the presence of short-wavelength-sensitive cones and a functional pathway to the visual cortex by 4-6 weeks of age.     

Analysis of visual modulation sensitivity: two components in flicker perception.

An analysis of human psychophysical flicker thresholds is developed from the increment threshold technique of Stiles (1939). Two independent detection components are required to account for all available modulation sensitivity data. These components are differentiated by properties corresponding to the difference between sustained and transient units in the cat retina.     

Opponent-processing effects on the field spectral sensitivity of pattern-elicited electroretinograms.

Field spectral sensitivities of the pattern-elicited electroretinogram (PERG) were obtained from two subjects using a modified version of Stiles' two-color increment threshold procedure. A 540 nm checkerboard test pattern of 38 degrees dia was alternated at 8 Hz on a uniform adapting field of the same size. Test intensity, field wavelength and field intensity were varied parametrically. The derived field spectral sensitivity does not resemble that of any individual class of cones; it roughly approximates the photopic luminosity function V lambda, but with sensitivity dips at 540 and 600 nm, which may be associated with an adaptation of the red/green (R/G) opponent site. Thus, it is proposed that the PERG reflect activities in both the luminance and the R/G opponent channels.     

Visibility of chromatic flicker upon spectrally mixed adapting fields.

Adding steady green light to a low-frequency flickering red field or adding steady red light to a low-frequency green field reduced the flicker threshold. The red and green adapting lights act in a cancellative manner, lowering the threshold. This is contrary to a model of independent Π-mechanisms acting like fundamental color mechanisms. When the flicker frequency was increased to 12 Hz, red and green adapting fields acted approximately additively in raising the threshold. At this frequency the spectral field sensitivity of the middle-wavelength π₄ and long-wavelength π₅ mechanisms was measured with the two color increment threshold method of Stiles. The spectral functions were then used to estimate the flicker sensitivities (Weber fractions) of π₄ and π₅ at 12 Hz, which were found to be similar.     

Visual masking and the contrast-flash effect

We have compared the effect of disk-shaped masking flashes, 2° or 8° dia, to the effect of an annular contrast-flash, 2–8° dia, on the threshold for a 2° test flash. At all stimulus onset asynchronies and stimulus intensities studied, the threshold curves for the annulus are similar to those for the masking disks. We have evaluated the possibility that the annulus acts like a masking disk due to light scattered from the annulus to its center, and find that when stimulus onsets are synchronous, scattered light can account for the annulus data. When the test onset precedes the annulus onset by 100 msec, scattered light cannot account for the results.