The all-cone retina of the garter snake: spectral mechanisms and photopigment

Summary The retina of the garter snake contains 3 morphologically distinct classes of cone photoreceptor. The spectral mechanisms in the retinas of garter snakes (Thamnophis sirtalis and T. marcianus) were studied by recording a retinal gross potential, the electroretinogram, using a flicker photometric procedure. Spectral sensitivity functions recorded with stimuli presented at high temporal frequency (62.5 Hz) are broadly peaked in the region of 550–570 nm. These functions remain spectrally invariant (a) in the face of significant changes in stimulus pulse rate (8–62.5 Hz), (b) whether the eye is light or dark adapted, and (c) under conditions of intense chromatic adaptation. It is concluded that the garter snake has only a single class of cone pigment. The results from a curve fitting analysis suggests that this pigment has peak absorbance at about 556 nm.     

Alouatta Trichromatic Color Vision: Cone Spectra and Physiological Responses Studied with Microspectrophotometry and Single Unit Retinal Electrophysiology

The howler monkeys (Alouatta sp.) are the only New World primates to exhibit routine trichromacy. Both males and females have three cone photopigments. However, in contrast to Old World monkeys, Alouatta has a locus control region upstream of each opsin gene on the X-chromosome and this might influence the retinal organization underlying its color vision. Post-mortem microspectrophotometry (MSP) was performed on the retinae of two male Alouatta to obtain rod and cone spectral sensitivities. The MSP data were consistent with only a single opsin being expressed in each cone and electrophysiological data were consistent with this primate expressing full trichromacy. To study the physiological organization of the retina underlying Alouatta trichromacy, we recorded from retinal ganglion cells of the same animals used for MSP measurements with a variety of achromatic and chromatic stimulus protocols. We found MC cells and PC cells in the Alouatta retina with similar properties to those previously found in the retina of other trichromatic primates. MC cells showed strong phasic responses to luminance changes and little response to chromatic pulses. PC cells showed strong tonic response to chromatic changes and small tonic response to luminance changes. Responses to other stimulus protocols (flicker photometry; changing the relative phase of red and green modulated lights; temporal modulation transfer functions) were also similar to those recorded in other trichromatic primates. MC cells also showed a pronounced frequency double response to chromatic modulation, and with luminance modulation response saturation accompanied by a phase advance between 10–20 Hz, characteristic of a contrast gain mechanism. This indicates a very similar retinal organization to Old-World monkeys. Cone-specific opsin expression in the presence of a locus control region for each opsin may call into question the hypothesis that this region exclusively controls opsin expression.     

Critical flicker detection frequency and double pulse recognition threshold do not depend on the spectral properties of stimuli

We have measured the critical flicker detection frequency (CFDF) and double pulse recognition threshold (DPT) using three LEDs with power peaks at 460, 525 and 625 nm for target illumination. Brightness equalization was performed by customized heterochromatic flicker photometry (cHFP). Reference luminance levels were 170 cd/m² (blue LED, 60 subjects), 4 cd/m² (green LED, 20 subjects), and 1 cd/m² (green LED, 20 subjects). The measurement at 1 cd/m² was preceded by 15 min of dark adaptation. The angle of view for the target was 3°, and the duration of stimuli was 1 ms. An experimental pulse generator with three channels and a projector was used. No differences in CFDF at different spectral properties of stimulus were observed at all three levels of luminance. Thus, it is concluded that temporal vision resolution does not depend on the spectral properties of visual stimuli.     

Colour and luminance interactions in the visual perception of motion

We sought to determine the extent to which red-green, colour-opponent mechanisms in the human visual system play a role in the perception of drifting luminance-modulated targets. Contrast sensitivity for the directional discrimination of drifting luminance-modulated (yellow-black) test sinusoids was measured following adaptation to isoluminant red-green sinusoids drifting in either the same or opposite direction. When the test and adapt stimuli drifted in the same direction, large sensitivity losses were evident at all test temporal frequencies employed (1-16 Hz). The magnitude of the loss was independent of temporal frequency. When adapt and test stimuli drifted in opposing directions, large sensitivity losses were evident at lower temporal frequencies (1-4 Hz) and declined with increasing temporal frequency. Control studies showed that this temporal-frequency-dependent effect could not reflect the activity of achromatic units. Our results provide evidence that chromatic mechanisms contribute to the perception of luminance-modulated motion targets drifting at speeds of up to at least 32 degrees s(-1). We argue that such mechanisms most probably lie within a parvocellular-dominated cortical visual pathway, sensitive to both chromatic and luminance modulation, but only weakly selective for the direction of stimulus motion.     

The Spatial Properties of L- and M-Cone Inputs to Electroretinograms That Reflect Different Types of Post-Receptoral Processing

We studied the spatial arrangement of L- and M-cone driven electroretinograms (ERGs) reflecting the activity of magno- and parvocellular pathways. L- and M-cone isolating sine wave stimuli were created with a four primary LED stimulator using triple silent substitution paradigms. Temporal frequencies were 8 and 12 Hz, to reflect cone opponent activity, and 30, 36 and 48 Hz to reflect luminance activity. The responses were measured for full-field stimuli and for different circular and annular stimuli. The ERG data confirm the presence of two different mechanisms at intermediate and high temporal frequencies. The responses measured at high temporal frequencies strongly depended upon spatial stimulus configuration. In the full-field conditions, the L-cone driven responses were substantially larger than the full-field M-cone driven responses and also than the L-cone driven responses with smaller stimuli. The M-cone driven responses at full-field and with 70° diameter stimuli displayed similar amplitudes. The L- and M-cone driven responses measured at 8 and 12 Hz were of similar amplitude and approximately in counter-phase. The amplitudes were constant for most stimulus configurations. The results indicate that, when the ERG reflects luminance activity, it is positively correlated with stimulus size. Beyond 35° retinal eccentricity, the retina mainly contains L-cones. Small stimuli are sufficient to obtain maximal ERGs at low temporal frequencies where the ERGs are also sensitive to cone-opponent processing.     

Development of contrast sensitivity and acuity of the infant colour system

We have monitored the development of infant colour vision by measuring chromatic contrast sensitivity and acuity in eight young infants over a period of 6 months. Steady-state visual evoked potentials (VEPS) were recorded in response to both chromatic (red-green) and luminance (red-black or green-black) patterns that were reversed in contrast over time. For most infants, no response could be obtained to chromatic stimuli of any size or contrast before 5 weeks of age, although luminance stimuli of 20% contrast gave reliable responses at that age. When responses to chromatic stimuli first appeared, they could be obtained only with stimuli of very low spatial frequency, 20 times lower than the acuity for luminance stimuli. Both contrast sensitivity and acuity for chromatic stimuli increased steadily, more rapidly than for luminance stimuli. As the spectral selectivities of infant cones are similar to those of adults, the difference in rate of development of luminance and chromatic contrast sensitivity and acuity stimuli probably reflects neural development of the infant colour system.     

The Electroretinogram of a Diurnal Gecko

Using the electroretinogram as the criterion of retinal activity the flicker fusion frequency, course of dark adaptation, and spectral sensitivity of the pure cone retina of the diurnal gecko, Phelsuma inunguis, were investigated. Both the curve relating flicker fusion frequency to stimulus intensity and that relating the amplitude of the flicker response to stimulus intensity showed a break as the intensity was increased. The dark adaptation curve was that typical of cone retinae; there was no break, adaptation was relatively rapid, and there was a total increase of sensitivity of only about 3 log units. The spectral sensitivity curve showed two maxima, a major one at about 560 mµ and another at about 460 mµ. Chromatic adaptation with red and blue lights demonstrated the presence of two independent mechanisms. Although red adaptation could not have had a direct effect on the pigment responsible for the "blue" mechanism the sensitivity of this mechanism was depressed by red adaptation. The possible relationships of the two mechanisms are discussed.     

Visual capacities of the owl monkey (Aotus trivirgatus): temporal contrast sensitivity.

Aotus monkeys were tested in a forced-choice discrimination task to determine their ability to discriminate sinusoidally flickering lights varying in temporal frequency and luminance contrast. Under conditions of moderate light adaptation this primate is maximally sensitive to lights flickering at 10 Hz while the highest frequency they can discriminate is about 42 Hz. At very low light levels (10(-5) ft L) maximum sensitivity is for 2.2--5 Hz flicker. The highest flicker rate that could be discriminated under these conditons was about 29 Hz. In comparison to humans tested in the same situation. Aotus monkeys show relatively lower sensitivity to temporal flicker under conditions of light adaptation but relatively higher sensitivity at very low light levels.     

Different temporal structure for form versus surface cortical color systems--evidence from chromatic non-linear VEP

Physiological studies of color processing have typically measured responses to spatially varying chromatic stimuli such as gratings, while psychophysical studies of color include color naming, color and light, as well as spatial and temporal chromatic sensitivities. This raises the question of whether we have one or several cortical color processing systems. Here we show from non-linear analysis of human visual evoked potentials (VEP) the presence of distinct and independent temporal signatures for form and surface color processing. Surface color stimuli produced most power in the second order Wiener kernel, indicative of a slowly recovering neural system, while chromatic form stimulation produced most power in the first order kernel (showing rapid recovery). We find end-spectral saturation-dependent signals, easily separable from achromatic signals for surface color stimuli. However physiological responses to form color stimuli, though varying somewhat with saturation, showed similar waveform components. Lastly, the spectral dependence of surface and form color VEP was different, with the surface color responses almost vanishing with yellow-grey isoluminant stimulation whereas the form color VEP shows robust recordable signals across all hues. Thus, surface and form colored stimuli engage different neural systems within cortex, pointing to the need to establish their relative contributions under the diverse chromatic stimulus conditions used in the literature.     

An account of responses of spectrally opponent neurons in macaque lateral geniculate nucleus to successive contrast

Coloured surfaces in the normal environment may be brighter or dimmer than the mean adaptation level. Changes in the firing rate of cells of the parvocellular layers of macaque lateral geniculate nucleus were studied with such stimuli; chromatic mixtures briefly replaced a white adaptation field. This paradigm is therefore one of successive contrast. Families of intensity-response curves for different wavelengths were measured. When taking sections at different luminance ratios through these families of curves, strongly opponent cells displayed spectrally selective responses at low luminance ratios, while weakly opponent cells had higher chromatic thresholds and responded well to stimuli at higher luminance ratios, brighter than the adaptation field. Strength of cone opponency, defined as the weight of the inhibitory cone mechanism relative to the excitatory one, was thus related to the range of intensity in which cells appeared to operate most effectively. S-cone inputs, as tested with lights lying along tritanopic confusion lines, could either be excitatory or inhibitory. Families of curves for different wavelengths can be simulated mathematically for a given cell by a simple model by using known cone absorption spectra. Hyperbolic response functions relate cone absorption to the output signals of the three cone mechanisms, which are assumed to interact linearly. Parameters from the simulation provided estimates of strength of cone opponency and cone sensitivity which were shown to be continuously distributed. Cell activity can be related to cone excitation in a trichromatic colour space with the help of the model, to give an indication of suprathreshold coding of colour and lightness.     

How Long Depends on How Fast—Perceived Flicker Dilates Subjective Duration

How do humans perceive the passage of time and the duration of events without a dedicated sensory system for timing? Previous studies have demonstrated that when a stimulus changes over time, its duration is subjectively dilated, indicating that duration judgments are based on the number of changes within an interval. In this study, we tested predictions derived from three different accounts describing the relation between a changing stimulus and its subjective duration as either based on (1) the objective rate of changes of the stimulus, (2) the perceived saliency of the changes, or (3) the neural energy expended in processing the stimulus. We used visual stimuli flickering at different frequencies (4–166 Hz) to study how the number of changes affects subjective duration. To this end, we assessed the subjective duration of these stimuli and measured participants'' behavioral flicker fusion threshold (the highest frequency perceived as flicker), as well as their threshold for a frequency-specific neural response to the flicker using EEG. We found that only consciously perceived flicker dilated perceived duration, such that a 2 s long stimulus flickering at 4 Hz was perceived as lasting as long as a 2.7 s steady stimulus. This effect was most pronounced at the slowest flicker frequencies, at which participants reported the most consistent flicker perception. Flicker frequencies higher than the flicker fusion threshold did not affect perceived duration at all, even if they evoked a significant frequency-specific neural response. In sum, our findings indicate that time perception in the peri-second range is driven by the subjective saliency of the stimulus'' temporal features rather than the objective rate of stimulus changes or the neural response to the changes.     

Perception of heterochromatic flicker by honeybees: a behavioural study

Freely flying honeybees were trained to discriminate a stimulus consisting of two alternating chromatic lights (heterochromatic flicker) from a steady mixture of the same two lights, using 3 different pairs of lights: blue-UV, UV-green, and green-UV. With each light pair, training to the heterochromatic flicker was conducted at several flicker frequencies, using experimentally naive bees in each training. In subsequent tests, the trained bees were given a choice between the two lights that constituted the flicker, presented steady, as well as between either of them and the steady mixture. We find that bees trained to particular frequencies of heterochromatic flicker prefer one of the component lights over the other as well as over the steady mixture, suggesting that the colour they perceive in the heterochromatic flicker to which they have been trained is shifted in the direction of one of the lights contained in the flicker. The colour shift occurs at flicker frequencies that depend on the pair of lights used. We propose that the shift is generated by an effect similar to the Brücke-Bartley phenomenon known from human vision. This effect is based on the enhancement of the photoreceptors' response upon onset of stimulation, causing an intermittent light to appear brighter than a steady light of identical physical intensity. We propose that the degree of enhancement might differ among the 3 spectral classes of photoreceptor, causing the colour perceived in a heterochromatic flicker to differ from that perceived in a steady mixture of its two light components.     

Spectral sensitivity of vervet monkeys (Cercopithecus aethiops sabaeus) and the issue of catarrhine trichromacy

Several genera of platyrrhine monkeys show significant polymorphism of color vision. By contrast, catarrhine monkeys have usually been assumed to have uniform trichromatic color vision. However, the evidential basis for this assumption is quite limited. To study this issue further, spectral sensitivity functions were obtained from vervet monkeys (Cercopithecus aethiops sabaeus) using the technique of electroretinographic flicker photometry. Results from a chromatic adaptation experiment indicated that each of the twelve subjects had two classes of cone pigment in the 540/640 nm portion of the spectrum. That result strongly suggests that this species has routine trichromatic color vision. Comparison of the spectral sensitivity functions obtained from vervets and from similarly-tested humans further indicates that the cone complements of the two species are very similar. Results from this investigation add further support to the idea that there are fundamental differences in the genetic mechanisms underlying color vision in platyrrhine and catarrhine monkeys.     

Color and luminance contrasts attract independent attention

Paying attention can improve vision in many ways, including some very basic functions such as contrast discrimination, a task that probably reflects very early levels of visual processing. Electrophysiological, psychophysical, and imaging studies on humans as well as single recordings in monkey show that attention can modulate the neuronal response at an early stage of visual processing, probably by acting on the response gain. Here, we measure incremental contrast thresholds for luminance and color stimuli to derive the contrast response of early neural mechanisms and their modulation by attention. We show that, for both cases, attention improves contrast discrimination, probably by multiplicatively increasing the gain of the neuronal response to contrast. However, the effects of attention are highly specific to the visual modality: concurrent attention to a competing luminance, but not chromatic pattern, greatly impedes luminance contrast discrimination; and attending to a competing chromatic, but not luminance, task impedes color contrast discrimination. Thus, the effects of attention are highly modality specific, implying separate attentional resources for different fundamental visual attributes at early stages of visual processing.     

Cone pigments in a North American marsupial, the opossum (Didelphis virginiana)

Only two of the four cone opsin gene families found in vertebrates are represented in contemporary eutherian and marsupial species. Recent genetic studies of two species of South American marsupial detected the presence of representatives from two of the classes of cone opsin genes and the structures of these genes predicted cone pigments with respective peaks in the ultraviolet and long-wavelength portions of the spectrum. The Virginia opossum (Didelphis virginiana), a profoundly nocturnal animal, is the only marsupial species found in North America. The prospects for cone-based vision in this species were examined through recordings of the electroretinogram (ERG), a commonly examined retinal response to photic stimulation. Recorded under flickering-light conditions that elicit signals from cone photoreceptors, the spectral sensitivity of the opossum eye is well accounted for by contributions from the presence of a single cone pigment having peak absorption at 561–562 nm. A series of additional experiments that employed various chromatic adaptation paradigms were conducted in a search for possible contributions from a second (short-wavelength sensitive) cone pigment. We found no evidence that such a mechanism contributes to the ERG in this marsupial.     

Temporal acuity of honeybee vision: behavioural studies using flickering stimuli

ABSTRACT. Temporal resolution of freely-flying bees was measured by training bees, Apis mellifera (Linn.), to discriminate between a steady light and a flickering light. Two kinds of experiments were conducted: those using a homochromatic flicker, in which the intensity of the flickering light varied periodically with time; and ones using a heterochromatic flicker, in which the colour of the flickering light varied periodically. In either case, the time-averaged properties (intensity and colour) of the flickering light matched those of the steady light, and the bees' ability to discriminate between the two stimuli was measured for various flicker frequencies. The results indicate that bees perform poorly in the homochromatic flicker experiments, regardless of the colour of the light (u.v., blue or green), but well in those with heterochromatic flicker. Heterochromatic flicker experiments using various pairwise combinations of the colours U.V., blue and green (corresponding to the three known spectral receptor-types in the bee's retina) reveal that temporal resolution is much better when blue is one of the component colours, than when it is not. The simplest interpretation of the results is in terms of colour channels possessing different response speeds. Heterochromatic flicker promises to be a useful tool in investigating the temporal properties of colour vision in bees.     

Visual sensitivity of ground squirrels to spatial and temporal luminance variations

Summary We have investigated the visual sensitivity of the California ground squirrel (Speromphilus beecheyi) to spatial and temporal luminance patterns. Spatial contrast sensitivity functions were determined in behavioral discrimination experiments in which the stimuli were sinusoidally-modulated luminance gratings. These squirrels were found to be maximally sensitive to spatial frequencies of about 0.7 cycles/ degree (c/d), and they are unable to discriminate gratings whose frequencies exceed 4 c/d. Similar results were obtained in electrophysiological experiments when the visually evoked cortical potential (VECP) was recorded from anesthetized squirrels. A third experiment involved tests of the ability of ground squirrels to discriminate square-wave gratings of much higher luminance (340 cd/m²). The finest gratings which were discriminable at this luminance level did not exceed 3.9–4.3 c/d and, thus, we conclude that the maximal spatial resolution of the California ground squirrel is about 4 c/d (corresponding to a bar separation of 7.5). In another behavioral experiment the abilities of ground squirrels to discriminate sinusoidally flickering lights (mean luminance = 3.4 cd/m²) was measured. The results show that ground squirrels are maximally sensitive to lights flickering at a rate of about 18 Hz, and that the highest rates that are still discriminable are slightly above 60 Hz.Abbreviations c/d cycles/degree - CFF critical flicker frequency - VECP visually evoked cortical potential This research was supported by Grant EY 00105 from the National Eye Institute. We thank David Birch who participated in some preliminary behavioral experiments and Kenneth Long who provided the histological material from which measurements of receptor spacing were made.     

Photopigments underlying color vision in ringtail lemurs (Lemur catta) and brown lemurs (Eulemur fulvus)

A recent examination of color vision in the ringtail lemur produced evidence that these prosimians could make color discriminations consistent with a diagnosis of trichromatic color vision. However, it was unclear if this behavior reflected the presence of three classes of cone or whether lemurs might be able to utilize signals from rods in conjunction with those from only two classes of cone. To resolve that issue, spectral sensitivity functions were obtained from ringtail lemurs (Lemur catta) and brown lemurs (Eulemur fulvus) using a noninvasive electrophysiological procedure, electroretinographic flicker photometry. Results from experiments involving chromatic adaptation indicate that these lemurs routinely have only a single class of cone photopigment in the middle to long wavelengths (peak sensitivity of about 545 nm); they also have a short-wavelengthsensitive cone pigment with peak of about 437 nm. The earlier behavioral results are suggested to have resulted from the ability of lemurs to jointly utilize signals from rods and cones. The cone pigment complements of these lemurs differ distinctly from those seen among the anthropoids. © 1993 Wiley-Liss, Inc.     

Cone photopigments in nocturnal and diurnal procyonids

Summary Procyonids are small, New World carnivores distributed among some 6 genera. Electroretinogram (ERG) flicker photometry was used to measure the spectra of the cone photopigments for members of two nocturnal species, the raccoon (Procyon lotor) and the kinkajou (Potos flavus), and a diurnal species, the coati (Nasua nasua). Each of the 3 has a class of cone photopigment with maximum sensitivity in the middle to long wavelengths. The spectral positioning of this cone is different for the three. Whereas the raccoon and kinkajou are monochromatic, the diurnal coati is a dichromat having an additional class of cone photopigment with peak sensitivity close to 433 nm.Abbreviations ERG electroretinogram - SWS short wavelength sensitive     

Transfer of stimulus control from a TFT to CRT screen

The use of television and computer screens for presenting stimuli to animals is increasing as it is non-invasive and can provide precise control over stimuli. Past studies have used cathode ray tube (CRT) screens; however, there is some evidence that these give different results to non-flickering thin film transistor (TFT) screens. Hens’ critical flicker fusion frequency ranges between 80 and 90 Hz - above standard CRT screens. Thus, stimuli presented on CRT screens may appear distorted to hens. This study aimed to investigate whether changing the flicker rate of CRT screens altered hens’ discrimination. Hens were trained (in a conditional discrimination) to discriminate between two stimuli on a TFT (flickerless) screen, and tested with the stimuli on a CRT screen at four flicker rates (60, 75, 85, and 100 Hz). The hens’ accuracy generally decreased as the refresh rate of the CRT screen decreased. These results imply that the change in flicker rate changed the appearance of the stimuli enough to affect the hens’ discrimination and stimulus control is disrupted when the stimuli appear to flicker.