Spectral sensitivity and colour vision in the ground-dwelling sciurids: results from golden mantled ground squirrels and comparisons for five species

Electroretinogram of the Cone-Dominant Thirteen-Lined Ground Squirrel during Euthermia and Hibernation in Comparison with the Rod-Dominant Brown Norway Rat

Purpose

The majority of small animal species used in research are nocturnal, with retinae that are anatomically and functionally dissimilar from humans, complicating their use as disease models. Herein we characterize the retinal structure and electrophysiological function of the diurnal, cone-dominant 13-lined ground squirrel (13-LGS) retina during euthermia and in hibernation.

Methods

Full-field electroretinography (ERG) was performed in 13-LGS and Brown Norway (BN) rat models to establish baseline values for retinal function in each species, including following intravitreal injection of pharmacologic agents to selectively block the contributions of ON- and OFF-bipolar cells. The effect of hibernation-associated retinal remodeling on electrophysiological function was assessed in 13-LGS during torpor and emergence, with correlative histology performed using transmission electron microscopy.

Results

Under light-adapted conditions, the a-, b-, and d-wave amplitude of the 13-LGS was significantly greater than that of the BN rat. Retinal function was absent in the 13-LGS during hibernation and correlated to widespread disruption of photoreceptor and RPE structure. Remarkably, both retinal function and structure recovered rapidly on emergence from hibernation, with ERG responses reaching normal amplitude within 6 hours.

Conclusions

ERG responses for both BN rats and 13-LGS reflect the relative proportions of cone photoreceptors present within the retinae, indicating that the cone-dominant 13-LGS may be a potentially useful model for studying human central retinal function and disease. That retinal remodeling and restoration of electrophysiological function occurs rapidly on emergence from hibernation implies the 13-LGS may also be a useful tool for studying aspects of retinal physiology and recovery from injury.

Ground squirrel - A cool model for a bright vision

The great evolutionary biologist, Theodosius Dobzhansky, once said, "Nothing in biology makes sense except in the light of evolution." Vision, no doubt, is a poster child for the work of evolution. If it has not already been said, I would humbly add that "Nothing in biology makes sense except in the context of metabolism." Marrying these two thoughts together, when one chooses an animal model for vision research, the ground squirrel jumps out immediately for its unique cone dominant retina, which has evolved for its diurnal lifestyle, and for hibernation-an adaptation to unique metabolic challenges encountered during its winter sojourn.

Photopigments and the dimensionality of animal color vision

Early color-matching studies established that normal human color vision is trichromatic. Subsequent research revealed a causal link between trichromacy and the presence in the retina of three classes of cone photopigments. Over the years, measurements of the photopigment complements of other species have expanded greatly and these are frequently used to predict the dimensionality of an animal's color vision. This review provides an account of how the linkage between the number of active photopigments and the dimensions of human color vision developed, summarizes the various mechanisms that can impact photopigment spectra and number, and provides an across-species survey to examine cases where the photopigment link to the dimensionality of color vision has been claimed. The literature reveals numerous instances where the human model fails to account for the ways in which the visual systems of other animals exploit information obtained from the presence of multiple photopigments in support of their behavior.

S cones: Evolution, retinal distribution, development, and spectral sensitivity

S cones expressing the short wavelength-sensitive type 1 (SWS1) class of visual pigment generally form only a minority type of cone photoreceptor within the vertebrate duplex retina. Hence, their primary role is in color vision, not in high acuity vision. In mammals, S cones may be present as a constant fraction of the cones across the retina, may be restricted to certain regions of the retina or may form a gradient across the retina, and in some species, there is coexpression of SWS1 and the long wavelength-sensitive (LWS) class of pigment in many cones. During retinal development, SWS1 opsin expression generally precedes that of LWS opsin, and evidence from genetic studies indicates that the S cone pathway may be the default pathway for cone development. With the notable exception of the cartilaginous fishes, where S cones appear to be absent, they are present in representative species from all other vertebrate classes. S cone loss is not, however, uncommon; they are absent from most aquatic mammals and from some but not all nocturnal terrestrial species. The peak spectral sensitivity of S cones depends on the spectral characteristics of the pigment present. Evidence from the study of agnathans and teleost fishes indicates that the ancestral vertebrate SWS1 pigment was ultraviolet (UV) sensitive with a peak around 360 nm, but this has shifted into the violet region of the spectrum (>380 nm) on many separate occasions during vertebrate evolution. In all cases, the shift was generated by just one or a few replacements in tuning-relevant residues. Only in the avian lineage has tuning moved in the opposite direction, with the reinvention of UV-sensitive pigments.

Color vision sensitivity in normally dichromatic species and humans

Spectral-sensitivity functions for large, long-duration increments presented on a photopic white background indicate that wavelength-opponent mechanisms mediate detection in both normal and dichromatic humans. Normal humans exhibit high color-vision sensitivity as they discriminate the color of spectral flashes at detection-threshold intensities. However, dichromatic humans require stimuli up to about 0.4 log units above detection intensity to see certain colors. This low color-vision sensitivity in human dichromats may be an abnormal condition involving a defect in postreceptoral color processing. To test this hypothesis, we determined color-discrimination thresholds in normally dichromatic species: chipmunk, 13-lined ground squirrel, and tree shrew. For comparison, we also tested humans with normal and abnormal (deutan) color vision with the same apparatus and methods. Animals were trained to perform spatial two-choice discrimination tasks for food reward. Detection thresholds were determined for increments of white, 460 nm, 540 nm, 560 nm, 580 nm, 500 nm/long-pass, and 500 nm/short-pass on white backgrounds of 1.25 cd/m2, 46 cd/m2, and 130 cd/m2. Animals were also trained to respond to the colored increments when paired with the white increment when both were at equally detectable intensities Color-discrimination thresholds were determined by dimming stimulus pairs (coloredvs. white) until the subjects could no longer make the discriminations. Results indicated that the normally dichromatic species could discriminate colored stimuli from white at a mean intensity of 0.1 (±0.1) log units above detection threshold. The ability of normally dichromatic species to discriminate color near detection-threshold intensity is consistent with increment spectral-sensitivity functions that indicate detection by wavelength-opponent mechanisms. In keeping with previous studies, normal human trichromats discriminated color near detection-threshold intensities but humans with deutan color vision required suprathreshold intensities to discriminate the color of middle and long wavelengths. This high color-vision sensitivity of normally dichromatic species suggests that the low color-vision sensitivity in dichromatic humans is an abnormal condition and indicates a possible defect in their postreceptoral color-vision processing.

Separate blue and green cone networks in the mammalian retina

The distinct absorbance spectra of the cone photopigments form the basis of color vision, but ultrastructural and physiological evidence shows that mammalian cones are electrically coupled. Coupling between cones of the same spectral type should average voltage noise in adjacent photoreceptors and improve the ability to resolve low-contrast spatial patterns. However, indiscriminate coupling between spectral types could compromise color vision by smearing chromatic information across channels. Here we show, by measuring the junctional conductance between green-green and blue-green cone pairs in slices from the dichromatic ground-squirrel retina, that green-green cone pairs are routinely coupled with an average conductance of 220 pS, whereas coupling is undetectable in blue-green cone pairs. Together with a lack of tracer coupling and the selective localization of connexin proteins, our results show that signals in blue and green cones are processed separately in the photoreceptor layer.

Optic nerve head morphology of the Eastern gray squirrel, Sciurus carolinensis

The objective of this study was to characterize the morphology of the Eastern gray squirrel optic nerve head, using histochemical techniques and scanning electron microscopy, in order to make comparisons with other mammalian species. Eyes of three Eastern gray squirrels were enucleated, fixed in 10% neutral buffered formalin, and embedded in paraffin. Sections through the optic nerve head were made in the three different planes, and staining was accomplished with hematoxylin and eosin and Gomori's trichrome stain. Stained sections were examined using light microscopy. The surface of the optic nerve head and the peripapillary retina were evaluated using field emission scanning electron microscopy. The anterior optic nerve head of the Eastern gray squirrel was found to be horizontally elongated and tapers to a more conventional round shape after it exits the eye. Retinal nerve fiber axons converge on the optic nerve head from all directions. Anterior to the lamina cribrosa, the mean (+/- SD) optic nerve axon bundle diameter was 27 +/- 11 microm. The lamina cribrosa measured 363 microm dorsal to ventral and 3.60 mm nasal to temporal. The thickness of the lamina was 125 micro m anterior to posterior with a mean (+/- SD) laminar pore size of 45 +/- 12 microm. The optic nerve head measured 3.65 mm nasal to temporal and 330 microm dorsal to ventral. Trichrome staining indicated the presence of collagen in the lamina cribrosa. This is the first detailed description of the squirrel optic nerve head. It has an unusual shape among mammals with a dramatic horizontal elongation. The purpose of this shape is not apparent. The presence of collagen in the scleral lamina cribrosa indicates a similar extracellular matrix composition as in other species.

Animal colour vision--behavioural tests and physiological concepts

Over a century ago workers such as J. Lubbock and K. von Frisch developed behavioural criteria for establishing that non-human animals see colour. Many animals in most phyla have since then been shown to have colour vision. Colour is used for specific behaviours, such as phototaxis and object recognition, while other behaviours such as motion detection are colour blind. Having established the existence of colour vision, research focussed on the question of how many spectral types of photoreceptors are involved. Recently, data on photoreceptor spectral sensitivities have been combined with behavioural experiments and physiological models to study systematically the next logical question: 'what neural interactions underlie colour vision?' This review gives an overview of the methods used to study animal colour vision, and discusses how quantitative modelling can suggest how photoreceptor signals are combined and compared to allow for the discrimination of biologically relevant stimuli.

An animal model for studying cone function in retinal detachment

In people, retinal detachment often leads to a significant loss in cone-based vision. Most of the animal models commonly used for studying the consequences of retinal detachment have rod-dominated retinas. The purpose of this investigation was to evaluate the possibility that the ground squirrel, a rodent with a heavily cone-dominated retina, might provide a useful model for studying cone function in retinal detachment. Corneal ERGs were recorded from ground squirrels for large-field temporal modulations presented on a computer-controlled color monitor. Modulations were chosen to selectively stimulate either of the two classes of cone found in the ground squirrel retina. Under these test conditions, large and reliable cone ERGs could be readily recorded. In animals in which the retina had been surgically detached, the loss of cone signal was directly related to the number of cones in the detachment zone relative to the total cone population and that relationship did not differ for short-wavelength sensitive (S) and middle-wavelength sensitive (M) cones. Surgical reattachment produced a progressive recovery of cone-based signals. The ground squirrel seems likely to provide a useful animal model for studying the dynamics of cone function in retinal detachment and subsequent events.

Modelling the mosaic organization of rod and cone photoreceptors with a minimal-spacing rule

The mosaic of photoreceptors is regarded as a prime example of the precise control of cellular positioning in the vertebrate nervous system. This study was undertaken with the idea that understanding the intrinsic geometrical features of photoreceptor mosaics is a necessary step to unveil the biological mechanisms governing their formation. We show in the retina of the ground squirrel that the arrays of both the rods and S cones are non-random, but that nothing more than a simple minimal-spacing rule constraining receptor positioning is sufficient to account for the spatial organization of both mosaics. The size of this 'exclusion zone' is an intrinsic characteristic of each cell type, and it is simply the difference in the size of this domain that accounts for the regularity of the S cone array and the irregularity of the rod array at identical density. Consequently, regularity in receptor mosaics is produced by two independent biological events, one embodying the exclusion zone, and another specifying the local density of a given receptor type.

The distribution and nature of colour vision among the mammals

1. An oft-cited view, derived principally from the writings of Gordon L. Walls, is that relatively few mammalian species have a capacity for colour vision. This review has evaluated that proposition in the light of recent research on colour vision and its mechanisms in mammals. 2. To yield colour vision a retina must contain two or more spectrally discrete types of photopigment. While this is a necessary condition, it is not a sufficient one. This means, in particular, that inferences about the presence of colour vision drawn from studies of photopigments, the precursors of photopigments, or from nervous system signals must be accepted with due caution. 3. Conjoint signals from rods and cones may be exploited by mammalian nervous systems to yield behavioural discriminations consistent with the formal definition of colour vision. Many mammalian retinas are relatively cone-poor, and thus there are abundant opportunities for such rod/cone interactions. Several instances were cited in which animals having (apparently) only one type of cone photopigment succeed at colour discriminations using such a mechanism. it is suggested that the exploitation of such a mechanism may not be uncommon among mammals. 4. Based on ideas drawn from natural history, Walls (1942) proposed that the receptors and photopigments necessary to support colour vision were lost during the nocturnal phase of mammalian history and then re-acquired during the subsequent mammalian radiations. Contemporary examination of photopigment genes along with the utilization of better techniques for identifying rods and cones suggest a different view, that the earliest mammals had retinas containing some cones and two types of cone photopigment. Thus the baseline mammalian colour vision is argued to be dichromacy. 5. A consideration of the broad range of mammalian niches and activity cycles suggests that many mammals are active during photic periods that would make a colour vision capacity potentially useful. 6. A systematic survey was presented that summarized the evidence for colour vision in mammals. Indications of the presence and nature of colour vision were drawn both from direct studies of colour vision and from studies of those retinal mechanisms that are most closely associated with the possession of colour vision. Information about colour vision can be adduced for species drawn from nine mammalian orders.(ABSTRACT TRUNCATED AT 400 WORDS)

Photopigments of dogs and foxes and their implications for canid vision

Electroretinogram (ERG) flicker photometry was used to examine the photopigment complements of representatives of four genera of Canid: domestic dog (Canis familiaris), Island gray fox (Urocyon littoralis), red fox (Vulpes vulpes), and Arctic fox (Alopex lagopus). These four genera share a common cone pigment complement; each has one cone pigment with peak sensitivity of about 555 nm and a second cone pigment with peak at 430-435 nm. These pigment measurements accord well with the conclusions of an earlier investigation of color vision in the dog, and this fact allows some predictions about color vision in the wild canids. An additional set of measurements place the peak of the dog rod pigment at about 508 nm.

Duplicity theory and ground squirrels: linkages between photoreceptors and visual function

The presence of rod and cone photoreceptors has traditionally been linked to well-defined classes of visual capacity by the generalization known as duplicity theory. This paper summarizes results obtained from studies of vision and the visual system in ground squirrels (Spermophilus sp.) that reveal instances where structure/function linkages depart from expectations based in duplicity theory. The details of these exceptions are reviewed and their possible mechanisms discussed.

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.

Photocurrents of cone photoreceptors of the golden-mantled ground squirrel

1. Visual transduction in photoreceptors of the ground squirrel, Citellus lateralis, was studied by recording membrane current from individual cones in small pieces of retina. 2. Brief flashes of light produced transient reductions of the dark current; saturating response amplitudes were up to 67 pA. A flash strength of about 11,000 photons microns-2 at lambda max was required to give a half-saturating response. The stimulus-response relation was well fitted by an exponential saturation curve. Responses below 20% of maximum behaved linearly. 3. The response to a dim flash in most cells had a time to peak of 20-30 ms and resembled the impulse response of a series of five low-pass filters. 4. The variance of the dim-flash response amplitude put an upper limit of 80 fA on the size of the single photon response. Estimates based on the effective collecting area suggest the single photon response to be of the order of 10 fA. 5. Flash responses of squirrel cones usually lacked the undershoot observed in primate cones, although in about 1/3 of the cells a small undershoot developed during recording. 6. Background lights slightly shortened the time to peak of the flash response and reduced the integration time. 7. Spectral sensitivity measurements showed two classes of cones with peak sensitivities at about 520 and 435 nm. Rod sensitivity peaked near 500 nm. Spectral univariance was obeyed by all three classes of cells. 8. The shapes of the spectral sensitivity curves of the rod and both types of cones were similar to each other when plotted on a log wave number scale, but differed significantly from similar plots of monkey and human cone spectra. 9. The kinetics and sensitivity of flash responses of the blue- and green-sensitive cones were indistinguishable.

Morphology of ganglion cells which project to the dorsal lateral geniculate and superior colliculus in the ground squirrel

We wished to determine whether retinal ganglion cells that have axons terminating in the dorsal lateral geniculate and/or the superior colliculus have specific sizes of somata, comprising only part of the entire size range of ganglion cell somata. If so, then perhaps the specific functional types described by Michael might be associated with morphological types based on soma size. HRP was injected into either the superior colliculus (SC) or dorsal lateral geniculate nucleus (LGd) of thirteen-lined ground squirrels. Soma diameter of labeled ganglion cells was measured and the relation between cell size and frequency determined. After SC injections HRP-filled cells were mostly small and medium-sized. They ranged in diameter from 3 to 14 microns and the mean diameter of labeled neurons was 7.35 microns. Cells labeled after SC injections were often distributed as doublets or triplets in the retina. After LGD injections the majority of labeled cells were medium and large-sized. They ranged from 4 to 18 microns in diameter with a mean of 9.1 microns and were more regularly spaced within the retinal region of labeled cells. Thus, the present results provide reason to believe that functional classes of ganglion cells in ground squirrels may be correlated with particular morphological types.

Retinal inputs to the geniculate relay cells in the eastern chipmunk (Tamias sibiricus asiaticus): a comparison between color and non-color sensitive cells

Single unit recordings were made from the relay cells of the lateral geniculate nucleus in the eastern chipmunk. Of 362 relay cells, 47 cells (13%) were classified as color sensitive and the rest as non-color sensitive cells. Non-color sensitive cells were further classified into 5 subclasses: off-phasic, on-phasic, on-off-phasic, on-tonic and uncommon types. Within the color sensitive cells there were 3 subclasses; blue excited and green inhibited (+B-G), blue inhibited and green excited (-B+G), and blue excited (+B) cells. Retinal afferents to color sensitive relay cells had the following characteristics: ganglion cells of their origin were distributed in the central high density areas of the retina and axonal conduction velocities were in the intermediate range, though they were somewhat slow in +B cells.

Sensitivity of the human neonate to short- and long-wavelength stimuli

Increment thresholds for long- and short-wavelength stimuli were determined under both white and orange adaptation. The forced-choice preferential looking technique (FPL) was used to test neonates shortly after birth and again at 2 months postnatal. Results were compared to an adult control group. Neonates, 2-month-olds, and adults showed a change in relative sensitivity to the 617 and 453 nm test stimuli with a change in chromatic adaptation, indicating the existence of at least 2 separately adaptable chromatic mechanisms at birth. These results are tentatively discussed in terms of the functioning of rods and short-, middle- and long-wavelength cones. A decrease in all thresholds with age is discussed with regard to attentional differences and postnatal neural development in the visual system.

Characterization of the color related receptor mosaic in the ground squirrel retina

Light microscopic and histochemical studies reveal that the retina of the European ground squirrel (Citellus citellus L.) contains a mosaic pattern of two cone types and a small population of rods. A minority (7%) of the cones can be characterized by their ellipsoids having larger diameters and increased staining density over the majority population. Exposure to green light selectively elicited intense NBT-diformazan labeling in the major population of cones while the larger diameter cone type was labeled after blue illumination. The two cone subpopulations are probably the blue and green cone types of ground squirrel protanopic color vision.

Distribution and soma size of ganglion cells in the retina of the eastern chipmunk (Tamias sibiricus asiaticus)

Topographic distribution and soma size of ganglion cells were studied in Nissl-stained, whole-mounted retinas of the eastern chipmunk. High density areas in the central retina were elongated horizontally, making up the visual streak. The total count of ganglion cells was estimated as 410,000. Throughout the retina soma size of ganglion cells showed a uimodal distribution, although a distinct population of large cells was found in the dorso-temporal periphery.

Development of spectral mechanisms in the ground squirrel retina following lid opening

The retina of the California ground squirrel (Spermophilus beecheyi) contains three classes of photopigments (lambda max = 440, 500, 525 nm). From optic nerve recordings it was previously discovered that the effectiveness of signals from the 440 nm cone increase gradually over the weeks following lid opening. In this experiment several features of the electroretinogram (ERG) were examined to assess developmental changes in signals originating in the outer retina. As judged by threshold sensitivity, suprathreshold responsivity, or adaptability, the contribution of the 525 nm mechanism reaches its adult level two to three weeks after lid opening. The 500 nm mechanism appears to have a similar developmental time course. The development of the 440 nm mechanism was tracked using a chromatic adaptation measure. So indexed, the time required for this mechanism to reach its adult status was much greater than that for the other two mechanisms (70-80 days vs 40-45 days of age). The relatively slow development of effectiveness of the 440 spectral mechanism appears to reflect events occurring in the outer retina.

A reweighting of receptor mechanisms in the ground squirrel retina: PIII and B-wave spectral sensitivity functions

PIII and b-wave spectral sensitivity functions were measured under various adapted conditions in the 13-lined ground squirrel. The PIII was isolated by intravitreal injection of sodium L-aspartate. Under the conditions studied, the PIII spectral sensitivity function always fitted the absorption spectrum of a 518 +/- 3 mm Dartnall nomogram. This suggests that of the three photoreceptors ('rod-like', 'blue' cone and 'green' cone) present in the 13-lined ground squirrel the green cones are the overwhelming majority. The dark-adapted b-wave had two components; a b1-wave with a spectral sensitivity that fitted a 516 +/- 1 nm Dartnall nomogram and a b2-wave with a spectral sensitivity that fitted a 502 +/- 4 nm Dartnall nomogram. The light-adapted b-wave had a spectral sensitivity that fitted a 516 +/- 2 nm Dartnall nomogram.

Rod photoreceptors and scotopic vision in ground aquirrels

Ground squirrel retinas contain a relatively small complement of rods (5--10% of all photoreceptors) which are thought to provide the basis for a weak scotopic visual capacity. In a previous investigation of the California ground squirrel (Spermophilus beecheyi) involving the recording of a retinal gross potential, the electroretinogram (ERG), electrophysiological evidence for a viable scotopic signal could be obtained from some, but not all of the ground squirrels examined. To further pursue the possibility that there is a structural/functional discrepancy in the relationship between rod photoreceptors and scotopic vision in the ground squirrel, several experiments involving electrophysiological, behavioral, and anatomical observations have been conducted. We found that although about one-third of the ERGs recorded from a large sample of California ground squirrels lack those characteristics which would indicate the presence of a viable scotopic signal, the retinas of all the squirrels appear to contain the same small population of rod photoreceptors. Additional experiments on the golden-mantled ground squirrel (Spermophilus lateralis), including behavioral as well as ERG measurements and anatomical observations, lead to this same conclusion.