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PubMed1999

Multifocal lenses compensate for chromatic defocus in vertebrate eyes.

The focal length of the vertebrate eye is a function of wavelength, i.e. the eye suffers from longitudinal chromatic aberration. Chromatic defocus is a particularly severe problem in eyes with high light-gathering ability, since depth of field is small due to a pupillary opening that is large in relation to the focal length of the eye. Calculations show that in such eyes only a narrow spectral band of light can be in focus on the retina. For the major part of the visual spectrum, spatial resolution should be limited by the optics of the eye and far lower than the resolving power achievable by the retinal cone photoreceptor mosaic. To solve this problem, fishes with irises unresponsive to light have developed lenses with multiple focal lengths. Well-focused images are created at the wavelengths of maximum absorbance of all spectral cone types. Multifocal lenses also appear to be present in some terrestrial species. In eyes with mobile irises, multifocal lenses are correlated with pupil shapes that allow all zones of the lens, with different refractive powers, to participate in the imaging process, irrespective of the state of pupil constriction.

Animals↗
PubMed2006

Chromatic pattern-reversal electroretinograms (ChPERGs) are spared in multiple system atrophy compared with Parkinson's disease.

Idiopathic Parkinson's disease (IPD) patients have abnormal visual evoked potentials (VEPs) and pattern electroretinograms (PERGs), attributed to dopaminergic transmission deficiency in visual pathway, probably the retina. VEP abnormalities are not reported in multiple system atrophy (MSA). The aim of this study was to investigate and compare chromatic (Ch) red-green (R-G) and blue-yellow (B-Y), and luminance yellow-black (Y-Bk) PERGs in patients with MSA and IPD. We investigated 6 MSA patients (mean age: 62+/-7.4 years) not undergoing any pharmacological treatment, as well as 12 early IPD patients (mean age: 60.1+/-8.3 years) and 12 age-matched normal observers. ChPERGs were recorded monocularly in response to full-field equiluminant R-G, B-Y and Y-Bk horizontal gratings. In MSA only responses to R-G stimuli showed minimal insignificant changes (slight but not significant amplitude reduction without any significant latency delay); no significant abnormality was detected for B-Y and luminance Y-Bk stimuli. By contrast, in IPD all responses were reduced in amplitude and delayed in latency, above all for B-Y stimuli. Present data indicate that both chromatic and achromatic PERGs are virtually unaffected in MSA, whereas in early IPD they are clearly impaired, suggesting different pathogenic retinal mechanisms and a useful simple tool for distinguishing MSA from IPD.

Aged↗
PubMed1975

Spatial and chromatic properties of neurons subserving foveal and parafoveal vision in rhesus monkey.

The response properties of neurons in the region of striate cortex subserving central retina (0 degrees-2 degrees) and in a region of representation of parafoveal retina (4 degrees-7 degrees) were studied in unanesthetized paralyzed macaque monkeys. Neurons sensitive to the orientation of the stimulus in the visual field (simple, complex, and hypercomplex), and neurons lacking orientation selectivity (concentric, and a new class termed uniform) were found. In foveal cortex non-oriented cells were more numerous, and orientation sensitive cells had less strict spatial stimulus requirements than in parafoveal cortex. Most neurons received a monocular input, either exclusively or very predominantly. Three types of neurons were recognized on the basis of their responses to chromatic stimuli. (1) Luminosity neurons (about half the population) gave the same qualitative response to all effective wayelengths and had a spectral sensitivity similar to that of the macaque, determined behaviorally. Cells with all spatial types of receptive fields, except simple, occurred in this group. (2) Spectrally-treated neurons also responded in the same manner to different wavelengths, but over a narrower range than luminosity neurons, and their maximal sensitivity was shifted toward one or the other end of the visible spectrum. All tuned neurons had uniform or complex receptive field. (3) Spectrally-opponent neurons were either excited or inhibited by long wavelengths and responded in the opposite manner to short wavelengths. For cells with uniform or complex receptive fields the two opponent systems were coextensive. Simple or concentric neurons often had dual-opponent organization. The distribution of functional types among different cortical layers was similar in parafoveal and foveal cortex. The functional attributes of ocular dominance and orientation sensitivity were found to be statistically independent dimensions of cortical organization. On the other hand, the correlation between spatial and chromatic properties did not vary between different cytoarchitectonic layers, a finding suggesting that these neuronal properties depend on conjoined projectional and intracortical connecting mechanisms.

Animals↗
PubMed1988

Molecular characterization and evolution of sequences encoding light-harvesting components in the chromatically adapting cyanobacterium Fremyella diplosiphon.

The major light-harvesting complex in eukaryotic red algae and prokaryotic cyanobacteria is the phycobilisome, a water-soluble complex located on the outer surface of the photosynthetic membranes and composed of both pigmented phycobiliproteins (85%) and non-pigmented linker (15%) polypeptides. The phycobiliproteins are encoded by a gene family and exhibit varying degrees of sequence homology (25 to 55%). Some cyanobacteria can maximize the absorption of prevalent wavelengths of light by adjusting the phycobiliprotein composition of the phycobilisome, a process called complementary chromatic adaptation. In the chromatically adapting species Fremyella displosiphon, there are at least two sets of phycocyanin genes; one is transcribed as two red light-induced transcripts and the other is encoded on a single transcript present in both red and green light. We have determined the complete nucleotide sequences of both sets of phycocyanin subunit genes and their associated 5' and 3' regulatory regions. Based on S1 nuclease protection experiments, the transcripts (1600 and 3800 bases) encoding the inducible phycocyanin subunits have the same 5' end, and possible mechanisms for their synthesis are presented. The 5' end of the 1500-base transcript encoding the constitutive phycocyanin subunits was determined and revealed an Escherichia coli-like "-10" and "-35" region, and sequences near the transcription initiation site homologous to the analogous region of the phycocyanin gene set of Anabaena sp. 7120. Determination of the 3' ends of the transcripts encoding both F. diplosiphon phycocyanin gene sets revealed regions of potential secondary structure that may be important for transcription termination and/or transcript stability. In addition, the sequence of an open reading frame (encoding a 30 kDa polypeptide), located 3' to the constitutive phycocyanin gene set in F. diplosiphon and highly conserved in at least three cyanobacterial species, is presented. The same high degree of sequence homology between the two F. diplosiphon PC alpha and PC beta sequences (85 and 77%, respectively) was found at both the nucleotide and amino acid levels, and similar results were obtained for interspecies comparisons. Implications of these homologies with regard to the evolution of phycobiliprotein subunits are discussed.

Amino Acid Sequence↗
PubMed1995

Parvocellular and magnocellular contributions to visual evoked potentials in humans: stimulation with chromatic and achromatic gratings and apparent motion.

Psychophysical evidence suggests that two major parallel pathways, the parvocellular (P) and the magnocellular (M) pathways, exist in humans. We herein report that responses specific to the P and M systems can be recorded in human visual evoked potentials (VEPs) by using the appropriate stimuli. The onset of isoluminant chromatic (red-green) and high contrast achromatic sinusoidal gratings were used for stimulating the P-system. A chromatic stimulation evoked a characteristic negative wave (N1) with peak latencies around 120 msec. The amplitude showed an inverse U-shaped function as a function of spatial frequency with a peak at 2 c/deg. In contrast, VEPs to achromatic (black-white) gratings showed different spatial frequency characteristics with a peak at 5.3 c/deg. By varying the luminous intensity ratio between the red and green gratings, N1 was found to reach a maximum during isoluminant stimulation. An apparent motion display was used for stimulating the M-system. The speed of alternation (i.e., the interstimulus interval (ISI)) was varied to record both the transient and steady-state VEPs. Transient VEPs showed triphasic waves with the major positive peak (P1) at around 120 ms. Steady-state VEPs were quasi-sinusoidal waveforms, depending on the ISI, and were quite stable across all subjects. There was a also high correlation between the motion threshold and the VEP amplitude. The above observations indicate that characteristic potentials may distinguish between these two parallel visual systems in humans. Thus, the combined use of isoluminant color and high contrast achromatic gratings and an apparent motion display is considered to be useful for evaluating both systems electrophysiologically.

Adolescent↗
PubMed1982

Chromatic dispersion of the ocular media.

Measurements of chromatic dispersion of aqueous and vitreous humors, cornea and lens of the eye are sparse and incomplete. The wavelength variation in refractive index of the ocular media of cow, pig, frog (Rana pipiens), chicken, rock bass (Ambloplites rupestris), albino rat and cat as well as human lenses was determined by means of Abbe and Pulfrich refractometry. While the humors are somewhat less dispersive than water, the cornea is more dispersive at short wavelengths. In general, the lens is significantly more dispersive than water with dispersion increasing asymptotically at the blue end of the spectrum. The exaggerated dispersion taking place at short wavelengths should be taken into account in calculations of chromatic aberration.

Adolescent↗
PubMed1983

The chromatic Cornsweet effect.

The Cornsweet effect was measured using equiluminous chromatic gradients as well as with an achromatic gradient. The chromatic Cornsweet effect is smaller than the achromatic effect.

Color Perception↗
PubMed1983

Aspheric curvatures, refractive indices and chromatic aberration for the rat eye.

Thicknesses and curvatures of the optic components of enucleated eyes were determined in both transverse and sagittal planes. The cornea and lens surfaces are closer approximations to spheres than are the retina and choroid. Refractive indices of the cornea, aqueous and vitreous humours were obtained by two techniques at eight wavelengths across the visible spectrum. Photography of light beams passing through the crystalline lens immersed in albumin gave the "back vertex power", and thus (assuming homogeneity) the "equivalent" refractive index. With this assumption, which is supported by additional modelling, calculations for the eyes of six individual rats show chromatic aberration across the spectrum of 5.8 (5.5-6.0) diopter. Longitudinal chromatic aberration may make a significant contribution to image blur.

Animals↗
PubMed1985

The magnitude of longitudinal chromatic aberration of the human eye between 458 and 633 nm.

The longitudinal chromatic aberration (LCA) of the human eye was determined between wavelengths 458 and 633 nm for ten observers by incorporating Argon and Helium-Neon Lasers into a Badal optometer system such that tonic (or "dark focus") resting positions of accommodation could be measured under darkroom conditions. A mean chromatic range of 1.87 D was found between 488 and 633 nm. The range increased slightly to 1.91 D when the experiment was repeated under cycloplegia on a subset of three subjects. Additional wavelengths (458 and 476 nm) for a further subset (four subjects) increased the range to 2.65 D. This magnitude of LCA would support recent predictions based on Abbe and Pulfrich refractometry analyses of the dispersion of the human crystalline lens and ocular media.

Accommodation, Ocular↗
PubMed1986

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.

Color Perception↗
PubMed1987

Dioptric and non-dioptric stimuli for accommodation: target size alone and with blur and chromatic aberration.

The frequency response of the accommodative system (0.05-1 Hz) was determined for various combinations of stimuli: changing target size was presented alone, together with defocus blur, and with both defocus blur and chromatic aberration. A high-speed infrared optometer monitored accommodation while the subject viewed the target in a Badal optometer. Target size was varied sinusoidally and blur was provided by moving the target towards and away from the subject at the same frequency. Chromatic aberration was controlled by using either monochromatic (590 nm) or white (3300 K) light. The target was presented under open-loop conditions when size was the only stimulus. We find that besides the conventional dioptric stimuli, changes in target size that result in changes in apparent distance can have substantial effects on accommodation.

Accommodation, Ocular↗
PubMed1987

A central binocular mechanism affects chromatic adaptation.

Two experiments explored the role of central binocular mechanisms in color perception. The first experiment examined the effect of adapting to simultaneous, binocularly fused fields. Each eye adapted to a slowly flickering (0.5 Hz) long-wavelength light. The two eyes were adapted either inphase (both eyes stimulated at the same moment) or out-of-phase (only one eye stimulated at any given moment). Both adapting procedures shifted equilibrium yellow toward longer wavelengths, but a significantly greater shift was found when adapting light stimulated both eyes simultaneously. This reveals that a central binocular mechanism affects chromatic adaptation. The second experiment tested whether the binocular mechanism could shift equilibrium yellow measurements made with both eyes (identical, binocularly fused fields presented to each eye) outside of the range of measurements established by left-eye monocular viewing and right-eye monocular viewing. Differences were found between monocular left-eye and monocular right-eye color appearance under conditions of moderate chromatic adaptation, but binocularly fused measurements fell within the range established by the monocular results. This is consistent with the view that central mechanisms serve to keep the two eyes in balance, rather than systematically alter color appearance from colors perceived under monocular viewing.

Adaptation, Ocular↗
PubMed1990

The influence of stimulus size on newborns' discrimination of chromatic from achromatic stimuli.

We used an habituation procedure to explore newborns' ability to detect successive changes in luminance and based on those data, their ability to discriminate chromatic from achromatic stimuli. Newborns were very insensitive to successive changes in luminance: after habituating to a white square of given luminance, they showed no evidence of dishabituating when the luminance of the square increased or decreased by 0.37 log cd/m2, but dishabituated only to the next larger difference of 0.53 log cd/m2. Moreover, after newborns were habituated to a series of white squares that varied widely in luminance, they did not react when the luminance of the square was increased as much as 0.71 log cd/m2. In the color vision experiments, infants were habituated to a series of white squares of varying luminance and then tested with a chromatic square and with a white square of novel luminance. The size of the squares was also varied. The results showed that newborns discriminated 8 deg red (lambda peak = 650 nm) and 16 deg green (lambda peak = 540 nm) squares from white but required a larger stimulus (16 deg) to demonstrate the discrimination of yellow (lambda peak = 585 nm) from white. In addition, newborns showed no evidence of discriminating a 32 deg blue (lambda peak = 470 nm) square from white. Thus, although the results imply that newborns are at least dichromats, they also show that their color vision mechanisms are immature, particularly those operating in the blue and yellow spectral regions.

Color Perception↗
PubMed1990

Discrimination of luminance and chromaticity differences by dichromatic and trichromatic monkeys.

Dichromatic and trichromatic representatives from two genera of platyrrhine monkeys that show widespread color vision polymorphism (Saguinus--tamarins, Saimiri--squirrel monkeys) were tested for their abilities to make increment-threshold and flicker discriminations based on luminance and chromaticity differences. The details of these tests were arranged to attempt to emphasize the relative contributions to visual behavior of non-opponent and spectrally-opponent neural mechanisms. The results indicate that dichromatic and trichromatic monkeys differ only trivially on tests where performance is based on the contributions of non-opponent mechanisms, that the contribution of spectrally opponent mechanisms to the "brightness signal" is very similar in trichromatic and dichromatic monkeys, and that in increment-threshold discriminations where there are both chromaticity and luminance cues some test wavelengths yield superior performance for trichromats while others appear to favor the dichromat.

Animals↗
PubMed1990

Texture segregation with luminance and chromatic contrast.

Preattentive texture discrimination was investigated using low spatial frequency texture elements. The contrast between the texture elements and the background was either purely luminance or purely chromatic, or some combination of both these types of contrast. The threshold to discriminate correctly the location of a different textured region was obtained from each subject, as was each subject's threshold to detect the elements of the texture. Using the ratio of texture to element detection as a measure of the effectiveness of texture discrimination, little difference could be found between the perception of luminance or chromatic texture. However, there were large and significant variations among subjects with otherwise normal colour vision.

Adaptation, Ocular↗
PubMed1992

The influence of chromatic aberration on the static accommodative response.

Previous measurements of static accommodation have consistently shown steady state errors over most of the range; the response lags below the stimulus and, at low levels, the response leads the stimulus. A series of experiments is presented in which the longitudinal and, for the first time, transverse chromatic aberrations of the eye were varied and the resultant stimulus-response functions of accommodation were measured. The results show that the steady state error of accommodation is not influenced by manipulations of the magnitude or the direction of either longitudinal or transverse chromatic aberration. This indicates that a particular wavelength is not preferentially focussed on the retina as a function of stimulus level and supports the negative feedback theory of accommodation.

Accommodation, Ocular↗
PubMed1992

Color perception within a chromatic context: changes in red/green equilibria caused by noncontiguous light.

We measured changes in the color appearance of one light caused by another light presented in a well-separated region. Observers viewed a 1 degrees test field superimposed on a 3 degrees, 540 or 660 nm adapting field (32 or 320 td). The change in appearance due to noncontiguous light was determined by surrounding the 3 degrees adapting field with a continguous 3 degrees i.d., 5 degrees o.d. ring of either 32 or 320 td. The ring was 540, 660 nm or achromatic (tungsten-halogen "white"). The test was an admixture of 549 and 660 nm light, and varied from 6 to 1000 td. The observer adjusted the ratio of 549 to 660 nm test light so the test appeared neither reddish nor greenish. A 540 or 660 nm ring had a chromatic inducing effect on the small test that mimicked a simple surround contiguous with the test. Results with an achromatic ring were more complex: an isolated achromatic ring (no adapting field present) had virtually no effect on the color appearance of the test, but the same achromatic ring surrounding a chromatic adapting field shifted the test toward the color appearance of the adapting light (e.g. introducing a "white" ring surrounding a "green" adapting field shifted the test toward greenness). A thin pencil-width band of "white" light superimposed on a larger 5 degrees adapting field had an effect similar to a "white" 3-5 degrees ring. These results demonstrate (1) strong effects of the remote noncontiguous lights and (2) that the change in color appearance they cause is not a simple function of only the light in the noncontinguous region. The change depends on other lights in view. The visual processes revealed in these experiments are considered in terms of inferred illumination and surface reflectances of objects in natural scenes.

Adaptation, Ocular↗
PubMed1992

Heterochromatic Fusion Nystagmus: its use in estimating chromatic equiluminance in humans and monkeys.

The use of chromatic patterns that are equated for luminance has become increasingly popular in psychophysical and neurophysiological studies of visual processing. The currently available techniques for equating different colors for brightness rely upon human reports of perceptual events that are reduced at some luminance ratio. We report here the results of a study using a technique we have recently developed that produces a vivid and compelling motion percept only at isoluminance. That is, unlike previous methods, this technique relies upon a perceptual event (motion) that actually becomes more salient at isoluminance. We also observed that the optokinesis generated by the moving pattern mirrors the perceptual reports at all luminance ratios. If used in this manner, the technique can provide an estimate of chromatic isoluminance in a variety of species and can be used to corroborate a human subject's perceptual experience.

Animals↗