Cone Ratio Research Articles

Can spatial resolution reveal individual differences in the L:M cone ratio?

We measured spatial resolution in the parafovea for targets designed to isolate either the long-wavelength (L) or the middle-wavelength (M) cones. Landolt C optotypes were presented for 100ms on a calibrated monitor at an eccentricity of 5° to the left or right of fixation. There were large individual differences in the ratio of the resolution obtained with L targets to that obtained with M targets, and we suggest that these differences reflect variations in the relative sampling densities of L and M cones in the parafovea. In Experiment 1, we measured contrast thresholds for targets of varying size. Among 10 unselected observers, there was a threefold variation in the ratio of the contrast thresholds for the smallest targets. In Experiments 2 and 3, we held contrast constant and we varied size, in order to establish the minimal target that could be discriminated for each of the two classes of cone. In Experiment 2, two groups of observers, selected on the basis of their settings on a flicker-photometric test, showed a highly significant difference in the ratio of the M and L acuities on the spatial task. In Experiment 3, female carriers of protan or deutan deficiencies, classified only on the basis of their sons’ phenotypes, also showed a large difference in the ratio of their acuities for M and L targets. In all three experiments, there was a strong correlation between the ratio of M and L spatial acuities and a flicker-photometric measure of relative sensitivity to long- and middle-wavelength light.

Generalized symmetric weight assignment technique: Incorporating managerial preferences in data envelopment analysis using a penalty function

In this paper, we generalize the Symmetric Weight Assignment Technique to incorporate all managerial preferences in Data Envelopment Analysis (DEA). This is a method that promotes managerial preferences, while not changing the feasibility region of the nominal DEA model, unlike standard techniques such as cone ratio and assurance region that potentially yield infeasible problems. We discuss how this generalization is motivated by a real word problem where the decision maker's preference is not precisely known. We also discuss how we are using our generalization in solving a workforce allocation problem for the United States Navy.

Measuring the Balanced Efficiency of Decision Making Units

Cone ratios, assurance regions and similar data envelopment analysis (DEA) approaches measuring the efficiency of decision making units (DMU) restrict their ability to “show themselves in the most favorable light”. Indeed they limit the relations or the multipliers of inputs and outputs as efficiency factors and thus induce a certain balance among them. We develop an alternative approach which – based on the conventional DEA methodology and additionally to the usual efficiency score – assesses a degree of balance or balanced efficiency for each DMU. For that purpose we assume that a region within the data envelopment can be specified in which all DMUs are 100% balanced. In a case study, measuring the balanced effectiveness of German business schools’ research performance, we obtain plausible results providing informative insights into German business research. This confirms the reasonable use of the new method.

Individual differences in cone ratio: Measurements by counterphase modulation photometry and by spatial acuity

Individual differences in cone ratio: Measurements by counterphase modulation photometry and by spatial acuity

Noninvasive imaging of the human rod photoreceptor mosaic using a confocal adaptive optics scanning ophthalmoscope

The rod photoreceptors are implicated in a number of devastating retinal diseases. However, routine imaging of these cells has remained elusive, even with the advent of adaptive optics imaging. Here, we present the first in vivo images of the contiguous rod photoreceptor mosaic in nine healthy human subjects. The images were collected with three different confocal adaptive optics scanning ophthalmoscopes at two different institutions, using 680 and 775 nm superluminescent diodes for illumination. Estimates of photoreceptor density and rod:cone ratios in the 5°–15° retinal eccentricity range are consistent with histological findings, confirming our ability to resolve the rod mosaic by averaging multiple registered images, without the need for additional image processing. In one subject, we were able to identify the emergence of the first rods at approximately 190 μm from the foveal center, in agreement with previous histological studies. The rod and cone photoreceptor mosaics appear in focus at different retinal depths, with the rod mosaic best focus (i.e., brightest and sharpest) being at least 10 μm shallower than the cones at retinal eccentricities larger than 8°. This study represents an important step in bringing high-resolution imaging to bear on the study of rod disorders.

Transmission of colour and acuity signals by parvocellular cells in marmoset monkeys

The red-green axis of colour vision evolved recently in primate evolutionary history. Signals serving red-green colour vision travel together with signals serving spatial vision, in the parvocellular (PC) division of the subcortical visual pathway. However, the question of whether receptive fields of PC pathway cells are specialized to transmit red-green colour signals remains unresolved. We addressed this question in single-cell recordings from the lateral geniculate nucleus of anaesthetized marmosets. Marmosets show a high proportion of dichromatic (red-green colour-blind) individuals, allowing spatial and colour tuning properties of PC cells to be directly compared in dichromatic and trichromatic visual systems. We measured spatial frequency tuning for sine gratings that provided selective stimulation of individual photoreceptor types. We found that in trichromatic marmosets, the foveal visual field representation is dominated by red-green colour-selective PC cells. Colour selectivity of PC cells is reduced at greater eccentricities, but cone inputs to centre and surround are biased to create more selectivity than predicted by a purely 'random wiring' model. Thus, one-to-one connections in the fovea are sufficient, but not necessary, to create colour-selective responses. The distribution of spatial tuning properties for achromatic stimuli shows almost complete overlap between PC cells recorded in dichromatic and trichromatic marmosets. These data indicate that transmission of red-green colour signals has been enabled by centre-surround receptive fields of PC cells, and has not altered the capacity of PC cells to serve high-acuity vision at high stimulus contrast.

A realistic large-scale model of the retinal cone mosaic

Event Abstract Back to Event A realistic large-scale model of the retinal cone mosaic Yoshimi Kamiyama1*, Daiki Sone1 and Shiro Usui2 1 Aichi Prefectural University, Information Science and Technology, Japan 2 RIKEN Brain Science Institute, Japan The physiology and anatomy of the retina are relatively well known. Physiological studies of the retina have uncovered a number of cellular and subcellular mechanisms such as the characteristics of the ion channels found in retinal neurons. Anatomical studies have revealed the morphological principles hidden in the structure of the retina. It is as yet impossible to reconstruct a complete model of the retina. But it is now possible to identify some computational operations that the retina performs and to relate them to specific physiological and anatomical mechanisms, on the basis of neuroinformatics. In the present study, a realistic model of the retinal cone mosaic is proposed. The spatial arrangement of the photoreceptors, i.e. photoreceptor mosaic, samples the continuous image of the world that is focused on the retina and transforms the image into a discrete array of signals that is transmitted to higher stages in the visual information processing. Color vision is mediated by three types of cone photoreceptors, the L, M and S cones. These cones have the function of converting absorbed photons into neural signals with different peak sensitivities at long (L), medium (M) and short (S) wavelengths. The cones distribute nonuniformly: cone density decreases with distance from the fovea, the L/M cone ratio are highly variable between individuals, and S cones are absent from the center of the fovea. We developed a large-scale computational model of the cone photoreceptors based on these physiological and anatomical characteristics. The model incorporated the spectral sensitivities of three types of cones, as well as the nonuniform spatial distributions of cones. The cone mosaic was generated by a stochastic algorithm to reproduce the nonuniformity. The chromatic light response was modeled by the spectral sensitivity and the ordinary differential equations of the membrane dynamics. The present model covers a visual angle of 60 degrees with two million cones. The model was implemented on the Riken Integrated Cluster of Clusters with 1024 nodes. In simulation, the model reproduced the nonuniform distribution of cones similar to the anotomical data. The model also well reproduced the chromatic light responses of the cones. In conclusion, the present model makes it possible to analyze how color information is processed in the retinal cone mosaic. Figure 1 Keywords: digital atlasing, Large scale modeling Conference: 4th INCF Congress of Neuroinformatics, Boston, United States, 4 Sep - 6 Sep, 2011. Presentation Type: Demo Presentation Topic: Digital atlasing Citation: Kamiyama Y, Sone D and Usui S (2011). A realistic large-scale model of the retinal cone mosaic. Front. Neuroinform. Conference Abstract: 4th INCF Congress of Neuroinformatics. doi: 10.3389/conf.fninf.2011.08.00120 Copyright: The abstracts in this collection have not been subject to any Frontiers peer review or checks, and are not endorsed by Frontiers. They are made available through the Frontiers publishing platform as a service to conference organizers and presenters. The copyright in the individual abstracts is owned by the author of each abstract or his/her employer unless otherwise stated. Each abstract, as well as the collection of abstracts, are published under a Creative Commons CC-BY 4.0 (attribution) licence (https://creativecommons.org/licenses/by/4.0/) and may thus be reproduced, translated, adapted and be the subject of derivative works provided the authors and Frontiers are attributed. For Frontiers’ terms and conditions please see https://www.frontiersin.org/legal/terms-and-conditions. Received: 17 Oct 2011; Published Online: 19 Oct 2011. * Correspondence: Dr. Yoshimi Kamiyama, Aichi Prefectural University, Information Science and Technology, Nagakute, Japan, kamiyama@ist.aichi-pu.ac.jp Login Required This action requires you to be registered with Frontiers and logged in. To register or login click here. Abstract Info Abstract The Authors in Frontiers Yoshimi Kamiyama Daiki Sone Shiro Usui Google Yoshimi Kamiyama Daiki Sone Shiro Usui Google Scholar Yoshimi Kamiyama Daiki Sone Shiro Usui PubMed Yoshimi Kamiyama Daiki Sone Shiro Usui Related Article in Frontiers Google Scholar PubMed Abstract Close Back to top Javascript is disabled. Please enable Javascript in your browser settings in order to see all the content on this page.

The effect of cone ratio on the separation efficiency of small diameter hydrocyclones

In this study, the effect of cone ratio (ratio of the vortex finder diameter to the apex diameter, D o/D u) on by-pass was investigated for small diameter hydrocyclones. Experimental studies were carried out in a closed circuit laboratory test rig for a wide range of operational parameters, such as diameter of cyclone, vortex finder and apex, inlet pressure and solids content of feed pulp. In the tests, 25.4 mm and 50.8 mm diameter cyclones were used with different vortex and apex units. The results showed that the cone ratio is an important design parameter for cyclones.

L‐ and M‐cone input to 12Hz and 30Hz flicker ERGs across the human retina

We recorded L- and M-cone isolating ERGs from human subjects using a silent substitution technique at temporal rates of 12 and 30 Hz. These frequencies isolate the activity of cone-opponent and non-opponent post-receptoral mechanisms, respectively. ERGs were obtained using a sequence of stimuli with different spatial configurations comprising; (1) circular stimuli of different sizes which increased in 10° steps up to 70°diameter, or (2) annular stimuli with a 70° outer diameter but with different sized central ablations from 10° up to 60°. L- and M-cone isolating ERGs were obtained from five colour normal subjects using a DTL fibre electrode. Fourier analysis of the ERGs was performed and we measured the amplitude of the first harmonic of the response. For 12 Hz ERGs the L:M cone response amplitude ratio (L:M(ERG)) was close to unity and remained stable irrespective of the spatial configuration of the stimulus. The maintenance of this balanced ratio points to the existence of cone selective input across the human retina for the L-M cone opponent mechanism. For 30 Hz the L:M(ERG) ratio was greater than unity but varied depending upon which region of the retina was being stimulated. This variation we consider to be a consequence of the global response properties of M-cone ERGs rather than representing a real variation in L:M cone ratios across the retina.

Epigenetic control of expression of the human L‐ and M‐ pigment genes

Epigenetics alters gene expression by chromatin modification without changing the sequence of DNA. DNA methylation is an essential signal for epigenetic gene regulation. Methylation of cytosine bases at CpG dinucleotides in DNA results in chromatin condensation resulting in suppression of gene expression. DNA methylation has been shown to play important roles in cell differentiation, genomic imprinting and X-chromosome inactivation. We compared the CpG methylation patterns of the promoters of the L-opsin gene (OPN1LW) and the M-opsin gene (OPN1MW), plus a DNase I hypersensitive (DHS) site located about 8 kb (kilobases) upstream of the OPN1LW gene. Comparisons were made using the human retinoblastoma cell line WERI, which expresses the L and M opsin genes when treated with thyroid hormone (T3), and a lymphoblastoid cell line GM06990 that does not express these genes. The results showed that the great majority of the 14 CpGs located within the proximal 200 bp (base pairs) of each promoter, plus 20 bp of the 5'-untranslated region, were hypo-methylated in WERI-Rb-1 cells, whether or not treated with T3, but almost totally methylated in the lymphoblastoid cell line. Three of the CpGs that are located beyond 200 bp from the transcription start site of OPN1LW were hyper-methylated in both WERI and lymphoblastoid cells. Significant differential methylation was also observed within the DHS region (24 CpGs). This DHS region contains a highly conserved motif that binds CCCTC-binding factor (CTCF), referred to as a 'chromatin insulator or boundary element', that has been shown to regulate gene expression at several genome locations. The results suggest that DNA methylation is likely to contribute to regulation of expression of the L- and M-opsin genes during differentiation, as well as to the retinal L:M cone ratio. In addition, thyroid hormone induction of the opsin genes does not appear to alter DNA methylation.

Effect of pine cone ratio on the wettability and surface roughness of particleboard

In this study, the effect of pine cone ratio on wettability and surface roughness of particleboards was examined. Contact angles of water on the produced samples were measured with a goniometer. The surface roughness of the samples was determined with a fine stylus tracing technique. Particleboards made from 100% wood particle had the lowest average contact angle (95.6°), but the highest was for the particleboards containing 50% pine cone (116.3°). Average surface roughness was higher for samples containing a higher amount of pine cone in the mixture. The smoothest surface (9.77 μm Ra) was observed when panels were produced using 100% wood particles. On the other hand, the roughest surface (15.50 μm Ra) was found for the samples containing 50% cone particles in the mixture. Rmax and Rz parameters had similar trends to the Ra values. Increasing the pine cone ratio in the mixture negatively affected the contact angle and surface roughness parameters of the particleboard.

Estimation of L: M Cone Ratio from ERG, Adaptive Optics, and Color Naming Methods

Estimation of L: M Cone Ratio from ERG, Adaptive Optics, and Color Naming Methods

The span of cone ratios and color naming

It has been suggested that ratios of photoreceptor catches for the light reflected by pairs of surfaces play an important role in color vision (Foster & Nascimento, 1994). We provide results from simulations showing that L, M and S ratios obtained from surface pairs consisting of a colored surface and an achromatic surface of the same luminosity reach their extreme values for a few specific colored surfaces, namely red, green, blue and yellow surfaces. There is a striking correspondence between the colors singled out in this way and the ones singled out in cross-cultural studies of color naming (Regier, Kay & Cook, 2005). These results will be compared with a related approach proposed in Philipona & O'Regan (in press), which further addressed the question of unique hues and hue cancellation. References - Nascimento Foster. (1994). Relational colour constancy from invariant cone-excitation ratios. Proc. R. Soc. Lond. B. 257, 115-121. - Kay Regier Cook. (2005). Focal colors are universal after all. Proc. Natn. Acad. Sci. USA. 102, 8386-8391. - O'Regan Philipona (in press). Color naming, unique hues and hue cancellation predicted from singularities in reflection properties. Vis. Neurosci.

A diagnostic tool for basic daily quality assurance of a tomotherapy Hi•Art machine

To investigate and evaluate the use of an in‐house developed diagnostic software tool using the imaging detector data for a quick daily quality assurance check of the output (dose) and lateral profile (cone) of a tomotherapy Hi•Art system. The Hi•Art treatment system is a radiation therapy machine for delivering intensity modulated radiation therapy (IMRT) in a helical fashion with an integrated CT scanner used for improved patient positioning before treatment. Since the system was developed specifically for IMRT, fat fields can be obtained by modulating the beam and therefore the fattening filter could be omitted. Because of this, the field has a cone‐like profile in both lateral and transversal directions. Patients are treated in a helical fashion with a tight pitch and a constant gantry rotation speed, while modulation is performed by a binary MLC. Consequently dose output per time‐unit (dose rate) as well as the shape of the cone‐profile are very important for correct patient treatment and should be closely monitored. However, using the company‐provided initial tools and conventional dosimetry, this can be a time consuming daily procedure. The aim of this work is to develop a fast, automated method of quality assurance based on the detector signal. A software tool called “tomocheck” running on the operation station has been developed to evaluate the output (dose rate) and the lateral cone profile (energy) of the Hi•Art system, comparing actual output and cone profile with a reference (previously approved against ionization chamber measurements). This is done by using the data of the 640 on‐board detector array that are directly retrieved and processed after a specific QA procedure. The detector file consists of the CT detector data and the three monitoring dose chamber readings over a time period of 200 sec. To evaluate the method, the system was benchmarked against ionization chamber measurements and classical IMRT QA methods. Action levels (final status “NOT ACCEPTED”) for dose ratio as well as the cone ratio are set to ±2%. The QA tool was introduced for daily QA in May 2007. For the following 24 months, a total of 931 morning checks was made on both tomotherapy machines. In 42 cases the check status was “NOT ACCEPTED”. In 34 cases the dose ratio (DR) was out of tolerance. The corrected cone ratio (CCR) was outside of specification tolerance in 8 cases. The tomocheck data was related to the ionization chamber measurements for the IMRT plan indicating a close relationship between the CCR and the off‐axis measurements. Average dose ratio against the mean value of the on‐ and off‐axis IC measurement indicates that this parameter is a good interpretation of the dose output. This tool makes it possible to perform an easy‐to‐use and fast basic daily quality assurance check featuring an output as well as an energy evaluation. Ideally this tool should offer also the combined dosimetry check of jaw width, couch speed, leaf latency, output, leaf/gantry synchrony, and lasers. This will be investigated in the future.PACS 87.55Qr

Normality of colour vision in a compound heterozygous female carrying protan and deutan defects

Background: Inherited red‐green colour vision defects are quite common, affecting one in 12 males, but are less common in women, affecting about one in 250. Because red‐green defects are X‐linked, nearly 15 per cent of females are heterozygous carriers of red‐green colour deficiency. In addition, about one in 150 females are ‘double carriers’, where both of their X chromosomes have L/M gene arrays encoding a red‐green defect. If a woman carries the same type of colour vision defect on each X‐chromosome, she will be red‐green colour deficient, whereas if she carries opposing defects (protan versus deutan) on each X chromosome, she will have normal colour vision, owing to the process of X‐inactivation. These women are referred to as compound heterozygotes, though very few have been reported. Questions remain about whether the colour vision capacity of these women is comparable to that of ‘normal’ trichromats.Methods: We examined a compound heterozygote carrier of both protanopia and deuteranomaly. We also examined male members of her family representing both forms of red‐green defect carried by the female proband. Complete colour vision testing was done, including Rayleigh matches, pseudoisochromatic plates, unique hue measurements and 100‐Hue tests. Flicker‐photometric ERG estimates of L : M cone ratio were obtained, as were Medmont C100 settings.Results: Genetic analyses provided direct confirmation of compound heterozygosity. The compound heterozygote showed Schmidt's sign, consistent with an extreme skew in her L : M cone ratio and usually associated with protan carrier status.Conclusion: Apart from Schmidt's sign, we found the colour vision of the compound heterozygote to be indistinguishable from that of a normal trichromat.

Chromatic perceptive field sizes measured at 10° eccentricity along the horizontal and vertical meridians

The different hemifields in the retina are known to vary in photoreceptor density as well as in the number of photoreceptors converging onto one ganglion cell. The effect of these differences among the retinal hemifields at 10 degrees retinal eccentricity was investigated using a color-naming procedure to derive perceptive field sizes for the hue terms of blue, green, yellow, and red. Color-naming data were obtained under two conditions: (1) after a bleach condition, chosen to minimize rod contribution, and (2) after 30 min dark adaptation, chosen to maximize rod contribution. Perceptive field sizes measured in the bleach condition were consistent with degree of neural convergence of cones to ganglion cells across the retina rather than differences in cone density. Rod densities relative to cone densities correlated with the size of perceptive fields in the no-bleach condition, i.e., the greater the rod:cone ratio, the larger the perceptive field.

The topography of cone photoreceptors in the retina of a diurnal rodent, the agouti (Dasyprocta aguti)

The presence, density distribution, and mosaic regularity of cone types were studied in the retina of the diurnal agouti, Dasyprocta aguti. Longwave-sensitive (L-) and shortwave-sensitive (S-) cones were detected by antibodies against the respective cone opsins. L- and S-cones were found to represent around 90 and 10% of the cone population, respectively. There was no evidence for L- and S-opsin coexpression in agouti cones. L-cone densities were highest, up to 14,000/mm2, along a horizontal visual streak located about 2-3 mm dorsal to the optic nerve, and the L-cone distribution showed a dorsoventral asymmetry with higher densities in ventral (about 10,000/mm2) than in dorsal (about 4000/mm2) retinal regions. This L-cone topography parallels the agouti's ganglion cell topography. S-cones had a peak density of 1500-2000/mm2 in the central retinal region but did not form a visual streak. Their distribution also showed a dorsoventral asymmetry with densities around 600/mm2 in dorsal and around 1000/mm2 in ventral retinal regions. The patterning of cone arrays was assessed by the density recovery profile analysis. At all eccentricities evaluated, the S-cone mosaic less efficiently packed than the L-cone mosaic. Rod densities ranged from 47,000/mm2 in peripheral to 64,000/mm2 in central retina, and rod:cone ratios were 4:1-9:1. The comparatively low rod density and high cone proportion appear well adapted to the diurnal lifestyle of the agouti.

Retinal neurogenesis and ontogenetic changes in the visual system of the brown banded bamboo shark, Chiloscyllium punctatum (hemiscyllidae, elasmobranchii)

The development of the eye of the oviparous brown banded bamboo shark, Chiloscyllium punctatum, was monitored from egg deposition through adulthood. The order and timing of retinal cell differentiation were assessed by light and transmission electron microscopy. As in other vertebrates, the ganglion cells are the first to differentiate, in this case by 81 days post-egg deposition (dpd). The order then deviates from what is typically quoted for vertebrates, with the Müller and amacrine cells differentiating morphologically around the same time, followed by the bipolar cells (101 dpd) and finally the horizontal cells and photoreceptors (124 dpd). The neural retina is fully differentiated and synaptic connections are formed approximately 1 month prior to hatching, which occurs at about 158 dpd. The mature retina is duplex, with a peak rod to cone ratio of approximately 12:1. The eye and lens of C. punctatum continue to grow throughout life and become less aspherical with growth; the equatorial (nasotemporal) lens diameter is 12% larger than the axial (anterior-posterior) lens diameter in embryos and 8% larger in adults. Access to developmental stages and the protracted gestational period of C. punctatum make it a highly valuable model for developmental studies of the visual system. This study also provides an evolutionary perspective on retinal neurogenesis in an elasmobranch.

What limits chromatic sensitivity in normal and colour deficient observers?

Abstract Purpose Variability in chromatic sensitivity was assessed in normal trichromats (n=133) and in subjects with deutan (n= 106) and protan (n=48) deficiency. The purpose of this study was to quantify variability within normal colour vision and to determine how this impacts the assessment of minimum colour vision requirements within occupational environments. Methods Rayleigh matches were modelled with the aim of explaining the locations of match midpoints and ranges. Model parameters included the wavelength of peak sensitivity of cone photopigments, the photopigment optical density and the noise amplitude in the red/green colour channel. In order to avoid the suprathreshold, perceptual effects of extreme L:M cone ratios on colour vision, selective postreceptoral amplification of cone signals is needed. The noise is also amplified and this causes corresponding changes in red/green threshold sensitivity. Results We measured midpoint and matching range in anomaloscope matches and red/green and yellow/blue thresholds under conditions that isolate the use of colour signals. A subset of subjects with deutan deficiency that exhibited less common Nagel matches were selected for genetic analysis of their cone pigment genes to predict the corresponding peak wavelength separation of their two, long‐wavelength cone pigments. Conclusion The model predicts accurately the midpoint and the range for the spectral differences specified by the genes. The model also predicts the “normal” matches made by some subjects that rely on two hybrid genes and therefore exhibit red/green thresholds outside the normal range, typical of mild deuteranomaly. The anomaloscope is therefore not by itself sufficient to classify with certainty the colour deficiency involved.

Avoiding commitment: cathemerality among primates

Cathemeral animals distribute their activity more or less uniformly between the dark and light phases of the 24-hour day. This activity pattern is quite widely observed among major mammal taxa, although the number of species involved is relatively low. Among primates, cathemerality is seen principally, though not uniquely, in the strepsirhine (“lower primate”) family Lemuridae; it is ubiquitous in the genus Eulemur, among species of which it seems to be a stable activity pattern with a history going back at least 8 – 12 million years. Nocturnal primates have visual systems that accentuate sensitivity to light over visual acuity, showing large cornea size, high rod:cone ratios and large numbers of photoreceptors relative to ganglion cells. Diurnal forms do the reverse, while Eulemur species are intermediate in these respects, suggesting substantial accommodation of the visual system to the cathemeral lifestyle. Studies of one Eulemur species have revealed that its cathemerality is underpinned by a robust endogenous circadian timing mechanism with a free-running periodicity of over 25 hours. The lemurs appear to be entrained to the 24-hour day by the light/dark cycle, plausibly with dusk as the primary zeitgeber. The causes of cathemerality have been sought in such environmental influences as predator avoidance, energy conservation, dietary quality, and interspecies feeding competition. All of these factors seem to be relevant to the cathemeral lifestyle, but none appears to be a crucial determinant. Nothing is known of the underlying neural mechanisms that facilitate the lemurs' accommodation to the extremes to which 24-hour activity exposes them.