Saccadic oscillations facilitate ocular perfusion from the avian pecten

THE evolution of the eye is constrained by two conflicting requirements--good vascular perfusion of the retina, and an optical path through the retina that is unobstructed by blood vessels. Birds are interesting in that they have higher metabolic rates and thicker retinas than mammals, but have no retinal blood vessels. Nutrients and oxygen must thus reach the neurons of the inner retina either from the choroid through 300 micron of metabolically very active retina, or from the pecten, a pleated vascular structure protruding from the head of the optic nerve into the vitreous chamber, and more than a centimetre away from some retinal neurons. Despite the diffusional distance involved, several lines of evidence indicate that the pecten is the primary source of nutrients for the inner retina: the presence of an oxygen gradient from pecten to retina, the large surface area produced by macroscopic folds and by microscopic infoldings of the luminal and external surfaces of the capillary endothelium, extrusion of circulating fluorescein, high content of carbonic anhydrase and alkaline phosphatase, and retinal impairments after pecten ablation. Another peculiarity of birds, their saccadic oscillations, occur with a large cyclotor-sional component during every saccadic eye movement. In different species, saccades, which occur at intervals of 0.5-40 s, have up to 13 oscillations with frequencies of 15-30 Hz and ampliá-tudes of about 10 degrees. Therefore, as much as 12% of some birds' total viewing time may be subject to the image instability caused by the oscillations. Using fluorescein angiography, we show here that during every saccade, the pecten acts as an agitator which propels perfusate towards the central retina much more effectively than is observed during intersaccadic intervals.

Slow binocular rivalry in bipolar disorder

BACKGROUND

The rate of binocular rivalry has been reported to be slower in subjects with bipolar disorder than in controls when tested with drifting, vertical and horizontal gratings of high spatial frequency.

METHOD

Here we assess the rate of binocular rivalry with stationary, vertical and horizontal gratings of low spatial frequency in 30 subjects with bipolar disorder, 30 age- and sex-matched controls, 18 subjects with schizophrenia and 18 subjects with major depression. Along with rivalry rate, the predominance of each of the rivaling images was assessed, as was the distribution of normalized rivalry intervals.

RESULTS

The bipolar group demonstrated significantly slower rivalry than the control, schizophrenia and major depression groups. The schizophrenia and major depression groups did not differ significantly from the control group. Predominance values did not differ according to diagnosis and the distribution of normalized rivalry intervals was well described by a gamma function in all groups.

CONCLUSIONS

The results provide further evidence that binocular rivalry is slow in bipolar disorder and demonstrate that rivalry predominance and the distribution of normalized rivalry intervals are not abnormal in bipolar disorder. It is also shown by comparison with previous work, that high strength stimuli more effectively distinguish bipolar from control subjects than low strength stimuli. The data on schizophrenia and major depression suggest the need for large-scale specificity trials. Further study is also required to assess genetic and pathophysiological factors as well as the potential effects of state, medication, and clinical and biological subtypes.

The distribution and morphological characteristics of serotonergic cells in the brain of monotremes

The distribution and cellular morphology of serotonergic neurons in the brain of two species of monotremes are described. Three clusters of serotonergic neurons were found: a hypothalamic cluster, a cluster in the rostral brainstem and a cluster in the caudal brainstem. Those in the hypothalamus consisted of two groups, the periventricular hypothalamic organ and the infundibular recess, that were intimately associated with the ependymal wall of the third ventricle. Within the rostral brainstem cluster, three distinct divisions were found: the dorsal raphe nucleus (with four subdivisions), the median raphe nucleus and the cells of the supralemniscal region. The dorsal raphe was within and adjacent to the periaqueductal gray matter, the median raphe was associated with the midline ventral to the dorsal raphe, and the cells of the supralemniscal region were in the tegmentum lateral to the median raphe and ventral to the dorsal raphe. The caudal cluster consisted of three divisions: the raphe obscurus nucleus, the raphe pallidus nucleus and the raphe magnus nucleus. The raphe obscurus nucleus was associated with the dorsal midline at the caudal-most part of the medulla oblongata. The raphe pallidus nucleus was found at the ventral midline of the medulla around the inferior olive. Raphe magnus was associated with the midline of the medulla and was found rostral to both the raphe obscurus and raphe pallidus. The results of our study are compared in an evolutionary context with those reported for other mammals and reptiles.

The distribution and morphological characteristics of catecholaminergic cells in the brain of monotremes as revealed by tyrosine hydroxylase immunohistochemistry

The present study describes the distribution and cellular morphology of catecholaminergic neurons in the CNS of two species of monotreme, the platypus (Ornithorhynchus anatinus) and the short-beaked echidna (Tachyglossus aculeatus). Tyrosine hydroxylase immunohistochemistry was used to visualize these neurons. The standard A1-A17, C1-C3 nomenclature was used for expediency, but the neuroanatomical names of the various nuclei have also been given. Monotremes exhibit catecholaminergic neurons in the diencephalon (A11, A12, A13, A14, A15), midbrain (A8, A9, A10), rostral rhombencephalon (A5, A6, A7), and medulla (A1, A2, C1, C2). The subdivisions of these neurons are in general agreement with those of other mammals, and indeed other amniotes. Apart from minor differences, those being a lack of A4, A3, and C3 groups, the catecholaminergic system of monotremes is very similar to that of other mammals. Catecholaminergic neurons outside these nuclei, such as those reported for other mammals, were not numerous with occasional cells observed in the striatum. It seems unlikely that differences in the sleep phenomenology of monotremes, as compared to other mammals, can be explained by these differences. The similarity of this system across mammalian and amniote species underlines the evolutionary conservatism of the catecholaminergic system.

The distribution and morphological characteristics of cholinergic cells in the brain of monotremes as revealed by ChAT immunohistochemistry

The present study employs choline acetyltransferase (ChAT) immunohistochemistry to identify the cholinergic neuronal population in the central nervous system of the monotremes. Two of the three extant species of monotreme were studied: the platypus (Ornithorhynchus anatinus) and the short-beaked echidna (Tachyglossus aculeatus). The distribution of cholinergic cells in the brain of these two species was virtually identical. Distinct groups of cholinergic cells were observed in the striatum, basal forebrain, habenula, pontomesencephalon, cranial nerve motor nuclei, and spinal cord. In contrast to other tetrapods studied with this technique, we failed to find evidence for cholinergic cells in the hypothalamus, the parabigeminal nucleus (or nucleus isthmus), or the cerebral cortex. The lack of hypothalamic cholinergic neurons creates a hiatus in the continuous antero-posterior aggregation of cholinergic neurons seen in other tetrapods. This hiatus might be functionally related to the phenomenology of monotreme sleep and to the ontogeny of sleep in mammals, as juvenile placental mammals exhibit a similar combination of sleep elements to that found in adult monotremes.

Spontaneous and stimulus-evoked intrinsic optical signals in primary auditory cortex of the cat

Spontaneous and tone-evoked changes in light reflectance were recorded from primary auditory cortex (A1) of anesthetized cats (barbiturate induction, ketamine maintenance). Spontaneous 0.1-Hz oscillations of reflectance of 540- and 690-nm light were recorded in quiet. Stimulation with tone pips evoked localized reflectance decreases at 540 nm in 3/10 cats. The distribution of patches "activated" by tones of different frequencies reflected the known tonotopic organization of auditory cortex. Stimulus-evoked reflectance changes at 690 nm were observed in 9/10 cats but lacked stimulus-dependent topography. In two experiments, stimulus-evoked optical signals at 540 nm were compared with multiunit responses to the same stimuli recorded at multiple sites. A significant correlation (P < 0.05) between magnitude of reflectance decrease and multiunit response strength was evident in only one of five stimulus conditions in each experiment. There was no significant correlation when data were pooled across all stimulus conditions in either experiment. In one experiment, the spatial distribution of activated patches, evident in records of spontaneous activity at 540 nm, was similar to that of patches activated by tonal stimuli. These results suggest that local cerebral blood volume changes reflect the gross tonotopic organization of A1 but are not restricted to the sites of spiking neurons.

Colour vision in billfish

Members of the billfish family are highly visual predatory teleosts inhabiting the open ocean. Little is known about their visual abilities in detail, but past studies have indicated that these fishes were likely to be monochromats. This study, however, presents evidence of two anatomically distinct cone types in billfish. The cells are arranged in a regular mosaic pattern of single and twin cones as in many fishes, and this arrangement suggests that the different cone types also show different spectral sensitivity, which is the basis for colour vision. First measurements using microspectrophotometry (MSP) revealed a peak absorption of the rod pigment at 484 nm, indicating that MSP, despite technical difficulties, will be a decisive tool in proving colour vision in these offshore fishes. When hunting, billfish such as the sailfish flash bright blue bars on their sides. This colour reflects largely in ultraviolet (UV) light at 350 nm as revealed by spectrophotometric measurements. Billfish lenses block light of wavelengths below 400 nm, presumably rendering the animal blind to the UV component of its own body colour. Interestingly, at least two prey species of billfish have lenses transmitting light in the UV waveband and are therefore likely to perceive a large fraction of the UV peak found in the blue bar of the sailfish. The possible biological significance of this finding is discussed.

Morphology of pyramidal neurones in cytochrome oxidase modules of the S-I bill representation of the platypus

The primary somatosensory cortex of the platypus (Ornithorhynchus anatinus) is characterized by a distinct array of functionally specific cytochrome oxidase (CO) modules, forming alternate CO-rich and CO-poor bands. In the current study, we undertook to establish whether the cellular morphology of layer V pyramidal neurones reflects this modular organization. To this end, we injected neurones with Lucifer Yellow in 250 microm thick, flat-mounted cortical slices and processed the tissue to reveal a light-stable reaction product. By aligning blood vessels in serial sections in which we injected individual neurones with sections processed for CO, we were able to establish the exact location of injected cells with respect to the pattern of CO bands. Pyramidal neurones in the CO-poor bands (which are responsive to both mechano- and electroreceptive stimuli) had basal dendritic fields that were larger than those in the CO-rich bands. The large basal dendritic fields of layer V pyramidal neurones in the CO-poor bands may allow for integration of a greater number of more diverse inputs, thus allowing for averaging of stimuli to improve the signal-to-noise ratio or enhance spatial discrimination of peripheral stimuli. In some instances, neurones located within approximately 100 microm of the boundaries of the CO bands had dendritic fields that appeared to conform to the CO bands, the dendrites preferentially arborizing within a single band and avoiding the neighbouring band. However, the bias was not absolute, as we observed many examples of cells with dendrites that crossed the boundary between bands. Furthermore, many cells had dendrites that showed distinct dendritic bias that bore no obvious relationship to the CO boundaries.

Interhemispheric switching mediates perceptual rivalry

BACKGROUND

Binocular rivalry refers to the alternating perceptual states that occur when the images seen by the two eyes are too different to be fused into a single percept. Logothetis and colleagues have challenged suggestions that this phenomenon occurs early in the visual pathway. They have shown that, in alert monkeys, neurons in the primary visual cortex continue to respond to their preferred stimulus despite the monkey reporting its absence. Moreover, they found that neural activity higher in the visual pathway is highly correlated with the monkey's reported percept. These and other findings suggest that the neural substrate of binocular rivalry must involve high levels, perhaps the same levels involved in reversible figure alternations.

RESULTS

We present evidence that activation or disruption of a single hemisphere in human subjects affects the perceptual alternations of binocular rivalry. Unilateral caloric vestibular stimulation changed the ratio of time spent in each competing perceptual state. Transcranial magnetic stimulation applied to one hemisphere disrupted normal perceptual alternations when the stimulation was timed to occur at one phase of the perceptual switch, but not at the other. Furthermore, activation of a single hemisphere by caloric stimulation affected the perceptual alternations of a reversible figure, the Necker cube.

CONCLUSIONS

Our findings suggest that interhemispheric switching mediates perceptual rivalry. Thus, competition for awareness in both binocular rivalry and reversible figures occurs between, rather than within, each hemisphere. This interhemispheric switch hypothesis has implications for understanding the neural mechanisms of conscious experience and also has clinical relevance as the rate of both types of perceptual rivalry is slow in bipolar disorder (manic depression).

Prey capture and accommodation in the sandlance, Limnichthyes fasciatus (Creediidae; Teleostei)

The eyes of the sandlance, Limnichthyes fas ciatus (Creediidae, Teleostei) move independently and possess a refractive cornea, a convexiclivate fovea and a non-spherical lens giving rise to a wide separation of the nodal point from the axis of rotation of the eye much like that of a chameleon. To investigate this apparent convergence of the visual optics in these phylogenetically disparate species, we examine feeding behaviour and accommodation in the sandlance with special reference to the possibility that sandlances use accommodation as a depth cue to judge strike length. Frame-by-frame analysis of over 2000 strikes show a 100% success rate. Explosive strikes are completed in 50 ms over prey distances of four body lengths. Close-up video confirms that successful strikes can be initiated monocularly (both normally and after monocular occlusion) showing that binocular cues are not necessary to judge the length of a strike. Additional means of judging prey distance may also be derived from partallax information generated by rotation of the eye as suggested for chameleons. Using photorefraction on anaesthetised sandlances, accommodative changes were induced with acetylcholine and found to range between 120 D and 180 D at a speed of 600-720 D s(-1). The large range of accommodation (25% of the total power) is also thought to be mediated by corneal accommodation where the contraction of a unique cornealis muscle acts to change the corneal curvatures.

Sleep in the platypus

We have conducted the first study of sleep in the platypus Ornithorhynchus anatinus. Periods of quiet sleep, characterized by raised arousal thresholds, elevated electroencephalogram amplitude and motor and autonomic quiescence, occupied 6-8 h/day. The platypus also had rapid eye movement sleep as defined by atonia with rapid eye movements, twitching and the electrocardiogram pattern of rapid eye movement. However, this state occurred while the electroencephalogram was moderate or high in voltage, as in non-rapid eye movement sleep in adult and marsupial mammals. This suggests that the low-voltage electroencephalogram is a more recently evolved feature of mammalian rapid eye movement sleep. Rapid eye movement sleep occupied 5.8-8 h/day in the platypus, more than in any other animal. Our findings indicate that rapid eye movement sleep may have been present in large amounts in the first mammals and suggest that it may have evolved in pre-mammalian reptiles.

Electroreception in monotremes

I will briefly review the history of the bill sense of the platypus, a sophisticated combination of electroreception and mechanoreception that coordinates information about aquatic prey provided from the bill skin mechanoreceptors and electroreceptors, and provide an evolutionary account of electroreception in the three extant species of monotreme (and what can be inferred of their ancestors). Electroreception in monotremes is compared and contrasted with the extensive body of work on electric fish, and an account of the central processing of mechanoreceptive and electroreceptive input in the somatosensory neocortex of the platypus, where sophisticated calculations seem to enable a complete three-dimensional fix on prey, is given.

Convergence of specialised behaviour, eye movements and visual optics in the sandlance (Teleostei) and the chameleon (Reptilia)

Chameleons have a number of unusual, highly specialised visual features, including telescopic visual optics with a reduced lens power, wide separation of the eye's nodal point from the axis of rotation, a deep-pit fovea, rapid pre-calculated strikes for prey based on monocular depth judgements (including focus), and a complex pattern of partially independent alternating eye movements. The same set of features has been acquired independently by a teleost, the sandlance Limnichthyes fasciatus. Despite its underwater lifestyle, this fish displays visual behaviour and rapid strikes for prey that are remarkably similar to those of the chameleon [1]. In a direct comparison of the two species, we have revealed other, previously unsuspected, similarities, such as corneal accommodation, which was unknown in teleosts, as well as bringing together, for the first time, data collected from both species. The sandlance is the only teleost, among thousands studied, that has corneal refraction, corneal accommodation and reduced lens power, as well as sharing the other specialised optical features seen in chameleons. The independent eye movement pattern in the sandlance is also unusual and similar to that of the chameleon. The selection pressures that have produced this remarkable example of convergence may relate to common visual constraints in the life styles of these two phylogenetically disparate species.

A 'sticky' interhemispheric switch in bipolar disorder?

Despite years of research into bipolar disorder (manic depression), its underlying pathophysiology remains elusive. It is widely acknowledged that the disorder is strongly heritable, but the genetics are complex with less than full concordance in monozygotic twins and at least four susceptibility loci identified. We propose that bipolar disorder is the result of a genetic propensity for slow interhemispheric switching mechanisms that become 'stuck' in one or the other state. Because slow switches are also 'sticky' when compared with fast switches, the clinical manifestations of bipolar disorder may be explained by hemispheric activation being 'stuck' on the left (mania) or on the right (depression). Support for this 'sticky' interhemispheric switching hypothesis stems from our recent observation that the rate of perceptual alternation in binocular rivalry is slow in euthymic subjects with bipolar disorder (n = 18, median = 0.27 Hz) compared with normal controls (n = 49, median = 0.60 Hz, p < 0.0005). We have presented evidence elsewhere that binocular rivalry is itself an interhemispheric switching phenomenon. The rivalry alternation rate (putative interhemispheric switch rate) is robust in a given individual, with a test-retest correlation of more than 0.8, making it suitable for genetic studies. The interhemispheric switch rate may provide a trait-dependent biological marker for bipolar disorder.

The development of the electroreceptors of the platypus (Ornithorhynchus anatinus)

A series of developmental stages of the platypus were examined to obtain an anatomical description of the development of the periphery of the electroreceptive system. Putative electroreceptors, composed of modified mucous glands, were observed to appear at 10 days post hatching (p.h.). The typical striped arrangement of peripheral electroreceptors in the platypus was seen at 12 days p.h. The arrangement of the stripes was modified during development with a range of additions and divisions of stripes occurring until the adult pattern is obtained, approximately 6 months p.h. After appearing at 10 days p.h., the number of electroreceptors increases rapidly until sometime between 24 and 28 days p.h. when there is massive death of electroreceptors, the number present at 28 days p.h. being 60% of the number present at 24 days p.h. This massive death of receptors is coincident with the appearance of other sensory structures in the epidermis of the bill skin, the push-rod mechanoreceptors and the sensory serous glands. Histological examination of a range of developmental stages demonstrated poorly differentiated innervation at 28 days p.h., which became differentiated and reached the adult configuration between 11 weeks p.h. and 6 months p.h., the time at which nestling platypuses leave the burrow. Lamination of the cells lining the duct of the electroreceptors showed a similar developmental profile. This study indicates that the electroreceptive system of the developing platypus is not functional, in a similar manner to the adult, until it is time for the platypus to leave the nesting burrow. However, the system may be functional in the developing platypus, and may be used speculatively in the location of the mammary region for suckling.

The sensory world of the platypus

Vision, audition and somatic sensation in the platypus are reviewed. Recent work on the eye and retinal ganglion cell layer of the platypus is presented that provides an estimate of visual acuity and suggests that platypus ancestors may have used vision, as well as the bill organ, for underwater predation. The combined electroreceptor and mechanoreceptor array in the bill is considered in detail, with special reference to the elaborate cortical structure, where inputs from these two sensory arrays are integrated in a manner that is astonishingly similar to the stripe-like ocular dominance array in primate visual of cortex, that integrates input from the two eyes. A new hypothesis, along with supporting data, is presented for this combined mechanoreceptive-electroreceptive complex in platypus cortex. Bill mechanoreceptors are shown to be capable of detecting mechanical waves travelling through the water from moving prey. These mechanical waves arrive after the electrical activity from the same prey, as a function of distance. Bimodal cortical neurones, sensitive to combined mechanical and electrical stimulation, with a delay, can thus signal directly the absolute distance of the prey. Combined with the directional information provided by signal processing of the thousands of receptors on the bill surface, the stripe-like cortical array enables the platypus to use two different sensory systems in its bill to achieve a complete, three-dimensional 'fix' on its underwater prey.

Some related aspects of platypus electroreception: temporal integration behaviour, electroreceptive thresholds and directionality of the bill acting as an antenna

This paper focuses on how the electric field from the prey of the platypus is detected with respect to the questions of threshold determination and how the platypus might localize its prey. A new behaviour in response to electrical stimuli below the thresholds previously reported is presented. The platypus shows a voluntary exploratory behaviour that results from a temporal integration of a number of consecutive stimulus pulses. A theoretical analysis is given, which includes the threshold dependence on the number of receptors and temporal integration of consecutive stimuli pulses, the close relationships between electrical field decay across the bill, electroreceptive thresholds and directionality of the platypus bill acting as an antenna. It is shown that a lobe shape, similar to that which has been measured, can be obtained by combining responses in a specific way from receptors sensing the electric field decay across the bill. Two possible methods for such combinations are discussed and analysed with respect to measurements and observed behaviour of the platypus. A number of factors are described which need to be considered when electroreceptive thresholds are to be determined. It is shown that some information about the distance to the source is theoretically available from the pattern of field decay across the platypus's bill. The paper includes a comparative analysis of radar target tracking and platypus prey localization.

Distribution and putative function of autonomic nerve fibres in the bill skin of the platypus (Ornithorhynchus anatinus)

The electroreceptors located in the bill skin of the platypus are modified secretory glands. The electroreceptive nerve terminals form bare endings in close proximity to the duct of these glands. In this study, we describe the autonomic innervation of the glands and a separate specialized autonomic innervation of the epidermal portion of the glandular duct. A range of immunohistochemical labels showed that the gland cells of the electroreceptors have a non-noradrenergic (putative parasympathetic) innervation. Phalloidin labelling revealed a 'sphincter' of epidermal luminal cells that labelled strongly for actin. These actin-dense keratinocytes were seen to have a noradrenergic (putative sympathetic) innervation. Fine-diameter sensory fibres containing substance P (presumably C-fibre thermoreceptors or polymodal nociceptors) were observed to terminate in the superficial epidermis surrounding the pore of the gland. When the bill of the platypus is dry these pores were closed. However, when room temperature water was washed over the bill, the pores opened. It is proposed that this autonomic and sensory innervation, along with the actin sphincter, mediates the opening and closing of the pores. By doing this, the platypus prevents the desiccation of the bare electrosensory nerve terminals when it is out of the water, and it may also be a way to regulate the impedance of the internal electrical circuit presented to the water at the pores.

The development of the external features of the platypus (Ornithorhynchus anatinus)

The present study describes the post-hatching development of the external features of the platypus. Specimens range in age from the day of hatching through to 6 months old, and provide the first comprehensive view of the developmental sequence of these features. Various features, those specific to the platypus, those specific to monotremes and those shared with marsupials and eutherians, are described. Features specific to the platypus, including the bill and webbing of the forefeet, are seen to develop precociously. Many features show similarities to marsupials, although marsupials show differential development both in timing and in morphology. The developmental progression is related to the age, in days, although the exact age of the specimens is unclear, and relies on ages given to the specimens at the time of collection, sometimes as long as 70 years ago. Despite this, the progression of development of these features suggests that the ageing given is relatively accurate.

Monotremes and the evolution of rapid eye movement sleep

Early studies of the echidna led to the conclusion that this monotreme did not have rapid eye movement (REM) sleep. Because the monotremes had diverged from the placental and marsupial lines very early in mammalian evolution, this finding was used to support the hypothesis that REM sleep evolved after the start of the mammalian line. The current paper summarizes our recent work on sleep in the echidna and platypus and leads to a very different interpretation. By using neuronal recording from mesopontine regions in the echidna, we found that despite the presence of a high-voltage cortical electroencephalogram (EEG), brainstem units fire in irregular bursts intermediate in intensity between the regular non-REM sleep pattern and the highly irregular REM sleep pattern seen in placentals. Thus the echidna displays brainstem activation during sleep with high-voltage cortical EEG. This work encouraged us to do the first study of sleep, to our knowledge, in the platypus. In the platypus we saw sleep with vigorous rapid eye, bill and head twitching, identical in behaviour to that which defines REM sleep in placental mammals. Recording of the EEG in the platypus during natural sleep and waking states revealed that it had moderate and high-voltage cortical EEGs during this REM sleep state. The platypus not only has REM sleep, but it had more of it than any other animal. The lack of EEG voltage reduction during REM sleep in the platypus, and during the REM sleep-like state of the echidna, has some similarity to the sleep seen in neonatal sleep in placentals. The very high amounts of REM sleep seen in the platypus also fit with the increased REM sleep duration seen in altricial mammals. Our findings suggest that REM sleep originated earlier in mammalian evolution than had previously been thought and is consistent with the hypothesis that REM sleep, or a precursor state with aspects of REM sleep, may have had its origin in reptilian species.

Base-compositional biases and the bat problem. III. The questions of microchiropteran monophyly

Using single-copy DNA hybridization, we carried out a whole genome study of 16 bats (from ten families) and five outgroups (two primates and one each dermopteran, scandentian, and marsupial). Three of the bat species represented as many families of Rhinolophoidea, and these always associated with the two representatives of Pteropodidae. All other microchiropterans, however, formed a monophyletic unit displaying interrelationships largely in accord with current opinion. Thus noctilionoids comprised one clade, while vespertilionids, emballonurids, and molossids comprised three others, successively more closely related in that sequence. The unexpected position of rhinolophoids may be due either to the high AT bias they share with pteropodids, or it may be phylogenetically authentic. Reanalysis of the data with varying combinations of the five outgroups does not indicate a rooting problem, and the inclusion of many bat lineages divided at varying levels similarly discounts long branch attraction as an explanation for the pteropodid-rhinolophoid association. If rhinolophoids are indeed specially related to pteropodids, many synapomorphies of Microchiroptera are called into question, not least the unitary evolution of echolocation (although this feature may simply have been lost in pteropodids). Further, a rhinolophoid-pteropodid relationship--if true--has serious implications for the classification of bats. Finally, among the outgroups, an apparent sister-group relation of Dermoptera and Primates suggests that flying lemurs do not represent the ancestors of some or all bats; yet, insofar as gliding of the type implemented in dermopterans is an appropriate model for the evolution of powered mammalian flying, the position of Cynocephalus in our tree indirectly strengthens the argument that true flight could have evolved more than once among bats.

Base-compositional biases and the bat problem. II. DNA-hybridization trees based on AT- and GC-enriched tracers

We conducted a series of parallel DNA-hybridization experiments on a small group of bats (species of Pteropus, Rhinolophus, Noctilio and Pteronotus) and outgroups (Lemur, Cynocephalus, Didelphis), using whole-genome labels and tracers made from extracts enriched with AT and two levels of GC content. FITCH (additive phylogenetic trees) topologies were constructed from the four sets of comparisons, indexed as both delta Tmode and delta NPHs (normalized percentage of hybridization). Based on our previous work showing that the shared AT bias of pteropodids and some microchiropterans may affect the rank-ordering of taxa based on either AT- or GC-rich labels, our expectation was that the resulting trees would show differing topologies when generated from tracers made with the variously enriched DNA extracts. Whereas there was some variation among the trees, most of them grouped the bats together, and almost all paired the representative megachiropteran and rhinolophoid microchiropteran as sister-taxa in contrast to the other microchiropterans. As the pteropodid-rhinolophoid relationship is an unexpected and unlikely one, we attribute this association to an AT bias that was not obviated even by our most GC-rich labels, and suggest that such a bias may compromise the truth of some molecular trees. Accordingly, we believe the broader issue of bat monophyly remains unresolved by DNA-hybridization and probably also by gene-sequencing studies.

Base-compositional biases and the bat problem. I. DNA-hybridization melting curves based on AT- and GC-enriched tracers

We explored the interordinal relationships of mammals using DNA-DNA hybridization, with particular reference to the much-debated problem of whether the megabats and microbats are more closely related to each other than the megabats are to primates. To try to improve resolution when taxa are distantly related and the melting points of hybrids are low and difficult to distinguish, we increased the GC content of DNA by a fractionation method that used the same melting-point apparatus used in the hybridization studies. When we used GC-rich DNA as the tracer to make hybrids, the melting point of the self-hybrid shifted to a higher temperature as expected, but the behaviour of heterologous hybrids varied with the taxa being compared. When the melting point of the heterologous hybrid also shifted to a higher temperature so that the two compared taxa maintained the same or proportional distance, we called this 'following behaviour', because the heterologous hybrid made with GC-rich tracer 'followed' the GC-rich self-hybrid to higher temperatures. We also commonly saw anomalous behaviour, where the melting point of the heterologous hybrid shifted to a lower temperature when compared with an AT-rich hybrid. In these anomalous cases, the distance measured between the taxa increased markedly as a result of GC-enrichment, indicating that an underestimate of distance may have resulted from AT bias in DNA. This inference was supported by the finding that it was rare to observe a decrease in measured distance between taxa using GC-rich DNA, but very common to find an increase as would be expected from the generally higher AT contents of eutherian DNAs. Moreover, the most extreme cases, where distances changed most using GC-rich DNA, were usually those involving comparisons between taxa known to have the most extreme AT-biases among mammals, such as the megabats and rhinolophoid (including megadermatid) microbats. Our results show consistent underestimates of measured differences between eutherian taxa with extreme AT-biases.

The echidna Tachyglossus aculeatus combines REM and non-REM aspects in a single sleep state: implications for the evolution of sleep

Placental and marsupial mammals exist in three states of consciousness: waking, non-REM sleep, and REM sleep. We now report that the echidna Tachyglossus aculeatus, a representative of the earliest branch of mammalian evolution (the monotremes), does not have the pattern of neuronal activity of either of the sleep states seen in nonmonotreme mammals. Echidna sleep was characterized by increased brainstem unit discharge variability, as in REM sleep. However, the discharge rate decreased and the EEG was synchronized, as in non-REM sleep. Our results suggest that REM and non-REM sleep evolved as a differentiation of a single, phylogenetically older sleep state. We hypothesize that the physiological changes that occur during postnatal sleep development parallel certain aspects of the changes that have occurred during the evolution of sleep-waking states in mammals.

Ultrastructure, number, distribution and innervation of electroreceptors and mechanoreceptors in the bill skin of the platypus, Ornithorhynchus anatinus

The platypus is presently the only mammal demonstrated to use electroreception to obtain food. The electroreceptive system of the platypus is far more complex than that of its close relative the echidna. This paper presents an anatomical study of the basis of electroreception in the platypus. The innervation of the bill by the trigeminal nerve is described, as are three sensory structures, associated with food gathering, within the bill skin. There are 40,000 mucous gland electroreceptors found in the bill skin of the platypus. The papillary portion of each of these sensory mucous glands is modified to accommodate electrosensory nerve terminals. In contrast to fish electroreceptors, the electrosensory terminals of the platypus are not associated with a sensory cell. These mucous gland electroreceptors are arranged in a series of parasagittal stripes on the bill. This array suggests a basis for the ability of the platypus to quickly and accurately locate the origin of an electrical stimulus. A push-rod mechanoreceptor, similar in morphology to Eimer's organ of the mole, and bill-tip organs in birds, was also found in the bill skin. The slightly differing morphology of these mechanoreceptors when compared to their avian and talpid counterparts suggests that this is another example of convergent evolution, with the common need to provide a solution to increasing tactile sensitivity on bare rhinarial skin. These push-rods are found to be most dense around the labial margins of the bill, with a marked decrease in density towards the middle and caudal portions of the bill. The distribution of the push-rods is similar to the distribution of the third sensory structure found on the bill, the sensory serous gland. Although less numerous than the mechanoreceptors (46,500 mechanoreceptors compared with 13,500 sensory serous glands), these sensory serous glands have a similar distribution and similar changes in density. These concurrent distributions argue for some functional correlation of these two sensory structures. The papillary region of the serous gland is modified in a manner similar to that of the mucous gland electroreceptor to accommodate sensory input. The sensory terminals of the serous glands are very similar to those of the mucous gland electroreceptors, and so it is presumed that these sensory serous glands are a type of electroreceptor that might be involved in detection of electrical signals at close quarters where the mechanoreceptors are also engaged.

Unusual pattern of retinogeniculate projections in the controversial primate Tarsius

Studies of simian and prosimian primates have demonstrated a remarkable interspecific constancy in the pattern of innervation of the magnocellular and parvocellular layers of the dorsal lateral geniculate nucleus (dLGN) by retinal afferents. The characteristic organization of this nucleus in primates, as well as its apparent phylogenetic stability, have led to the proposal that the distinct laminar arrangement of the dLGN is one of the diagnostic characters that define the Order Primates. Here, we describe the distribution of retinal afferents to the dLGN of Tarsius, the single contemporary member of an ancient lineage of primates. In this genus, the more superficial layer of the dLGN receives projections from the ipsilateral eye, a unique situation among the members of the Order Primates. This observation adds support to the proposal that Tarsius does not share a more recent common ancestry with simians as compared with lemuriform and lorisiform primates.

Nerve terminals of mucous gland electroreceptors in the platypus (Ornithorhynchus anatinus)

Platypus mucous gland electroreceptors differ from electroreceptors described for fish in that they lack an associated specialized sensory cell. Thus a bare nerve terminal is used to detect electrical stimuli, and also to generate local and action potentials. Previous studies have identified these terminals (an average of 16 per mucous gland), but had not shown whether the terminals have direct contact with the duct of the mucous gland. This poses the problem of how the electrical stimulus reaches the nerve terminals. This study demonstrates the portions of the nerve terminals responsible for electroreception, and shows how these portions use the surrounding epidermal tissue to overcome the combined problems of lacking a sensory cell and making physical contact with the conducting medium in the duct of the gland. A terminal axonal filament is described which accommodates for these problems, the terminal filament provides a low-resistance pathway for the electrical stimuli, and is embedded with its proximal and distal portions in high and low resistance epidermis, respectively. Lateral interactions occur between adjacent terminal filaments via a plexus that is directed circumferentially around the duct from the proximal portion of the terminal filament. These circumferential arbors form an interconnecting ring between all 16 terminal filaments, and may be used to lower the signal-to-noise ratio of the electroreceptor and thus enhance overall sensitivity.

Genomic evolution. Flying DNA

The extremely high AT content of bat DNA complicates the reconstruction of bat phylogeny from DNA sequence data, but may help throw light on genomic evolution.

Organization of the second visual area in the megachiropteran bat Pteropus

The organization of peristriate cortex was studied in nine flying foxes (genus Pteropus). Based on receptive field mapping and architectonic data, we report on the organization of the second visual area (V2). V2 forms a continuous belt 2-4 mm wide bordering V1 anteriorly. In each hemisphere, V2 contains a precisely organized representation of the entire contralateral visual field. The vertical meridian of the visual field (VM), and a short strip of the ipsilateral hemifield are represented at the posterior border of V2, with V1. The area centralis is represented approximately at the center of the posterior border of V2. At each mediolateral level, progressively more peripheral portions of the visual field are represented as V2 is crossed from posterior to anterior. The representation of the upper quadrant is continuous, and confined to the lateral half of V2. In contrast, the representation of the lower quadrant is split along a line running from the temporal edge of the field of vision to the optic disk. As a result of this arrangement, the portions of the lower quadrant close to the VM are represented medially, and those away from the VM laterally in V2. The entire representation of the horizontal meridian is located in lateral V2, and is not split between medial and lateral V2 as in primates. The linear cortical magnification factor (CMF) decays by a factor of 3-5 from the central to the peripheral representation. The CMF is anisotropic, and equal distances in the visual field are magnified twice as much parallel to the V1/V2 border than perpendicular to this border. Moreover, points in the lower quadrant are magnified relative to symmetrical points in the upper quadrant. V2 is histologically distinct from all surrounding areas in both cytochrome oxidase- and Nissl-stained sections. These results suggest that V2 is an homologous area common to all archontans, and imply that much of the variability reported among mammals may be due to technical factors, rather than true species differences.

Retinotopic organization of the primary visual cortex of flying foxes (Pteropus poliocephalus and Pteropus scapulatus)

The representation of the visual field in the occipital cortex was studied by multiunit recordings in seven flying foxes (Pteropus spp.), anesthetized with thiopentone/N2O and immobilized with pancuronium bromide. On the basis of its visuotopic organization and architecture, the primary visual area (V1) was distinguished from neighboring areas. Area V1 occupies the dorsal surface of the occipital pole, as well as most of the tentorial surface of the cortex, the posterior third of the mesial surface of the brain, and the upper bank of the posterior portion of the splenial sulcus. In each hemisphere, it contains a precise, visuotopically organized representation of the entire extent of the contralateral visual hemifield. The representation of the vertical meridian, together with 8-15 degrees of ipsilateral hemifield, forms the anterior border of V1 with other visually responsive areas. The representation of the horizontal meridian runs anterolateral to posteromedial, dividing V1 so that the lower visual quadrant is represented medially, and the upper quadrant laterally. The total surface area of V1 is about 140 mm2 for P. poliocephalus, and 110 mm2 for P. scapulatus. The representation of the central visual field is greatly magnified relative to that of the periphery. The cortical magnification factor decreases with increasing eccentricity, following a negative power function. Conversely, receptive field sizes increase markedly with increasing eccentricity, and therefore the point-image size is approximately constant throughout V1. The emphasis in the representation of the area centralis in V1 is much larger than that expected on the basis of ganglion cell counts in flat-mounted retinas. Thus, a larger degree of convergence occurs at the peripheral representations in the retino-geniculo-cortical pathway, in comparison with the central representations. The marked emphasis in the representation of central vision, the wide extent of the binocular field of vision, and the relatively large surface area of V1 reflect the importance of vision in megachiropterans.

Common principle of guidance by echolocation and vision

1. Using echolocation, bats move as gracefully as birds through the cluttered environment, suggesting common principles of optic and acoustic guidance. We tested the idea by analysing braking control of bats (Macroderma gigas) flying through a narrow aperture with eyes covered and uncovered. 2. Though braking control would seem to require rapid detection of distance and velocity and computation of deceleration, simpler control is possible using the tau function of any sensory variable S that is a power function of distance to aperture. Tau function of S is tau (S) = S/S (the dot means time derivative). Controlled braking is achievable by keeping tau (S) constant. 3. Previous experiments indicated the tau (S) constant procedure is followed by humans and birds in visually controlling braking. Analysis of the bats' flight trajectories indicated they too followed the braking procedure using echolocation. 4. The tau function of echo-delay or of echo-intensity or of angle subtended by directions of echoes from two points on the approach surface could be used to control braking. Aperture size was modulated during flight on some trials in an attempt to test between these possibilities, but the results were inconclusive.

The primary structure of the hemoglobin from the tomb bat (Taphozous georgianus, Microchiroptera)

The primary structures of the alpha- and beta-chains of the single hemoglobin component from the tomb bat (Taphozous georgianus, Microchiroptera) are presented. After chain separation by reversed-phase HPLC the sequences could be determined by automatic gas and liquid phase Edman degradation of the chains and their tryptic peptides. The alpha- and beta-chains differ from human hemoglobin by 14 and 18 replacements, respectively. Compared to the total number of amino-acid exchanges, the exchange rate in the interhelical regions of the alpha-chains is surprisingly high (25%). It seems unlikely that substitutions at contact positions affect the oxygen binding properties of the hemoglobin.

Retinal topography in the koala (Phascolarctos cinereus)

Nissl-stained retinal wholemounts were used to investigate the topographical organization of the ganglion cell layer of the koala (Phascolarctos cinereus); the visual resolution limit of this animal was subsequently estimated from retinal ganglion cell density data. Two types of cells could be differentiated on the basis of their size and staining characteristics: a subpopulation of presumed ganglion cells, consisting of medium to large cells with Nissl substance in the cytoplasm and pale uniformly staining nuclei, and a further subpopulation of small, densely staining cells. The latter group were presumed to be neuroglia and displaced amacrine cells. Iso-density contour maps were prepared from total cell counts and also counts of presumed ganglion cells; in all cases, the density of cells was greatest in the inferior retina where there was an area of peak density occurring as a poorly developed, horizontal streak that extended across the inferior retina. The inferior position of the streak in the koala contrasts with reports of the superior position of streaks in other marsupials. Peak cell densities of 2370 cells/mm2 and 1480 cells/mm2 were recorded for the total cell population and the presumed ganglion cell subpopulation, respectively. The latter value is equivalent to a visual resolution of 2.4 cycles/degree, based on sampling theory and a square packing paradigm, placing the koala close in visual performance to two other marsupials, the Australian Northern native cat and the American Virginia opossum.

The primary structure of the hemoglobin from the Australian ghost bat (Macroderma gigas, Microchiroptera)

The Australian ghost bat (Macroderma gigas, Microchiroptera) has two hemoglobin components in the ratio 3:2. They share identical beta-chains and differ by three replacements in the alpha-chains. The primary structures of all three chains are presented. They could be separated by high-performance liquid chromatography. The sequences were determined by automatic liquid and gas phase Edman degradation of the chains and their tryptic peptides. The two alpha-chains show 18 and 19 and the beta-chains 15 exchanges compared to human alpha- and beta-chains, respectively. The divergent evolution of Macroderma gigas and Megaderma lyra, two representatives of the family Megadermatidae, is discussed. An influence of replacements at functionally important positions on the hemoglobin oxygen affinity seems unlikely.

Cone photoreceptors lacking oil droplets in the retina of the echidna, Tachyglossus aculeatus (Monotremata)

The echidna, Tachyglossus aculeatus, a monotreme mammal, is thought to possess an all-rod retina (O'Day, 1952). This study provides anatomical evidence for the presence of cone-like photoreceptors in the retina of the echidna. The cones, which constitute 10-15% of the photoreceptors, have all of the ultrastructural characteristics previously shown in the cones of placental mammals, and, like cones of other animals (Blanks & Johnson, 1984), they bind peanut agglutinin. Unlike the cones of another monotreme, the platypus, the cones of the echidna retina do not possess oil droplets. Twin cones, pairs of cones in which there is no obvious difference in the size, shape, or ultrastructural features of the members of a pair, are common. The density of cones varies from 9000 cells/mm2 in the superior periphery to 22,000 cells/mm2 in the central retina. Nearest-neighbor analysis suggests that the cone mosaic in the echidna retina results from the presence of single and twin cones in a relatively regular array.

Origins of descending spinal pathways in prehensile birds: do parrots have a homologue to the corticospinal tract of mammals?

In mammals, the supraspinal descending projections that influence distal limb muscles are the rubrospinal and corticospinal tracts. The former, which is found in other vertebrates, shows greater somatotopy in mammals that are 'dextrous' (e.g. monkeys) than those that are not (e.g. opossums). Similarly, the corticospinal tract, which is found only in mammals, has more extensive connections (i.e. direct corticomotoneural) in mammals that are dextrous than in mammals that are not. Descending spinal pathways have been described in 'non-dextrous' avian species (chickens, ducks, geese and pigeons), and the purpose of this study was to determine if there are any differences in the origins of descending projections to the spinal cord in 'dextrous' or prehensile parrots (sulphur-crested cockatoo, Cacatua galerita, and eastern rosella, Platycerus eximius). True Blue or wheat germ agglutinin-horseradish peroxidase was injected into the lumbar or cervical spinal cord. The distribution of retrogradely labelled cells was similar to that previously reported for non-prehensile birds. We found no evidence of any direct spinal projections from the telencephalon (including any pathway homologous to the corticospinal tract of mammals), nor any specialized anatomical organization of the descending pathways that could account for the pedal dexterity of these species.

Phylogenetic relations between microbats, megabats and primates (Mammalia: Chiroptera and Primates)

We examine the paraphylectic hypothesis of bat origins, both in the light of previous discussions, and in the light of new evidence from our analyses of neurological traits and wing morphology. Megabats share with primates a variety of complex details in the organization of neural pathways that have not been found in any other mammalian group, particularly not in microbats. The features previously used to link microbats and megabats have been examined and found to be questionable bases for support of a monophyletic origin. In particular, morphological analyses of the musculoskeletal adaptations associated with the flight apparatus are consistent with two separate origins of the mammalian wing. Taken together, these analyses suggest that megabats evolved from an early branch of the primate lineage. This branch was comprised of moderate-sized, phytophagous gliders, of which the other living descendants are the dermopterans. Microbats, in contrast, probably evolved much earlier from small, agile insectivores whose forelimbs had long metacarpals in relation to their phalanges.

Quantitative comparison of the limits on visual spatial resolution set by the ganglion cell layer in twelve species of reef teleosts

A diverse range of retinal specializations are examined in twelve species of reef teleosts and estimates of the spatial resolution of neurons within the ganglion cell layer calculated using Matthiessen's ratio. Upper limits of between 4 and 27 cycles per degree were found to facilitate acute vision into frontal and eccentric space, utilizing temporal and nasal area centralis, respectively. Upper limits of between 3 and 20 cycles per degree were found in horizontal areas of acute vision across the retinal meridian. These areas are thought to be used for panoramic vision and may, in one species, indicate the relative importance of this region in comparison to the temporal area centralis. Comparisons are made between ganglion cell acuity and other morphological and behavioural acuities calculated in previous studies.

Quantitative analysis of the retinal ganglion cell layer and optic nerve of the barn owl Tyto alba

The visual capacity of the common barn owl (Tyto alba) was studied by quantitative analysis of the retina and optic nerve. Cell counts in the ganglion cell layer of the whole-mounted retina revealed a temporal area centralis with peak cell density of 12,500 cells/mm2 and a horizontal streak of high cell density extending from the area centralis into the nasal retina. Integration of the ganglion cell density map gave an estimated total of 1.4 million cells for the ganglion cell layer. Electron microscopy of a single, complete section of the optic nerve revealed a bimodal fiber diameter spectrum (modes at 0.3 and 0.9 microns; bin width = 0.2 microns), with diameters ranging from 0.15 microns (unmyelinated) to 6.05 microns (myelinated, sheath included). The total axon count for the optic nerve was estimated from sample counts to be about 680,000 axons (25% unmyelinated). Therefore, roughly half of the cells in the retinal ganglion cell layer do not send axons into the optic nerve. With certain assumptions, the data predict a visual spatial acuity for barn owls on the order of 8 cycles/degree, a value similar to the known behaviorally measured acuities of masked owls (10 cycles/degree) and domestic cats (6 cycles/degree).

Retinal ganglion cell topography in teleosts: a comparison between Nissl-stained material and retrograde labelling from the optic nerve

The retinal topography of cells within the ganglion cell layer of three teleost species is examined in Nissl-stained material in which all neuronal elements containing Nissl substance in the cytoplasm are counted. A topographic comparison is made with retrogradely labelled ganglion cells to differentiate the proportion of nonganglion cells not possessing an axon joining the optic nerve. In the three species studied 92%, 80%, and 66% were found to be the maximum proportion of true ganglion cells in the area centralis, horizontal streak, and periphery, respectively. The proportion of nonganglion cells in the total population of cells counted was 24%. The major contribution to this discrepancy is from peripheral nonspecialized regions of the retina. There is little difference in both topography and peak densities of retinal ganglion cells between the two techniques. The soma areas of both populations are analysed, with the homogeneous nonganglion cell population possessing cells between 5 and 15 micron2 and the heterogeneous ganglion cell soma between 5 and 68 micron2, increasing in size with eccentricity.

The primary structure of the hemoglobin from the grey-headed flying fox (Pteropus poliocephalus) and the black flying fox (P. alecto, Megachiroptera)

The primary structures of the hemoglobins of two Flying Foxes of the genus Pteropus are presented. Both comprise two components: in P. alecto hemoglobin two alpha-chains at a ratio of 1:1 and two beta-chains at a ratio of 4:1 were detected. The hemoglobin of P. poliocephalus comprises one alpha-chain and two beta-chains, the latter at a ratio of 1:1. The globin chains were separated by high-performance liquid chromatography and the sequences determined by automatic liquid and gas phase Edman degradation of the chains and their tryptic peptides. Compared with human hemoglobin, the alpha-chains of P. alecto and P. poliocephalus show 18 and 19 exchanges, respectively, whereas in the beta-chains 16/17 substitutions are found in both cases. In the alpha-chains of P. alecto, one exchange involves an alpha 1/beta 1-contact. In the beta-chains of both species one heme-, one alpha 1/beta 2- and two alpha 1/beta 1-contacts are exchanged. The relevant side chains are the same in both species. The functional and systematic aspects of these findings are discussed.