The number, morphology, and distribution of retinal ganglion cells and optic axons in the Australian lungfish Neoceratodus forsteri (Krefft 1870)

Australian lungfish Neoceratodus forsteri may be the closest living relative to the first tetrapods and yet little is known about their retinal ganglion cells. This study reveals that lungfish possess a heterogeneous population of ganglion cells distributed in a horizontal streak across the retinal meridian, which is formed early in development and maintained through to adult stages. The number and complement of both ganglion cells and a population of putative amacrine cells within the ganglion cell layer are examined using retrograde labelling from the optic nerve and transmission electron-microscopic analysis of axons within the optic nerve. At least four types of retinal ganglion cells are present and lie predominantly within a thin ganglion cell layer, although two subpopulations are identified, one within the inner plexiform and the other within the inner nuclear layer. A subpopulation of retinal ganglion cells comprising up to 7% or the total population are significantly larger (> 400 mu m(2)) and are characterized as giant or alpha-like cells. Up to 44% of cells within the retinal ganglion cell layer represent a population of presumed amacrine cells. The optic nerve is heavily fasciculated and the proportion of myelinated axons increases with body length from 17% in subadults to 74% in adults. Spatial resolving power, based on ganglion cell spacing, is low (1.6-1.9 cycles deg(-1), n = 2) and does not significantly increase with growth. This represents the first detailed study of retinal ganglion cells in sarcopterygian fish, and reveals that, despite variation amongst animal groups, trends in ganglion cell density distribution and characteristics of cell types were defined early in vertebrate evolution.

The variable colours of the fiddler crab Uca vomeris and their relation to background and predation

Colour changes in fiddler crabs have long been noted, but a functional interpretation is still lacking. Here we report that neighbouring populations of Uca vomeris in Australia exhibit different degrees of carapace colours, which range from dull mottled to brilliant blue and white. We determined the spectral characteristics of the mud substratum and of the carapace colours of U. vomeris and found that the mottled colours of crabs are cryptic against this background, while display colours provide strong colour contrast for both birds and crabs, but luminance contrast only for a crab visual system. We tested whether crab populations may become cryptic under the influence of bird predation by counting birds overflying or feeding on differently coloured colonies. Colonies with cryptically coloured crabs indeed experience a much higher level of bird presence, compared to colourful colonies. We show in addition that colourful crab individuals subjected to dummy bird predation do change their body colouration over a matter of days. The crabs thus appear to modify their social signalling system depending on their assessment of predation risk.

Synaptogenesis in the mushroom body calyx during metamorphosis in the honeybee Apis mellifera: An electron microscopic study

The goals of this study are to determine relationships between synaptogenesis and morphogenesis within the mushroom body calyx of the honeybee Apis mellifera and to find out how the microglomerular structure characteristic for the mature calyx is established during metamorphosis. We show that synaptogenesis in the mushroom body calycal neuropile starts in early metamorphosis (stages P1-P3), before the microglomerular structure of the neuropile is established. The initial step of synaptogenesis is characterized by the rare occurrence of distinct synaptic contacts. A massive synaptogenesis starts at stage P5, which coincides with the formation of microglomeruli, structural units of the calyx that are composed of centrally located presynaptic boutons surrounded by spiny postsynaptic endings. Microglomeruli are assembled either via accumulation of fine postsynaptic processes around preexisting presynaptic boutons or via ingrowth of thin neurites of presynaptic neurons into premicroglomeruli, tightly packed groups of spiny endings. During late pupal stages (P8-P9), addition of new synapses and microglomeruli is likely to continue. Most of the synaptic appositions formed there are made by boutons (putative extrinsic mushroom body neurons) into small postsynaptic profiles that do not exhibit presynaptic specializations (putative intrinsic mushroom body neurons). Synapses between presynaptic boutons characteristic of the adult calyx first appear at stage P8 but remain rare toward the end of metamorphosis. Our observations are consistent with the hypothesis that most of the synapses established during metamorphosis provide the structural basis for afferent information flow to calyces, whereas maturation of local synaptic circuitry is likely to occur after adult emergence.

Sensory Processing in Aquatic Environments

Research on sensory processing or the way animals see, hear, smell, taste, feel and electrically and magnetically sense their environment has advanced a great deal over the last fifteen years. This book discusses the most important themes that have emerged from recent research and provides a summary of likely future directions. The book starts with two sections on the detection of sensory signals over long and short ranges by aquatic animals, covering the topics of navigation, communication, and finding food and other localized sources. The next section, the co-evolution of signal and sense, deals with how animals decide whether the source is prey, predator or mate by utilizing receptors that have evolved to take full advantage of the acoustical properties of the signal. Organisms living in the deep-sea environment have also received a lot of recent attention, so the next section deals with visual adaptations to limited light environments where sunlight is replaced by bioluminescence and the visual system has undergone changes to optimize light capture and sensitivity. The last section on central co-ordination of sensory systems covers how signals are processed and filtered for use by the animal. This book will be essential reading for all researchers and graduate students interested in sensory systems.

Biological polarized light reflectors in stomatopod crustaceans

Body parts that can reflect highly polarized light have been found in several species of stomatopod crustaceans (mantis shrimps). These polarized light reflectors can be grossly divided into two major types. The first type, usually red or pink in color to the human visual system, is located within an animal’s cuticle. Reflectors of the second type, showing iridescent blue, are located beneath the exoskeleton and thus are unaffected by the molt cycle. We used reflection spectropolarimetry and transmission electron microscopy (TEM) to study the reflective properties and the structures that reflect highly polarized light in stomatopods. For the first type of reflector, the degree of polarization usually changes dramatically, from less than 20% to over 70%, with a change in viewing angle. TEM examination indicates that the polarization reflection is generated by multilayer thin-film interference. The second type of reflector, the blue colored ones, reflects highly polarized light to all viewing angles. However, these reflectors show a slight chromatic change with different viewing angles. TEM sections have revealed that streams of oval-shaped vesicles might be responsible for the production of the polarized light reflection. In all the reflectors we have examined so far, the reflected light is always maximally polarized at around 500 nm, which is close to the wavelength best transmitted by sea water. This suggests that the polarized light reflectors found in stomatopods are well adapted to the underwater environment. We also found that most reflectors produce polarized light with a horizontal e-vector. How these polarized light reflectors are used in stomatopod signaling remains unknown.

Gaze angle: a possible mechanism of visual stress in virtual reality headsets

It is known that some Virtual Reality (VR) head-mounted displays (HMDs) can cause temporary deficits in binocular vision. On the other hand, the precise mechanism by which visual stress occurs is unclear. This paper is concerned with a potential source of visual stress that has not been previously considered with regard to VR systems: inappropriate vertical gaze angle. As vertical gaze angle is raised or lowered the 'effort' required of the binocular system also changes. The extent to which changes in vertical gaze angle alter the demands placed upon the vergence eye movement system was explored. The results suggested that visual stress may depend, in part, on vertical gaze angle. The proximity of the display screens within an HMD means that a VR headset should be in the correct vertical location for any individual user. This factor may explain some previous empirical results and has important implications for headset design. Fortuitously, a reasonably simple solution exists.

Attentional modulation of neural responses to action observation: Implications for models of the human 'mirror' system

In primates, the observation of meaningful, goaldirected actions engages a network of cortical areas located within the premotor and inferior parietal lobules. Current models suggest that activity within these regions arises relatively automatically during passive action observation without the need for topdown control. Here we used functional magnetic resonance imaging to determine whether cortical activit)' associated with action observation is modulated by the strategic allocation of selective attention. Normal observers viewed movie clips of reach-to-grasp actions while performing an easy or difficult visual discrimination at the fovea. A wholebrain analysis was performed to determine the effects of attentional load on neural responses to observed hand actions. Our results suggest that cortical areas involved in action observation are significantiy modulated by attentional load. These findings have important implications for recent attempts to link the human action-observation system to response properties of "mirror neurons" in monkeys.

The fMRI response of the LGN and V1 to cardinal red-green, blue-yellow, and achromatic visual stimuli

We compared the responsiveness of the LGN and the early retinotopic cortical areas to stimulation of the two cone-opponent systems (red - green and blue - yellow) and the achromatic system. This was done at two contrast levels to control for any effect of contrast. MR images were acquired on seven subjects with a 4T Bruker MedSpec scanner. The early visual cortical areas were localised by phase encoded retinotopic mapping with a volumetric analysis (Dumoulin et al, 2003 NeuroImage 18 576 - 587). We initially located the LGN in four subjects by using flickering stimuli in a separate scanning session, but subsequently identified it using the experimental stimuli. Experimental stimuli were sine-wave counterphasing rings (2 Hz, 0.5 cycle deg-1), cardinal for the selective activation of the L/M cone-opponent (RG), S cone-opponent (BY), and achromatic (Ach) systems. A region of interest analysis was performed. When presented at equivalent absolute contrasts (cone contrast = 5% - 6%), the BOLD response of the LGN is strongest to isoluminant red - green stimuli and weakest to blue - yellow stimuli, with the achromatic response falling in between. Area V1, on the other hand, responds best to both chromatic stimuli, with the achromatic response falling below. The key change from the LGN to V1 is a dramatic boost in the relative blue - yellow response, which occurred at both contrast levels used. This greatly enhanced cortical response to blue - yellow relative to the red - green and achromatic responses may be due to an increase in cell number and/or cell response between the LGN and V1. We speculate that the effect might reflect the operation of contrast constancy across colour mechanisms at the cortical level.

Cervical mobilisation: concurrent effects on pain, sympathetic nervous system activity and motor activity

Recent findings that spinal manual therapy (SMT) produces concurrent hypoalgesic and sympathoexcitatory effects have led to the proposal that SMT may exert its initial effects by activating descending inhibitory pathways from the dorsal periaqueductal gray area of the midbrain (dPAG). In addition to hypoalgesic and sympathoexcitatory effects, stimulation of the dPAG in animals has been shown to hal e a facilitatory effect on motor activity. This study sought to further investigate the proposal regarding SMT and the FAG by including a test of motor function in addition to the variables previously investigated, Using a condition randomised, placebo-controlled, double blind, repeated measures design, 30 subjects with mid to lon er cervical spine pain of insidious onset participated in the study. The results indicated that the cervical mobilisation technique produced a hypoalgesic effect as revealed by increased pressure pain thresholds on the side of treatment (P = 0.0001) and decreased resting visual analogue scale scores (P = 0.049). The treatment technique also produced a sympathoexcitatory effect with an increase in skin conductance (P < 0.002) and a decrease in skin temperature (P = < 0.02). There was a decrease in superficial neck flexor muscle activity (P < 0.0002) at the lower levels of a staged cranio-cervical flexion test. This could imply facilitation of the deep neck flexor muscles with a decreased need for co-activation of the superficial neck flexors, The combination of all findings,would support the proposal that SMT may, at least initially, exert part of its influence via activation of the PAG, (C) 2000 Harcourt Publishers Ltd.