Modeling the effects of enclosure size on geometry learning

Several recent studies have shown that chickens, fish, and humans trained to find a reward in a corner of a rectangular enclosure with distinctive features rely more on the geometry of the enclosure in small enclosures and rely more on the features in large enclosures. Here, these results are modeled using a recent associative model of geometry learning [Miller, N.Y., Shettleworth, S.J., 2007. Learning about environmental geometry: an associative model. J. Exp. Psychol. Anim. B 33, 191–212]. By adjusting the salience of either geometric or featural information or both the model is capable of reproducing much of the data on the effects of enclosure size on geometry learning.     

Theoretical implications of the size principle of motoneurone recruitment

There are a limited number of ways by which an orderly recruitment of motoneurones by size might occur in a population of similar neurones activated by synapses with invariant average properties and uniform distribution: (i) The smaller motoneurones might have lower voltage thresholds, or, if spherical, current thresholds that increase more rapidly than the square of the diameter, or faster than the inverse of input resistance, (ii) Smaller motoneurones might receive a higher uniform density of afferent boutons than larger cells, (iii) Larger cells might show a disproportionately large increase in soma diameter compared with smaller cells, thus having a smaller ratio of soma to dendrite input resistances.In particular, a size principle does not automatically arise from cells receiving a constant density of afferent terminals, even if the afferents end preferentially on the motoneurone dendrites, and despite the fact that individual synapses generate larger EPSPs in smaller cells.     

Synapse elimination,the size principle,and Hebbian synapses

Synapse elimination at the vertebrate neuromuscular junction reduces a polyinnervated population of muscle fibers to a monoinnervated state. The function of this developmental phenomenon (if any) is unproven. A theoretical analysis of Hebbian (correlation) rules connecting presynaptic and postsynaptic activity and synaptic strength at the neuromuscular junction is presented. The following points are demonstrated: (1) Correlational competition leads to the reduction of polyinnervation to a stable monoinnervated state; (2) the competition gives rise to the size principle over a wide range of the plausible parameter space; (3) over a significant subrange, the competition selectively eliminates topographically incorrect synapses; and (4) in cases in which topographic projection errors overwhelm the system, both error correction and the development of the size principle are disrupted. Correlational competition may explain contradictory experimental results concerning the effects of stimulating or silencing subpopulations of motor neurons. It may also explain an otherwise puzzling instance of a breakdown in the size principle seen in humans undergoing neural regeneration. Taken together, these findings suggest a novel hypothesis for the function of synapse elimination at the neuromuscular junction: the establishment of the size principle. © 1995 John Wiley & Sons, Inc.     

Electron states of semiconductor quantum ring with geometry and size variations

Heterostructured semiconductor quantum rings (QR) are studied by a three-dimensional model in a single sub-band approach with energy dependent electron effective mass. Nonlinear confinement energy problem is numerically solved by the finite element method. Effects of boundary conditions corresponding to various QR/substrate configurations are analyzed. General relation for the size dependence of the QR confined energy is derived. The contribution to confinement energy due to the presence of external magnetic field is compared with the effect of variation of QR geometrical parameters. Applicability of the model is tested by comparison with experimental data.     

Viewing geometry determines how vision and haptics combine in size perception

Vision and haptics have different limitations and advantages because they obtain information by different methods. If the brain combined information from the two senses optimally, it would rely more on the one providing more precise information for the current task. In this study, human observers judged the distance between two parallel surfaces in two within-modality experiments (vision-alone and haptics-alone) and in an intermodality experiment (vision and haptics together). In the within-modality experiments, the precision of visual estimates varied with surface orientation, as expected from geometric considerations; the precision of haptic estimates did not. An ideal observer that combines visual and haptic information weights them differently as a function of orientation. In the intermodality experiment, humans adjusted visual and haptic weights in a fashion quite similar to that of the ideal observer. As a result, combined size estimates are finer than is possible with either vision or haptics alone; indeed, they approach statistical optimality.     

Co-Expression of VAL- and TMT-Opsins Uncovers Ancient Photosensory Interneurons and Motorneurons in the Vertebrate Brain

The functional principle of the vertebrate brain is often paralleled to a computer: information collected by dedicated devices is processed and integrated by interneuron circuits and leads to output. However, inter- and motorneurons present in today''s vertebrate brains are thought to derive from neurons that combined sensory, integration, and motor function. Consistently, sensory inter­motorneurons have been found in the simple nerve nets of cnidarians, animals at the base of the evolutionary lineage. We show that light-sensory motorneurons and light-sensory interneurons are also present in the brains of vertebrates, challenging the paradigm that information processing and output circuitry in the central brain is shielded from direct environmental influences. We investigated two groups of nonvisual photopigments, VAL- and TMT-Opsins, in zebrafish and medaka fish; two teleost species from distinct habitats separated by over 300 million years of evolution. TMT-Opsin subclasses are specifically expressed not only in hypothalamic and thalamic deep brain photoreceptors, but also in interneurons and motorneurons with no known photoreceptive function, such as the typeXIV interneurons of the fish optic tectum. We further show that TMT-Opsins and Encephalopsin render neuronal cells light-sensitive. TMT-Opsins preferentially respond to blue light relative to rhodopsin, with subclass-specific response kinetics. We discovered that tmt-opsins co-express with val-opsins, known green light receptors, in distinct inter- and motorneurons. Finally, we show by electrophysiological recordings on isolated adult tectal slices that interneurons in the position of typeXIV neurons respond to light. Our work supports “sensory-inter-motorneurons” as ancient units for brain evolution. It also reveals that vertebrate inter- and motorneurons are endowed with an evolutionarily ancient, complex light-sensory ability that could be used to detect changes in ambient light spectra, possibly providing the endogenous equivalent to an optogenetic machinery.     

The best-fit size and geometry of metal ions for coordination with nitrogen-donor macrocycles

The technique of calculating the strain energy of metal ion complexes as a function of metal to ligand bond length (Hancock and McDougall, J. Am. Chem. Soc., 102 (1980) 6553) is used to study best-fit sizes of metal ions for coordinating with tetraaza and triaza macrocyles. In addition to varying the metal to ligand bond length in the calculations, different coordination geometries of the metal ion are also examined. The metal to nitrogen (M-N) bond lengths, and coordination geometries, that give lowest energies for several N-donor macrocyles, are calculated by molecular mechanics, and 16-aneN₄ (1,5,9,13-tetraazacyclohexadecane) is found, contrary to popular belief, to coordinate best with very small metal ions, with lowest energy occurring for a slightly flattened tetrahedral metal ion of M-N length = 1.81 Å. The best-fit size and geometry for coordination in 12-aneN₄ (1,4,7,10-tetraazacyclotetradecane) is an M-N length of 2.15 Å and square pyramidal geometry, and with cyclam (1,4,8,11-tetraazacyclotetradecane) an M-N length of 2.06 Å and planar coordination that is approximately square.     

The kakapo Mutation Affects Terminal Arborization and Central Dendritic Sprouting of Drosophila Motorneurons

The lethal mutation l(2)CA4 causes specific defects in local growth of neuronal processes. We uncovered four alleles of l(2)CA4 and mapped it to bands 50A-C on the polytene chromosomes and found it to be allelic to kakapo (Prout et al. 1997. Genetics. 146:275– 285). In embryos carrying our kakapo mutant alleles, motorneurons form correct nerve branches, showing that long distance growth of neuronal processes is unaffected. However, neuromuscular junctions (NMJs) fail to form normal local arbors on their target muscles and are significantly reduced in size. In agreement with this finding, antibodies against kakapo (Gregory and Brown. 1998. J. Cell Biol. 143:1271–1282) detect a specific epitope at all or most Drosophila NMJs. Within the central nervous system of kakapo mutant embryos, neuronal dendrites of the RP3 motorneuron form at correct positions, but are significantly reduced in size. At the subcellular level we demonstrate two phenotypes potentially responsible for the defects in neuronal branching: first, transmembrane proteins, which can play important roles in neuronal growth regulation, are incorrectly localized along neuronal processes. Second, microtubules play an important role in neuronal growth, and kakapo appears to be required for their organization in certain ectodermal cells: On the one hand, kakapo mutant embryos exhibit impaired microtubule organization within epidermal cells leading to detachment of muscles from the cuticle. On the other, a specific type of sensory neuron (scolopidial neurons) shows defects in microtubule organization and detaches from its support cells.     

Amplifiers and the handicap principle in sexual selection: a different emphasis

Population genetic models have shown that female choice is a potential cause of the evolution of male display. In these models the display is assumed to be the immediate object of female choice. Here I present an explicit genetic model that shows that male display can evolve as a consequence of female choice even when the display is not the immediate object of choice. When females initially base their preferences on the existence of variance in a cue that is correlated with male viability, a rare display can evolve to fixation if it amplifies the previously recognized differences in males, (i.e. if it increases the resolution power of females with respect to the original cue). By definition, amplifying displays (or amplifiers) increase mating success of the more viable males and decrease mating success of the less viable males. Therefore, the higher the frequency of the preferred, more viable males, the more likely it is that amplifiers will evolve to fixation. The evolution of an amplifier is further facilitated by a genetic association that is built up between the amplifier allele and the more viable allele. If the expression of the amplifier is limited to the more viable males, the amplifier will evolve to fixation provided only that the change in total fitness to the more viable males (higher mating success, lower viability), is positive.     

Vaccine delivery: a matter of size, geometry, kinetics and molecular patterns

Researchers working on the development of vaccines face an inherent dilemma: to maximize immunogenicity without compromising safety and tolerability. Early vaccines often induced long-lived protective immune responses, but tolerability was a major problem. Newer vaccines have very few side effects but can be of limited immunogenicity. One way to tackle this problem is to design vaccines that have all the properties of pathogens with the exception of causing disease. Key features of pathogens that can be mimicked by vaccine delivery systems are their size, shape and surface molecule organization. In addition, pathogen-associated molecular patterns can be used to induce innate immune responses that promote adaptive immunity. In this Review, we discuss the approaches currently being used to optimize the delivery of antigens and enhance vaccine efficacy.     

Nucleolar size in lymphocytes and haemocytes of different species

The number of nucleoli in a cell and nucleolar area vary according to the cell. We compared nucleoli in mammalian circulating lymphocytes and insect circulating haemocytes. An increased nucleolar coefficient correlated with a lowered nucleoli size. The smaller nucleolar size in mammalian lymphocytes indicates a lower proteosynthetic cellular activity in both mammalian lymphocytes and insect haemocytes. Moreover, in insect haemocytes, the smaller size of the nucleoli may reflect a lowered potential to transform into another cell type.     

Body size, locomotion, and long bone cross-sectional geometry in indriid primates.

The geometry of the midshaft cross-sections of the femur and humerus of five indriid species was analysed. Internal (marrow cavity) and external diameters were measured on X-rays in the anteroposterior (a-p) and mediolateral (m-l) planes; cross-sectional areas, second moments of area, and section moduli were calculated using formulae for a hollow ellipse. Cortical thickness, robusticity indices (relating external diameters to the length of the bones), and a-p/m-l shape variables were also calculated. Model II regression was supplemented by analyses of correlation between size and shape. Indriids are saltatory, i.e., their locomotion is dominated by the hind limbs. Accordingly, the femur is more rigid than the humerus, and it shows a consistent difference between the a-p and m-l planes in measures related to bending strength. Cortical thickness varies considerably both within and across species. The type specimen of the new species Propithecus tattersalli is virtually indistinguishable from P. verreauxi on the basis of its long bone cross-sectional geometry. Femoral robusticity is uncorrelated with size, but humeral robusticity decreases significantly with increasing size. Femoral shape variables (a-p/m-l) are all negatively correlated with body size, indicating that m-l dimensions of the femur increase at a faster rate than do a-p dimensions. The highly loaded plane of movement seems to be more reinforced in the smaller species. Contrary to static biomechanical scaling predictions of positive allometry, all cross-sectional parameters scale relatively close to isometry. It is concluded that either changes in locomotor performance must compensate for the weight-related increase in forces and moments or that the larger-bodied animals operate appreciably closer to the limits of their safety margins.     

The Unc-8 and Sup-40 Genes Regulate Ion Channel Function in Caenorhabditis Elegans Motorneurons

Two Caenorhabditis elegans genes, unc-8 and sup-40, have been newly identified, by genetic criteria, as regulating ion channel function in motorneurons. Two dominant unc-8 alleles cause motorneuron swelling similar to that of other neuronal types in dominant mutants of the deg-1 gene family, which is homologous to a mammalian gene family encoding amiloride-sensitive sodium channel subunits. As for previously identified deg-1 family members, unc-8 dominant mutations are recessively suppressed by mutations in the mec-6 gene, which probably encodes a second type of channel component. An unusual dominant mutation, sup-41 (lb125), also co-suppresses unc-8 and deg-1, suggesting the existence of yet another common component of ion channels containing unc-8 or deg-1 subunits. Dominant, transacting, intragenic suppressor mutations have been isolated for both unc-8 and deg-1, consistent with the idea that, like their mammalian homologues, the two gene products function as multimers. The sup-40 (lb130) mutation dominantly suppresses unc-8 motorneuron swelling and produces a novel swelling phenotype in hypodermal nuclei. sup-40 may encode an ion channel component or regulator that can correct the osmotic defect caused by abnormal unc-8 channels.     

Egg size and clutch size in two Daphnia species grown at different food levels

The effect of three different Scenedesmus food concentrations(0.04, 0.2 and 1 mg C l^–1) on maternal investment wasstudied in two cladoceran species of similar size, Daphnia pulicariaand D.hyalina. It was observed that as food concentration decreased(between 1 and 0.2 mg C l^–1), there was an increase insize, protein content, lipid content, carbon and mass of theegg, while, at the same time, the clutch size of the femalesbecame smaller. Such an increase in ‘per offspring investment’was reflected in an increase in body length, body carbon andbody mass of neonates as the food available for females decreased.However, in D.pulicaria this tendency was not maintained downat 0.04 mg C l^–1 in which there was a decrease of theegg characteristics mentioned above. Although, there are notavailable all the egg and neonate parameters of D.hyalina at0.04 mg C l^–1, the body length of the neonates was largerthan at 0.2 and 1 mg C l^–1. These results show that, asfood diminishes, these two cladoceran species are able to respondby decreasing clutch size, but increasing the size of egg, therebyincreasing the probability of neonate survival. This tendencyis probably maintained until the food concentration is too lowand the females have to reduce the energy allocated for reproduction.