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- Neuroscience x
Thomas W. Cronin, N. Justin Marshall, and Roy L. Caldwell
The predatory stomatopod crustaceans, or mantis shrimp, are among the most attractive and dynamic creatures living in the sea. Their special features include their powerful raptorial appendages, used to kill, stun, or disable other animals (whether predators, prey, or competitors), and their highly specialized compound eyes. Mantis shrimp vision is unlike that of any other animal and has several unique features. Their compound eyes are optically triple, each having three separate regions that produce overlapping visual fields viewing certain regions of space. They have the most diverse set of spectral classes of receptors ever described in animals, with as many as 16 types in a single compound eye. These receptors are based on a highly duplicated set of opsin molecules paired with strongly absorbing photostable filters in some photoreceptor types. The receptor set includes six ultraviolet types, all spectrally distinct, many themselves tuned by photostable filters. There are as many as eight types of polarization receptors of up to three spectral classes (including an ultraviolet class). In some species, two sets of these receptors analyze circularly polarized light, another unique capability. Stomatopod eyes move independently, each capable of visual field stabilization, image foveation and tracking, or scanning of image features. Stomatopods are known to recognize colors and polarization features and evidently use these in predation and communication. Altogether, mantis shrimps have perhaps the most unusual vision of any animal.
Jose M. Alonso and Harvey A. Swadlow
The thalamocortical pathway is the main route of sensory information to the cerebral cortex. Vision, touch, hearing, taste, and balance all depend on the integrity of this pathway that connects the thalamic structures receiving sensory input with the cortical areas specialized in each sensory modality. Only the ancient sense of smell is independent of the thalamus, gaining access to cortex through more anterior routes. While the thalamocortical pathway targets different layers of the cerebral cortex, its main stream projects to the middle layers and has axon terminals that are dense, spatially restricted, and highly specific in their connections. The remarkable specificity of these thalamocortical connections allows for a precise reconstruction of the sensory dimensions that need to be most finely sampled, such as spatial acuity in vision and sound frequency in hearing. The thalamic axon terminals also segregate topographically according to their stimulus preferences, providing a simple principle to build cortical sensory maps: neighboring values in sensory space are represented by neighboring points within the cortex.
Thalamocortical processing is not static. It is continuously modulated by the brain stem and corticothalamic feedback based on the level of attention and alertness, and during sleep or general anesthesia. When alert, visual thalamic responses become stronger, more reliable, more sustained, more effective at sampling fast changes in the scene, and more linearly related to the stimulus. The high firing rates of the alert state make thalamocortical synapses chronically depressed and excitatory synaptic potentials less dependent on temporal history, improving even further the linear relation between stimulus and response. In turn, when alertness wanes, the thalamus reduces its firing rate, and starts generating spike bursts that drive large postsynaptic responses and keep the cortex responsive to sudden stimulus changes.
Kerrianne Ryan and Ian A. Meinertzhagen
Urochordates are chordate siblings that comprise the following marine invertebrates: the sessile Ascidiaceae, or sea squirts; planktonic Larvacea; and the pelagic salps, doliolids, and pyrosomes (collectively the Thaliacea), each more beautiful than the next. Tadpole larvae of ascidians and adult larvaceans both have a body plan that is chordate, with a notochord and dorsal, tubular nervous system that forms from a neural plate. Thalaciacea have a ganglion developed from a tubular structure, which has been compared to the vertebrate mes-metencephalic region, and while salps have well developed eyes, other anterior brain components are absent, and the connections within their central nervous system, as well as the neurobiology of other Thaliacea are all little reported. The ascidian tadpole larva is extensively reported, especially in the model species Ciona intestinalis, as is the caudal nerve cord in the larvacean Oikopleura dioica.
Chordate features that share proposed homology with vertebrate features include ciliary photoreceptors that hyperpolarize to light, descending decussating motor pathways that resemble Mauthner cell pathways, coronet cells in the ascidian larva and saccus vasculosus of fishes, the neural canal’s Reissner’s fiber; secondary mechanoreceptors that resemble hair cells; and ascidian bipolar cells that are like dorsal root ganglion cells.
Sabine Kastner and Timothy J. Buschman
Natural scenes are cluttered and contain many objects that cannot all be processed simultaneously. Due to this limited processing capacity, neural mechanisms are needed to selectively enhance the information that is most relevant to one’s current behavior and to filter unwanted information. We refer to these mechanisms as “selective attention.” Attention has been studied extensively at the behavioral level in a variety of paradigms, most notably, Treisman’s visual search and Posner’s paradigm. These paradigms have also provided the basis for studies directed at understanding the neural mechanisms underlying attentional selection, both in the form of neuroimaging studies in humans and intracranial electrophysiology in non-human primates. The selection of behaviorally relevant information is mediated by a large-scale network that includes regions in all major lobes as well as subcortical structures. Attending to a visual stimulus modulates processing across the visual processing hierarchy with stronger effects in higher-order areas. Current research is aimed at characterizing the functions of the different network nodes as well as the dynamics of their functional connectivity.
Pedro Martínez, Volker Hartenstein, and Simon G. Sprecher
The emergence and diversification of bilateral animals are among the most important transitions in the history of life on our planet. A proper understanding of the evolutionary process will derive from answering such key questions as, how did complex body plans arise in evolutionary time, and how are complex body plans “encoded” in the genome? the first step is focusing on the earliest stages in bilaterian evolution, probing the most elusive organization of the genomes and microscopic anatomy in basally branching taxa, which are currently assembled in a clade named Xenacoelomorpha. This enigmatic phylum is composed of three major taxa: acoel flatworms, nemertodermatids, and xenoturbellids. Interestingly, the constituent species of this clade have an enormously varied set of morphologies; not just the obvious external features but also their tissues present a high degree of constructional variation. This interesting diversity of morphologies (a clear example being the nervous system, with animals showing different degrees of compaction) provides a unique system in which to address outstanding questions regarding the parallel evolution of genomes and the many morphological characters encoded by them. A systematic exploration of the anatomy of members of these three taxa, employing immunohistochemistry, in situ hybridization, and high-throughput transmission electron microscopy, will provide the reference framework necessary to understand the changing roles of genes and gene networks during the evolution of xenacoelomorph morphologies and, in particular, of their nervous systems.