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Scientific Understanding of Consciousness |
Visual Circuitry Motion Detection
Nature, 500, 154–155 (08 August 2013) Accurate maps of visual circuitry Richard H. Masland Departments of Ophthalmology and Neurobiology, Harvard Medical School, Boston, Massachusetts 02114, USA. [paraphrase] The mammalian retina contains more than 60 different types of neuron, each of which has a distinct morphology and carries out a different function. Within the retina, photoreceptor cells sense light, and their output is processed by amacrine, horizontal and bipolar cells. Downstream, roughly 20 different types of retinal ganglion cell transmit the final coded signal — 20 different representations of the visual input — to the brain. Unsurprisingly, therefore, sorting out neuronal connectivity in the retina has been a daunting task. Researchers now report a connectome (a list of all synaptic connections) for an inner layer of the mouse retina. They achieve this by serial tissue sectioning and electron microscopy, followed by digital reconstruction of cells within the virtual three-dimensional solid that results. The analysis reveals patterns of connections that could account for the stimulus selectivity of two types of ganglion cell. More fundamentally, the reconstruction, which contains 950 neurons allows definitive classification of the types of bipolar cell. With only a slight refinement, the new classification matches extremely well with the existing understanding of these cells, which was based largely on the identification of molecular markers using light microscopy. Researchers now provide enormously more precise descriptions of bipolar-cell structure and by effectively acting as a positive control, increasing confidence that analysis of the amacrine and ganglion cell types, which have resisted classification by previous techniques, will be equally definitive. And this is just the beginning: once these cell types are classified, the same basic methods should allow the synaptic connections among them to be deciphered. Researchers now report progress on a classic problem of neural computation — the detection of visual motion. Their test system is the eye of the fruitfly, an animal that must rapidly navigate during flight and that is especially effective at dodging predators (doubters are invited to swat one). It is easy to make simple models of motion detection, but pinning the mechanism to precise neural events has been much harder. Whereas photoreceptor cells cannot detect direction, downstream neurons called tangential cells are robustly tuned to the direction of movement. Somewhere in between lies the neural mechanism that creates the directional discrimination, but the crucial neurons, called T4 and T5, are too small for ordinary electrical recording. Researchers got around this difficulty by recording activity optically, using an indicator protein introduced into the cells by genetic techniques. Researchers demonstrate that T4 and T5 detect visual movement, with subsets of each being selective for one of four cardinal directions: upward, downward, front to back, and back to front. Furthermore, these cells are sensitive to opposite visual contrasts — T4 cells respond to light ON and so are sensitive to light edges, whereas T5 cells respond to light OFF and are sensitive to dark edges. The authors' genetic-knockout experiments not only confirm this optical observation but also show that T4 and T5 are the sole pathways mediating these functions, with no other cells being able to step in and carry the message. Thus, a fly initially breaks down moving visual inputs into a total of eight components: bright edges moving up, down, forward or backward, and dark edges moving along the same four axes. [end of paraphrase]
Nature, 500, 212–216 (08 August 2013) A directional tuning map of Drosophila elementary motion detectors Max Planck Institute of Neurobiology, 82152 Martinsried, Germany Matthew S. Maisak, Juergen Haag, Georg Ammer, Etienne Serbe, Matthias Meier, Aljoscha Leonhardt, Tabea Schilling, Armin Bahl, Dierk F. Reiff, Elisabeth Hopp & Alexander Borst Janelia Farm Research Campus, Ashburn, Virginia 20147, USA Gerald M. Rubin & Aljoscha Nern Institute of Molecular Pathology, 1030 Vienna, Austria Barry J. Dickson Institute Biology, Albert-Ludwigs University, 79085 Freiburg, Germany. Dierk F. Reiff [paraphrase] The extraction of directional motion information from changing retinal images is one of the earliest and most important processing steps in any visual system. In the fly optic lobe, two parallel processing streams have been anatomically described, leading from two first-order interneurons, L1 and L2, via T4 and T5 cells onto large, wide-field motion-sensitive interneurons of the lobula plate. Therefore, T4 and T5 cells are thought to have a pivotal role in motion processing; however, owing to their small size, it is difficult to obtain electrical recordings of T4 and T5 cells, leaving their visual response properties largely unknown. We circumvent this problem by means of optical recording from these cells in Drosophila, using the genetically encoded calcium indicator GCaMP5. Here we find that specific subpopulations of T4 and T5 cells are directionally tuned to one of the four cardinal directions; that is, front-to-back, back-to-front, upwards and downwards. Depending on their preferred direction, T4 and T5 cells terminate in specific sublayers of the lobula plate. T4 and T5 functionally segregate with respect to contrast polarity: whereas T4 cells selectively respond to moving brightness increments (ON edges), T5 cells only respond to moving brightness decrements (OFF edges). When the output from T4 or T5 cells is blocked, the responses of postsynaptic lobula plate neurons to moving ON (T4 block) or OFF edges (T5 block) are selectively compromised. The same effects are seen in turning responses of tethered walking flies. Thus, starting with L1 and L2, the visual input is split into separate ON and OFF pathways, and motion along all four cardinal directions is computed separately within each pathway. The output of these eight different motion detectors is then sorted such that ON (T4) and OFF (T5) motion detectors with the same directional tuning converge in the same layer of the lobula plate, jointly providing the input to downstream circuits and motion-driven behaviours. [end of paraphrase]
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