Topographic Map Formation Research Articles

Self-organizing semantic maps

Self-organized formation of topographic maps for abstract data, such as words, is demonstrated in this work. The semantic relationships in the data are reflected by their relative distances in the map. Two different simulations, both based on a neural network model that implements the algorithm of the selforganizing feature maps, are given. For both, an essential, new ingredient is the inclusion of the contexts, in which each symbol appears, into the input data. This enables the network to detect the "logical similarity" between words from the statistics of their contexts. In the first demonstration, the context simply consists of a set of attribute values that occur in conjunction with the words. In the second demonstration, the context is defined by the sequences in which the words occur, without consideration of any associated attributes. Simple verbal statements consisting of nouns, verbs, and adverbs have been analyzed in this way. Such phrases or clauses involve some of the abstractions that appear in thinking, namely, the most common categories, into which the words are then automatically grouped in both of our simulations. We also argue that a similar process may be at work in the brain.

Conservation of receptive-field properties of superior colliculus cells after developmental rearrangements of retinal input

The formation of topographic maps requires not only that afferents synapse with the appropriate targets, but that the spatial relationships between the afferents be maintained. During development, in addition to the formation of the topographic map, the connectivity patterns responsible for the receptive-field properties of the target cells are being formed. The extent of interaction between these two processes is unknown. The present study addresses this question by manipulating afferent/target ratios during development, thus altering the topography of the map, and studying the effects of this alteration on the receptive-field properties of single target cells in the adult. Partial unilateral lesions of the superior colliculus (SC) were made in neonatal hamsters. These lesions result in a compression of the retinotopic map onto the remaining collicular fragment. Single cells were recorded from the superficial gray layer of the SC in the adult in response to visual stimuli. Receptive-field properties observed in lesioned animals were compared to those in normal animals and in sham operates. Receptive-field properties were largely unaffected by the change in the topographic map. There was no difference in the receptive-field size of single tectal cells of lesioned and unlesioned animals. Stimulus velocity and stimulus size tuning functions remained the same. This raises the possibility that, rather than the expected increase in convergence of retinal ganglion cells (RGC) onto single collicular cells, single SC cells receive input from ganglion cells representing the same amount of retinal area as in unlesioned animals. The excess ganglion cells created by the partial target removal would then project elsewhere and/or reduce their arbor within the SC. Regardless of the mechanism, it is clear from our results that circuitry in the retinotectal system of the hamster can compensate for conditions of increased afferent availability and thus maintain receptive-field properties.

Formation of topographic maps.

The catalogue of data presented here form many systems demonstrates that multiple mechanisms are involved in the formation of topographic maps. We are not yet in a position to explain why a particular mechanism appears to dominate in some situations and not in others. Certain generalizations can be made, however. First, at least some form of chemospecificity can be invoked to help explain connectivity in all of the experiments we have cited. Often, the differential identities of a population of neurons can be reflected in an orderly pattern of axon outgrowth and in the actively maintained preservation of neighbor relations as the axons grow toward their targets; such orderly arrangements are not obligatory, but, where present, they facilitate the speedy establishment of orderly maps when the axons reach their target nuclei. Within a terminal zone, chemospecific cues may dominate and constrain a given axon to terminate in a specific location, but axon-axon interactions commonly supercede chemospecific matching. At least two forms of axon-axon interaction occur, one based on some sort of biochemical properties related to the axon's embryological identity and another based on the axons' electrical activity. Tasks for the future are to identify the cellular bases of each of these mechanisms and to understand the situations in which each is manifested.

Magnification, receptive-field area, and "hypercolumn" size in areas 3b and 1 of somatosensory cortex in owl monkeys.

Magnification, receptive-field area, and "hypercolumn" size in areas 3b and 1 of somatosensory cortex in owl monkeys.M Sur, M M Merzenich, and J H KaasM Sur, M M Merzenich, and J H KaasPublished Online:01 Aug 1980https://doi.org/10.1152/jn.1980.44.2.295MoreSectionsPDF (3 MB)Download PDF ToolsExport citationAdd to favoritesGet permissionsTrack citations ShareShare onFacebookTwitterLinkedInWeChat Previous Back to Top Next Download PDF FiguresReferencesRelatedInformation Cited ByTactile innervation densities across the whole bodyGiulia Corniani and Hannes P. Saal19 October 2020 | Journal of Neurophysiology, Vol. 124, No. 4Biases in tactile localization by pointing: compression for weak stimuli and centering for distributions of stimuliJack Brooks, Tatjana Seizova-Cajic, and Janet L. 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Formation of topographic maps and columnar microstructures in nerve fields.

Topographic connections are found in many parts of the vertebrate nervous systems, known for example as retinotopy. The self-organizing ability of Hebb type modifiable synapses plays an important role in forming, at least in refining, the topographic connections. We present a mathematical analysis of a revised version of the Willshaw-Malsburg model of topographic formation, solving the equations of synaptic self-organization coupled with the field equation of neural excitations. The equilibrium solutions are obtained and their stability is studied. It is proved that two cases exist depending on parameters. In one case, the smooth topographic organization is obtained as a stable equilibrium of the equations. In the other case, this solution becomes unstable, and instead the topographic organization with columnar microstructures appears. This might explain the columnar structures in the cerebrum. The theory is confirmed by computer simulated experiments.