Development of Continuous and Discrete Neural Maps

Howard Hughes Medical Institute, Department of Biological Sciences, Stanford University, Stanford, CA 94305, USA.
Neuron (Impact Factor: 15.05). 11/2007; 56(2):284-300. DOI: 10.1016/j.neuron.2007.10.014
Source: PubMed


Two qualitatively different kinds of neural map have been described: continuous maps exemplified by the visual retinotopic map, and discrete maps exemplified by the olfactory glomerular map. Here, we review developmental mechanisms of retinotopic and olfactory glomerular mapping and discuss underlying commonalities that have emerged from recent studies. These include the use of molecular gradients, axon-axon interactions, and the interplay between labeling molecules and neuronal activity in establishing these maps. Since visual retinotopic and olfactory glomerular maps represent two ends of a continuum that includes many other types of neural map in between, these emerging general principles may be widely applicable to map formation throughout the nervous system.

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    • "In the visual and somatosensory systems, the discrimination of stimuli located in different positions in the sensed environment is achieved by their representation in topographic (or continuous) maps in the brain. For example, neighboring activated retina cells, representing adjacent sources of light in the visual field, send projections to neighboring neurons in the thalamus and visual cortices, such that the ordering of sensory stimuli in the external world is represented by ordered maps of neural activity in the brain (Luo and Flanagan, 2007). In contrast, gustatory information is represented in a non-continuous, or discrete, fashion, where different taste qualities, such as sweet, bitter, umami, and salty, resulting from the detection of the corresponding tastants in the upper digestive system, are each represented by cohorts of activated neurons in discrete sub-areas of the primary taste cortex in the brain (Chen et al., 2011). "
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    • "The cellular and molecular mechanisms underlying many steps of mammalian brain circuit assembly have been explored in detail. Prominent examples include growth cone navigation through intermediate choice points (Dickson, 2002), topographic mapping (Luo and Flanagan, 2007; Cang and Feldheim, 2013), and laminar specificity (Huberman et al., 2010; Robles and Baier, 2012; Baier, 2013). Among the lesser understood steps involved in circuit assembly, however, is axon-target matching. "
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    • "In others, such as in the gustatory system, the arrangement corresponds to the qualities of the stimuli, such that similar stimuli are represented on nearby positions of the perceptual space. Not every topographic map is a continuous function according to modern usage of the term, although a continuous function satisfies the properties of a topographic map and historically they have been used synonymously (Luo & Flanagan, 2007). However, the same principles that guide the arrangement of neurons composing a topographic map also tend towards continuous functions. "
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