752 nature neuroscience • volume 3 no 8 • august 2000

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this area (A) should be a constant inde-pendent of θ, k, E, x and y. They calculat-ed the variance of A over a representativeset of values for all parameters, and tookthis variance, normalized by the mean, asa measure of the departure from unifor-mity of coverage — the smaller the valueof this quantity (they refer to this as C′),the more homogeneous the coverage. Tosee how optimal the coverage was for themaps obtained, the authors then distort-ed the arrangement of maps by progres-sively rotating, mirror-reflecting or shiftingone or more maps with respect to the oth-ers. Each such relative motion introducedentirely new local interrelations betweenthe different maps at any cortical point,even though the total distributions of val-ues for each map remained the same. Typ-ically, they found that each step of relativemotion led to progressively increased val-ues of C′ over the original, implying thatany departure from the real maps wors-ened the coverage.

Such an experimental confirmation ofoptimal coverage has important conse-quences for theories of neural develop-ment. Neural circuits in V1 undergo anextensive process of refinement in theimmature brain, driven by patterned neur-al activity, before taking their adult form.The results of Swindale et al. give strongsupport to models of cortical developmentthat propose, as the principle behind neur-al refinement, mechanisms that optimizethe fitting of multiple maps on the two-dimensional cortical surface by minimiz-ing the axonal connection lengths betweenneurons of similar RF properties5.

The present results also suggest a num-ber of interesting directions in which theauthors’ exploration of coverage could beextended. The present authors have used

as those examined by Swindale and col-leagues, point to a dramatic expansion incell number between retina and V1.Unlike the retina, where each major classof retinal ganglion cells forms a sheet ofclose-packed RFs tiling visual space withlittle overlap8, V1 has a 100- to 1,000- foldmultiplicity in the number of neuronsrepresenting each point in space if theneurons were solely engaged in repre-senting the local orientation, direction,ocular dominance and other elementaryfeatures that arise de novo in cortex. Thisexpansion in cell number could beresolved by our growing understandingthat V1 cells are involved in many tasksbeyond just extracting local elementaryfeatures: V1 neurons combine these sim-ple elements to extract complex featuressuch as smooth contours or corners ortexture boundaries, are capable of modi-fying their responses as we get better atperceptual discrimination tasks, and evenadjust their responses depending on thecontext of a specific task9.

1. Swindale, N. V., Shoham, D., Grinvald, A.,Bonhoeffer,T. & Hübener, M. Nat. Neurosci. 3,822–826 (2000).

2. Hubel, D. H. & Wiesel, T. W. Proc. R. Soc.Lond. B 198, 1–59 (1977).

3. Obermayer, K. & Blasdel, G. G. J. Neurosci. 13,4114–4129 (1993).

4. Hübener, M., Shoham, D., Grinvald, A. &Bonhoeffer, T. J. Neurosci. 17, 9270–9284(1997).

5. Swindale, N. V. Network 7, 161–247 (1996).

6. White, L. E., Bosking, W. H., Wiliams, S. M. &Fitzpatrick, D. J. Neurosci. 19, 7089–7099(1999).

7. Das, A. & Gilbert, C. D. Nature 387, 594–598(1997).

8. Cleland, B. G., Levick, W. R. & Wässle, H. J. Physiol. (Lond.) 248, 151–171 (1975).

9. Gilbert, C. D. Physiol. Rev. 78, 467–485 (1998).

only ‘rigid body’ changes in the maps —where each map is shifted, rotated orreflected as a rigid object. But it is also pos-sible to imagine distortions of one mapwith respect to the other that treat the mapslike rubber sheets, locally stretching orcompressing them while maintaining theouter boundaries and the overall statisticsunchanged. Because the authors would liketo conclude that real maps are optimalwhen compared against any possible dis-tortion, it would be necessary to includesuch local distortions along with the rigidbody changes explored empirically by theauthors. Extending the range of relativechanges in maps to include these local dis-tortions should support and strengthen theauthors’ conclusions because local relativedistortions between different maps are like-ly to degrade the local relationships thatexist between the different maps, and thusworsen the uniformity of coverage overall.This should extend their results to obtain amathematically complete exploration of afull range of distortions.

Also, the authors have assumed that thebase map of space is uniform6, and that onthis map, the cortical area devoted to anypoint x, y is of uniform size independent ofits position. Some recent results suggest,however, that the map of space has period-ic local inhomogeneities mirroring singu-larities in the map of orientation7. Therefore,the intersection of the cortical area x, y withthe cortical area for orietation, ocular dom-inance or spatial frequency would be differ-ent from the intersection obtained byassuming a uniform map of x, y. It wouldbe valuable to know whether coverageimproves with such maps of retinal position.

Finally, estimates of the number of V1neurons required for optimal coverage ofthe elementary response properties, such

Mapping Drosophila olfactory axonsThe primary olfactory organs of Drosophila, the third antennal segment and the maxillarypalp, carry chemosensory bristles whose neurons project to glomeruli of the antennallobe, analogous to the vertebrate olfactory bulb. In mammals, neurons expressing thesame olfactory receptors converge onto the same glomerulus. Whether this is true forinsects is unclear: although individual glomeruli in bees respond to specific odors, themolecular structure of the insect olfactory system had not been explored. Chess andcolleagues (pages 780-785, this issue) now address this question by using transgenic fliesin which a marker gene is driven by the regulatory sequences that normally contrololfactory receptor gene expression. This allowed the authors to visualize the projections ofindividual olfactory neurons. Axons from neurons expressing a given olfactory receptorgene converged onto a small number of glomeruli, in a pattern that was invariant betweenindividuals. This stereotypical wiring pattern presumably underlies the formation of an odotopic map in the antennal lobe. Given thepower of Drosophila genetics, it should now be possible to determine how these connections are established and maintained.

Kalyani Narasimhan

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