Atmospheric trace gases: linear relation between concentration and time.

that of the host eye only. Previous work on the neuromuscular and various other systems including the retinotectal projection also indicate no direct inhibitory effect of 1TX on axoplasmic transport (12) nor on the capacity offibers to grow, arborize, and form synapses (13). Histology on 12 representative retinas revealed no pathology. The formation of gross retinotopography-as shown by the growth of donor fibers to the posterior dorsal quadrant and by previous work on developing urodeles-is insensitive to TrX (15) and is thus pharmacologically distinguishable from column formation. To some extent, then, the mechanisms producing gross topography must be different from those giving rise to columns. Column formation by regenerating fibers in the mature goldfish parallels the normal development of cortical columns in neonatal mammals. In both, there is an initial stage of extensive overlap between fibers designated to segregate (2, 4). In both, segregation is modulated by impulse activity; that is, it is inhibited by its elimination (6) or retarded by its decrement (5). For goldfish, and therefore by -analogy for mammals, overlap is not a trivial consequence of developmental immaturity, that is, the role of activity is more than a nonspecific trophic factor for neural growth and differentiation. Neighboring ganglion cells in goldfish retina are positively coupled, exhibiting positively correlated spontaneous activity (15). In this light, the present results can be taken to support the idea that columns are generated according to the rule that fibers that fire together, terminate together. This rule, together with correlated spontaneous activity, could explain why column formation in the goldfish (16) or the monkey (2, 4) is not prevented by continuous darkness. Studies in cats and monkeys showing that monocular deprivation reduces the size of cortical ocular dominance columns corresponding to the deprived eye (4) has led to the idea that some competition between fibers, perhaps linked to impulse activity, underlies the formation of these columns. However, the finding that TTX blockade of one eye did not prevent its fibers from forming columns shows that blocked fibers still retain a significant capacity to compete with unblocked fibers for tectal space. RONALD L. MEYER Department ofDevelopmental and Cell Biology and Developmental Biology Center, University of California, Irvine 92717 References and Notes