COMPUTATIONAL STRUCTURE OF VISUAL CALLOSAL AXONS

We analysed the activation profiles obtained by simulating invasion of an orthodromic action potential in eleven anterogradely filled and se rially reconstructed terminal arbors of callosal axons originating and terminating in areas 17 and 18 of the adult cat. This was done in ord er to understand how geometry relates to computational properties of a xons. In the simulation, conduction from the callosal midline to the f irst bouton caused activation latencies of 0.9 - 3.2 ms, compatible wi th published electrophysiological values. Activation latencies of the total set of terminal boutons varied across arbors between 0.3 and 2.7 ms. Arbors distributed boutons in tangentially segregated terminal co lumns spanning one or, more often, several layers. Individual columns of one axon were frequently activated synchronously or else within a f ew hundred microseconds of each other. Synchronous activation of spati ally separate columns is achieved by: (i) long primary or secondary br anches of similar calibre running nearly parallel to each other for se veral millimetres; (ii) variations in the calibre of branches serially fed to separate columns by the same primary or secondary branch; (iii ) exchange of high-order or preterminal branches across columns. The l ong, parallel branches blatantly violate principles of axonal economy. Simulated alterations of the axonal arbors indicate that similar spat iotemporal patterns of activity could, in principle, be obtained by le ss axon-costly architectures. The structure of axonal arbors, therefor e, may not be determined solely by the type of spatiotemporal activati on profiles it achieves in the cortex but also by other constraints, i n particular those imposed by developmental mechanisms.