Organization of the cat's optic tract as assessed by single-axon recordings

The organization of the cat's optic tract was assessed from recordings of single axons during vertical electrode penetrations. To analyse both the order and the scatter of axons in the optic tract, units were pooled from electrode tracks that had evidently passed through nearly equivalent parts of the tract. Receptive field elevation, the most strongly ordered parameter, was primarily organized horizontally, increasing from posteromedial to anterolateral tract. Azimuth tended to increase from dorsal to ventral, but was only half as well organized vertically as elevation was horizontally. Axon type was also organized vertically, at least among axons that were identified as X or Y (contralateral axons of less than 20 degrees eccentricity). Axons of ON-center X-type were located mostly in the dorsal quarter, OFF-center X-type in the dorsal two-thirds, and Y-type (ON- and OFF-center) in the ventral two-thirds of the tract. Another difference between the horizontal and vertical axes was revealed by estimates that axons of one type, from one retinal locus, spread about twice as far vertically as horizontally in the tract. Torrealba et al. ('82) have proposed that the sequential addition of ingrowing axons to the pial surface is a major source of the order in the tract. The differences between horizontal and vertical order seen here suggest an extension of this proposal: the sequential addition of axons to the ventral surface generates the vertical order, while some other process arranges axons horizontally, along the ventral surface, according to receptive field elevation (or some similar one-dimensional retinal coordinate).