ESTIMATES OF THE CONVERGENCE OF ASSOCIATION PROJECTIONS FROM AREA-17 TO ROSTRAL AREA-18 IN THE CAT DEPEND ON INJECTION SITE SIZE

To estimate the degree of convergence of corticocortical projections f rom area 17 to the rostral part of area 18 in the cat, we injected the retrograde tracer diamidino yellow either at single or multiple adjac ent sites in area 18. We measured the diameters of the individual or l arger composite injections and the diameters of the labelled regions i n area 17; from these values we estimated convergence factors. Small i njections led to underestimates of the overall convergence of this pat hway. A maximal value for convergence (about 3 mm) was obtained with l arger composite injections (above about 3 mm in diameter). These resul ts indicate that some area 17-to-18 projections may arborize widely in area 18. Their detection may require that injected tracer covers a la rge portion of their arbors.