Cortical integration of parallel pathways in the visual system of primates

Injections of wheatgerm agglutinin conjugated with horseradish peroxidase (WGA-HRP) were placed in the middle temporal visual area (MT) of squirrel monkeys to reveal the distributions of interconnections with functionally distinct modules in areas 17 and 18. In agreement with previous reports, brain sections cut parallel to the surface of manually flattened cortex and reacted for cytochrome oxidase (CO) revealed CO dense blobs in area 17 and alternating CO dense thick and thin bands separated by CO light interbands in area 18. Alternate sections stained for myelin indicated that the CO light interblobs and interbands are more densely myelinated than the CO dense blobs and bands. Our major finding is that projections from MT to areas 17 and 18 are both to modules projecting to MT and modules projecting to other targets. In area 17, the cells in the middle layers projecting to injection sites in MT typically were distributed in several short merging and diverging rows, suggesting the convergence of projections from several matched orientation columns in area 17 to the restricted injection site in MT. Backward projections from MT to more superficial layers in area 17 were distributed more evenly across cortex and over a wider area of cortex. These terminations were dense throughout the interblob cortex which includes all orientation columns and neurons projecting to area 18, but were light over the blobs. As previously reported, neurons in area 18 projecting to MT were located in one set of the CO dense bands. However, these bands appeared to be thin rather than thick.(ABSTRACT TRUNCATED AT 250 WORDS)