Structure of reciprocal connections of visual cortical fields 17 and 18 in the cat

Microiontophoretic application of horseradish peroxidase to individual columns in fields 17 and 18 of the cat cortex was used to identify the distribution by area and layer of retrograde labeled cells in both fields. After application of marker to fields 17 or 18, the area of labeled cells in field 17 (in the tangential plane) was extended and orientated along the projection of the horizontal meridian of the field of vision. The area of labeled cells in field 18 in these cases was orientated along the projections of the vertical meridian. Similar differences in the organization of the connections of fields 17 and 18 were seen in the projection zone of the central 10 degrees of the field of vision at different elevations. Thus, the spatial distributions of internal and external connections in each field coincide and their orientations in fields 17 and 18 are mutually perpendicular. It is suggested that field 17 performs the more detailed analysis of information on the horizontal components of an image and communicates this to field 18, while field 18 is responsible for the more detailed analysis of information about the vertical components of the same image, communicating this to field 17.