Complete flatmounting of the macaque cerebral cortex.

The elaborate folding of the brain surface has posed a practical impediment to investigators engaged in mapping the areas of the cerebral cortex. This obstacle has been overcome partially by the development of methods to erase the sulci and gyri by physically flattening the cortex prior to sectioning. In this study, we have prepared a step-by-step atlas of the flatmounting process for the entire cerebral cortex in the macaque monkey. The cortex was dissected from the white matter, unfolded, and flattened in a single piece of tissue by making three relieving cuts. The flatmount was sectioned at 60-75 microm and processed for cytochrome oxidase (CO) or myelin. From animal to animal there was nearly a twofold variation in the surface area of individual cortical regions, and of the whole cortex. In each specimen, a close correlation was found between V1 surface area (mean = 1343 mm2), V2 surface area (mean = 1012 mm2), hippocampal area (mean = 181 mm2), and total cerebral cortex area (mean = 10,430 mm2). The complete pattern of CO stripes in area V2 was labeled clearly in several cases; the number of cycles of thick-pale-thin-pale stripes ranged from 26 to 34. Characteristic patterns of strong CO activity were encountered in areas V3, MT, auditory and somatosensory cortex. In some animals we made injections of a retrograde tracer, gold-conjugated cholera toxin B subunit, into area V2 to identify all sources of cortical input. In addition to previously described inputs, we identified three new regions in the occipitotemporal region that project to V2. Flatmounting the cerebral cortex is a simple, efficient method that can be used routinely for mapping areas and connections in the macaque brain, the most widely used primate model of the human brain.