Mapping the parts of higher level cortex
Understanding brain dynamics requires knowing what the parts of the brain are. Human neuroimaging has attempted using contrasts between high level cognitive tasks averaged across subjects in 3-D. Two problems are: (1) higher level tasks generate activity in multiple cortical areas, some of which adjoin each other; and (2) cross-subject 3-D averages must use blurring kernels close to the modal size of human cortical areas (1 cm) to overcome anatomical variation and variation in how subjects perform tasks. Even liberal statistical thresholds underestimate the area of cortex involved, and activation borders only accidentally represent cortical area borders. Another way to subdivide cortex is to find topological maps (e.g., retinotopic, head-centred, tonotopic, somatotopic, musculotopic, and movement-o-topic). Topological retinal maps were expected in V1 and early secondary visual areas based on non-human primate data. However, recent work in parietal, temporal, cingulate, and frontal cortex shows that these maps are present at higher levels, extending to the boundaries between modalities (e.g., VIP). This was not expected on the basis of work in animals because higher areas have larger receptive fields with a substantial degree of scatter. Independent manipulation of stimulus and attention shows that higher level maps are largely maps of attention. Three possible reasons why spatial maps might persist at high levels are: (1) intracortical connections are overwhelmingly local; (2) sensory space (retinal, frequency, skin position) is the most important feature for distinguishing events; and (3) cortical space remains a convenient way to allocate processing, even if it is not explicitly spatial (e.g., comprehension of time metaphors, motor representations of place of consonant articulation).