Cortical Maps: Data and Models

Half a century ago, the pioneering investigations of Mountcastle, Hubel and Wiesel began to reveal an intricate small-scale spatial mapping of functional properties, such as ocular dominance and orientation preference in the primary visual cortex. Since then, topographic ordering of function has been demonstrated in auditory, somatosensory, gustatory and primary motor cortices. But while maps seem to be ubiquitous in the primary sensory cortical areas, many questions about their significance remain. Might they simply be an epiphenomenon of development with no real functional significance? How widespread are maps in the cortex? For example are there maps of speech properties in Broca’s area? Are there maps in frontal cortex? It is known that in some species map development begins before birth and continues for some time afterward. To what extent are the details of the maps present in an individual dependent on their genetic endowment or on patterns of neural activity resulting from sensory experiences and motor behaviors (some of which may occur in utero)?

Despite the importance of these questions, the answers to them are unknown. If maps are a by-product of development and only present in a few special cases, then it might seem that there would be little point in persevering in studying them. However, this attitude might be naive. If maps are dismissed as epiphenomena then no further search for their function is likely to be made. But, if there is an important function, it may then be missed. Even if maps are epiphenomena, answering the second question – whether or not spatial order is ubiquitous – would still be important. Knowing that cortical functions are always likely to be related to the position, and possibly in a predictable way, would be enormously useful. For example, knowing the location of the fusiform face area is of practical importance in imaging experiments; knowing that neighboring areas are likely to be functionally related may lead more quickly to finding out what those functions are. Just as the physical locations of genes on the chromosome are functionally relevant, the location and spatial relations between different columns may be related to their function.

The extent to which maps are pre-specified by genetic instructions has no complete answer at present. Although the role of molecular labels in establishing retinotopy is well established it is not clear if molecular labels guide the formation of ocular dominance columns, although there is suggestive evidence that labels might be involved. Orientation columns are also a puzzle, because it seems they can develop in the absence of natural visual stimulation but it is not clear how this could happen. Retinal waves may not have enough structure nor are it easy for models based on them to explain how matched preferences can develop in the two eyes. Their periodic structure has also been especially hard to capture.

With these questions in mind, we propose a Research Topic collection of articles on the spatial organization of mammalian cerebral cortex. The scope will include developmental studies, theoretical and computational models, studies of adult organization, comparative studies, studies relating map structure to behavior, neuroethological perspectives, methodological advances and perspectives from neuroinformatics. Experimental techniques can include fMRI, meso-scale and cellular level imaging, molecular techniques and electrophysiology. The unifying theme is the presence of some relation between position and function in the cortex.