Retinotopy Multiple Cortical Visual Areas

The first application of fMRI-based research was in the domain of the early stages of visual processing. Indeed, the very first human fMRI study involved the demonstration that a region of the brain associated with early visual processing—occipital cortex in the calcarine fissure—yielded an NMR signal that varied as flashing lights were presented (or not) to a subject.

This demonstration was exciting, but the excitement was limited for several reasons. First, nothing "new" had been demonstrated about human visual cortex. Second, there were a host of technical concerns that, had they been correct, would have meant that the spatial resolution obtainable with fMRI would be seriously compromised. Finally, most of the following simple advances would not go beyond what we already know from (invasive) single cell recordings in nonhuman primates.

However, the development of fMRI in the ensuing years for the study of early visual processing addressed all these concerns and went far beyond them. First, retinopy was demonstrated for area V1 at a level of spatial resolution that exceeded any previously demonstrated with a noninvasive technique. Second, retinotopy was used to delineate multiple visual areas. Differences between the layout of human visual areas compared with those of other primate species were demonstrated, and new visual areas apparently unique to humans were claimed.

At the same time, some classic psychological effects, such as the motion aftereffect, were seen to be associated with detectable brain activity localized to specific parts of the cortex associated with visual motion processing.

The early dependence on the connection to known primate neurophysiology is being lessened. Several laboratories are now developing fMRI suites designed specifically to study nonhuman primates. The idea is to use the invasive technologies such as single cell recording, adapted for the MR environment, to obtain a deeper understanding of both the functional brain structures and the relationship between neural activity and hemodynamics using methods that would be unethical with human subjects.

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