Retinotopic mapping reveals extrastriate cortical basis of homonymous quadrantanopia

It has been conventionally assumed that cortically based quadrantic visual field deficits (homonymous quadrantanopias) are caused by lesions in striate cortex (V1), extending precisely to the horizontal meridian representation. A more recent model, supported by anatomic MRI evidence, consists of an exclusively extrastriate cortical basis (e.g. V2, V3, VP, V4v). Employing fMRI, we sought to distinguish between these models through retinotopic mapping of a patient with an upper right homonymous quadrantanopia. As expected, maps of the lower right quadrant and left hemifield were normal. The map corresponding to the impaired upper right quadrant was normal in V1 and V2, with little or no activity in VP and V4v. These results provide functional evidence that extrastriate cortical lesions can elicit homonymous quadrantanopias.