Feedback connections to ferret striate cortex: direct evidence for visuotopic convergence of feedback inputs

Interareal feedback connections are a fundamental aspect of cortical architecture, yet many aspects of their organization and functional relevance remain poorly understood. Previous studies have investigated the topography of feedback projections from extrastriate cortex to macaque area 17. We have extended this analysis to the ferret. We made restricted injections of cholera toxin B (CTb) into ferret area 17 and mapped the distribution of retrogradely labeled cells in extrastriate cortex. In addition to extensive label spreading within area 17, we found dense cell label in areas 18, 19, and 21 and the suprasylvian cortex and sparser connections from the lateral temporal and posterior parietal cortex. We made extensive physiological assessments of magnification factors in the extrastriate visual cortex and used these measures to convert the spread of labeled cortex in millimeters into a span in degrees of visual field. We also directly measured the visuotopic extents of receptive fields in the regions containing labeled cells in cases in which we made both CTb injections and physiological recordings in the same animals; we then compared the aggregate receptive field (ARF) of the labeled region in each extrastriate area with that of the injection site. In areas 18, 19, and 21, receptive fields of cells in regions containing labeled neurons overlapped those at the injection site but spanned a greater distance in visual space than the ARF of the injection site. The broad visuotopic extent of feedback connections is consistent with the suggestion that they contribute to response modulation by stimuli beyond the classical receptive field.