Abstract
Visual cortical areas in the adult mammalian brain are linked by a network of interareal feedforward and feedback circuits. We investigated the topography of feedback projections to ferret (Mustela putorius furo) area 18 from extrastriate areas 19, 21, and Ssy. Our objective was to characterize the anatomical organization of the extrastriate feedback pool to area 18. We also wished to determine if feedback projections to area 18 share similar features as feedback projections to area 17. We injected the tracer cholera toxin B subunit (CTb) into area 18 of adult ferrets to visualize the distribution and pattern of retrogradely labeled cells in extrastriate cortex. We find several similarities to the feedback projection to area 17: (i) Multiple visual cortical areas provide feedback to area 18: areas 19, 21, Ssy, and weaker inputs from posterior parietal and lateral temporal visual areas. Within each area a greater proportion of feedback projections arises from the infragranular than from the supragranular layers. (ii) The cortical area immediately rostral to area 18 provides the greatest proportion of total cortical feedback, and has the greatest peak density of cells providing feedback to area 18. (iii) The spacing (peak cell density and nearest neighbor distances) of cells in extrastriate cortex providing feedback to areas 17 and 18 are similar. However, peak density of feedback cells to area 18 is comparable in the supra- and infragranular layers, whereas peak density of feedback cells to area 17 is higher in the infragranular layers. Another prominent difference is that dorsal area 18 receives a cortical input that area 17 does not: from ventral cortex representing the upper visual field; this appears to be roughly 25% of the feedback input to area 18. Lastly, area 17 receives a greater proportion of cortical feedback from area 21 than from Ssy, whereas area 18 receives more feedback from Ssy than from area 21. While the organization of feedback projections from extrastriate cortex to areas 17 and 18 is broadly similar, the main difference in input topography might arise due to differences in visual field representations of the two areas.
Highlights
Mammalian visual cortical areas are linked via a system of feedforward (FF) and feedback (FB) circuits, which are thought to mediate different visual functions
By analogy with other studies, we refer to projections that arise from higher order visual areas and terminate in area 18 as feedback projections
We injected the bidirectional tracer Cholera toxin B subunit (CTb) into area 18 of 6 adult female ferrets, which resulted in retrogradely labeled feedback cells in extrastriate cortex
Summary
Mammalian visual cortical areas are linked via a system of feedforward (FF) and feedback (FB) circuits, which are thought to mediate different visual functions. Studies in primates (Kennedy and Bullier, 1985; Felleman and Van Essen, 1991), cats (Batardiere et al, 1998), and rodents. Prior studies have largely focused on the nature of FB inputs to area 17/V1 in adult primates (Rockland and Pandya, 1979; Zeki and Shipp, 1988), cats (Grant and Shipp, 1991; Batardiere et al, 1998), rodents (Coogan and Burkhalter, 1990; Yang et al, 2013), and ferrets (Cantone et al, 2005; Dell et al, 2019), yet detailed quantitative investigations focused on the organization of FB projections to area 18/V2 are scarce
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