Abstract
The largely dissociated laminar distribution patterns of feedforward (FFD) and feedback (FBK) cortical connnections in sensory areas of nonhuman primates (NHP) have figured prominently in ideas of network organization and cortical hierarchy. While convenient, this emphasis does not take full account of the actual anatomical complexity. There is a diversity of FFD and FBK neuronal subtypes (cf. the corticogeniculate (CG) projections, where the fact of multiple CG neuronal subtypes inspired ideas of parallel processing). There is a complex architecture of recurrent connections, where FFD and FBK connections interact together and with numerous other cortical and subcortical connections, in ways still to be elucidated. Importantly, the laminar patterns so apparent at the global level, are less so from the perspective of postsynaptic dendritic input maps. Both supragranular FFD and FBK projecting neurons have apical dendrites in layer 1, and both FFD and FBK neurons can directly access inputs to layer 4 (respectively, via basal dendrites of neurons in deep layer 3 and apical denrites of infragranular neurons). Here, I briefly discuss some of the basic anatomical questions regarding FFD- FBK processing, still unanswered at the cellular level, and propose that the corrrelation with a functional hierarchy is less evident than often suggested.
Highlights
INTRODUCTIONReviewed by: Francisco Clasca, Autonomous University of Madrid, Spain Stewart Shipp, University College London, United Kingdom
Edited by: Alessandra Angelucci, The University of Utah, United StatesReviewed by: Francisco Clasca, Autonomous University of Madrid, Spain Stewart Shipp, University College London, United KingdomReceived: 27 September 2021 Accepted: 06 January 2022 Published: 28 January 2022Citation: Rockland KS (2022) Notes on Visual Cortical Feedback and Feedforward Connections
Owing to sparsity of data in non-human primate (NHP) concerning the role of interneuron subpopulations in microcircuitry, these are not discussed
Summary
Reviewed by: Francisco Clasca, Autonomous University of Madrid, Spain Stewart Shipp, University College London, United Kingdom. Feedforward (FFD) – feedback (FBK) cortical processing needs to be considered in the context of whole-brain activation, including interactions with cortico-thalamo-cortical, callosal, and the excitatory and inhibitory intrinsic cortical circuits. My goal has been to briefly re-consider what have become key features of FFD-FFK connections in the early visual cortical pathway, with emphasis on the cellular and dendritic circuitry components. For detailed reports on visual cortical connectivity and physiological response properties (see Bullier, 2004; Douglas and Martin, 2007; Shipp, 2007, 2016; Markov et al, 2014a,b; Angelucci et al, 2017; Vanni et al, 2020; Vezoli et al, 2021, among others). A strict comparison of cortical FFB and FBK connections is better addressed in extrastriate areas V2, V3, V4, MT, or TEO. Much of the following discussion is written as applying to V2
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