Feedforward, feedback and inhibitory connections in primate visual cortex
Section snippets
Introduction and summary
Anatomical studies of cortical circuits have revealed enormous complexity. At most levels of analysis the neurons in the cortex appear to form widespread distributed networks. For example, the primate primary visual cortex (V1) receives connections from at least 4 other cortical areas and from numerous subcortical structures and also provides output to most of the same structures (Felleman & Van Essen, 1991). But relating the various components of these circuits to their function reveals that
Classical feedforward and feedback connections between visual cortical areas
Because the great majority of visual information reaching the cerebral cortex via the lateral geniculate nucleus of the thalamus (LGN) terminates in V1 (Benevento & Standage, 1982), and most (but not all) extrastriate cortical areas are dependent on V1 for their activation (Salin & Bullier, 1995), this area can be assigned to the lowest level in a hierarchy of visual cortical areas (Felleman & Van Essen, 1991). In contrast, other areas receiving input from V1 represent higher processing levels
Local feedforward and feedback connections in V1
The hierarchy of visual areas rests on a strong foundation. It is relatively straightforward to infer that V1 is the primary sensory area and most higher visual areas depend on this structure for their proper function. Understanding the flow of information within V1 is more circuitous, but much can be learned from the rules that emerged from studies of cortico-cortical connections. Because feedforward cortico-cortical connections terminate predominantly in layer 4, it can be inferred that this
A feedforward cortico-cortical pathway from layer 5 via the pulvinar nucleus of the thalamus
One of the more intriguing ideas to have emerged in recent years is the possibility that the classical cortico-cortical feedforward pathway has been overrated, or at the very least an important alternate pathway has been ignored (Guillery & Sherman, 2002). Guillery and Sherman propose that driving cortico-cortical communication can be mediated by layer 5 pyramidal neurons that project to the pulvinar nucleus, which in turn provides output to higher cortical areas (Fig. 3). They make the case
Inhibitory connections
Although there are a great diversity of inhibitory neuron types (e.g. see Kawaguchi & Kondo, 2002 for one recent review), the precise role of each cell type in the regulation of excitation in the cortex is not well understood. The purpose of this section of this review is not to discuss in detail each of these cell types or what is known about their roles in cortical circuits, but rather to discuss relationships between inhibitory connections and feedforward versus feedback excitatory
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