Figure 5.

The left hand panel is the canonical microcircuit based on Haeusler and Maass (2007), where we have removed inhibitory cells from the deep layers – because they have very little interlaminar connectivity. The numbers denote connection strengths (mean amplitude of PSPs measured at soma in mV) and connection probabilities (in parentheses) according to Thomson et al. (2002). The right panel shows the proposed cortical microcircuit for predictive coding, where the quantities of the previous figure have been associated with various cell types. Here, prediction error populations are highlighted in pink. Inhibitory connections are shown in red, while excitatory connections are in black. The dotted lines refer to connections that are not present in the microcircuit on the left (but see Figure 2). In this scheme, expectations (about causes and states) are assigned to (excitatory and inhibitory) interneurons in the supragranular layers, which are passed to infragranular layers. The corresponding prediction errors occupy granular layers, while superficial pyramidal cells encode prediction errors that are sent forward to the next hierarchical level. Conditional expectations and prediction errors on hidden causes are associated with excitatory cell types, while the corresponding quantities for hidden states are assigned to inhibitory cells. Dark circles indicate pyramidal cells. Finally, we have placed the precision of the feedforward prediction errors against the superficial pyramidal cells. This quantity controls the postsynaptic sensitivity or gain to (intrinsic and top-down) pre-synaptic inputs. We have previously discussed this in terms of attentional modulation, which may be intimately linked to the synchronisation of pre-synaptic inputs and ensuing postsynaptic responses (Fries et al 2001; Feldman and Friston, 2010).