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Fig. 16.1. Local microcircuit for lateral interactions. This circuit can potentially explain how lateral interactions can depend on the input contrast. A long-range lateral connection from an excitatory cell contacts two pyramidal excitatory cells (large black triangles) and one inhibitory cell (large circle). The inhibitory cell has a high threshold for activation, but strongly inhibits the pyramidal cells when activated. Weak excitation activates the pyramidal cells monosynaptically, and does not activate the inhibitory cell. However, strong excitation activates the inhibitory cell as well, causing a net inhibitory effect. In this manner, a single incoming excitatory long-range lateral connection could have inhibitory effects for strong stimuli (e.g. high-contrast patterns), and excitatory effects for weak stimuli. The SG model of cortical columns in PGLISSOM produces a similar effect, and can be seen as an abstraction of this circuitry at the columnar level. The excitatory synapses (shown as small triangles) adapt by Hebbian learning, but the inhibitory synapses (shown as small circles) are fixed in strength. Such learning can be approximated by direct Hebbian excitatory and inhibitory connections, as is done in PGLISSOM. Adapted from Weliky et al. (1995).