Figure 6.

Inhibitory circuits for contextual modulation. (a) In contrast to excitatory (E) neurons, the response of somatostatin (SOM) neurons continues to increase up to a plateau with stimulus size. (b) SOM neurons receive input across a large area and do not inhibit each other. As a consequence, their response increases, leading to increasing responses with stimulus size. By inhibiting nearby excitatory neurons when large stimuli are presented, SOM neurons mediate surround suppression. Panels a and b adapted from Adesnik et al. (2012). (c–e) Contextual modulation depends on properties of the stimulus in the surround. An iso-oriented surround suppresses the response, while a cross-oriented surround increases the response. This gives rise to the illusory perception that the central grating contrast in panel d is greater than in panel c. The disinhibitory circuit from vasoactive intestinal peptide (VIP) neurons onto SOM neurons mediates this context dependence, as the activation of VIP neurons in the cross-oriented condition inhibits the surround suppression that would otherwise be provided by SOM neurons. Panels c–e adapted from Keller et al. (2020a).