(A) A cross section of the functional circuitry underlying direction selectivity in ON-OFF DSGCs. DSGCs receive glutamatergic inputs from ON and OFF bipolar cells, and inputs from mirror symmetric populations of ON and OFF GABAergic/cholinergic starburst amacrine cells. In this study, the stimuli used emphasized the ON responses. Preferred-side starbursts (p-starburst) provide mainly cholinergic excitation, while null-side starbursts (n-starburst) provide cholinergic excitation and a dominant GABAergic inhibition (Lee et al., 2010; Yonehara et al., 2011). Thus, GABAergic/cholinergic signals mediated by starbursts are differentially transmitted to the DSGC. (B) Schematic depicting the asymmetric anatomical connectivity between starbursts and DSGCs (top view). Bona fide ‘wrap-around’ synaptic connections (circled in red) are made largely by null-side starbursts, enabling DS dendrites of starbursts to mediate a ‘null’ inhibition (Briggman et al., 2011). This contrasts with the symmetrical functional cholinergic connectivity depicted in (A). Paracrine or ‘volume’ transmission could make cholinergic signals agnostic to the specific synaptic connectivity (Briggman et al., 2011; Brombas et al., 2017). (C) In conventional models, direction selectivity in DSGCs is largely shaped by asymmetric postsynaptic GABAergic inhibition. Asymmetric inhibition is contingent on the DS release of GABA from dendrites of starbursts; and on the asymmetric wiring (B). By contrast, excitation is non-directional, mainly mediated by glutamate released from bipolar cells (Park et al., 2014; Yonehara et al., 2013). In this model, the cholinergic receptive field is co-extensive with the glutamatergic receptive field. ACh is usually thought to play a non-directional role (Ariel and Daw, 1982; Amthor et al., 1996; Chiao and Masland, 2002; Park et al., 2014), although some studies note amplitude differences between preferred and null-evoked cholinergic currents (as shown here) suggesting that ACh may directly contribute to direction selectivity (Lee et al., 2010; Pei et al., 2015). (D) In the revised model proposed here, cholinergic excitation is directional by virtue of its timing with GABAergic inhibition rather than by its response amplitude (C). The differences in the functional connectivity of GABA and ACh (A) predict that for preferred-direction motion, excitation would lead inhibition; and for null-direction motion, E/I would be activated together. In this study, the central hypothesis is that E/I offsets contribute to a parallel DS mechanism, which does not necessarily rely on the modulation of the peak amplitude of GABA or ACh inputs (in contrast to the model shown in C).