Supplementary Figure 2: Luminance-dependent response changes with and without GABA blockers.

(a) Stimulus protocol. SR: SR-95531 (gabazine), Pic.: picrotoxin. (b–e) Examples of luminance- and GABA-blocker-dependent response patterns in three OFF cells (b,d,f) and two ON cell (c,e). Left: Spike rates at ND7 and ND6 luminance levels with and without GABA blockers. Right: One possible circuit scheme each which is consistent with the observed responses. The five examples represent the following categories of observations: (b) Luminance-dependent response changes not influenced by GABA (observed in n = 3 units; the example shows appearing early ON response at ND6 under both control and drug condition). Such cells changed their response properties identically under control and drug conditions between ND7 and ND6. Thus, these luminance-dependent response changes were independent of GABAergic regulation. (c) Luminance-dependent GABAergic masking of responses (n = 3; example cell has a delayed ON response masked at ND7). In such cells, light responses differed at ND7 and ND6 under control conditions, but not in the presence of GABA blockers. This suggests that GABAergic inhibition masked a response at one light level. (d) Luminance-independent GABAergic masking of responses (n = 12; example: unmasked early response at ND7 and ND6). Such cells did not show any luminance-dependent changes, neither in control nor with GABA blockers, but their responses were different between control and drug conditions within each light level. This suggests that GABAergic inhibition regulated responses at both luminance levels. Potentially, these masked responses might be revealed at other brightness levels. Note that the same phenomenon applies to the early ON responses in f. (e) GABA-dependent stabilization of responses (n = 13; the example illustrates this effect for early OFF responses). Such cells with stable responses under control conditions had changing responses under drug conditions. Thus, those changing response themselves were GABA-independent, while at the same time GABA stabilized the responses during the luminance-switch under control conditions. Note that the same phenomenon applies to the delayed ON responses in f. (f) GABA-dependent disinhibition (n = 6, the example shows disappearance of delayed ON response with GABA blockers at ND6). While in all examples above GABA blockers revealed additional responses, in few cells responses disappeared in GABA blockers (n = 2 at ND7, n = 5 at ND6, of which 1 unit was affected at both NDs). This suggests luminance-dependent disinhibitory GABAergic mechanisms.

The phenomena described by these examples occurred in both ON and OFF cells. In some cells, we observed one phenomenon to the white step, and another phenomenon to the black step, highlighting the response asymmetry already observed in control conditions (Fig. 3). In summary, we found that the mechanism of GABAergic response regulation is highly diverse, and that it underlies some but not all luminance-dependent qualitative response changes.