[O I]/Hα vs. line ratios. Top: Electron-density-sensitive [S II]_λ 6717/[S II]_λ 6731 line ratio vs. shocks-proxy [O I]_λ 6300/Hα line ratio for IC1553. The dashed horizontal line represents the theoretical low-density limit for the [S II] doublet, corresponding to 1.469 (Osterbrock & Ferland 2006). Colours and symbols are the same as in Figure 5. Bottom: [O III]/Hβ vs. [O I]/Hα diagnostic diagram for the bins of IC1553 at the minor axis, centred in the biconical structure seen in the [N II]/Hα map (triangles) and in the region within 2.5 < MAD < 4.5 kpc (squares), coincident with the high-level SF region visible in the disc of the galaxy. Curves represent the hybrid model for 40% shocks (without precursor) and 60% star formation with Z = Z_⊙ and q = 10⁷ cm/s.