Table 2. Type I isobolographic analysis of interactions for non-parallel dose-response relationship lines between antiepileptic drugs (at the fixed drug dose ratio of 1:1) in the 6-Hz corneal stimulation-induced seizure model in mice.

Drug combination	ED50 exp (mg/kg)	n exp	L-ED50 add (mg/kg)	n add	U-ED50 add (mg/kg)	n add	I-index	Types of interaction
GBP + LCM	19.20 ± 5.78	8	29.33 ± 10.56	44	47.35 ± 11.56	44	0.65	additivity
LEV + LCM	1.73 ± 0.26 *	24	7.26 ± 3.36	44	11.73 ± 3.90	44	0.24	synergy
RTG + LCM	12.12 ± 1.81	24	12.45 ± 4.22	52	21.15 ± 4.82	52	0.97	additivity

The experimentally-derived median effective doses (ED_{50 exp} values in mg/kg ± S.E.M.) for two-drug mixtures were statistically compared to their respective theoretically calculated additive median effective doses (ED_{50 add} values in mg/kg ± S.E.M.) by the use of unpaired Student’s t-test with Welch’s correction as recommended elsewhere [60, 86–88]. n _exp−total number of animals used at those doses whose expected antiseizure effects ranged between 16% and 84% (i.e., 4 and 6 probits) for the experimental mixture; n _add−total number of animals calculated for the additive mixture of the drugs examined (n _add = n_{_antiepileptic drug_1} + n_{_antiepileptic drug_2}−4)

L-ED_{50 add} value calculated from the equation for the lower line of additivity; U-ED_{50 add} value calculated from the equation for the upper line of additivity; I-index–interaction index as a ratio of ED_{50 exp} and L-ED_{50 add} values

*P<0.05 vs. the respective ED_{50 add} value.