Figure 4.

The effect of chronic treatment with an antidepressant on behaviour and modelling of judgement bias. Rats assigned to the chronic antidepressant group experienced subcutaneous injections of fluoxetine (1.0 mg/kg) daily for three weeks, whilst control rats experienced daily subcutaneous injections of saline vehicle (0.0 mg/kg). Twice weekly test sessions were conducted one week prior to treatment (pre; sessions 1–2), for the three weeks during treatment (drug; sessions 3–8) and for one following the end of treatment (post; sessions 9–10). There were no significant differences between groups during the pre-drug period for any measure. (A) Rats in the fluoxetine-treated group had a significantly more positive change from baseline in cognitive bias index (CBI) compared to controls during the drug period only. (B) Analysing each week of treatment shows that the more positive change from baseline in CBI in the fluoxetine group did not occur immediately, instead only becoming apparent during weeks 2 and 3 of treatment. This difference was no longer apparent following cessation of drug treatment. (C/D/E/F) Behavioural data were modelled using the diffusion model and parameters corresponding to the relative starting point (zr), drift rate (v) and boundary separation (a) were analysed. Data shown are averages for all probe sessions during that experimental period. (C) Although not significant, chronic antidepressant treatment showed a tendency to cause a more positive zr. This was both compared to the groups own baseline, and compared to controls. (D) Analysis of this measure by week indicated that zr was more positive compared to baseline in weeks 2 and 3, the same weeks that there was a more positive change in CBI. (E) There were no differences in v between the control and fluoxetine-treated groups. (F) There were also no changes in boundary separation ($\alpha$). Change from baseline (average of the two pre-drug test sessions) measures were calculated to take into account individual differences in underlying bias. Data shown are for the midpoint tone only, and represent mean±SEM. Control group: n=9, fluoxetine-treated group: n=8. *p<0.05, ^#p≤0.09.