Figure 1: Task and behaviour.
(a) After initiating a trial with a nose poke and receiving an instructional odour, rats responded at one of two fluid wells for 1 or 3 drops of chocolate (choc) or vanilla (van) milk, delivered 500 ms after the well-poke. Two odours indicated forced choices, left or right; a third odour indicated free choice. Reward contingencies were stable across blocks of ∼60 trials, but switched in number of drops (dashed lines) or flavour (dotted lines) in four unsignalled transitions. Rewards in the two directions always differed in both number and flavour (only one of four possible block sequences is shown). (b) Chocolate and vanilla milk solutions were equally preferred in a 10-min consumption test conducted previously in a separate group of rats (mean±s.e.; n=6; t10=0.1, P=0.93). (c) Shown is the extent of unilateral neurotoxic lesions of orbitofrontal cortex (OFC) made in one group. (d) Control rats (sham lesions) were sensitive to the number of drops but not the flavour of reward (n=83 sessions). Number switches (left panel, d) had a large effect on choice rates independent of flavour. Flavour switches (middle panel, d) had no effect on choice rates for big vanilla to big chocolate or big chocolate to big vanilla switches. Line figures show average trial-by-trial choice rates; inset bar graphs compare average choice rates±s.e. in the 25 trials before block switches versus the 25 trials after. In the rightmost panels of d, reaction (reac.) time (odour offset to port exit; top panel) and accuracy (bottom panel) on forced-choice trials reflected number but not flavour. Scatter plots in each panel show rat-by-rat difference (diff) scores; the length of lines in each dimension represents the s.e. No rat showed a significant effect of flavour on any measure. (e) Rats with unilateral OFC lesions were also sensitive to number but not flavour (n=67 sessions). Their behaviour was not significantly different from shams, except that they had slower reaction times across all conditions and showed a greater effect of number on reaction time. Panels in e follow the same conventions as those in d. No individual rat showed a significant effect of flavour on any measure.
