1 Introduction A q A d A B q B d B [1,2,6,8] [23,26,42] [8,19,31] [19,29–31] [19] V (1) V = 1 1 + K d . 1 1 + ( Q / q ) , K [29] Q [19] V A V B (1) V A V B d B(50) (2) d B ( 50 ) = 1 K . 1 / ( 1 + ( Q / q B ) ) − 1 ( 1 + ( Q / q A ) ) 1 / ( 1 + ( Q / q A ) ) + d A . 1 + ( Q / q A ) 1 + ( Q / q B ) , d B(50) d A K [19] [26] 4 [2] (2) [12,14] [9] 2 Methods The experiment was carried out in accordance with UK Home Office regulations governing experiments on living animals. 2.1 Subjects Sixty experimentally naive female Wistar rats (Charles River UK) approximately 4 months old and weighing 250–300 g at the start of the experiment were used. They were housed individually under a constant cycle of 12 h light and 12 h darkness (light on 06:00–18:00 h), and were maintained at 80% of their initial free-feeding body weights throughout the experiment by providing a limited amount of standard rodent diet after each experimental session. Tap water was freely available in the home cages. 2.2 Surgery Disconnection Callosotomy [17] Disconnection callosotomy Sham lesion 2.3 Apparatus The rats were trained in standard operant conditioning chambers (CeNeS Ltd., Cambridge, UK) of internal dimensions 25 cm × 25 cm × 22 cm. One wall of the chamber contained a recess into which a peristaltic pump could deliver a 0.6 M sucrose solution. Two apertures situated 5 cm above and 2.5 cm to either side of the recess, through which motor-operated retractable levers could be inserted into the chamber. The levers could be depressed by a force of approximately 0.2 N. A 2.8-W lamp was mounted 2.5 cm above each lever; a third lamp was mounted 10 cm above the central recess. Six red light-emitting diodes were mounted in a row, 4 cm apart, 5 cm above the levers. The operant chamber was enclosed in a sound-attenuating chest; masking noise was generated by a rotary fan. An Acorn microcomputer programmed in Arachnid BASIC (CeNeS Ltd., Cambridge, UK), located in an adjoining room, controlled the schedules and recorded the behavioural data. 2.4 Behavioural training d A d B vs. d A d A d B d A d B d A d B Experimental sessions were carried out 7 days a week, at the same time each day, during the light phase of the daily cycle (between 08:00 and 14:00 h). 2.5 Histology At the end of the behavioural experiment, the rats were deeply anaesthetised with sodium pentobarbitone, and perfused transcardially with 0.9% sodium chloride, followed by 10% formol saline. The brains were removed from the skulls and fixed in formol saline for 1 week. Forty micrometer coronal sections were taken through the regions of the OPFC and AcbC (approximately from AP +5.0 to AP 0.0) using a freezing microtome. 2.5.1 Cresyl violet staining [26] 2.5.2 Immunocytochemistry [21] [2] 2 2 2 2 [34] 2.6 Data analysis Data from 5 of the 60 rats were discarded. Histological examination revealed two rats with misplaced excitotoxin-induced lesions; one rat in the disconnection + callosotomy group was discarded because of a failed callosotomy. Data from two further rats were discarded because they showed persistent exclusive responding on one lever. This left 13 rats in the sham-lesioned group, 14 in the disconnection group, 15 in the callosotomy group and 13 in the disconnection + callosotomy group. 2.6.1 Preference functions and linear indifference functions B B vs. d B d B(50) d B B B i j d B(50) d i d j d i B i B i B j [39] d B(50) vs d A r 2 d B(50) vs. d A [43] y y [43] 2.6.2 Psychophysical analysis of preference functions B B B d B d B(50) ɛ d B(50) ɛ d B(50) ɛ d B(25) d B(75) d B(25) d B(75) d B B B d B(50) 3 Results 3.1 Behavioural data 3.1.1 Preference functions and linear indifference functions B vs. d B Fig. 1 d B B d B d B(50) d A Fig. 2 d B(50) vs. d A r 2 [43] F P F P t P t P t P Fig. 3 F F P F F F r 2 3.1.2 Psychophysical analysis of preference functions Fig. 1 r 2 F F P F r 2 d B(50) d B(50)logist. d B(50)interp. d B(50)logist. vs. d B(50)interp. r d A F P Fig. 4 F P F P F ɛ Fig. 4 F P F F 3.2 Histology Fig. 5 OPFC AcbC Callosotomy 4 Discussion OPFC 13 AcbC 5 35 [40] [22] [10] d B [2,6–8,10,11,23,24,26,33,42] [2,23,24,26] d B d B d A B d B Fig. 1 d B(50) [11] [2,6–8,10,11,23,24,26,33,42] B d B [2] [18,19] B V A V B [2] [2] Fig. 1 ɛ d B(50) ɛ d B(50) [15,16,20,28] Fig. 4 ɛ [2] d B(50) [19,29] (2) K Q q A q B Q Q Q K Q K Q K [19] K K [32] [9,41] [23,26] [25] [6,8] [2] [37,38] [27] [4,36] [25] [3]