the majority of these studies were conducted with adult male sprague - dawley rats from a selective - breeding colony which has been previously described14 . selectively - bred rats from generations s18 , s20 and s21 were transported from the university of michigan ( ann arbor , mi ) to the university of washington ( seattle , wa ) for the fscv experiments . during each behavior session , chronically implanted microsensors , placed in the core of the nucleus accumbens , were connected to a head - mounted voltammetric amplifier for detection of dopamine by fscv23 . voltammetric scans were repeated every 100 ms to obtain a sampling rate of 10 hz . voltammetric analysis was carried out using software written in labview ( national instruments , austin , tx ) . on completion of the fscv experiments , recording sites were verified using standard histological procedures . to examine the effects of flupenthixol ( sigma , st . louis , missouri ; dissolved in 0.9% nacl ) on the performance of sign - tracking and goal - tracking behavior , rats received an injection ( i.p . ) of 150 , 300 or 600 g / kg of the drug one hour prior to pavlovian conditioning sessions 9 , 11 and 13 . doses of the drug were counterbalanced between groups and interspersed with saline injections ( i.p . , 0.9% nacl ; prior to sessions 8 , 10 , 12 and 14 ) to prevent any cumulative drug effects . to examine the effects of flupenthixol on the acquisition of sign - tracking and goal - tracking behavior , rats received an injection of either saline ( i.p . ) or 225 g / kg of the drug one hour prior to pavlovian conditioning sessions 17 . detailed methods and any associated references are available in the online version of the paper at www.nature.com/nature .