You will be presented with the full text of a biomedical research paper. Please write an abstract for this paper. Your response should include the abstract and no additional text.

Paper text:
all surgical and experimental procedures were in accordance with the national institutes of health guide for the care and use of laboratory animals and approved by the harvard institutional animal care and use committee .
we injected dat - cre and vgat - cre mice with adeno - associated virus carrying flex - chr2 into the vta and implanted a head plate and a microdrive containing six tetrodes and an optical fiber . while mice performed a classical conditioning task , we recorded spiking activity from vta neurons .
we delivered pulses of light to activate chr2 and classified neurons as dopaminergic , gabaergic or unidentified .
dopamine plays a key role in motivation and reward . 
 dopaminergic neurons in the ventral tegmental area ( vta ) signal the discrepancy between expected and actual rewards ( i.e. , reward prediction error , rpe)1 - 3 , but how they compute such signals is unknown . 
 we recorded the activity of vta neurons while mice associated different odour cues with appetitive and aversive outcomes . 
 we found three types of neurons based on responses to odours and outcomes : approximately half of the neurons ( type i , 52% ) showed phasic excitation after reward - predicting odours and rewards in a manner consistent with rpe coding . 
 the other half of neurons showed persistent activity during the delay between odour and outcome , that was modulated positively ( type ii , 31% ) or negatively ( type iii , 17% ) by the value of outcomes . 
 while the activity of type i neurons was sensitive to actual outcomes ( i.e. , when the reward was delivered as expected vs. unexpectedly omitted ) , the activity of types ii and iii neurons was determined predominantly by reward - predicting odours . 
 we  tagged  dopaminergic and gabaergic neurons with the light - sensitive protein channelrhodopsin-2 ( chr2 ) and identified them based on their responses to optical stimulation while recording . 
 all identified dopaminergic neurons were of type i and all gabaergic neurons were of type ii . 
 these results show that vta gabaergic neurons signal expected reward , a key variable for dopaminergic neurons to calculate rpe .