Relating genetic variations in dopamine brain transmission to task performance with and without rewards

To evaluate potential genetic influences on learning in young people with and without cerebral palsy (CP), we calculated individual dopamine-related gene scores and compared these to the ability to learn two different tasks, an implicit sequence learning task, the Serial Reaction Time Task (SRTT) and a probabilistic classification task, the Weather Prediction Task (WPT).  For the SRTT, 85% of trials had the same sequence (i.e., probable) which should lead to implicit learning; 15% were a different sequence (i.e., improbable), The SRTT was also administered in an unrewarded condition and a rewarded one known to increase circulating levels of dopamine, each consisting of 20 trials each.  Data are presented for each block for 4 different outcomes (unrewarded probable which is considered the baseline learning score; unrewarded improbable; rewarded probable which indicates the effect of rewards on learning, and rewarded improbable). There were two outcome measures for each set of blocks: Reaction Time and Error Rate.

For the WPT, the Feedback condition (rewarded) had 150 training trials during which they received rewards for accurate performance  Training for the Paired association condition consisted of showing them the cards and the outcome for each of the 150 trials. Then both the Feedback (FB) and the Paired Association (PA) conditions had a test which was used for data analysis. Both the proportion correct and reaction time were the outcome measures.   

Gene scores are presented individually and in a summed gene score for each participant.

All analyses reported in the paper were performed using these data or values computed from these data. The main analyses were to determine if learning occurred, whether it differed by participant groups, or whether it was improved with rewards.  Finally, the central hypothesis was tested which was on the influences of gene scores on learning with and without rewards. 

Methods
All task-related data were collected on computer programs which required participants to press certain keys to indicate responses and to do so as accurately and quickly as possible.  Summary scores were calculated for each set of trials for each condition. Outcomes for tasks included Reaction Time and Error Rate, (for SRTT) or proportion correct (for WPT).  Genetic data were collected through blood samples, sent to a company to perform the genetic analyses, from which we extracted the specific variants for each individual to calculate a combined dopamine gene score.  

Statistical analyses included repeated measures GLM to assess learning over the training period for the group as a whole and for subgroups in both the rewarded and unrewarded conditions.  The primary analysis also added gene score group (high or low) to the GLM.  Independent t-tests were also used to compare scores across groups, and correlations were performed to relate performance on the two tasks, relationship of gene scores to task performance, and speed vs. accuracy measures.