Figure 2.2 The Salience Attribution Test Subjects are presented with a fixation cross followed by a cue. They then have to respond to the solid square as quickly as possible. During 50% of trials, participants are rewarded with money for faster responses, with the probability of the reward signaled by the cue.
The SAT is a speeded-response game, with monetary reward, which measures responses to task-relevant and task-irrelevant cue features (Roiser et al. 2009; Roiser et al.
2010). These measures reflect incentive salience processing, described in Chapter 1, as the task-relevant cues are associated with a reward and task-irrelevant cues are not associated with reward. The “aberrant salience” measure from the SAT has displayed good face validity in previous results, with elevated scores both in high-schizotypy individuals, and in patients with schizophrenia suffering from delusions (Roiser et al. 2009). Furthermore, using a factor analysis Schmidt & Roiser (2009) demonstrated the psychometric construct validity of the SAT. In an fMRI study of the SAT, Roiser et al. (2010) found that cues associated with high relative to low reward probabilities elicited robust hemodynamic responses in brain networks implicated in motivational salience including the midbrain, in the vicinity of the ventral tegmental area, and regions targeted by its dopaminergic projections, i.e. medial dorsal thalamus, ventral striatum and prefrontal cortex. Adaptive salience was strongly correlated with responses in the medial dorsal thalamus and polar PFC, whilst participants who showed
greater aberrant salience exhibited greater dorsolateral PFC responses, and reduced medial temporal gyrus (MTG) responses. Data on the test-retest reliability of the measure are lacking.
As per the description of the task by Roiser et al.: During the task, participants respond to a probe (a black square) after seeing 1 of 4 categories of cues (blue animals, red animals, blue household objects, and red household objects), which vary along 2 dimensions (colour and form); see figure 2.2. Each cue set comprised 16 different pictures, each of which was presented once per block. Subjects receive monetary reward (£0.05 to £1.00) on 50% of trials, with more money available for faster responses. Feedback was given at the end of each trial.
The probability of reward varies along one of the cue dimensions (such that if colour was the task-relevant dimension if blue stimuli were reward 87.5% of the time, red stimuli would be rewarded 12.5% of the time), but not for the other (such that if form was the task-irrelevant dimension both animals and household objects would be rewarded 50% of the time). On rewarded trials where participants either made no response or responded after the probe had disappeared, the message “Missed: 5 pence” was displayed. If participants responded prematurely (<100ms after the onset of the probe), the message displayed was “Too early: 5 pence”. On rewarded trials where participants responded before the probe disappeared, but slower than their mean RT from the practice block, the message “Hit – good: 10 pence” was displayed. When participants responded more quickly than their mean practice RT, the message “Quick – very good: X pence” was displayed (for responses up to 1.5 standard deviations (SDs) faster than their practice mean RT) and “Very quick – excellent: X pence”
(for responses over 1.5 SDs faster than their practice mean RT). The reward was scaled according to X=10+90×(mean RT – trial RT)/(3×SDF), up to a maximum of £1.00. For example, a response 1 SDF faster than the mean was reinforced with 40 pence, a response 2
SDFs faster was reinforced with 70 pence, and any responses 3 SDFs or faster than the mean were reinforced with 100 pence. The money won on each trial was added to the participant's running total for that block, Y, which was displayed underneath the feedback: ‘Total – £Y’. On reinforced trials, a 0.5 s tone sounded, frequency: (300+(10×X)) Hz. At the end of each block, participants indicated, using 10 mm visual analogue scales (VAS), their estimate of the reinforcement probabilities for each of the four different CSs.
On the 50% of trials that were not rewarded, the message “Sorry – no money available”
was displayed, regardless of the speed of response. Participants were not informed of the contingencies between the different pictures and reward. Participants could earn a maximum of £20 on the test (minimum £5).
Category and reward probability contingencies are counterbalanced across participants and remained constant throughout the task. Two experimental sessions (64 trials each) were performed each session. The SAT provides measures of adaptive (relevant) and aberrant (irrelevant) motivational salience on the basis of visual analogue scale ratings (VAS; explicit salience) and reaction times (RTs; implicit salience).
Before the main task, participants performed a computerised tutorial, with neither rewards nor cues, on which they were required to respond as quickly as possible to the onset of the probe only. The tutorial featured example displays, written instructions and test trials before the main test. The tutorial was embedded with two practice sessions to familiarize
participants with the test and provide a measure of baseline response time (RT). During practice sessions, a fixation cross appeared at the beginning of each trial. Following a variable interval (minimum 0.5 s, maximum 1.5 s) the probe appeared, and participants responded by pressing a button as quickly as possible. Participants were instructed to try to respond as quickly as they were able to, and before the box disappeared. During the first practice session the probe was on the screen for randomized variable periods, with a maximum duration of 1.5 s, minimum duration 0.5 s and mean duration 1 s. Feedback was provided after 2 s as ‘Good’ if the participant responded before the box disappeared, ‘Try to respond faster’ if they responded after the box disappeared, ‘Too early’ if they responded before the box appeared, and ‘No key pressed’ if they made no response. On the second practice session, the mean probe duration was set to be the mean RT from the first, ensuring participants were responding as quickly as possible and to yoke task difficulty to individual performance, such that difficulty in the active task was calibrated on a participant-by-participant basis. The standard deviation (s.d.) of the fastest half of the trials (SDF) was also calculated, and was used to set the minimum and maximum probe durations for the second practice session (mean from first practice session±2×SDF). For the main test, the mean, minimum and maximum probe durations were calculated from the second practice session in the same way. No monetary reinforcement was provided during the practice sessions.