Skip to main content
. 2010 Mar 26;65A(6):639–647. doi: 10.1093/gerona/glq038

Figure 1.

Figure 1.

Digit symbol substitution test (DSST) and control condition tasks. Schematic representation of the DSST and control conditions tasks administered during the brain functional magnetic resonance imaging (fMRI) scanning session. fDSST = DSST modified for the fMRI protocol. Two examples are shown for the fDSST (matching: push right button; not matching : push left button) and for the control condition ( ‘R’: push right button; ‘L’: push left button). The participant sees on a computer screen one number–symbol matching pair (cue). After the cue disappears, an answer key (probe) appears containing a grid of four number–symbol matching pairs. The participant is instructed to push the right index finger button if the probe contains one number–symbol that matches the cue and to push the left index finger button if the probe does not contain any number–symbol that matches the cue. Instructions are to respond “as fast as you possibly can.” In the control condition, the probe is a grid of four “R-symbol” pairs or “L-symbol” pairs to which the participant is instructed to respond by pushing the right or the left button, respectively, using the index finger. Eye movements to visually scan the screen in the control condition and in the fDSST condition are estimated to account for non–task-specific brain activation (frontal eye field and visual cortex for eye movements and motor cortex for the index finger movements). In all trials, cues and a probe flashed on a black screen that had a 0.1-cm bright white fixation cross at the center, according to specific timing (cue: 1.0 seconds; pause: 0.2 seconds; probe: 3.6 seconds; pause: 1.2 seconds). The fDSST was presented using the block design, in which one block (eight trials per block) alternated with one block (eight trials per block) of a control condition. Each block lasted for 48 seconds, and the participants were reminded the instruction for 8 seconds at the start of each block. There were five fDSST blocks alternated with five control blocks for a total of 9 minutes and 20 seconds. No cue was repeated within the same block. The two task conditions (matching vs nonmatching) were randomized 1:1 across participants and block.