Trial unique delayed matching to sample (TU-DMTS) task

The trial unique delayed matching to sample (TU-DMTS) paradigm consisted of a training phase and a testing phase (Fig. ​(Fig.3).3). Each session consisted of 24 trials with a maximum session time of 30 min. Unique multicolored stimuli varying across six different colors were used for this task (Fig. ​(Fig.3).3). Each stimulus consisted of at least three colors with specific attention to avoid a red-green combination because of dichromacy in males and the possibility in females (Abreu et al., 2019; Moreira et al., 2015). During the sample phase, the subject was required to respond via touch (FR-1) to the sample stimulus presented at the center of the screen. Response to the sample stimulus initiated a 1-s delay period in which the screen was blank and there were no programmed consequences. After the delay, the choice phase was initiated in which two stimuli were presented simultaneously on either left or right sides of the screen. One of the stimuli was identical to the stimulus presented in the choice phase; the other was randomly selected from the pool of 50 training stimuli. Responses to the sample stimulus were rewarded with liquid reinforcer (marshmallow juice) followed by an 8-s ITI whilst a response to the alternative choice stimulus resulted in a white screen and no reward followed by an 8-s ITI. Each alternative choice stimulus was presented only once in a session and the order of presentation were randomized in the next training session. A correction trial (identical trial to the previous) was conducted as the subsequent trial for incorrect responses. Correction trials were repeated until the subject touched the correct stimulus. The DMTS training phase, which preceded TU-DMTS and was presented as a step-wise training paradigm, was similar to and an extension of the methods described by Nakamura and colleagues (2019). DMTS training was comprised of 12 steps (Fig. ​(Fig.3).3). During the initial step, a single correct sample stimulus was always presented across the 24 trials (1 × 24) and balanced and randomized across left and right sides. In the second step, two different sample stimuli were presented across 12 trials each (2 × 12). For the second step and subsequent step, there were two phases: block phase and random phase. During the block phase, one of the sample stimuli was presented for the first 12 trials (#1–12) while the second sample stimulus was presented for trials #13–24. During the random phase, sample stimuli were presented in a randomized order. Accuracy of ≥75% for 3 consecutive days resulted in advanced to the next step-phase session. In the third step, there were three sample stimuli presented across eight trials each (3 × 8) first in the block phase followed by the random phase. During subsequent steps of 4 × 6, 6 × 4, and 12 × 2, an addition phase was included (random block phase), which preceded the random trial phase and was set as presentation of sequential trials of the same sample stimulus but randomized each day as to which trials the block was presented within the test session. For each of the step-phase sessions, advancement criteria was set to ≥75% correct for 3 consecutive days. Novel sample stimuli were introduced in each of the different step sessions and paired with relatively novel, alternative choice stimuli randomly presented from a pool of 50 stimuli. Once subjects advanced through 2 × 12 fixed blocks, random blocks, and random trials, the trial unique (TU) phase of training was introduced. During TU, a novel set of 50 multicolored stimuli were introduced. Combinations of the sample and alternative choice stimuli changed each trial and each stimulus was presented only once in a session. A priori criteria to advance to delay phases required 3 consecutive days of ≥75% correct at the 1-sec delay. Once accuracy was determined for 1-s delay of the TU-DMTS, similar to the DMTP test as described above, longer delays were introduced every 3 sessions (e.g., 3, 4.5, 6, 9, 12, 17 s) until the subject demonstrated >20% reduction in the accuracy (% correct) from the baseline (average of the last 3 sessions in a 1-s delay).

Trial Unique Delayed Matching to Sample Task (TU-DMTS) for the assessment of recognition memory. The TU-DMTS paradigm consisted of a training phase and a testing phase. Unique multicolored stimuli varying across six different colors were used for this task. As illustrated in the left panel, during the sample phase, the subject was required to respond via touch (FR-1) to the sample stimulus presented at the center of the screen. Response to the sample stimulus initiated a 1-s delay period in which the screen was blank and there were no programmed consequences. After the delay, the choice phase was initiated in which two stimuli were presented simultaneously on either left or right sides of the screen. One of the stimuli was identical to the stimulus presented in the choice phase while the other was a novel stimulus. Responses to the correct stimulus were rewarded with liquid reinforcer (20% marshmallow juice) followed by an 8-s ITI. Training for TU-DMTS is initially conducted over the course of 12 training steps. During the initial 12-step training phase, as presented in the table (right panel), a single correct sample stimulus was always presented across the 24 trials (1 × 24) and balanced and randomized across left and right sides. In the second step, two correct sample stimuli were presented across 12 trials each (2 × 12). Subsequent steps introduced an additional correct stimulus (step 3 = 3 × 8 trials each; step 4 = 4 × 6 trials each; step 5 = 6 × 4 trials each, etc.). Once subject met a priori advancement criteria, they were transitioned onto the subsequent steps. Upon completion of all 12 training steps, subjects were advanced to the testing phase in which each of the 24 trials was a novel random stimulus and novel incorrect stimulus