The representational mechanism of implicit sequence learning is one of the basic problems in the field of implicit learning, and it remains unclear. Three key theories have been proposed to demonstrate the representational mechanism of implicit sequence learning: stimulus-stimulus association learning (S-S), response-response association learning (R-R), and stimulus-response association learning (S-R). Most studies of implicit sequence learning employed reaction time as a dependent variable to investigate the representational mechanism. However, using reaction time directly in the model may not be ideal as the measured reaction time has several limitations that may confuse the results. For example, the baseline of reaction time may vary across different age groups. In order to overcome the shortcomings of reaction time, the present research applied an eye movement tracking technique and used the saccadic response time as the dependent variable. In the current study, prosaccade and antisaccade trials in single or mixed tasks were investigated with eye movements, which were recorded using an EyeLink 1000 plus eye-tracker (SR Research inc., Canada). In a prosaccade trial, the subject was asked to look towards a newly appearing target, while in an antisaccade trial, a saccade of the subject to the location opposite to the appeared target was required. Three sets of experiments were conducted. In Experiment 1, forty (40) college students completed the mixed tasks which included both the prosaccade tasks (red target) and the antisaccade tasks (green target). The participants were randomly assigned to a stimulus sequence group (i.e. stimulus followed the sequence) or a response sequence group (i.e. response followed the sequence). In Experiment 2, thirty-eight (38) college students completed either the prosaccade tasks or antisaccade tasks by the instruction of experiment 2. In Experiment 3, two distractors which had one of the same features as the target (color/shape) were added in the trial.Thirty-eight (38) college students completed the distractor task or the no distractor task. The results showed that: (1) In the mixed saccadic tasks, there was implicit sequence learning in the stimulus sequence condition and in the response sequence condition; (2) in the single saccadic tasks, there was implicit sequence learning in the prosaccade condition and in the antisaccade condition. However, significant difference in the sequence learning scores between the mixed saccadic tasks and the single saccadic tasks was observed; (3) in the distractor tasks, there was implicit sequence learning in the distractor task condition and in the no distractor task condition. The results of the current three experiments indicate that the representational mechanism of implicit sequence learning includes learning of multiple sequences: stimulus-stimulus associations learning (S-S); response-response associations learning (R-R); and stimulus-response associations learning (S-R).
