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Background: In the psychological refractory period paradigm, the second response time is increased as the interval between the first and second stimulus is decreased. The response delay for the second stimulus is called the psychological refractory period. In contrast with these findings, some evidence of the backward crosstalk effect has shown that the first reaction time is affected by the second stimulus. The capacity sharing models, in which multiple stimuli can be processed simultaneously, unlike central bottleneck models, are able to provide some explanations for the changes in the reaction time to the first stimulus. However, sometimes these changes could be arisen from response grouping; the first response remains on the processing buffer until the response to the second stimulus is prepared, then both stimuli are responded simultaneously. The questions of this study were whether the second task difficulty and time interval manipulation would affect the first task response or not and in what extend changes are because of response grouping? Materials and Methods: Detection, discrimination and choice tasks were used as the second task in the psychological refractory period paradigm and the time interval between stimuli was 50, 100, 200, 400, or 1000 ms. Results: The type of the second stimulus and time interval between stimuli had an effect on the second-task and first-task reaction times. Conclusion: In the detection group, the first task response time changes were consistent with grouping between the two responses, but in the discrimination group the changes would arise from capacity limitation and were consistent with the backward crosstalk effect.