Temporal characteristics of primary-secondary message interference in a dichotic listening task.

Previous experiments have shown that during performance of a shadowing task the unattended message is processed at a semantic level. Four experiments are reported that deal with the question of how processing of the unattended message influences the difficulty of shadowing the attended message. Experiment I demonstrated that pupil dilation measures the difficulty associated with a shadowing task. In Experiment II it was shown that during shadowing of 20-item lists pupil dilation decreases as shadowing continues when either words or white noise are in the unattended channel. These results support the idea that it takes time for the attentional mechanism to focus on the attended message. In Experiments III and IV it was found that this decrease in difficulty of shadowing is much less when the attended and unattended messages come from the same stimulus category (letter-letter, digit-digit). The results indicate that, when the unattended message is from the same stimulus category as the attended message, it can interfere with the ability of the attentional system to focus on the attended message.