The effects of orientation and maintenance of attention on hemispheric asymmetry for speech perception.

The contributions of orientation and maintenance of attention to the measurement of ear asymmetry was investigated in 20 right handed subjects. The subject listened to dichotic lists and had to respond to a target word in either the male or the female voice only (focused attention), or both voices (divided attention). At random positions throughout the lists the voices changed ears. A switch in voices from one side of space to the other interfered with speech processing during focused attention, but facilitated the detection of targets during divided attention. The interference was greater for the left than the right ear during focused attention. Likewise, facilitation during divided attention was asymmetrical, with right targets receiving faster processing. The qualitative differences of attention strategy on ear asymmetry during dichotic listening may be due to hemispheric differences in preferred attentional modes.