Attention To Other Modalities A Selection beyond Vision

Beyond locations and objects in the visual modality, stimuli from other modalities can be selectively attended. This observation is evident from studies of selective attention in the 1950s and 1960s in which listeners attended one of two different speech signals arriving simultaneously in each ear. To determine how effectively attention could be restricted to one signal, listeners were required to repeat ("shadow") the speech in the attended channel. This shadowing procedure was the main behavioral technique for studying selective attention for decades. Studies of auditory attention supported early selection of stimuli; words spoken into the unattended ear are effectively filtered out. For example, if listeners are instructed to tap a key when they hear the word "tap," they almost always tap when the word is presented to the attended ear and almost never tap when the word is presented to the unattended ear. Words presented to the unattended ear are filtered at an early level of processing prior to word identification; if filtering occurred later, after word identification, listeners should have tapped when the word tap was spoken in the unattended ear. However, the early attentional filter appears to be leaky in that salient material on the unattended channel, such as the listener's name, can deter attention from the attended channel.

An early locus for auditory selective attention has been confirmed with ERP studies. The general procedure is similar to that used to study spatial attention (described previously). Listeners hear a sequence of auditory tone pips, half presented to the left ear and half to the right ear. Listeners pay attention to the tones in one ear and press a button whenever an infrequent target tone is presented to the attended ear. Many studies using this procedure have found that the early ERP waves are larger for tones presented in the attended ear than in the ignored ear, consistent with a sensory gain effect of auditory attention. This enhancement of the voltage amplitude occurs temporally early, with the effect beginning as early as 20 msec after stimulus onset, well within the period of sensory-level processing. Moreover, the attentional enhancement was present for both targets and nontargets presented in the attended ear, indicating that selection occurred before the stimuli were identified. In addition, magne-toencephalographic studies have shown that these effects arise in or near primary auditory cortex. The results from auditory attention experiments are similar with results from visual attention experiments in suggesting an early selection account of attention in both modalities.

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