Inattentional numbness and the influence of task difficulty.

Research suggests that clearly detectable stimuli can be missed when attention is focused elsewhere, particularly when the observer is engaged in a complex task. Although this phenomenon has been demonstrated in vision and audition, much less is known about the possibility of a similar phenomenon within touch. Across two experiments, we investigated reported awareness of an unexpected tactile event as a function of the difficulty of a concurrent tactile task. Participants were presented with sequences of tactile stimuli to one hand and performed either an easy or a difficult counting task. On the final trial, an additional tactile stimulus was concurrently presented to the unattended hand. Retrospective reports revealed that more participants in the difficult (vs. easy) condition remained unaware of this unexpected stimulus, even though it was clearly detectable under full attention conditions. These experiments are the first demonstrating the phenomenon of inattentional numbness modulated by concurrent tactile task difficulty.