Introduction

It has become something of a truism to say that the information-processing capacity of humans is limited, that at any given moment in time the nervous system is capable of fully processing only a fraction of the information that is potentially available. Sometimes the multifarious information arrives through a single sensory modality, but more often information arrives simultaneously through several modalities. Because of our limited capacity for processing information, mechanisms of selective attention provide a means to ensure or enhance the processing of some of the incoming stimulus information.

Given this framework, one of the central questions is the following: How selective is attention? How well can we process information about one perceptual dimension or coming from one sensory modality and ignore information in other dimensions or from other modalities? This chapter is concerned with selective attention in the face of cross-modal stimulation. In particular, the chapter focuses on the failure of selective attention that occurs when people are presented with compound or complex stimuli activating different sensory modalities and asked to respond to only one component.

These failures of selective attention are often characterized as “dimensional interactions,” a term that originated in research concerned with selective attention to component features or attributes of stimuli presented within a single modality. Dimensional interactions, or failures of selective attention, are readily observed in tasks of speeded classification—tasks that require subjects to classify each of several stimuli as quickly as possible. This chapter begins by outlining the main findings that have been obtained in studies of speeded classification of unimodal stimuli, primarily vision, where most research has been conducted. After reviewing the unimodal findings, the chapter continues with a more detailed analysis of interactions observed in speeded classification of cross-modal stimuli.