Mechanisms of human attention: event-related potentials and oscillations

Abstract

Electrophysiological and hemodynamical responses of the brain allow investigation of the neural origins of human attention. We review attention-related brain responses from auditory and visual tasks employing oddball and novelty paradigms. Dipole localization and intracranial recordings as well as functional magnetic resonance imaging reveal multiple areas involved in generating and modulating attentional brain responses. In addition, the influence of brain lesions of circumscribed areas of the human cortex onto attentional mechanisms are reviewed. While it is obvious that damaged brain tissue no longer functions properly, it has also been shown that functions of non-lesioned brain areas are impaired due to loss of modulatory influence of the lesioned area. Both early (P1 and N1) and late (P3) event-related potentials are modulated by excitatatory and inhibitory mechanisms.. Oscillatory EEG-correlates of attention in the alpha and gamma frequency range also show attentional modulation.

Introduction

Attention permits us to focus on a subset of the incoming sensory information, since we are not capable of processing all input. For example, at every moment of time, sensory information about the temperature of our left foot is available for processing by the brain. However, unless we focus our attention to our left foot, we will probably not evaluate this information unless something unexpected happens to our foot. Two different mechanisms of selective attention are responsible for this phenomenon. On the one hand, attentional mechanisms let us focus our attention to our left foot whenever we decide to do so. We will refer to this process as voluntary attention. On the other hand, if something unexpected happens to our left foot, let's say it suddenly gets cold, our brain automatically focusses attention to the sensory information of our left foot. One can then consciously perceive the changed sensory information and respond accordingly. We will refer to this mechanism of attention as automatic attention.

Such changes of attention can occur either within a modality or across modalities. These attentional mechanisms operate in all sensory modalities, but in the remainder of the text we will primarily discuss the auditory and visual domains. For the sake of simplicity we will use the term object for both visual and auditory events.

Attention can be directed either to one out of multiple objects, disregarding its spatial location, or to one out of multiple spatial locations, no matter which object is present at this location. Object attention refers to the process of attending to one object in the presence of other objects which are usually regarded as distractors, since they potentially distract our attention from the attended object. Spatial attention refers to attending to one out of multiple spatial locations, i.e. to attend to the right vs. the left half of the visual field.

Taken together, within the concept of attention, there are at least three differentiations of the involved mechanisms:

• Cross-modality switches vs. within-modality switches

• Automatic selection vs. voluntary selection

• Spatial-selective vs. object-selective

However, object-selective attention may be closely related to figure-ground-segregation. We will limit the scope of this review to the aspects of within-modality switches of attention. A thorough review on cross-modality switches of attention is given in this issue by Eimer.