ABSTRACT
Selective attention is, as far as perceptual processes are concerned, the ability of the brain to select a part of parallel competing sensory input for further processing or response selection and execution. Although psychological tests and psychophysical measurements have significantly contributed to our understanding of attention, the brain mechanisms responsible for attention can only be revealed by studying the brain itself. The first possibility to this end was opened by the advent of methodologies of electrical recording, which could be made either directly from the brain of experimental animals or of patients under surgery, or noninvasively with electrodes placed on the scalp. A large amount of information was obtained by means of electroencephalography (EEG) in the 1960s and has continued to accrue thereafter. More recently, attention research has been advanced by the advent of several new functional imaging techniques such as positron emission tomography (PET; ter-Pogossian, Phelps, Hoffman, & Mullani, 1975), single photon emission computed tomography (SPECT; Knoll, 1983), functional magnetic resonance imaging (fMRI; Belliveau et al., 1991), and magnetoencephalography (MEG; for reviews, see Hämäläinen, Hari, Ilmoniemi, Knuutila, & Lounasmaa, 1993; Hari, 1990; Näätänen, Ilmoniemi, & Alho, 1994). These functional imaging methods enable one to locate neural activation as well as changes in metabolism and blood flow accompanying neural activation. The EEG has been able to reveal sequences of activation with msec time resolution, but several questions have remained unanswered due to problems in separating signals arising from multiple sites simultaneously. The MEG can help solve some of these problems. It shows promise in locating brain electric sources with high spatial accuracy and good discrimination between multiple sources, thus permitting new studies aimed at identifying and quantifying brain processes underlying attention. It should be pointed out, however, that the EEG, particularly when used in combination with the MEG, remains a powerful tool for the study of cognitive brain functions.
