A long-standing question in attention research is the extent to which selection involves gates in the sensory stream and the extent to which they involve added secondary processes. Intermodal attention paradigms are useful for examining this issue since different modalities involve readily distinguished cortical regions. Evoked potential studies have identified an attention-related frontal negativity labeled the Nd in auditory attention studies. It has been suggested that it arises from modulation of the auditory cortex (compatible with gating mechanisms) or of the frontal cortex (compatible with secondary processes such as short-term memory buffers). Efforts to localize the Nd have been impaired by the finding that the Nd comprises multiple components. Some novel procedures utilizing principal components analysis, in conjunction with high-density 64-channel recordings, were used to address this issue. Results suggest that the major source of the early Nd (the portion of most interest) resides in the frontal cortex, supporting the secondary process view for this particular mechanism.

Recent PET studies have suggested a specific anatomy for feature identification, visual word forms and semantic associations. Our studies seek to explore the time course of access to these systems by use of reaction time and scalp electrical recording. Target detection times suggest that different forms of representation are involved in the detection of letter features, feature conjunctions (letters), and words. Feature search is fastest at the fovea and slows symmetrically with greater foveal eccentricity. It is not influenced by lexicality. Detecting a letter case (conjunction) shows a left to right search which differs between words and consonant strings. Analysis of scalp electrical distribution suggest an occipito-temporal distribution for the analysis of visual features (right sided) and for the visual word form discrimination (left sided). These fit with the PET results, and suggest that the feature related analysis begins within the first 100 millisec and the visual word form discriminates words from strings by about 200 msec. Lexical decision instructions can modify the computations found in both frontal and posterior areas.