Abstracts

To study spatio-temporal stages in word processing, both current source density in cortex laminars and subdural potentials were recorded. Laminar multiple microelectrodes and clinical macroelectrodes were chronically implanted in epilepsy patients undergoing subdural grid and depth implantation for seizure localization. The patients participated in several cognitive tasks during their normal alert status. Three of them were visual word recognition tasks, in which the patients were requested to decide if a word: 1) presented an animal or object longer than one foot; 2) rhymed in [ldquo]AY[rdquo]; or 3) ended in [ldquo]ED[rdquo] for its past tense. Another task was auditory version of the size judgment task, in which a computer speaker instead of a monitor presented the stimuli. In all four tasks, half of the words were presented during practice stages prior to test, so repetition effect was expected. Extensive overlapping responses were recorded in frontal, temporal, and occipital cortices. In visual cortex, the earliest response was peaking at [sim]140ms. For auditory stimuli, the earliest auditory response was in superior temporal cortex, peaking at [sim]100ms, and the largest response was in adjacent area [still in superior temporal cortex], starting from [sim]120ms and peaking at [sim]300ms. These responses were equivalent to new and repeated words. However, there were significant differential responses from [sim]500-1000ms to new versus repeated words, for both auditory and visual stimuli, mainly in medial and ventral temporal areas. Laminar electrodes in adjacent areas measured a sink and source complex, with a greater response to new words during the same time range. For both micro and macro recording, word repetition does not affect the initial processing stage but decreases later activity. These differential responses appeared to represent the N4-P3b. (Supported by NS18741 and NS44623.)