Cognitive evoked potentials during word and picture recognition

The N400 and P600 components of cognitive evoked potentials or event-related potentials (ERP) are context related and usually recordable after a priming stimulus or context consisting of words or any meaningful stimuli. All studies previous to this one used a sequential presentation of stimuli to elicit the ERPs. In this study, however, the picture and word stimuli were presented simultaneously. ERPs were recorded from 79 neurologically normal subjects including 62 right-handed, 10 left-handed and 7 subjects who were suffering from temporal lobe brain damage. Subjects were either asked to respond as to whether the stimuli were congruent, incongruent or non-sensical (semantic incongruency) or as to whether they were 'new' or 'old' (recognition memory). The number of correct responses and the reaction time were both measured. ERPs were recorded using an electrode cap which consisted of 16-channel scalp electrode montage (an enhanced international 10/20 electrode placement system) with linked mastoids as reference electrodes. Data was acquired and analysed using Neuroscan signal averaging and mapping software.In all the experimental conditions studied the tasks were performed accurately but reaction time was significantly longer for the incongruent stimuli. The same occurred for 'new' items as opposed to 'old' items. In the ERP recordings the N400 and P600 components were independent. The N400 peak was recorded maximally over the frontal locations while the P600 was mainly a parietal component. Differences were observed in their latency, amplitude and spatial distribution within the various test conditions.