Mapping early somatosensory evoked potentials in selective attention: critical evaluation of control conditions used for titrating by difference the cognitive P30, P40, P100 and N140.

Detailed procedures are described for the study of somatosensory event-related potentials (ERPs) to electric stimulation of fingers. Control responses to homogeneous (100%) series of identical stimuli (thus eliminating input mismatch) while the subject reads a novel (thus providing a distinct attention-capturing activity and maintaining vigilance level) are validated as reflecting the exogenous obligatory profiles required for assessing cognitive component in ERPs to target relevant stimuli. With these 'neutral' conditions, the control responses have a similar profile even at larger ISIs such as those separating the infrequent targets in Attention runs. Conversely, series of stimuli identical to those in control runs can elicit cognitive components in a 'Lie' experiment when the subject is induced to treat the stimuli like targets even though there is no discrimination involved. On this basis, the somatosensory P30, P40, P100 and N140 components appearing in the target profiles are considered genuine cognitive components. They have been analyzed with scatter displays, electronic subtraction, bit-mapped displays and with calculation of Z and dilation factors. The cognitive P30 and P40 reflect selective attention-related enhancements of the neural generators in receiving somatosensory cortex. The early parietal positivity P27 can thus be modulated separately from the frontal N30 component and is thought to be generated by a radial dipole in area 1. The later cognitive P100 and N140 reflect the invocation of distinct processors in conjunction with the behavioral use of the sensory input. The evolving topographical patterns of the P100 and N140 electrogeneses, revealed by bit-mapped data, suggest complex interactions between posterior parietal and prefrontal cortex whereby the sensory information is placed into spatial coordinate systems and matched with representations of relevant objects or relationships in space for target processing in the sequential tasks.