Richards, J.E. (2002). Cortical sources of the P1 / N1 validity effect in spatial cueing in young infants. International Society for Infancy Studies, Toronto, CA.
Infants have been shown to covertly move attention around in space without moving fixation. This is shown in the “spatial cueing” procedure in which a cue indicates the side of an upcoming target (“valid trial”), or is contralateral to the target (“invalid trial’). Infants localize the target more quickly if the cue and target are in the same location than when not (“facilitation”), though at certain stimulus-onset-asychrony durations target localization may be delayed (“inhibition of return”). Two prior studies have shown with scalp-recorded event-related-potentials (ERP) that there is an enhancement of the first positive component to target onset for the validly cued target. However, these studies were done with relative few electrodes (e.g., 10-20 system) and can not be used to infer the cortical sources for these ERP effects. The present study identifies cortical activity related to spatial cueing using 128-channel EEG recording.
The infants in this study were 14 or 20 weeks of age. A spatial cueing procedure was used in which a central stimulus was presented for two seconds and a “cue” stimulus was then presented in the periphery for 300 ms. Subsequently, a “target” stimulus was presented on the same side of the cue or the opposite side of the cue. Control trials were included in which a target was presented without a previous cue. The EEG was recorded with a “high-density” system using 124 scalp channels and two eye movement channels. The EEG was used to make event-related-potential averages around the time of the target onset and immediately preceding the saccade towards the target.
Traditional ERP analyses showed a “validity effect” in which there was an enhanced P1 ERP component to the target on the same side as the cue, compared to the P1 amplitude on the contralateral side or on control trials. “Brain electrical source analysis” (BESA, cortical source analysis) was used to identify the cortical areas involved in this ERP effect. Cortical areas were identified that were hypothesized to be involved in this effect, an electrical current source was hypothesized at that location, an electrical scalp distribution based on that cortical current source was generated, and the hypothesized and actual electrical distributions were compared. Two cortical sources could be identified that accounted for ERP activity occurring about 100 to 130 ms following target onset. First, there was a current source located around the calcarine fissure, in the primary visual cortex (Brodmann area 17). This activity was unrelated to the validity of the target and occurred whenever a target appeared. Second, there was a current source located in the fusiform gyrus (Brodmann area 19). This area was activated most highly for validly cued targets relative to either control trials (no cue) or invalidly cued targets (contralateral cue). This result suggests that the “P1 validity effect” in infants may be due to the enhancement of the “ventral stream” for processing objects rather than a function of “spatial attention”.