The Posterior Contralateral Negativity as a Temporal Indicator of Visuo-Spatial Processing

Abstract. The temporal segmentation of the processing stream between stimulus and response is a declared aim of many EEG-studies on cognitive processing. This goal, however, can only be reached when measures of high temporal sensitivity are available that are unequivocally assigned to a particular cognitive process. The present study evaluates the peak latency of the posterior contralateral negativity (PCN) as a temporal measure of visual spatial processing. In three experiments, we show systematic variability of the PCN latency with task demands. In visual search, PCN latency varied in the same direction as response times with the number of distractors. These effects, however, were smaller than the effects observed on response times, indicating that the process underlying the PCN might contribute to but did not determine response time effects. In contrast, in attentional cueing tasks and a stimulus localization task, where stimulus detection was the primarily addressed process, PCN latency varied to the same amount as manual response times. Thus, it is suggested that the latency of the PCN provides a reliable and valid temporal measure of target localization.

Attentional and intentional cueing in a Simon task: an EEG-based approach

Advance information about the location of a stimulus (attentional cueing) does not affect the Simon effect (a shortening of manual response times whenever the position of a stimulus that is irrelevant for the task corresponds to the side of the response). However, advance information about the side of a response (intentional cueing) enhances the Simon effect. At first sight, these well-established results contradict two important assumptions about the origin of the Simon effect: (a) the effect originates at least in part in a covert shift of visual attention that forces the preparation of a response towards the location of the attentional shift and (b) interference between stimulus location and response side takes place within a response selection stage. We replicated the behavioral finding in a study that measured event-related potentials (ERPs) of the EEG. ERPs indicated that the mechanisms causing the Simon effect remain widely unaffected by advance information. Clear evidence for both response preparation and attentional shifts in the cue-target interval was found. Additionally, ERPs suggested that the increment of the Simon effect by intentional cueing might be due to perceptual factors rather than to an alteration in the mechanisms involved in the generation of a regular Simon effect. The implications of these data for the role of attention and of response selection in Simon tasks are discussed.