Evoked potentials and central processing of visual information

ERP components related to stimulus selection processes

Event-related potential (ERP) components associated with target stimulus selection in a double discrimination visual task were studied. The experimental paradigm consisted in the presentation of low intensity stimuli that varied according to two physical features: geometrical form (squares and circles) and location (a spot in different positions inside the stimulus). Subjects performed 3 tasks on these stimuli: control task in which they looked passively at the stimuli, and 2 discrimination tasks, in which they had to respond to a certain stimulus (a specific conjunction of form and spot location). The early components (P1 and N1) obtained in the control and discrimination tasks were associated with sensory analysis of simple stimulus features. Relevance of a particular feature modified the latency and/or the area of these components. The longer-latency components (N2 and P3) were elicited only in the discrimination tasks. N2 was associated with target stimulus selection because its area was significantly larger for target stimuli and because its "offset" latency correlated with choice reaction time. Results are discussed and contrasted with various models of target selection.

A technique for on-line classification of evoked potentials into two groups according to subjective interpretation of the stimulus

A technique is described for on-line classification of evoked potentials (EPs) into distinct groups for separate averaging according to a criterion available after the beginning of the EPs. It is suitable for analysing psychophysiological correlations (relationship between EPs and subjective perception). The system consists of a slightly modified NTA 512 B (Hungarian) average response computer (however, any CAT-computer-like device can be used in a similar way) and of an additional device. It can be used in all cases when the number of subjective judgements according to which EPs have to be classified can be limited to two (or three, when, by slightly changing the system, all quarters of memory are used).