Martin Eimer

THE N2PC COMPONENT AS AN INDICATOR OF ATTENTIONAL SELECTIVITY

Event-related potentials (ERPs) were recorded during visual discrimination tasks in which stimulus arrays were presented that contained one lateral target and 3 (experiment 1) or one (experiments 2 and 3) non-targets. In experiments 1 and 2, targets differed from non-targets with respect to their form or their color. In experiment 3, word pairs were presented, with targets differing from non-targets with respect to their content. Subjects were required to respond to the identity of the target. In all experiments, an enhanced negativity was elicited at posterior electrodes contralateral to the location of the target. In the form discrimination tasks, this effect was present in the N1, N2, and P3 time intervals. In the color discrimination tasks, it was confined to the N2 time range. In the word discrimination task (experiment 3), this effect could only be observed over the left posterior hemisphere. It is argued that these lateralized negativities reflect the N2pc component that is assumed to indicate attentional filtering processes during visual search tasks. The present results extend this assumption by showing that this component is also elicited when targets are presented together with just one non-target item. It is argued that the N2pc may reflect the attentional selection of task-relevant stimuli.

Effects of masked stimuli on motor activation: behavioral and electrophysiological evidence

Three experiments investigated the influence of unperceived events on response activation. Masked primers were presented before a target. On compatible trials, primes and targets were identical; on incompatible trials, opposite responses were assigned to them. Forced-choice performance indicated that prime identification was prevented by the masking procedure, but overt performance and motor activation as mirrored by the lateralized readiness potential (LRP) were systematically influenced by the prime. The direction of these effects was unexpected: Performance costs for compatible and performance benefits for incompatible trials were obtained relative to a neutral trial condition. The LRP revealed a sequential pattern of motor activation. A partial activation of the response corresponding to the prime was followed by a reverse activation pattern. It is argued that these effects primarily reflect an inhibition of the response initially triggered by the prime.

Stimulus-Response Compatibility and Automatic Response Activation: Evidence From Psychophysiological Studies

Effects of dimensional overlap between stimuli and responses on partial response activation were investigated within a priming paradigm with the help of event-related potentials. The likely position of a target stimulus (requiring a left or a right reaction) was indicated by an arrow precue. To test whether automatic response activation processes are triggered by the cue, the lateralized readiness potential was computed. It was found that responses congruent to the direction of the cue were activated about 200 ms after cue onset. This early process was unaffected by specific cue-response contingencies and was completely missing when a nonspatial (color) cue was used. A second response activation phase was observed, which was partially controlled by specific response instructions and subjective expectancies. It is concluded that when stimuli and responses overlap with respect to spatial attributes, automatic response activation process are triggered, which may later be replaced by the activation of an expected response.

The lateralized readiness potential as an on-line measure of central response activation processes

The lateralized readiness potential (LRP) is an electrophysiological indicator of the central activation of motor responses. Procedures for deriving the LRP on the basis of event-related brain potential (ERP) waveforms obtained over the left and right motor cortices are described, and some findings are summarized that show that the LRP is likely to reflect activation processes within the motor cortex. Two experiments investigating spatial S-R compatibility effects are reported that demonstrate that, because of systematic overlaps of motor and nonmotor asymmetries, LRP waveforms derived by the double subtraction method cannot always be interpreted unequivocally in terms of response activation. Such confounds can be detected when LRP waveforms are compared with difference waveforms obtained by the double subtraction method from ERPs elicited at other lateral scalp sites.

ERP effects of intermodal attention and cross-modal links in spatial attention

Effects of intermodal attention AND of cross-modal links in spatial attention on visual and auditory event-related potentials (ERPs) were investigated in two experiments where participants had to attend to one stimulus modality (audition or vision) to respond to infrequently presented targets whenever these were presented at a relevant location (indicated by a cue). The ERP effects of intermodal attention (measured by comparing the ERPs elicited by visual and auditory stimuli when the respective modality was relevant or irrelevant) were differently distributed in vision and audition, suggesting that intermodal attention operates by a selective modulation of modality-specific areas. Similar ERP effects of spatial attention (measured by comparing the ERPs to stimuli at cued and uncued locations) were elicited at midline electrodes in vision and audition. With one notable exception, these effects were also present when attention was directed within the other modality, suggesting the existence of cross-modal links between vision and audition in the control of transient spatial attention.

S-R COMPATIBILITY AND RESPONSE SELECTION

In serial stage models, perception and action are usually thought to be linked to each other by an S-R translation mechanism. However, phenomena of S-R compatibility suggest a more direct relationship between perceptual and action domains. We discuss behavioral and psychophysiological evidence that irrelevant stimulus information automatically activates response codes, but then decays over time. In a series of reaction time studies and electrophysiological experiments, we investigated both temporal and functional properties of the assumed automatic response activation process. We found that the amount of interference due to irrelevant spatial information depends upon how long its availability precedes that of the information relevant for response selection. This indicates that response activation decays rather quickly. If response-relevant and irrelevant spatial information are simultaneously available, electrophysiological measurements show that automatic activation of the spatially corresponding response rises soon after stimulus onset, but then dissipates and gets replaced by the activation of the response indicated by the relevant stimulus attribute. We conclude that these findings do not support a pure translation account, but rather suggest the presence of two parallel and (at least partially) independent routes from perception to action: A direct route, allowing for automatic activation of response codes if stimulus and response features overlap, and an indirect route linking S and R codes in an arbitrary manner. Via the direct route responses may be primed independent of task-specific contingencies, while the correct response is selected via the indirect route. This use suggests that (a) the transmission of stimulus information to response stages does not (fully) depend on task relevance and that (b) different stimulus features can be transmitted asynchronously and independently from one another.

Explicit and implicit learning of event sequences: Evidence from event-related brain potentials

Event-related brain potentials (ERPs) were recorded during a serial reaction time (RT) task, where single deviant items seldom (Experiment 1) or frequently (Experiment 2) replaced 1 item of a repeatedly presented 10-item standard sequence. Acquisition of sequence knowledge was reflected in faster RTs for standard as compared with deviant items and in an enhanced negativity (N2 component) of the ERP for deviant items. Effects were larger for participants showing explicit knowledge in their verbal reports and in a recognition test. The lateralized readiness potential indicated that correct responses were activated with shorter latencies after training. For deviant items, participants with explicit knowledge showed an initial activation of the incorrect but expected response. These findings suggest that the acquisition of explicit and implicit knowledge is reflected in different electrophysiological correlates and that sequence learning may involve the anticipatory preparation of responses.

ATTENTIONAL MODULATIONS OF EVENT-RELATED BRAIN POTENTIALS SENSITIVE TO FACES

Event-related brain potentials (ERPs) were recorded in response to centrally and peripherally presented faces and chairs under conditions where one stimulus category was attended and the other unattended. It was studied whether selective attention affects ERP components sensitive to the presence of faces. When compared with chairs, faces elicited larger N1 amplitudes at lateral temporal electrodes and a midline positivity in the same latency range. The latter effect was only found for central faces. Attention to centrally presented faces was reflected in enhanced posterior N1 amplitudes. This effect may be related to an attentional modulation of processing within face-specific brain areas. It was not elicited by chairs or peripheral faces. Beyond 200msec post-stimulus, a category-unspecific attentional negativity was found at all recording sites for centrally and peripherally presented face and nonface stimuli.

Uninformative symbolic cues may bias visual-spatial attention: behavioral and electrophysiological evidence

Event-related brain potentials and response latencies were measured in an experiment where centrally presented arrow cues were followed by left or right visual target stimuli. In one condition, target location was indicated by the cues with 75% validity. In another condition, the precues were uninformative with respect to target location. Faster response times and larger negativities in the ERPs at midline electrodes were measured for targets at cued locations following informative cues, but also with uninformative precues. This indicates that visual-spatial attention may be biased involuntarily by central symbolic precues.

An event-related potential (ERP) study of transient and sustained visual attention to color and form

Event-related potential (ERP) effects of transient and sustained non-spatial visual attention were investigated in an experiment where subjects were instructed to attend to the color or form of visual stimuli in order to detect infrequently presented targets with the relevant feature. The to-be-attended feature was either varied in a trial-by-trial fashion (transient attention) or was kept constant for an entire experimental block (sustained attention). Both transient and sustained attention resulted in a larger negativity for attended as compared to unattended stimuli between 200 and 300 ms post-stimulus. This effect was more pronounced for sustained attention than for transient attention and larger for attention to color than for attention directed to stimulus form. In the sustained, but not in the transient attention condition, color attention resulted in larger positivities for attended stimuli between 150 and 200 ms and in the P3 time range. These effects are interpreted as evidence for the existence of non-spatial attentional selection processes that are more effective under sustained than under transient attention conditions and are different from processes of visual-spatial attention. Moreover, the results indicate a special status for sustained attention to stimulus color.