Alexandra Muller-Gass

Evidence for the auditory P3a reflecting an automatic process: elicitation during highly-focused continuous visual attention.

The P3a is an event-related potential (ERP) component believed to reflect an attention-switch to task-irrelevant stimuli or stimulus information. The present study concerns the automaticity of the processes underlying the auditory P3a. More specifically, we investigated whether the auditory P3a is an attention-independent component, that is, whether it can still be elicited under highly-focused selective attention to a different (visual) channel. Furthermore, we examined whether the auditory P3a can be modulated by the demands of the visual diversion task. Subjects performed a continuous visual tracking task that varied in difficulty, based on the number of objects to-be-tracked. Task-irrelevant auditory stimuli were presented at very rapid and random rates concurrently to the visual task. The auditory sequence included rare increments (+10 dB) and decrements (-20 dB) in intensity relative to the frequently-presented standard stimulus. Importantly, the auditory deviant stimuli elicited a significant P3a during the most difficult visual task, when conditions were optimised to prevent attentional slippage to the auditory channel. This finding suggests that the elicitation of the auditory P3a does not require available central capacity, and confirms the automatic nature of the processes underlying this ERP component. Moreover, the difficulty of the visual task did not modulate either the mismatch negativity (MMN) or the P3a but did have an effect on a late (350-400 ms) negativity, an ERP deflection perhaps related to a subsequent evaluation of the auditory change. Together, these results imply that the auditory P3a could reflect a strongly-automatic process, one that does not require and is not modulated by attention.

Perceptual and cognitive task difficulty has differential effects on auditory distraction

When a task-irrelevant feature of an auditory stimulus varies at rare and unpredictable times, the processing of this change interferes with the processing of task-relevant stimulus information. The present study investigated whether this distraction effect is modulated by the difficulty of the auditory task. Event-related potentials (ERPs) and behavioral responses were recorded while subjects classified stimuli based on their temporal dimension. In one condition, the task was made more difficult by decreasing the perceptual discriminability (temporal distinctiveness) of the stimuli. In a second condition, the difficult task involved an increase in memory load: subjects were asked to assess the duration of the current compared to that of the previous stimulus. The occurrence of an infrequent task-irrelevant change in the pitch of the stimulus caused distraction in all task conditions. Following this change, performance deteriorated, and a distinct P3a component was visible in the ERP. Importantly, the extent of this distraction effect was significantly enhanced during the high memory load task, but not during the difficult perceptual task. It may be that the attentional resources afforded to the stimuli, rather than task difficulty, affected the extent of the distraction response. When the processing requirements of a task demand more highly focused attention for stimulus processing, the processing of the distracting information embedded within this stimulus may inadvertently also benefit from this attention.

Automatic detection of lexical change: an auditory event-related potential study

We investigated the detection of rare task-irrelevant changes in the lexical status of speech stimuli. Participants performed a nonlinguistic task on word and pseudoword stimuli that occurred, in separate conditions, rarely or frequently. Task performance for pseudowords was deteriorated relative to words, suggesting unintentional lexical analysis. Furthermore, rare word and pseudoword changes had a similar effect on the event-related potentials, starting as early as 165 ms. This is the first demonstration of the automatic detection of change in lexical status that is not based on a co-occurring acoustic change. We propose that, following lexical analysis of the incoming stimuli, a mental representation of the lexical regularity is formed and used as a template against which lexical change can be detected.

Multiple P3 evidence of a two-stage process in word gender decision

All French nouns must be assigned to one of two grammatical genders: masculine or feminine. Participants used either the superordinate labels masculin/féminin or the singular indefinite articles un/une to classify French target nouns. Reaction time to the labels masculin/féminin was about 200 ms longer than to the un/une labels. When the indefinite articles were used, a single P3 peak of the event-related potential was elicited. When superordinate labels were used, a double-peaked positivity was observed. The latency of the initial P3 in the masculin/féminin trials was not significantly different from that in the un/une trials. The second positive wave peaked ∼300 ms following the first. An explanation consistent with these data is that subjects used a two-stage process to classify the nouns appearing with superordinate labels.

The extent of processing of near-hearing threshold stimuli during natural sleep.

OBJECTIVES Event-related potential (ERPs) provide an exquisite means to monitor the extent of processing of external stimulus input during sleep. The processing of relatively high intensity stimuli has been well documented. Sleep normally occurs in much less noisy environments. The present study therefore employed ERPs to examine the extent of processing of very low intensity (near-hearing threshold) stimuli. DESIGN Brief duration 1000 Hz auditory tone bursts varying in intensity at random from -5 to +45 dB from normative hearing level (nHL) in 10 dB steps were presented every 1.5 to 2.5 s when the subject was awake and reading a book and again during all-night sleep. SUBJECTS n = 10 healthy young adults. MEASUREMENTS AND RESULTS In the waking state, the auditory stimuli elicited a negative-going deflection, N1, peaking at about 100 ms, followed by a smaller positivity, P2, peaking at about 180 ms. N1-P2 gradually decreased in amplitude with decreases in stimulus intensity and remained visible at near-hearing threshold levels. During NREM sleep, the amplitude of N1 was at baseline level and was reduced to only 15% to 20% of its waking amplitude during REM sleep. P2 was much larger in sleep than in wakefulness. Importantly, during sleep, P2 could be reliably elicited by the auditory stimuli to within 15 dB of threshold. During NREM, a large amplitude negativity peaking at about 350 ms was elicited by the higher intensity stimuli. This N350 was much reduced in amplitude during REM sleep. A significant N350 was not, however, elicited when stimuli intensity levels were below 25 dB nHL. CONCLUSIONS Auditory stimuli that are only slightly above hearing threshold appear to be processed extensively during a 200 to 400 ms interval in both NREM and REM sleep. The nature of this processing is, however, very different compared to the waking state.

The effects of total sleep deprivation on semantic priming: event-related potential evidence for automatic and controlled processing strategies.

There is general consensus that performance on a number of cognitive tasks deteriorates following total sleep deprivation. At times, however, subjects manage to maintain performance. This may be because of an ability to switch cognitive strategies including the exertion of compensatory effort. The present study examines the effects of total sleep deprivation on a semantic word priming task. Word priming is unique because it can be carried out using different strategies involving either automatic, effortless or controlled, effortful processing. Twelve subjects were presented with word pairs, a prime and a target, that were either highly semantically associated (cat…dog), weakly associated (cow…barn) or unassociated (apple…road). In order to increase the probability of the use of controlled processing following normal sleep, the subjects task was to determine if the target word was semantically related to the prime. Furthermore, the time between the offset of the prime and the onset of the target was relatively long, permitting the use of an effortful, expectancy-predictive strategy. Event-related potentials (ERPs) were recorded from 64 electrode sites. After normal sleep, RTs were faster and accuracy higher to highly associated targets; this performance advantage was also maintained following sleep deprivation. A large negative deflection, the N400, was larger to weakly associated and unassociated targets in both sleep-deprived and normal conditions. The overall N400 was however larger in the normal sleep condition. Moreover, a long-lasting negative slow wave developed between the offset of the prime and the onset of the target. These physiological measures are consistent with the use of an effortful, predictive strategy following normal sleep but an automatic, effortless strategy following total sleep deprivation. A picture priming task was also run. This task benefits less from the use of a predictive strategy. Accordingly, in this task, ERPs following the target did not differ as a function of the amount of sleep.

Mismatch negativity to inclusions of noise within complex spectrotemporal sound patterns.

Event-related potentials to complex spectrotemporal sound patterns were recorded during non-attend conditions. A high-intensity noise deviant elicited a large negative wave. This is probably a combined mismatch negativity (MMN) and N2b since it was followed by a large amplitude frontal positive wave (P3a). When the noise intensity was reduced, a smaller amplitude negative wave was elicited. This probably corresponds to a true MMN since it was not followed by the positive wave. These findings are consistent with previous research in which N2b is elicited in non-attend conditions when the deviant stimulus was particularly difficult to ignore. These findings demonstrate that complex spectrotemporal sound patterns are represented in auditory memory and that inclusions of noise within these complex stimuli can be passively discriminated.

Neuroanatomical correlates of categorizing emotional valence.

Categorization is fundamental to cognition, and evidence suggests that categorizing emotional stimuli holds a privileged position in human information processing. According to theories on embodied emotion, the subjective emotional feeling elicited by a stimulus plays a causal role in its categorization. Using functional MRI, we tested the hypothesis that categorizing emotional stimuli in terms of valence would activate structures involved in valence-specific experience of emotion. On each trial, two pictures from the International Affective Picture System were presented successively. Upon viewing the second picture, participants categorized it as belonging to the same valence category as or a different valence category from the first picture. Categorization activated an exclusively left-lateralized set of regions implicated in taxonomic categorization (i.e. judging whether two items are of the same kind) including the middle temporal gyrus and precuneus, as well as the posterior cingulate cortex. Critically, for negative pictures categorization activated structures that underlie the experience of negative emotions (anterior insula, left orbitofrontal cortex), whereas for positive pictures categorization activated structures that underlie the experience of positive emotions (dorsomedial and ventromedial prefrontal cortex). Consistent with predictions derived from theories on embodied emotion, these results suggest that experience of emotion contributes to categorizing emotional valence.

The processing of infrequently-presented low-intensity stimuli during natural sleep: An event-related potential study

Event-related potentials (ERPs) provide an exquisite means to measure the extent of processing of external stimuli during the sleep period. This study examines ERPs elicited by stimuli with physical characteristics akin to environmental noise encountered during sleep. Brief duration 40, 60 or 80 dB sound pressure level (SPL) tones were presented either rapidly (on average every two seconds) or slowly (on average every 10 seconds). The rates of presentation and intensity of the stimuli were similar to those observed in environmental studies of noise. ERPs were recorded from nine young adults during sleep and wakefulness. During wakefulness, the amplitude of an early negative ERP, N1, systematically increased as intensity level increased. A later positivity, the P3a, was apparent following the loudest 80 dB stimulus regardless of the rate of stimulus presentation; it was also apparent following the 60 dB stimulus, when stimuli were presented slowly. The appearance of the N1-P3a deflections suggests that operations of the central executive controlling ongoing cognitive activity was interrupted, forcing subjects to become aware of the obtrusive task-irrelevant stimuli. The auditory stimuli elicited very different ERP patterns during sleep. During non-rapid eye movement (NREM) sleep, the ERP was characterized by an enhanced (relative to wakefulness) early positivity, P2, followed by a very prominent negativity, the N350. Both deflections systematically varied in amplitude with stimulus intensity level; in addition, N350 was much larger when stimuli were presented at slow rates. The N350, a sleep-specific ERP, is thought to reflect the inhibition of processing of potentially sleep-disrupting stimulus input. During rapid eye movement (REM) sleep, a small amplitude N1 was apparent in the ERP, but only for the loudest, 80 dB stimulus. A small (nonsignificant) P3a-like deflection was also visible following the 80 dB stimulus, but only when stimuli were presented slowly. The findings of the present study offer, on one hand, an explanation of the means by which consciousness is prevented during sleep but also, on the other hand, an explanation of how sleep can be disrupted and possibly reversed, leading to an awakening.

Event-related potential measures of gap detection threshold during natural sleep.

OBJECTIVE The minimum time interval between two stimuli that can be reliably detected is called the gap detection threshold. The present study examines whether an unconscious state, natural sleep affects the gap detection threshold. METHODS Event-related potentials were recorded in 10 young adults while awake and during all-night sleep to provide an objective estimate of this threshold. These subjects were presented with 2, 4, 8 or 16ms gaps occurring in 1.5 duration white noise. RESULTS During wakefulness, a significant N1 was elicited for the 8 and 16ms gaps. N1 was difficult to observe during stage N2 sleep, even for the longest gap. A large P2 was however elicited and was significant for the 8 and 16ms gaps. Also, a later, very large N350 was elicited by the 16ms gap. An N1 and P2 was significant only for the 16ms gap during REM sleep. SIGNIFICANCE ERPs to gaps occurring in noise segments can therefore be successfully elicited during natural sleep. The gap detection threshold is similar in the waking and sleeping states.