Showing papers in "Psychophysiology in 2004"

The selective processing of briefly presented affective pictures: an ERP analysis.

Abstract: Recent event-related potential (ERP) studies revealed the selective processing of affective pictures. The present study explored whether the same phenomenon can be observed when pictures are presented only briefly. Toward this end, pleasant, neutral, and unpleasant pictures from the International Affective Pictures Series were presented for 120 ms while event related potentials were measured by dense sensor arrays. As observed for longer picture presentations, brief affective pictures were selectively processed. Specifically, pleasant and unpleasant pictures were associated with an early endogenous negative shift over temporo-occipital sensors compared to neutral images. In addition, affective pictures elicited enlarged late positive potentials over centro-parietal sensor sites relative to neutral images. These data suggest that a quick glimpse of emotionally relevant stimuli appears sufficient to tune the brain for selective perceptual processing.

Automatic removal of eye movement and blink artifacts from EEG data using blind component separation

Abstract: Signals from eye movements and blinks can be orders of magnitude larger than brain-generated electrical potentials and are one of the main sources of artifacts in electroencephalographic (EEG) data. Rejecting contaminated trials causes substantial data loss, and restricting eye movements/blinks limits the experimental designs possible and may impact the cognitive processes under investigation. This article presents a method based on blind source separation (BSS) for automatic removal of electroocular artifacts from EEG data. BBS is a signal-processing methodology that includes independent component analysis (ICA). In contrast to previously explored ICA-based methods for artifact removal, this method is automated. Moreover, the BSS algorithm described herein can isolate correlated electroocular components with a high degree of accuracy. Although the focus is on eliminating ocular artifacts in EEG data, the approach can be extended to other sources of EEG contamination such as cardiac signals, environmental noise, and electrode drift, and adapted for use with magnetoencephalographic (MEG) data, a magnetic correlate of EEG.

The music of speech: Music training facilitates pitch processing in both music and language

Abstract: The main aim of the present experiment was to determine whether extensive musical training facilitates pitch contour processing not only in music but also in language. We used a parametric manipulation of final notes' or words' fundamental frequency (F0), and we recorded behavioral and electrophysiological data to examine the precise time course of pitch processing. We compared professional musicians and nonmusicians. Results revealed that within both domains, musicians detected weak F0 manipulations better than nonmusicians. Moreover, F0 manipulations within both music and language elicited similar variations in brain electrical potentials, with overall shorter onset latency for musicians than for nonmusicians. Finally, the scalp distribution of an early negativity in the linguistic task varied with musical expertise, being largest over temporal sites bilaterally for musicians and largest centrally and over left temporal sites for nonmusicians. These results are taken as evidence that extensive musical training influences the perception of pitch contour in spoken language.

ERP components associated with successful and unsuccessful stopping in a stop-signal task.

Abstract: The primary aim of this study was to examine how response inhibition is reflected in components of the event-related potential (ERP), using the stop-signal paradigm as a tool to manipulate response inhibition processes. Stop signals elicited a sequence of N2/P3 components that partly overlapped with ERP components elicited by the reaction stimulus. N2/P3 components were more pronounced on stop-signal trials than on no-stop-signal trials. At Cz, the stop-signal P3 peaked earlier on successful than on unsuccessful stop trials. This finding extends the horse race model by demonstrating that the internal response to the stop signal (as reflected in stop-signal P3) is not constant, but terminates at different moments in time on successful and unsuccessful stop trials. In addition, topographical distributions and dipole analysis of high density EEG recordings indicated that different cortical generators were involved in P3s elicited on successful and unsuccessful stop-signal trials. The latter results suggest that P3 on successful stop-signal trials not only reflects stop-signal processing per se, but also efficiency of inhibitory control.

Context dependence of the event-related brain potential associated with reward and punishment.

Abstract: The error-related negativity (ERN) is an event-related brain potential elicited by error commission and by presentation of feedback stimuli indicating incorrect performance. In this study, the authors report two experiments in which participants tried to learn to select between response options by trial and error, using feedback stimuli indicating monetary gains and losses. The results demonstrate that the amplitude of the ERN is determined by the value of the eliciting outcome relative to the range of outcomes possible, rather than by the objective value of the outcome. This result is discussed in terms of a recent theory that holds that the ERN reflects a reward prediction error signal associated with a neural system for reinforcement learning.

The stability of resting frontal electroencephalographic asymmetry in depression.

Abstract: Although resting frontal electroencephalographic (EEG) alpha asymmetry has been shown to be a stable measure over time in nonclinical populations, its reliability and stability in clinically depressed individuals has not been fully investigated. The internal consistency and test–retest stability of resting EEG alpha (8–13 Hz) asymmetry were examined in 30 women diagnosed with major depression at 4-week intervals for 8 or 16 weeks. Asymmetry scores generally displayed good internal consistency and exhibited modest stability over the 8- and 16-week assessment intervals. Changes in asymmetry scores over this interval were not significantly related to changes in clinical state. These findings suggest that resting EEG alpha asymmetry can be reliably assessed in clinically depressed populations. Furthermore, intraclass correlation stability estimates suggest that although some traitlike aspects of alpha asymmetry exist in depressed individuals, there is also evidence of changes in asymmetry across assessment occasions that are not closely linked to changes in depressive severity. Descriptors: Frontal EEG asymmetry, Depression, Risk, Psychometric reliability and stability

Regional cerebral blood flow correlates with heart period and high-frequency heart period variability during working-memory tasks: Implications for the cortical and subcortical regulation of cardiac autonomic activity

Abstract: The aim of the present study was to characterize the functional relationships between behaviorally evoked regional brain activation and cardiac autonomic activity in humans. Concurrent estimates of regional cerebral blood flow (rCBF; obtained by positron emission tomography), heart period, and high-frequency heart period variability (HF-HPV; an indicator of cardiac parasympathetic activity) were examined in 93 adults (aged 50-70 years) who performed a series of increasingly difficult working-memory tasks. Increased task difficulty resulted in decreased heart period (indicating cardioacceleration) and decreased HF-HPV (indicating decreased cardiac parasympathetic activity). Task-induced decreases in heart period and HF-HPV were associated with concurrent increases and decreases in rCBF to cortical and subcortical brain regions that are speculated to regulate cardiac autonomic activity during behavioral processes: the medial-prefrontal, insular, and anterior cingulate cortices, the amygdala-hippocampal complex, and the cerebellum. These findings replicate and extend a small number of functional neuroimaging studies that suggest an important role for both cortical and subcortical brain systems in human cardiac autonomic regulation.

Parsing the late positive complex: Mental chronometry and the ERP components that inhabit the neighborhood of the P300

Abstract: Falkenstein, Hohnsbein, and Hoorman (1994) suggested that common measures of P300 latency confound a "P-SR" component whose latency corresponds to stimulus evaluation time and a "P-CR" component whose latency varies with response-selection time, thus casting doubt on work in mental chronometry that relies on P300 latency. We report here a replication and extension of Falkenstein et al. (1994) using a high-density 129-electrode montage with 11 subjects. Spatiotemporal PCA was used to extract the components of the ERP. A centroid measure is also introduced for detecting waveform-timing changes beyond just peak latency. In terms of componentry, we argue that the P-SR and the P-CR, correspond to the P3a/Novelty P3 and the P300, respectively. Conceptually, we dispute the proposed distinction between stimulus evaluation and response selection. We suggest a four-stage ERP model of information processing and place the P3a and the P300 in this framework.

Simple, effective countermeasures to P300-based tests of detection of concealed information

Abstract: We found countermeasures to protocols using P300 in concealed information tests. One, the "six-probe" protocol, in Experiment 1, uses six different crime details in one run. The countermeasure: generate covert responses to irrelevant stimuli for each probe category. Hit rates were 82% in the guilty group; 18% in the countermeasure group. The average reaction time (RT) distinguished these two groups, but with overlap in RT distributions. The "one-probe" protocol, in the second experiment, uses one crime detail as a probe. Here, one group was run in 3 weeks as a guilty group, a countermeasure group, and again as in Week 1. COUNTERMEASURE: Covert responses to irrelevant stimuli. In Week 1, hit rate was 92%. In Week 2, it was 50%. In Week 3, 58%. There was no overlap in the irrelevant RT distribution in Week 2: Countermeasure use was detectable. However, in Week 3, the RT distributions resembled those of Week 1; test-beaters could not be caught. These studies have shown that tests of deception detection based on P300 amplitude as a recognition index may be readily defeated with simple countermeasures that can be easily learned.

Memory load and the cognitive pupillary response in aging.

Abstract: The effect of memory load on the cognitive pupillary response among 16 young adults and 16 older adults was investigated. Mean pupil dilation and reaction time were measured during a Sternberg memory-search task, which involved six levels of memory load. A classic interaction pattern was obtained in which the reaction times of the elderly participants increased more as a function of memory load than the reaction times of the young participants. In the encoding phase of the experiment, mean dilation increased with memory load. No age differences were observed here. In the search phase of the experiment, however, mean pupil dilation was considerably greater in the young than in the elderly participants. Moreover, mean dilation of the older participants was not sensitive to memory load, whereas mean dilation increased as a function of memory load in the young participants. The results suggest that the usefulness of the pupillary response as a correlate of subtle fluctuations in memory load diminishes with old age.

Detection of synchronized oscillations in the electroencephalogram: An evaluation of methods

Abstract: The signal averaging approach typically used in ERP research assumes that peaks in ERP waveforms reflect neural activity that is uncorrelated with activity in the ongoing EEG. However, this assumption has been challenged by research suggesting that ERP peaks reflect event-related synchronization of ongoing EEG oscillations. In this study, we investigated the validity of a set of methods that have been used to demonstrate that particular ERP peaks result from synchronized EEG oscillations. We simulated epochs of EEG data by superimposing phasic peaks on noise characterized by the power spectrum of the EEG. When applied to the simulated data, the methods in question produced results that have previously been interpreted as evidence of synchronized oscillations, even though no such synchrony was present. These findings suggest that proposed analysis methods may not effectively disambiguate competing views of ERP generation.

The error-related negativity as a state and trait measure: motivation, personality, and ERPs in response to errors.

Abstract: This study examines changes in the error-related negativity (ERN/Ne) related to motivational incentives and personality traits. ERPs were gathered while adults completed a four-choice letter task during four motivational conditions. Monetary incentives for finger and hand accuracy were altered across motivation conditions to either be equal or favor one type of accuracy over the other in a 3:1 ratio. Larger ERN/Ne amplitudes were predicted with increased incentives, with personality moderating this effect. Results were as expected: Individuals higher on conscientiousness displayed smaller motivation-related changes in the ERN/Ne. Similarly, those low on neuroticism had smaller effects, with the effect of Conscientiousness absent after accounting for Neuroticism. These results emphasize an emotional/evaluative function for the ERN/Ne, and suggest that the ability to selectively invest in error monitoring is moderated by underlying personality.

Accuracy of heartbeat perception is reflected in the amplitude of the heartbeat-evoked brain potential.

Abstract: Neurotransmission from the heart to the brain results in a heartbeat-evoked potential (HEP). In this study, the influence of the ability to detect one's heartbeats based on the HEP was examined. According to their results in a heartbeat perception task, subjects were classified as good (n=18) or poor (n=26) heartbeat perceivers. EEG, EOG, and ECG were recorded while participants attended to their heartbeats. The R-wave of the ECG served as a trigger for EEG averaging. In the latency range of 250-350 ms after the ECG R-wave, the HEP amplitude at the right central location was significantly higher in good heartbeat perceivers. A significantly positive correlation was observed between the heartbeat perception score and the mean HEP amplitude. Our results confirm that the accuracy of heartbeat perception is reflected in the amplitude of the HEP. Thus, the HEP may be a suitable research tool for the study of brain processes related to visceral perception.

Changes in rolandic mu rhythm during observation of a precision grip.

Abstract: We recorded 128-channel EEG from 16 participants while they observed, imitated, and self-initiated the precision grip of a manipulandum. Mu rhythm amplitudes were significantly lower during observation of a precision grip than during observation of a simple hand extension without object interaction. Scalp topographies for subtractions of observation, imitation, and execution conditions from the control condition showed a high degree of congruence, supporting the notion of a human observation–execution matching system. Surface Laplacian transformations suggest that the decrease in mu amplitude during precision grip observation reflects desynchronization of mu rhythm generators in the sensorimotor cortex. These results support the hypothesis that sensorimotor cortex is a neural substrate involved in the representation of both self- and other-generated actions and show the mu rhythm is sensitive to subtle changes in observed motor behavior.

Heart rate and skin conductance analysis of antecendents and consequences of decision making.

Abstract: The current study examined the pattern of heart rate and skin conductance changes preceding risky choices and following outcome for bad, moderate, and good performers on an analogue of the Iowa gambling task (Bechara, Damasio, Damasio, & Anderson, 1994). The task required a choice between four options; two options were followed by a high reward and, unpredictably, an even higher loss (disadvantageous options) and two other options were followed by a small reward but the unpredictable loss was also small (advantageous options). Anticipatory heart rate slowing and skin conductance level were higher preceding disadvantageous relative to advantageous options, but only for good performers. In contrast, heart rate slowed and skin conductance level increased following loss relative to reward outcomes, and these changes were similar for all performance groups. These findings were interpreted to suggest that decision-making impairments in bad performers arise from a weak somatic response generated by secondary inducers (i.e., somatic markers), rather than a weak somatic response generated by primary inducers of reward and punishment.

Awareness of sleepiness when driving

Abstract: The extent to which sleepy drivers are aware of sleepiness has implications for the prevention of sleep-related crashes, especially for drivers younger than 30 years old who are most at risk. Using a real car interactive simulator, we report on EEG, subjective sleepiness, and lane drifting (sleepiness-related driving impairment) from 38 sleep-restricted, healthy young adults undergoing nontreatment control conditions from three (unpublished) investigations using the same experimental protocols for assessing various drinks intended to alleviate sleepiness. Participants drove 2 h during midafternoon under monotonous conditions. For all studies, subjective sleepiness and EEG activity indicative of sleepiness were highly correlated, with both changing concomitantly, along with lane drifting. Drivers had knowledge of their physiological sleepiness. There were indications that sugar content of these drinks may additionally affect sleepiness. Language: en

Brain electrocortical activity during and after exercise: A quantitative synthesis

Abstract: Investigators of brain electrocortical responses to exercise have interpreted increased activity, or frontal hemispheric asymmetry, in the alpha frequency band as indicative of relaxation or a change in affect. However, few studies compared alpha activity with other frequencies and within or across hemispheres. To clarify the cumulative evidence in this area, we provide a quantitative review of the effects of exercise on brain electrocortical activity according to frequency bands and recording sites. Fifty-eight effects from 18 studies and 282 participants were retrieved. The mean effect size was moderately large (0.54 SD, 95% CI: 0.43 to 0.65) but heterogeneous. Compared to before exercise, alpha activity was greater immediately after and during exercise when expressed as absolute power but not as relative to power in other frequency bands; delta, theta, and beta activity also increased (0.38 to 0.75 SD). Effects did not differ significantly by recording sites. The cumulative evidence does not indicate that change in brain electrocortical activity after exercise is specific to alpha activity or hemispheric site.

Respiratory sinus arrhythmia as an index of emotional response in young adults.

Abstract: The relationship between respiratory sinus arrhythmia (RSA) and valence and arousal remains unclear. In the present study, the associations between emotion responses and tonic or task-related changes in RSA were assessed. Specifically, the sensitivities of changes in interbeat interval, RSA, and skin conductance to the valence and arousal values of emotional stimuli were examined. This study also explored the association between tonic RSA and subjective, expressive, and physiological emotional responses. Response measures were collected from 56 adults during baseline and film-viewing periods. Tonic RSA was not significantly related to any of the response measures. Increased skin conductance and decreased RSA were associated with arousal independent of valence. Interbeat interval was related to affective valence and not arousal. These findings suggest that RSA may be a useful adjunct to skin conductance measures in assessing emotional arousal.

What's special about personally familiar faces? A multimodal approach.

Abstract: Dual-route models of face recognition suggest separate cognitive and affective routes. The predictions of these models were assessed in recognition tasks with unfamiliar, famous, and personally familiar faces. Whereas larger autonomic responses were only triggered for personally familiar faces, priming effects in reaction times to these faces, presumably reflecting cognitive recognition processes, were equal to those of famous faces. Activation of stored structural representations of familiar faces (face recognition units) was assessed by recording the N250r component in event-related brain potentials. Face recognition unit activation increased from unfamiliar over famous to personally familiar faces, suggesting that there are stronger representations for personally familiar than for famous faces. Because the topographies of the N250r for personally and famous faces were indistinguishable, a similar network of face recognition units can be assumed for both types of faces.

Stimulus modality, perceptual overlap, and the go/no-go N2.

Abstract: Stimuli that elicit a prepotent but incorrect response are typically associated with an enhanced electrophysiological N2 that is thought to index the operation of a control process such as inhibition or conflict detection. However, recent studies reporting the absence of the N2 modulation in go/no-go tasks involving auditory stimuli challenge this view: It is not clear why inhibition or conflict detection should be sensitive to the modality of the stimulus. Here we present electrophysiological data from a go/no-go task suggesting that the relative size of the N2 modulation in visual and auditory tasks depends on the perceptual overlap between the go and no-go stimuli. Stimuli that looked similar but sounded different were associated with a typical visual N2 modulation and the absence of an auditory N2 modulation, consistent with previous findings. However, when we increased the perceptual overlap between the auditory stimuli, a large no-go N2 was observed. These findings are discussed in terms of existing hypotheses of the N2, and clarify why previous studies have not found an N2 modulation in auditory go/no-go tasks.

Where to Q in PEP.

Abstract: The onset of ventricular depolarization defines the start of the preejection period (PEP), which is commonly used as an index of myocardial contractility and sympathetic control of the heart. Although the fiducial point for this onset has traditionally been the onset of the Q wave of the electrocardiogram, other measurement points have also been used in the literature, including the peak of the Q wave (i.e., the onset of the R wave). Conceptual, physiological, and empirical considerations addressing the reliability and validity of these alternative metrics support the application of the Q-wave peak/R-wave onset as the fiducial point for PEP measures.

Recognition of affective prosody: continuous wavelet measures of event-related brain potentials to emotional exclamations.

Abstract: The affective state of a speaker can be identified from the prosody of his or her speech. Voice quality is the most important prosodic cue for emotion recognition from short verbal utterances and nonverbal exclamations, the latter conveying pure emotion, void of all semantic meaning. We adopted two context violation paradigms-oddball and priming-to study the event-related brain potentials (ERP) reflecting this recognition process. We found a negative wave, the N300, in the ERPs to contextually incongruous exclamations, and interpreted this component as analogous to the well-known N400 response to semantically inappropriate words. The N300 appears to be a real-time psychophysiological measure of spontaneous emotion recognition from vocal cues, which could prove a useful tool for the examination of affective-prosody comprehension. In addition, we developed a new ERP component detection and estimation method that is based on the continuous wavelet transform (CWT), does not rely on visual inspection of the waveforms, and yields larger statistical difference effects than classical methods.

Repetition and semantic priming of nonwords: implications for theories of N400 and word recognition.

Abstract: ERPs were elicited by two types of orthographically legal, pronounceable nonwords. One nonword set was derived from and resembled real words, whereas the other set did not. Nonwords derived from related root words elicited N400 semantic priming effects similar to those obtained for words, indicating semantic activation of the root words. N400 repetition priming effects from nonderived nonwords were similar to those obtained for words. The elicitation of N400 by only derived nonwords would have suggested it was generated by the activation of word meanings, per se. However, both types of nonwords produced an N400, and an N400 priming effect. Because nonderived nonwords are not associated with meaning, the N400 cannot be generated by semantic activation per se. Rather, the N400 appears to be generated by orthographic/phonological analysis and is attenuated by the top-down feed of semantic information to the orthographic/phonological level.

Comparison of sensory gating to mismatch negativity and self-reported perceptual phenomena in healthy adults.

Abstract: To better understand the possible functional significance of electrophysiological sensory gating measures, response suppression of midlatency auditory event related potential (ERP) components was compared to the mismatch negativity (MMN) and to self-rated indices of stimulus filtering and passive attention-switching phenomena in an agerestricted sample of healthy adults. P1 sensory gating, measured during a paired-click paradigm, was correlated with MMN amplitude, measured during an acoustic oddball paradigm (intensity deviation). Also, individuals that exhibited less robust P1 suppression endorsed higher rates of ‘‘perceptual modulation’’ difficulties, whereas component N1 suppression was more closely related to ‘‘over-inclusion’’ of irrelevant sounds into the focus of attention. These findings suggest that the ERP components investigated are not redundant, but correspond to distinctFpossibly relatedFpre-attentive processing systems. Descriptors: Auditory evoked potentials, Sensory gating, Attention, Auditory perception, Auditory stimulation

Menstrual cycle phase effects on prepulse inhibition of acoustic startle.

Abstract: Prepulse inhibition (PPI) represents an attenuation of the startle reflex following the presentation of a weak prepulse at brief intervals prior to the startle eliciting pulse. It has been shown that increases in striatal dopamine levels decrease PPI; because dopamine release is sensitive to estrogen, it is likely that PPI varies across the menstrual cycle. Cross-sectional studies looking at estrogen effects suggest that this may be true. In this study, we compare effects of menstrual phase on PPI in a between-group design (men, follicular phase women, and luteal phase women) as well as a within-subjects design (women across the two phases). The study found a between-group as well as a within-subjects effect of phase on PPI. PPI in follicular phase women did not differ significantly from PPI in men. However, PPI was reduced in luteal women compared to follicular women. These data provide evidence that ovarian hormones affect sensorimotor gating.

Event‐related potentials associated with sound discrimination versus novelty detection in children

Abstract: In children, deviant sounds in an oddball paradigm elicit a mismatch negativity (MMN) indexing discrimination of sound change and late difference negativity (LDN) with unknown functional significance. Salient sounds elicit an ERP index if orienting, P3a, and a late negative component, Nc. We compared children's responses elicited by moderate sound changes and novel sounds to examine the relationships between MMN and LDN, and LDN and Nc. Two components of the Nc, the Nc1 and Nc2, were identified. The scalp topography of LDN differed from those of the MMN and Nc1. Children's early P3a appeared mature but late P3a lacked frontal predominance. The findings suggested that LDN is not linked with either the sensory or attentional processing. It might reflect cognitive, albeit preattentive, processing of sound change. The Nc1 appears to reflect cognitive attentive processing of salient stimuli and the Nc2 might reflect reorienting after distraction.

Randomization tests for ERP topographies and whole spatiotemporal data matrices.

Abstract: In ERP studies, the comparison of topographies (multichannel measurements) or whole spatiotemporal data matrices (multichannel time series of measurements), the classical statistical tests very often cannot be used. It is argued that, for these comparisons, randomization tests are an excellent alternative. It is also argued that the randomization test is superior to another resampling method, the bootstrap, because exact probability statements (e.g., p values) can be made. A review is given of the literature on randomization tests designed for electrophysiological data. New randomization tests are presented and applied to two data sets, one coming from a psychopharmacological experiment and the other from an ERP experiment in visual word recognition.

The effect of erroneous responses on response monitoring in patients with major depressive disorder: a study with event-related potentials.

Abstract: Perceived failure is reported to have detrimental effects on subsequent performance in patients with major depressive disorder. We investigated the error-related negativity (ERN)/error negativity (Ne), an electrophysiological correlate of response monitoring, using a 64-channel EEG. Sixteen patients with DSM-IV major depressive disorder and 16 matched controls participated in an Eriksen flanker task with continuous performance feedback that signaled monetary reward. Compared to controls, patients with major depressive disorder showed a less negative ERN/Ne in error trials following error trials. This result might reflect impaired response monitoring processes in major depressive disorder resulting from an underactivity in a central reward pathway and/or a deficit in strategic reasoning.

Electrophysiological evidence for delayed mismatch response in infants at-risk for specific language impairment.

Abstract: The present study investigated whether delayed auditory processing typically found in children with specific language impairment (SLI) can already be observed in the event-related potentials of 2-month-old infants. Infants with and without a family history of SLI were tested in a passive auditory oddball paradigm with CV-syllables differing in vowel duration. For the long syllable, a positive mismatch response occurred in the difference wave between deviant and standard. Its amplitude was higher in infants during quiet sleep than in awake infants, although its peak latency remained unaffected by alertness. Awake infants showed an adultlike mismatch negativity preceding the positivity. Risk for SLI was reflected in the latency of the positive mismatch response, which was delayed in infants with risk compared to infants without risk. This latency difference suggests that 2-month-old infants at risk for SLI are already affected in processing an auditory stimulus change of duration.

Early neural activity in Necker-cube reversal: Evidence for low-level processing of a gestalt phenomenon

Abstract: Normally we experience the visual world as stable. Ambiguous figures provide a fascinating exception: On prolonged inspection, the "Necker cube" undergoes a sudden, unavoidable reversal of its perceived front-back orientation. What happens in the brain when spontaneously switching between these equally likely interpretations? Does neural processing differ between an endogenously perceived reversal of a physically unchanged ambiguous stimulus and an exogenously caused reversal of an unambiguous stimulus? A refined EEG paradigm to measure such endogenous events uncovered an early electrophysiological correlate of this spontaneous reversal, a negativity beginning at 160 ms. Comparing across nine electrode locations suggests that this component originates in early visual areas. An EEG component of similar shape and scalp distribution, but 50 ms earlier, was evoked by an external reversal of unambiguous figures. Perceptual disambiguation seems to be accomplished by the same structures that represent objects per se, and to occur early in the visual stream. This suggests that low-level mechanisms play a crucial role in resolving perceptual ambiguity.