Visual awareness of low-contrast stimuli is reflected in event-related brain potentials

Visual awareness was studied in 11 subjects by using coherent and scrambled objects as stimuli. The stimuli were presented near the subjective perceptual threshold. Explicitly recognized stimuli elicited a specific negative ERP deflection peaking at 460 ms. Our reaction time experiment suggests that this visual awareness negativity (VAN) is similar to that found previously at a shorter latency. We propose that VAN might reflect the exact moment of becoming aware of the task-relevant stimulus properties and provide an online marker of the temporal dynamics of visual awareness.     

Timing of the earliest ERP correlate of visual awareness

The earliest reliably occurring event-related brain potential (ERP) correlate of visual awareness (visual awareness negativity, VAN) emerges after 100 ms and peaks between 200 and 300 ms from stimulus onset. In a study using low-contrast stimuli, VAN was significantly delayed, peaking at 460 ms (V. Ojanen, A. Revonsuo, & M. Sams, 2003). In that study physical differences between the conscious and nonconscious stimuli may have confounded the results. Here we explored whether VAN is similarly delayed for physically identical stimuli. We presented low-contrast stimuli near an individually determined subjective contrast threshold. A delayed VAN peaked at 400 ms at occipito-temporal sites to subjectively perceived stimuli. Our results support the interpretation that VAN is the earliest ERP correlate of phenomenal visual awareness. The electrophysiological processes eliciting VAN may become delayed as a function of the difficulty of the early perceptual discrimination.     

A Pool of Pairs of Related Objects (POPORO) for Investigating Visual Semantic Integration: Behavioral and Electrophysiological Validation

Semantic processing of verbal and visual stimuli has been investigated in semantic violation or semantic priming paradigms in which a stimulus is either related or unrelated to a previously established semantic context. A hallmark of semantic priming is the N400 event-related potential (ERP)--a deflection of the ERP that is more negative for semantically unrelated target stimuli. The majority of studies investigating the N400 and semantic integration have used verbal material (words or sentences), and standardized stimulus sets with norms for semantic relatedness have been published for verbal but not for visual material. However, semantic processing of visual objects (as opposed to words) is an important issue in research on visual cognition. In this study, we present a set of 800 pairs of semantically related and unrelated visual objects. The images were rated for semantic relatedness by a sample of 132 participants. Furthermore, we analyzed low-level image properties and matched the two semantic categories according to these features. An ERP study confirmed the suitability of this image set for evoking a robust N400 effect of semantic integration. Additionally, using a general linear modeling approach of single-trial data, we also demonstrate that low-level visual image properties and semantic relatedness are in fact only minimally overlapping. The image set is available for download from the authors' website. We expect that the image set will facilitate studies investigating mechanisms of semantic and contextual processing of visual stimuli.     

Unnoticed regularity violation elicits change-related brain activity

Event-related brain electric activity (ERP) was investigated to unnoticed visual changes. The orientation of grid elements (vertical or horizontal) changed after the presentation of 10–15 identical stimuli. The grid patterns were task irrelevant, and were presented in the background of a shape discrimination task. During the first half of the session, participants were unaware of the stimulus change. However, in comparison to the ERPs to the fifth identical stimuli, stimulus change elicited posterior negativities in the 270–375 ms range (visual mismatch negativity, vMMN). With participants instructed on the stimulus change, negativities emerged with earlier onset and with wider distribution. When stimulus change was preceded by only two identical stimuli, there were no such ERP effects. As the results show, a longer sequence of identical unattended stimuli may establish the memory representation of stimulus regularity, and violation of regularity is indicated by posterior negative ERP components (vMMN).     

Temporal recalibration to tactile–visual asynchronous stimuli

Here we demonstrate that the perceptual system adapts to tactile–visual temporal asynchronies (i.e., temporal recalibration). Participants were exposed to a train of tactile and visual stimuli with a constant time lag (either −100 ms, 0 ms, or 100 ms; with negative values indicating that the tactile stimulus came first). Following exposure, they were presented tactile–visual test stimulus pairs and judged whether the tactile or the visual stimulus was presented first (Temporal Order Judgement). Results show that subjective simultaneity (the PSS) was shifted in the direction of the exposure lag. The results fit reports on auditory–visual temporal recalibration and indicate that the brain adapts to temporal incongruencies between modalities in general.     

Chronic stress impairs learning and hippocampal cell proliferation in senescence-accelerated prone mice

The anterior N2 is a component of the event-related brain potential (ERP) elicited by visual novel stimuli. Previous studies have reported that the stimuli that were viewed for longer periods of time elicited a larger anterior N2 than the stimuli viewed for shorter periods of time. To scrutinize this relationship between the ERP and viewing duration in response to visual materials, 18 university students were asked to look at various random polygons one-by-one for as long as they wished. ERPs time-locked to stimulus onset were averaged separately for three levels of complexity (12-, 24-, and 48-sided polygons). We found that the more complex the stimulus, the larger the anterior negativity (N2, 200-300 ms) and the posterior positivity (late positive potential [LPP], 400-800 ms), and the longer the viewing duration. However, when ERPs were calculated separately for the stimuli viewed for longer or shorter than the median viewing time of each participant at each complexity level, no amplitude differences were found in either component. These results suggest that the previously reported correlation between the anterior N2 and visual duration is spurious and produced by a third variable, namely, the perceptual demand of the eliciting stimulus such as complexity.     

Earthquake experience interference effects in a modified Stroop task: An ERP study

The effects of the modified Stroop task on ERP were investigated in 20 subjects who had experienced the Sichuan earthquake and a matched control group. ERP data showed that Incongruent stimuli elicited a more negative ERP deflection (N300–450) than did Congruent stimuli between 300 and 450 ms post-stimulus in the earthquake group but not found in the control group, and the N300–450 might reflect conflict monitor (the information of color and meaning do not match) in the early phase of perception identification due to their sensitivity to the external stimulus. Then, Incongruent stimuli elicited a more negative ERP deflection than did Congruent stimuli between 450 and 650 ms post-stimulus in both the groups. Dipole source analysis showed that the N450–650 was mainly generated in the ACC contributed to this effect in the control group, which might be related to monitor and conflict resolution. However, in the earthquake group, the N450–650 was generated in the thalamus, which might be involved in inhibiting and compensating of the ACC which may be related to conflict resolution process.     

Probability-independent and -dependent ERPs reflecting visual change detection

In ERP studies, two posterior components with different polarities have been identified as ERP correlates of visual change detection. To compare these components in terms of sensitivity to the preceding stimulus sequence, two peripheral stimuli of different colors (red and blue) were presented with equal (50:50) or different probabilities (20:80 or 80:20), while 12 participants performed shape discrimination at a central location. A posterior positivity at around 90-140 ms was observed with similar amplitude to all stimuli immediately preceded by a different stimulus. In contrast, a posterior negativity at around 140-180 ms was observed to increase in amplitude with increasing number of preceding different stimuli. These results suggest the existence of probability-independent and -dependent change processing in the human visual system. The functional significance is discussed in terms of memory-based comparison and stimulus-specific refractoriness.     

The effect of repetition of infrequent familiar and unfamiliar visual patterns on components of the event-related brain potential

This experiment examined changes in the waveforms of the event-related brain potential (ERP) during repeated presentations of infrequent-familiar and infrequent-unfamiliar visual patterns. The EEG waveforms were averaged separately for each presentation of the two types of stimuli across different stimulus blocks. Principal components analysis and baseline-to-peak measurement identified three ERP components, a P3, P4 and a slow wave of diminishing positivity at anterior, central and posterior scalp sites. Unfamiliar stimuli evoked components of greater positivity and P3 waves of longer latencies than familiar stimuli. Repetition of these stimuli was associated with a progressive diminution of the amplitude of the three components and a shortening of P3 latency. For the slow wave the decline in amplitude was more pronounced during repetition of unfamiliar than familiar stimuli. These effects are interpreted as signs of alterations in cognitive processes that play a role during orienting and perceptual learning.     

Perceptual learning: psychophysical thresholds and electrical brain topography.

We studied perceptual learning by determining psychophysical discrimination thresholds for visual hyper acuity targets (vernier stimuli) as a function of stimulus orientation. One aim was to relate perceptual improvements to changes of electrophysiological activity of the human brain. A group of 43 healthy adults participated in a psychophysical experiment where vernier thresholds for vertical and horizontal vernier targets were compared. In 16 subjects thresholds were measured for each orientation twice at an interval of 25 min. Between threshold estimations, evoked brain activity was recorded from 30 electrodes over the occipital brain areas while the subjects observed appearance and disappearance of supra-threshold vernier offsets. Mean evoked potentials were computed for the first and second 600 stimulus presentations, and the scalp topography of electrical brain activity was analyzed. Vertically oriented stimuli yielded significantly better performance than horizontal targets, and thresholds were significantly lower in the second half of the experiment, i.e. after prolonged viewing of stimuli. The improvements in discrimination performance were specific for stimulus orientation and did not generalize. Learning effects were also observed with electrical brain activity, and field strength of the potentials increased significantly as a function of time. Scalp topography of the evoked components was significantly affected indicating a shift of activation between different neuronal elements induced by perceptual learning.     

Can I trust in what I see? EEG evidence for a cognitive evaluation of perceptual constructs

Environmental information available to our senses is incomplete and to varying degrees ambiguous. It has to be disambiguated in order to construct stable and reliable percepts. Ambiguous figures are artificial examples where perception is maximally unstable and alternates between possible interpretations. Tiny low‐level changes can disambiguate an ambiguous figure and thus stabilize percepts. The present study compares ERPs evoked by ambiguous stimuli and disambiguated stimulus variants across three visual categories: geometry (Necker cube), motion (stroboscopic alternative motion stimulus, SAM) and semantics (Boring's old/young woman). We found that (a) disambiguated stimulus variants cause stable percepts and evoke two huge positive ERP excursions (Cohen's effect sizes 1–2), (b) the amplitudes of these ERP effects are inversely related to the degree of stimulus ambiguity, and (c) this pattern of results is consistent across all three tested visual categories. This generality across visual categories points to mechanisms at a very abstract (cognitive) level of processing. We discuss our results in the context of a high‐level Bayesian inference unit that evaluates the reliability of perceptual processing results, given a priori incomplete, ambiguous sensory information. The ERP components may reflect the outcome of this reliability estimation.     

Visual working memory capacity and stimulus categories: a behavioral and electrophysiological investigation

It has recently been suggested that visual working memory capacity may vary depending on the type of material that has to be memorized. Here, we use a delayed match-to-sample paradigm and event-related potentials (ERP) to investigate the neural correlates that are linked to these changes in capacity. A variable number of stimuli (1–4) were presented in each visual hemifield. Participants were required to selectively memorize the stimuli presented in one hemifield. Following memorization, a test stimulus was presented that had to be matched against the memorized item(s). Two types of stimuli were used: one set consisting of discretely different objects (discrete stimuli) and one set consisting of more continuous variations along a single dimension (continuous stimuli). Behavioral results indicate that memory capacity was much larger for the discrete stimuli, when compared with the continuous stimuli. This behavioral effect correlated with an increase in a contralateral negative slow wave ERP component that is known to be involved in memorization. We therefore conclude that the larger working memory capacity for discrete stimuli can be directly related to an increase in activity in visual areas and propose that this increase in visual activity is due to interactions with other, non-visual representations.     

The effect of odour priming on cortical EEG and visual ERP responses.

Two studies are reported that were designed to examine the way in which subjects, when primed with an olfactory stimulus, make perceptual judgements regarding subsequent congruent/incongruent information. The paradigm used was based on a modified version of a standard ERP semantic priming procedure. Subjects watched a computer screen and made a decision as to whether an image was congruent or incongruent with a priming olfactory stimulus. Results of the two studies showed significant effects in the N400 component of the visual ERP between congruent and incongruent stimuli are reported when a complex malodour accord was used as a prime but not when a complex pleasant odour accord was used as a prime.     

Toward a Functional Categorization of Slow Waves

This study is concerned with slowly varying, long-duration brain event-related potential (ERP) components, referred to as Slow Wave activity. Slow Wave activity can be observed in the epoch following P3b, suggesting that it reflects further processing invoked by increased task demands, beyond the processing that underlies P3b. The present experiment was designed to distinguish Slow Wave activity related to specific types of task demands which arise during difficult perceptual (pattern recognition) and conceptual (arithmetic) mental operations. Three late ERP components that respond differentially in amplitude to manipulation of perceptual and conceptual difficulty were identified: 1) A P3b, with a topography focused about P_z, evidently related to the subjective categorization of easy and difficult conceptual operations, that increased when the subjective low-probability operation was performed; 2) A longer latency, centroparietal positive Slow Wave that increased directly with perceptual difficulty but was not affected by conceptual difficulty; 3) A very long latency negative Slow Wave, broadly distributed over centroposterior scalp, that increased directly with conceptual difficulty while its onset was delayed when perceptual difficulty increased.     

THE INTERACTION OF RESPONSES IN THE BRAIN TO SEMANTIC STIMULI

Long time-constant EEG recording during paired stimuli has led to the discovery of the contingent negative variation or expectancy wave (Walter, 1964). This effect is produced when a conditional stimulus signals that an imperative stimulus demanding action, decision, or attention will follow at a short, constant time interval. Symbolic and meaningful stimuli were presented to subjects tachistoscopically, and the evoked responses in the brain were electronically averaged. The cerebral evoked responses to such psychological stimuli are more complex than to flashes. A slow negative DC potential shift (CNV) was seen during the interval between an auditory ready signal and the visual exposure if recognition of the stimulus was required, or if it was interesting. Following the visual exposure, a slow positive DC shift occurred. The method has been developed to study the brain responses to psychological stimuli. The amplitude of the responses relates to the information content and subjective factors rather than to the physical strength of the stimulus.     

Distraction of task-relevant information processing by irrelevant changes in auditory, visual, and bimodal stimulus features: A behavioral and event-related potential study

Distractibility with auditory, visual, and bimodal stimulus changes was investigated using an audio-visual distraction paradigm. Participants were asked to discriminate between equiprobable short and long audio-visual stimuli. Infrequently, the auditory, the visual, or both parts of the stimuli changed. These rare deviations (deviants) were irrelevant for the actual task. The influence of the three types of deviant stimuli on the processing of task-relevant information was assessed with behavioral and event-related potential (ERP) measures assuming that bimodal deviants would lead to an increase in distraction. Behavioral and ERP results did not support this assumption, as reaction time (RT) prolongation and components amplitudes did not differ significantly for auditory and bimodal deviants. It is suggested that a maximal threshold of distraction accounts for these results. In addition, the processing of bimodal deviations was assessed. Audio-visual interactions were found following modality-specific deviance detection suggesting that integration only occurs with involuntary attention switching to task-irrelevant changes.     

Distortion of ERP averages due to overlap from temporally adjacent ERPs: Analysis and correction

In studies of event-related potentials (ERPs), short interstimulus intervals (ISIs) are often employed to investigate certain neural or psychological phenomena. At short ISIs, however, the ERP responses to successive stimuli may overlap, thereby distorting the ERP averages. This paper describes a signal processing approach for analyzing the distortion of ERP averages due to such overlap. In general, the distortion is modeled in terms of mathematical convolutions of the ERP waveform elicited by each type of adjacent stimulus with the corresponding distribution in time of those stimuli relative to the averaging epoch. Using this framework, a number of implications of ERP overlap for experimental design and interpretation are examined, with special emphasis given to selective attention paradigms. It is shown that the possibility of confound due to ERP overlap is widespread in short-ISI experiments, and even the widely used procedure of stimulus randomization does not necessarily control for differential distortion of the ERPs to attended versus inattended stimuli. Problems due to ERP overlap can be particularly serious in short-ISI studies that examine how ERPs (and associated perceptual processes) are influenced by the nature of the preceding stimulus (i.e., stimulus sequence effects). A set of algorithms is presented for estimating and removing the residual distortion due to response overlap from recorded ERP averages. The use of these algorithms, collectively termed the Adjacent Response (Adjar) Technique, can alleviate many of the overlaprelated problems that arise when short ISIs are used, thereby enhancing the power of the ERP technique.     

Electrophysiological evidence of visual encoding deficits in a cross-modal attentional blink paradigm

Two experiments are reported in which two target stimuli, T1 and T2, were presented at variable stimulus onset asynchronies (SOAs). In Experiment 1, T1 and T2 were visual stimuli embedded in a rapid serial visual presentation (RSVP) stream of distractors. Participants were asked to report T1 and T2 at the end of the stream. In Experiment 2, T1 was an auditory stimulus, and T2 a visual stimulus embedded in an RSVP stream. Participants made a speeded discriminative response to T1, and reported T2 at the end of the stream. An attentional blink (AB) effect was observed in both experiments: T2 report suffered at short SOA compared to long SOA. During the AB, the amplitude of the P300 component of the event-related potential (ERP) locked to T2 onset was sensibly reduced in both experiments. Behavioral and ERP results were very similar across the two experiments. Implications for models of the AB effect are discussed.     

Neurodynamic Studies on Emotional and Inverted Faces in an Oddball Paradigm

The detection of a change in a face stimulus was studied in an oddball paradigm. Event-related potentials (ERPs) and MEG responses to face stimuli were recorded in four conditions: 1) happy standard, neutral deviant; 2) neutral standard, neutral deviant; 3) inverted happy standard, inverted neutral deviant; 4) inverted neutral standard, inverted neutral deviant. In all conditions, the target was a face with glasses. Neutral deviants elicited a negative deflection (with a maximum around 280 ms) in ERP and MEG responses, an effect similar to auditory mismatch negativity. Face inversion diminished deviance-related negativity, implying an important role of face recognition in the observed effect. Emotional content and larger physical differences between stimuli in conditions 1 and 3 compared to conditions 2 and 4 did not show statistically significant effect on the neutral-deviant-related negativity.     

Visual extinction of similar and dissimilar stimuli: Evidence for level-dependent attentional competition

Repetition blindness (RB) is the failure to report a visual stimulus presented shortly after a first occurrence of the same stimulus (Kanwisher, 1987). A similar phenomenon is that visual extinction, the failure to identify a contralesional stimulus presented simultaneously with an ipsilesional stimulus, increases with increasing similarity between the contralesional and ipsilesional stimulus (Baylis, Driver, & Rafal, 1993). We report a patient who, after a right parietal stroke, presented increased extinction for letters in repeated (e.g., A + A) than in unrepeated (e.g., T + U) displays. Increased extinction due to RB was observed in all experimental conditions probing item identification and varied between 5.4% and 40.6% across conditions. RB was unaffected by temporal modulation of the display, but was significantly reduced when stimuli grouped by a surrounding contour. Identification of contralesional repeated and unrepeated letters could be enhanced by auditory cues presented prior to the visual display. These results suggest that perceptual processing of extinguished stimuli that are similar to the stimulus presented on the preserved side is relatively unimpaired, but that the patient fails to ascribe to the stimulus a separate identity, supporting the distinction between type recognition and token individuation (Kanwisher, 1987). The extinction patterns for similar and dissimilar stimuli indicate that competition for attentional selection does not only occur at low (perceptual) levels, but also at higher processing levels, suggesting the presence of attentional competition on different levels of analysis.