Extent and time‐course of competition in visual cortex between emotionally arousing distractors and a concurrent task

Emotionally arousing cues automatically attract attentional resources, which may be at the cost of processing task‐related information. Of central importance is how the visual system resolves competition for processing resources among stimuli differing in motivational salience. Here, we assessed the extent and time‐course of competition between emotionally arousing distractors and task‐related stimuli in a frequency‐tagging paradigm. Steady‐state visual evoked potentials (ssVEPs) were evoked using random‐dot kinematograms that consisted of rapidly flickering (8.57 Hz) dots, superimposed upon emotional or neutral distractor pictures flickering at 12 Hz. The time‐varying amplitude of the ssVEP evoked by the motion detection task showed a significant reduction to the task‐relevant stream while emotionally arousing pictures were presented as distractors. Competition between emotionally arousing pictures and moving dots began 450 ms after picture onset and persisted for an additional 2600 ms. Competitive effects of the overlapping task and picture stream revealed cost effects for the motion detection task when unpleasant pictures were presented as distractors between 450 and 1650 ms after picture onset, where an increase in ssVEP amplitude to the flickering picture stimulus was at the cost of ssVEP amplitude to the flickering dot stimulus. Cost effects were generalized to all emotionally arousing contents between 1850 and 3050 ms after picture onset, where the greatest amount of competition was evident for conditions in which emotionally arousing pictures, compared to neutral, served as distractors. In sum, the processing capacity of the visual system as measured by ssVEPs is limited, resulting in prioritized processing of emotionally relevant cues.     

Snake fearfulness is associated with sustained competitive biases to visual snake features: Hypervigilance without avoidance

The extent and time course of competition between a specific fear cue and task-related stimuli in early human visual cortex was investigated using electrophysiology. Steady-state visual evoked potentials (ssVEPs) were evoked using random-dot kinematograms that consisted of rapidly flickering (8.57 Hz) dots moving randomly, superimposed upon emotional or neutral distractor pictures. Participants were asked to detect intervals of coherently moving dots, ignoring the distractor pictures that varied in hedonic content. Women reporting high or low levels of snake fear were recruited from a large sample of healthy college students, and snake pictures served as fear-relevant distractors. The time-varying amplitude of the ssVEP evoked by the motion detection task showed significant reduction when viewing emotionally arousing, compared to neutral, distractors, replicating previous studies. For high-fear participants, snake distractors elicited a sustained attenuation of task evoked ssVEP amplitude, greater than the attenuation prompted by other unpleasant arousing content. These findings support a hypothesis that fear cues prompt sustained hypervigilance rather than perceptual avoidance.     

Pre-stimulus alpha power affects vertex N2-P2 potentials evoked by noxious stimuli

It is well known that scalp potentials evoked by nonpainful visual and auditory stimuli are enhanced in amplitude when preceded by pre-stimulus low-amplitude alpha rhythms. This study tested the hypothesis that the same holds for the amplitude of vertex N2-P2 potentials evoked by brief noxious laser stimuli, an issue of interest for clinical perspective. EEG data were recorded in 10 subjects from 30 electrodes during laser noxious stimulation. The artifact-free vertex N2-P2 complex was spatially enhanced by surface Laplacian transformation. Pre-stimulus alpha power was computed at three alpha sub-bands according to subject's individual alpha frequency peak (i.e. about 6-8Hz for alpha 1, 8-10Hz for alpha 2 and 10-12Hz for alpha 3 sub-band). Individual EEG single trials were divided in two sub-groups. The strong-alpha sub-group (high band power) included halfway of all EEG single trials, namely those having the highest pre-stimulus alpha power. Weak-alpha sub-group (low band power) included the remaining trials. Averaging procedure provided laser evoked potentials for both trial sub-groups. No significant effect was found for alpha 1 and alpha 2 sub-bands. Conversely, compared to strong-alpha 3 sub-group, weak-alpha 3 sub-group showed vertex N2-P2 potentials having significantly higher amplitude (p<0.05). These results extend to the later phases of pain processing systems the notion that generation mechanisms of pre-stimulus alpha rhythms and (laser) evoked potentials are intrinsically related and subjected to fluctuating "noise". That "noise" could explain the trial-by-trial variability of laser evoked potentials and perception.     

Cortical augmenting in alexithymic subjects after unpleasant acoustic stimulation

OBJECTIVE: Alexithymia has a high prevalence in patients with chronic or somatoform pain disorders. Various investigations demonstrated a hypersensitivity and low tolerance of alexithymics to touch and painful stimulation. It was argued that alexithymic characteristics may lead an individual to augment stimuli in order to prevent ignoring stimuli that might be dangerous. Therefore, there may be a relation to the reducer-augmenter construct. To test this hypothesis, cortical reactivity in response to unpleasant acoustical stimulation was measured. METHODS: Nineteen high alexithymics and 18 low alexithymics were stimulated with five series of acoustic stimuli (white noise) of different intensity levels. Evoked potentials were recorded at electroencephalogram electrodes Fz, Cz, and Pz. RESULTS: With ascending stimulus intensities, both groups showed increasing P1-N1 and N1-P2 peak-to-peak amplitudes. High alexithymics had higher P1-N1 amplitudes and a stronger intensity-dependent slope of P1-N1 amplitude. In contrast to that, N1-P2 amplitude of low alexithymics tended to decrease in response to the highest stimulus intensity level. CONCLUSION: High alexithymic subjects seem to behave like cortical augmenters. This may be due to a disturbed transmarginal inhibition, which may normally serve as a protective physiological mechanism.     

Predictability of the target stimulus for sensory-guided movement modulates early somatosensory cortical potentials.

OBJECTIVE: To investigate the role of sensory modulation in the control of sensory-guided behaviour. Specifically, we hypothesized that early somatosensory evoked potentials (SEPs) would be facilitated during performance of continuous sensory-guided movement requiring sustained attention. METHODS: Median nerve SEPs were elicited via electrical stimulation and recorded from scalp electrodes while subjects performed tasks requiring continuous sensory-motor transformations. Subjects received a predictable (rhythmic amplitude modulation) or unpredictable (random amplitude modulation) amplitude varying tactile stimulus (frequency constant at 20 Hz) delivered to the tip of the index finger either alone or with the requirement to track it by modulating the isometric grip force produced by the opposite hand. RESULTS: Early SEP (N20-P27) amplitudes were differentially modulated during unpredictable tracking compared to sensory-motor controls. Specifically, N20 amplitudes were attenuated and P27 amplitudes were enhanced during sensory-guided tracking. CONCLUSIONS: Sustained attention to task-relevant sensory stimuli differentially modulates areas within primary somatosensory cortex (S1) during a continuous sensory-motor transformation. SIGNIFICANCE: These data have implications for understanding the role of attention in regulating somatosensory cortices during sensory-motor behaviour.     

ANESTHESIA AND TEMPERATURE EFFECTS ON SOMATOSENSORY EVOKED POTENTIALS PRODUCED BY TRAIN STIMULI

Objective: To determine the effect of anesthesia, temperature, and stimulus characteristics on the response of upper extremity somatosensory evoked responses (SSEP) to repetitive stimulation. Methods: Pairs and trains of electrical stimuli were used to elicit the upper extremity SSEP, and the amplitudes of the N20-P22, N13, and Erb s point potentials produced by each stimulus were measured. The ratio of the amplitude of the response to each stimulus to that produced by the first stimulus in a given train was computed. Recordings were also made directly from the cortical surface in a single patient. Results: There were only minimal effects of anesthesia, temperature, or stimulus characteristics on the amplitude ratios for the N13 and Erb s point potentials. The N20-P22 amplitude ratio demonstrated facilitation with interstimulus intervals in the 20 100 ms range and was markedly reduced with hypothermia. The degree of facilitation was strongly dependent on isoflurane and nitrous oxide concentrations. In response to stimulation with trains of four stimuli, increased amplitudes of the N20-P22 complex were seen maximally with the second response while the third and fourth responses were of lower amplitude. Conclusions: There are strong effects of anesthesia and temperature on the cortical responses to repetitive stimulation of the upper extremity. Speculations on the origin of these effects were made.     

Anesthesia and temperature effects on somatosensory evoked potentials produced by train stimuli

OBJECTIVE: To determine the effect of anesthesia, temperature, and stimulus characteristics on the response of upper extremity somatosensory evoked responses (SSEP) to repetitive stimulation. METHODS: Pairs and trains of electrical stimuli were used to elicit the upper extremity SSEP, and the amplitudes of the N20-P22, N13, and Erb's point potentials produced by each stimulus were measured. The ratio of the amplitude of the response to each stimulus to that produced by the first stimulus in a given train was computed. Recordings were also made directly from the cortical surface in a single patient. RESULTS: There were only minimal effects of anesthesia, temperature, or stimulus characteristics on the amplitude ratios for the N13 and Erb's point potentials. The N20-P22 amplitude ratio demonstrated facilitation with interstimulus intervals in the 20-100 ms range and was markedly reduced with hypothermia. The degree of facilitation was strongly dependent on isoflurane and nitrous oxide concentrations. In response to stimulation with trains of four stimuli, increased amplitudes of the N20-P22 complex were seen maximally with the second response while the third and fourth responses were of lower amplitude. CONCLUSIONS: There are strong effects of anesthesia and temperature on the cortical responses to repetitive stimulation of the upper extremity. Speculations on the origin of these effects were made.     

Defensive engagement and perceptual enhancement

We tested whether visual cortical sensitivity to external cues in the context of an acute defensive reaction is heightened or attenuated. A strong cardiac defense (fear) response was elicited by presenting an abrupt, loud acoustic stimulus following a 10-min period of quiescence. Electrocortical responses to aversive and neutral pictures following defensive stimulus onset were measured using dense-array EEG. Pictures were flickered at 12.5 Hz to evoke steady-state visual evoked potentials (ssVEP), which can be reliably extracted on the basis of single trials. Visual cortical activity indexing perceptual processing was substantially heightened when pictures were shown in temporal proximity to (i.e., 5 s after) the defense stimulus. Replicating previous studies, aversive visual stimuli were associated with enhanced ssVEP amplitude, compared to neutral stimuli. Acute defense facilitates visual perception of external cues and preserves accurate discrimination between threatening and safe cues.     

The influence of stimulus deviance on electrophysiologic and behavioral responses to novel events

This study investigated the role of stimulus deviance in determining electrophysiologic and behavioral responses to "novelty." Stimulus deviance was defined in terms of differences either from the immediately preceding context or from long-term experience. Subjects participated in a visual event-related potential (ERP) experiment, in which they controlled the duration of stimulus viewing with a button press, which served as a measure of exploratory behavior. Each of the three experimental conditions included a frequent repetitive background stimulus and infrequent stimuli that deviated from the background stimulus. In one condition, both background and deviant stimuli were simple, easily recognizable geometric figures. In another condition, both background and deviant stimuli were unusual/unfamiliar figures, and in a third condition, the background stimulus was a highly unusual figure, and the deviant stimuli were simple, geometric shapes. Deviant stimuli elicited larger N2-P3 amplitudes and longer viewing durations than the repetitive background stimulus, even when the deviant stimuli were simple, familiar shapes and the background stimulus was a highly unusual figure. Compared to simple, familiar deviant stimuli, unusual deviant stimuli elicited larger N2-P3 amplitudes and longer viewing times. Within subjects, the deviant stimuli that evoked the largest N2-P3 responses also elicited the longest viewing durations. We conclude that deviance from both immediate context and long-term prior experience contribute to the response to novelty, with the combination generating the largest N2-P3 amplitude and the most sustained attention. The amplitude of the N2-P3 may reflect how much "uncertainty" is evoked by a novel visual stimulus and signal the need for further exploration and cognitive processing.     

The effect of stimulus rate upon common peroneal, posterior tibial, and sural nerve somatosensory evoked potentials.

We examined the effect of stimulus rate on somatosensory evoked potentials (SEPs) following stimulation of the common peroneal nerve (CPN) at the knee, and the posterior tibial nerve (PTN) and sural nerve (SN) at the ankle. We measured the amplitude of P40-N50 and N50-P60 in the PTN-SEP and corresponding amplitude of CPN-SEP and SN-SEP at the rate of 2.3, 3.4, 4.1, and 5.1 Hz. When the stimulation rate was increased from 2.3 to 5.1 Hz, the P40-N50 amplitude decreased by 50% for the CPN-SEP and 20% for the PTN-SEP. Also, the N50-P60 amplitude was reduced by 30% in the CPN-SEP and 20% in the PTN-SEP. In contrast, this change in stimulus rate produced no significant amplitude decline in the SN-SEP. Blocking the peroneal nerve with lidocaine just distal to the stimulating electrodes eliminated the descending peroneal nerve volley and abolished the amplitude attenuation observed with the faster stimulus rate. The findings suggest that at higher rates of stimulation, the afferent volleys induced by the movements that follow mixed nerve stimulation interfere with the SEP produced by electrical activation of the sensory afferents. The interference is greater when the more proximal site of the mixed nerve is stimulated.     

Dysfunction of early-stage visual processing in schizophrenia: harmonic analysis

Schizophrenia is associated with severe neurocognitive deficits that constitute a core feature of the disorder. Deficits have been most extensively studied in relationship to higher-order processes. This study evaluated the integrity of early visual processing in order to evaluate the overall pattern of visual dysfunction in schizophrenia. Steady-state visual-evoked potentials (ssVEPs) were recorded over the occipital cortex (Oz) in patients with schizophrenia and schizoaffective disorder (N=26) and in age-matched comparison volunteers (N=22). Two stimuli were used: windmill-dartboard and partial-windmill, which are contrast-reversing ( approximately 4 Hz), radial patterns with dominant low spatial-frequency content. Each stimulus was presented for 1 min. Fourier analysis was performed on the ssVEP data to extract the relevant temporal frequency (i.e., harmonic) components. Magnitude-squared coherence (MSC) was computed to estimate the relative signal level for each frequency component. The patients showed reduced amplitude and coherence of second harmonic responses in both conditions, but intact first harmonic responses in the windmill-dartboard condition. This finding of a differential deficit may indicate a significant loss in the magnocellular pathway, which contributes to the generation of the second harmonic component under these conditions. Early sensory deficits may lead to impairments in subsequent stages of processing.     

Latencies of evoked potentials to flicker and to pattern speedily estimated by simultaneous method.

This note describes a method for rapidly measuring the latency of a steady-state visual evoked potential (EP). Three sinewaves of different frequencies (F1, F2 and F3 c/sec) are summed, and the resulting waveform modulates either light intensity (for flicker stimulation) or spatial contrast (for pattern stimulation). Three evoked potentials of frequencies F1, F2 and F3 c/sec respectively are stimultaneously recorded by running three Fourier analysers in parallel. The outputs of the three analysers are presented in a single polar plot so as directly to display the three values of EP amplitude and phase. Practical methods for generating the pattern and flicker stimuli are described.     

Phase synchronization of the ongoing EEG and auditory EP generation.

OBJECTIVE: We investigated the role of phase synchronization of the spontaneous electroencephalogram (EEG) in auditory evoked potential (EP) generation in a sample of healthy individuals. METHODS: Auditory responses were obtained from 20 healthy subjects following a double stimulus paradigm, using two identical tone bursts (S1 and S2) separated by 0.5s. Single-trial auditory evoked potentials were decomposed into sinusoidal, exponentially decaying/increasing components using the piecewise Prony method (PPM). Pre- and post-stimulus phase histograms were compared to determine the degree of phase synchronization produced by auditory stimulation. RESULTS: Analysis of single responses revealed that the S1 stimuli produced phase synchronization in the 2-8Hz frequency range, with little or no concomitant amplitude increase. A significantly reduced phase effect was seen in response to S2 stimuli. CONCLUSIONS: Stimulus-induced phase synchronization of the ongoing EEG is a major mechanism for the generation of auditory EP components with a latency in the 50-250ms range. SIGNIFICANCE: The fact that the EP components accessed here are generated through phase synchronization implies that the ensemble-averaged EP will not resemble the single trial response, and it would certainly be misleading to consider the single trial response as an amplitude-scaled version of the ensemble average.     

Functional dissociation of the early and late portions of human K-complexes

We analyzed K-complexes (KCs) evoked during sleep stage II by the subject's own name and by other names. KCs were composed either of four consecutive waves (full KCs, N2-P3-N3-P4) or of the two first components only (N2-P3). The amplitude of the late phase of KCs (N3-P4) was identical to all stimuli; conversely, own names enhanced selectively the N2-P3 waves, whether they were or not part of a full KC. Two independent phenomena appear to coexist during a full KC, one being connected to the physical characteristics of the stimulus (N3-P4) and the other to its intrinsic significance. This latter may appear either within a full KC or in isolation, and in this case it is reminiscent of the N200-P300 complex observed in wakefulness.     

Tagging cortical networks in emotion: A topographical analysis

Viewing emotional pictures is associated with heightened perception and attention, indexed by a relative increase in visual cortical activity. Visual cortical modulation by emotion is hypothesized to reflect re‐entrant connectivity originating in higher‐order cortical and/or limbic structures. The present study used dense‐array electroencephalography and individual brain anatomy to investigate functional coupling between the visual cortex and other cortical areas during affective picture viewing. Participants viewed pleasant, neutral, and unpleasant pictures that flickered at a rate of 10 Hz to evoke steady‐state visual evoked potentials (ssVEPs) in the EEG. The spectral power of ssVEPs was quantified using Fourier transform, and cortical sources were estimated using beamformer spatial filters based on individual structural magnetic resonance images. In addition to lower‐tier visual cortex, a network of occipito‐temporal and parietal (bilateral precuneus, inferior parietal lobules) structures showed enhanced ssVEP power when participants viewed emotional (either pleasant or unpleasant), compared to neutral pictures. Functional coupling during emotional processing was enhanced between the bilateral occipital poles and a network of temporal (left middle/inferior temporal gyrus), parietal (bilateral parietal lobules), and frontal (left middle/inferior frontal gyrus) structures. These results converge with findings from hemodynamic analyses of emotional picture viewing and suggest that viewing emotionally engaging stimuli is associated with the formation of functional links between visual cortex and the cortical regions underlying attention modulation and preparation for action. Hum Brain Mapp, 2012. © 2011 Wiley Periodicals, Inc.     

The laminar and temporal structure of stimulus information in the phase of field potentials of auditory cortex

Recent studies have shown that the phase of low-frequency local field potentials (LFPs) in sensory cortices carries a significant amount of information about complex naturalistic stimuli, yet the laminar circuit mechanisms and the aspects of stimulus dynamics responsible for generating this phase information remain essentially unknown. Here we investigated these issues by means of an information theoretic analysis of LFPs and current source densities (CSDs) recorded with laminar multi-electrode arrays in the primary auditory area of anesthetized rats during complex acoustic stimulation (music and broadband 1/f stimuli). We found that most LFP phase information originated from discrete "CSD events" consisting of granular-superficial layer dipoles of short duration and large amplitude, which we hypothesize to be triggered by transient thalamocortical activation. These CSD events occurred at rates of 2-4 Hz during both stimulation with complex sounds and silence. During stimulation with complex sounds, these events reliably reset the LFP phases at specific times during the stimulation history. These facts suggest that the informativeness of LFP phase in rat auditory cortex is the result of transient, large-amplitude events, of the "evoked" or "driving" type, reflecting strong depolarization in thalamo-recipient layers of cortex. Finally, the CSD events were characterized by a small number of discrete types of infragranular activation. The extent to which infragranular regions were activated was stimulus dependent. These patterns of infragranular activations may reflect a categorical evaluation of stimulus episodes by the local circuit to determine whether to pass on stimulus information through the output layers.     

Tobacco effects on cortical evoked potentials to distracting stimuli

In a group of female smokers, cortical evoked potentials (EP) were averaged to low and high intensity auditory stimuli which were presented during both task and non-task conditions and in separate smoking and non-smoking sessions. Four individual EP peaks, P1,N1,P2,N2 and 3 peak-to-peak measures, P1-N2, N1-P2, N2-P2, were involved in amplitude analysis. Tobacco significantly interacted with intensity and task condition to produce both increases (P1, N1) and decreases (P2, P2-N2) in amplitudes of EP peaks. The results are discussed in relation to arousal and information processing.     

Habituation of the auditory evoked potential in a short interstimulus interval paradigm

The present experiment was carried out to investigate elicitation and habituation of the auditory event related potentials with stimulus trains utilizing a short interstimulus intervals (ISI) of 1500 ms. Scalp event related potentials elicited by auditory stimuli were recorded in 10 male subjects. Thirty auditory stimuli were presented binaurally over headphones to every subject with a duration of 1000 ms, each with a constant ISI of 1500 ms. No task relevance was given to the stimuli. Wave-forms were collected using a Pentium 100 computer. All analyses were carried out over the 30 trials. In each single trial event related potentials, latencies and amplitudes of N1-P2 components were analyzed separately for four frequency bands (0.3-70, 0.3-4, 4-7, 7-13 Hz). Trend effects were tested with linear-regression analyses (N1-P2 amplitude x stimuli number). We found that the amplitude from the first stimulus decreased reliably across trial blocks of the N1-P2 components and that it was directly affected by ISI. The relevance of these results for the habituation is discussed.     

Auditory evoked transient and sustained potentials in the human EEG: I. Effects of expectation of stimuli

The characteristics of auditory evoked transient and sustained potentials were recorded using trains of four-tone stimuli of 1-second duration (interstimulus interval = 1 second) presented once every minute. The subject either attentively expected the stimuli or ignored them while reading. The electroencephalogram was recorded from derivations Cz-Al and Fz-Al. Expectation of the stimuli was associated with increased amplitudes of the transient responses both at the first stimulus of the train and during stimulus repetition. In contrast, the sustained potential at the first stimulus of the train was unchanged or smaller when the subject expected the stimuli. During stimulus repetition, however, the amplitude of the potential was enhanced by expectation of the stimuli. The results support the hypothesis of two sustained potential components and stress the importance of stimulus repetition rate when sustained potentials are studied.