Effects of stimulus repetitions on the event-related potential of humans and rats

Adaptation patterns and their associations with mismatch negativity: An electroencephalogram (EEG) study with controlled expectations

Adaptation refers to the decreased neural response that occurs after repeated exposure to a stimulus. While many electroencephalogram (EEG) studies have investigated adaptation by using either single or multiple repetitions, the adaptation patterns under controlled expectations manifested in the two main auditory components, N1 and P2, are still largely unknown. Additionally, although multiple repetitions are commonly used in mismatch negativity (MMN) experiments, it is unclear how adaptation at different time windows contributes to this phenomenon. In this study, we conducted an EEG experiment with 37 healthy adults using a random stimulus arrangement and extended tone sequences to control expectations. We tracked the amplitudes of the N1 and P2 components across the first 10 tones to examine adaptation patterns. Our findings revealed an L-shaped adaptation pattern characterised by a significant decrease in N1 amplitude after the first repetition (N1 initial adaptation), followed by a continuous, linear increase in P2 amplitude after the first repetition (P2 subsequent adaptation), possibly indicating model adjustment. Regression analysis demonstrated that the peak amplitudes of both the N1 initial adaptation and the P2 subsequent adaptation significantly accounted for variance in MMN amplitude. These results suggest distinct adaptation patterns for multiple repetitions across different components and indicate that the MMN reflects a combination of two processes: the initial adaptation in the N1 and a continuous model adjustment effect in the P2. Understanding these processes separately could have implications for models of cognitive processing and clinical disorders.

Time course of visual attention in rats by atomic magnetometer

Atomic magnetometer (AM) is utilized to non-invasively detect event-related magnetic fields (ERMFs) evoked by visual stimuli in rats. The aim of this study was to investigate the relationship between N2-like amplitude and visual attention. To achieve this, we combined the AM with a visual stimulation system and employed the passive single-stimulus paradigm. By measuring the ERMFs at various inter-stimulus intervals (ISIs) with a sensitivity of 20 fT/[Formula: see text], we analyzed the effects of the ISI and the 'habituation' resulting from repeated stimuli on the N2-like amplitude. Our method serves as a valuable reference for studying the passive single-stimulus paradigm and the time course of mammalian attention.

Neural Markers of Auditory Response and Habituation in Phelan-McDermid Syndrome

Phelan-McDermid Syndrome (PMS) is a rare genetic disorder caused by deletion or sequence variation in the SHANK3 gene at terminal chromosome 22 that confers high likelihood of comorbid autism spectrum disorder (ASD). Whereas individuals with idiopathic ASD (iASD) can demonstrate diverse patterns of sensory differences, PMS is mainly characterized by sensory hyporesponsiveness. This study used electrophysiology and a passive auditory habituation paradigm to test for neural markers of hyporesponsiveness. EEG was recorded from 15 individuals with PMS, 15 with iASD, and 16 with neurotypical development (NT) while a series of four consecutive 1,000 Hz tones was repeatedly presented. We found intact N1, P2, and N2 event-related potentials (ERPs) and habituation to simple auditory stimuli, both in individuals with iASD and in those with PMS. Both iASD and PMS groups showed robust responses to the initial tone and decaying responses to each subsequent tone, at levels comparable to the NT control group. However, in PMS greater initial N1 amplitude and habituation were associated with auditory hypersensitivity, and P2 habituation correlated with ASD symptomatology. Additionally, further classification of the PMS cohort into genetic groupings revealed dissociation of initial P2 amplitude and habituation of N1 based on whether the deletions included additional genes beyond solely SHANK3 and those not thought to contribute to phenotype. These results provide preliminary insight into early auditory processing in PMS and suggest that while neural response and habituation is generally preserved in PMS, genotypic and phenotypic characteristics may drive some variability. These initial findings provide early evidence that the robust pattern of behavioral hyporesponsiveness in PMS may be due, at least in audition, to higher order factors.

Generalization of sustained neurophysiological effects of short-term auditory 13-Hz stimulation to neighboring frequency representation in humans

A fuller understanding of the effects of auditory tetanization in humans would inform better language and sensory learning paradigms, however, there are still unanswered questions. Here, we probe sustained changes in the event-related potentials (ERPs) to 1020Hz and 980Hz tones following a rapid presentation of 1020Hz tone (every 75 ms, 13.3Hz, tetanization). Consistent with the previous studies (Rygvold, et al., 2021, Mears & Spencer 2012), we revealed the increase in the P2 ERP component after tetanization. Contrary to other studies (Clapp et al., 2005; Lei et al., 2017) we did not observe the expected N1 increase after tetanization even in the experimental sequence identical to Clapp. et al., 2005. We detected a significant N1 decrease after tetanization. Expanding previous research, we showed that P2 increase and N1 decrease is not specific to the stimulus type (tetanized 1020Hz and non-tetanized 980Hz), suggesting the generalizability of tetanization effect to the not-stimulated auditory tones, at least to those of the neighboring frequency. The ERPs tetanization effects were observed for at least 30 min - the most prolonged interval examined, consistent with the duration of long-term potentiation, LTP. In addition, the tetanization effects were detectable in the blocks where the participants watched muted videos, an experimental setting that can be easily used in children and other challenging groups. Thus, auditory 13-Hz stimulation affects brain processing of tones including those of neighboring frequencies.

A Multimodal Neuroprosthetic Interface to Record, Modulate and Classify Electrophysiological Biomarkers Relevant to Neuropsychiatric Disorders

Most mental disorders, such as addictive diseases or schizophrenia, are characterized by impaired cognitive function and behavior control originating from disturbances within prefrontal neural networks. Their often chronic reoccurring nature and the lack of efficient therapies necessitate the development of new treatment strategies. Brain-computer interfaces, equipped with multiple sensing and stimulation abilities, offer a new toolbox whose suitability for diagnosis and therapy of mental disorders has not yet been explored. This study, therefore, aimed to develop a biocompatible and multimodal neuroprosthesis to measure and modulate prefrontal neurophysiological features of neuropsychiatric symptoms. We used a 3D-printing technology to rapidly prototype customized bioelectronic implants through robot-controlled deposition of soft silicones and a conductive platinum ink. We implanted the device epidurally above the medial prefrontal cortex of rats and obtained auditory event-related brain potentials in treatment-naïve animals, after alcohol administration and following neuromodulation through implant-driven electrical brain stimulation and cortical delivery of the anti-relapse medication naltrexone. Towards smart neuroprosthetic interfaces, we furthermore developed machine learning algorithms to autonomously classify treatment effects within the neural recordings. The neuroprosthesis successfully captured neural activity patterns reflecting intact stimulus processing and alcohol-induced neural depression. Moreover, implant-driven electrical and pharmacological stimulation enabled successful enhancement of neural activity. A machine learning approach based on stepwise linear discriminant analysis was able to deal with sparsity in the data and distinguished treatments with high accuracy. Our work demonstrates the feasibility of multimodal bioelectronic systems to monitor, modulate and identify healthy and affected brain states with potential use in a personalized and optimized therapy of neuropsychiatric disorders.

Isoflurane effects on the N1 and other long-latency auditory evoked potentials in Wistar rats

Long-latency auditory evoked potentials (LLAEPs) may help further advances in understanding consciousness under general anesthesia and promote more objective means of assessing sedation depth than conventional clinical signs. Among the LLAEP components, the auditory N1 shows promise as a measure of sedation depth and a marker of consciousness, but findings are so far inconclusive. Research with animals can help elucidate the effects of various anesthetics on the N1 and other LLAEPs, but investigations of LLAEPs under anesthesia in animals is lacking. To address this deficit, we examined the P1, N1, P2, and N2, along with their corresponding peak-to-peak complexes, in 10 Wistar rats anesthetized with 1.5-2 % isoflurane in pure oxygen and again after recovery. While under anesthesia, subdermal needle electrodes were inserted and secured for electroencephalographic (EEG) recordings. LLAEPs were assessed during a 20-min, passive, two-tone (500 ms inter-tone interval) paradigm with randomized short (1 s) and long (5 s) inter-pair intervals (IPIs). Overall, while the LLAEP peaks under isoflurane were less defined, they were not eliminated. The peak-to-peak amplitudes, particularly the P1-N1, were significantly smaller under isoflurane than during post-recovery. Our preliminary findings indicate that isoflurane produces global suppression across LLAEP components, presumably reflecting impaired integration of top-down and bottom-up attention and sensory systems under profound sedation with isoflurane.

Neural mechanism of priming in visual search

Selective attention affords scrutinizing items in our environment. However, attentional selection changes over time and across space. Empirically, repetition of visual search conditions changes attentional processing. Priming of pop-out is a vivid example. Repeatedly searching for the same pop-out search feature is accomplished with faster response times and fewer errors. We review the psychophysical background of priming of pop-out, focusing on the hypothesis that it arises through changes in visual selective attention. We also describe research done with macaque monkeys to understand the neural mechanisms supporting visual selective attention and priming of pop-out, and survey research on priming of pop-out using noninvasive brain measures with humans. We conclude by hypothesizing three alternative neural mechanisms and highlighting open questions.

Reduced Sensitivity to Non-Fear-Related Stimulus Changes in Panic Disorder

Panic disorder (PD) is associated with increased body vigilance and reduced cognitive resources directed at non-fear-related stimuli, particularly in the absence of stimulus-rich environments. To date, only few studies have investigated whether this deficit in PD is reflected in reduced mismatch negativity (MMN), an event-related potential indexing preattentive sensitivity to unexpected stimulus changes. We tested 35 patients affected by PD and 42 matched healthy controls in an oddball paradigm, using frequency and duration deviant stimuli to measure auditory MMN. PD patients displayed reduced duration MMN amplitudes in comparison to healthy controls. No group differences were detected for duration MMN latency, as well as frequency MMN indices. Results support the notion of reduced processing of non-fear-related stimuli in PD patients, particularly with regard to the preattentive processing of sound duration deviants. Additionally, our findings are in line with clinical studies reporting divergent deficits in preattentive processing of frequency and duration deviants.

Event-related potentials evoked by passive visuospatial perception in rats and humans reveal common denominators in information processing

In the human cortex, event-related potentials (ERPs) are triggered in response to sensory, cognitive or motor stimuli. Due to the inherent difficulties of conducting invasive mechanistic studies in human subjects, little is known as to the precise neurophysiological mechanisms that lead to their manifestation. By contrast, although much is known about synaptic and neural mechanisms that underlie information processing in rodents, very few studies have addressed to what extent ERPs are comparable in rodents and humans. Here, we explored this by triggering ERPs in both species during the passive observation of visuospatial imagery, shown in an oddball-like manner, using an experimental design that was equivalent. Several ERP-components were identified in the rodent cohort, corresponding, for example, to the human P1, N1, and P2. ERPs that are likely to reflect a rodent N2 and P300 were also detected. Deviance, as well as repetition effects were evident in both species, whereby rodent ERPs displayed more immediate response alterations to repeated stimuli and humans showed more gradual response shifts. These results indicate that humans and rodents may implement similar strategies for the passive perception and initial processing of visuospatial imagery, despite clear differences in their sensory and cognitive capacities.

Estimation of auditory steady-state responses based on the averaging of independent EEG epochs

The amplitude of auditory steady-state responses (ASSRs) generated in the brainstem of rats exponentially decreases over the sequential averaging of EEG epochs. This behavior is partially due to the adaptation of the ASSR induced by the continuous and monotonous stimulation. In this study, we analyzed the potential clinical relevance of the ASSR adaptation. ASSR were elicited in eight anesthetized adult rats by 8-kHz tones, modulated in amplitude at 115 Hz. We called independent epochs to those EEG epochs acquired with sufficiently long inter-stimulus interval, so the ASSR contained in any given epoch is not affected by the previous stimulation. We tested whether the detection of ASSRs is improved when the response is computed by averaging independent EEG epochs, containing only unadapted auditory responses. The improvements in the ASSR detection obtained with standard, weighted and sorted averaging were compared. In the absence of artifacts, when the ASSR was elicited by continuous acoustic stimulation, the computation of the ASSR amplitude relied upon the averaging method. While the adaptive behavior of the ASSR was still evident after the weighting of epochs, the sorted averaging resulted in under-estimations of the ASSR amplitude. In the absence of artifacts, the ASSR amplitudes computed by averaging independent epochs did not depend on the averaging procedure. Averaging independent epochs resulted in higher ASSR amplitudes and halved the number of EEG epochs needed to be acquired to achieve the maximum detection rate of the ASSR. Acquisition protocols based on averaging independent EEG epochs, in combination with appropriate averaging methods for artifact reduction might contribute to develop more accurate hearing assessments based on ASSRs.

No Effect of Transcranial Direct Current Stimulation of the Auditory Cortex on Auditory-Evoked Potentials

Transcranial direct current stimulation (tDCS) is a non-invasive brain stimulation technique to change cortical excitability. Its effects are shown for cognitive processing, and behavior in the motor and perceptual domains. However, evidence of tDCS effects in the perceptual domain particularly for auditory processing is rare. Therefore, and in the context of disturbances in auditory processing in psychiatric populations, e.g., in patients with auditory verbal hallucinations, we aimed to investigate the potential modulatory effect of tDCS on the excitability of left posterior temporal cortex in detail. We included 24 healthy participants in a crossover design, applying sham and anodal stimulation in two measurement sessions 1 week apart. Electroencephalography (EEG) was recorded while participants listened to tones before, during, and after stimulation. Amplitudes and latencies of P50, N100, and P200 auditory-evoked potentials (AEP) were compared between anodal and sham stimulation, and between time points before, during, and after tDCS. In contrast to previous studies, results demonstrate no significant differences between stimulation types or time points for any of the investigated AEP amplitudes or latencies. Furthermore, a topographical analysis did not show any topographical differences during peak time periods of the investigated AEP for stimulation types and time points besides a habituation effect. Thus, our results suggest that tDCS modulation of excitability of the left posterior temporal cortex, targeting the auditory cortex, does not have any effect on AEP. This is particularly interesting in the context of tDCS as a potential treatment for changed electrophysiological parameters and symptoms of psychiatric diseases, e.g., lower N100 or auditory verbal hallucinations in schizophrenia.

Simultaneous monitoring of electroencephalographic characteristics in animals subjected to behavioral tests: a preclinical investigation

Drug-induced changes in electroencephalographic (EEG) characteristics in animals may be used to predict central activity of drugs in humans. Previous studies have established that drugs affect EEG characteristics in humans and rodents in a similar manner. However, there has been little work to establish correlations between drug effects on behavioral and EEG characteristics in rats. In the current study, we have simultaneously monitored EEG characteristics during a novel object recognition task (NORT) or open field (OF) test in rats. EEG was monitored using telemetric device from epidural and hippocampal regions during the choice trial in the NORT after treatment with scopolamine (0.1 mg/kg, intraperitoneal) alone or in combination with donepezil (0.3 mg/kg, subcutaneous). Power changes across spectral frequency bands during exploration of novel and familiar object were assessed separately. Amphetamine (2 mg/kg, intraperitoneal) was used to monitor effects on locomotor activity and EEG changes in the OF test. In the NORT, scopolamine impaired object recognition, but no differences were observed in the power densities across spectral bands during exploration of novel and familiar objects. Treatment with donepezil reversed scopolamine-induced cognitive impairment, and the power density in the theta frequency band was increased during exploration of the novel object. In OF, amphetamine increased locomotion and produced an overall decrease in the power densities of all frequency bands. Overall, the results indicate that EEG characteristics are closely related to behavioral changes in the NORT and OF in rodents.

Emotional self-other voice processing in schizophrenia and its relationship with hallucinations: ERP evidence

Abnormalities in self-other voice processing have been observed in schizophrenia, and may underlie the experience of hallucinations. More recent studies demonstrated that these impairments are enhanced for speech stimuli with negative content. Nonetheless, few studies probed the temporal dynamics of self versus nonself speech processing in schizophrenia and, particularly, the impact of semantic valence on self-other voice discrimination. In the current study, we examined these questions, and additionally probed whether impairments in these processes are associated with the experience of hallucinations. Fifteen schizophrenia patients and 16 healthy controls listened to 420 prerecorded adjectives differing in voice identity (self-generated [SGS] versus nonself speech [NSS]) and semantic valence (neutral, positive, and negative), while EEG data were recorded. The N1, P2, and late positive potential (LPP) ERP components were analyzed. ERP results revealed group differences in the interaction between voice identity and valence in the P2 and LPP components. Specifically, LPP amplitude was reduced in patients compared with healthy subjects for SGS and NSS with negative content. Further, auditory hallucinations severity was significantly predicted by LPP amplitude: the higher the SAPS "voices conversing" score, the larger the difference in LPP amplitude between negative and positive NSS. The absence of group differences in the N1 suggests that self-other voice processing abnormalities in schizophrenia are not primarily driven by disrupted sensory processing of voice acoustic information. The association between LPP amplitude and hallucination severity suggests that auditory hallucinations are associated with enhanced sustained attention to negative cues conveyed by a nonself voice.

The effect of Ramadan fasting on spatial attention through emotional stimuli

Fasting can influence psychological and mental states. In the current study, the effect of periodical fasting on the process of emotion through gazed facial expression as a realistic multisource of social information was investigated for the first time. The dynamic cue-target task was applied via behavior and event-related potential measurements for 40 participants to reveal the temporal and spatial brain activities – before, during, and after fasting periods. The significance of fasting included several effects. The amplitude of the N1 component decreased over the centroparietal scalp during fasting. Furthermore, the reaction time during the fasting period decreased. The self-measurement of deficit arousal as well as the mood increased during the fasting period. There was a significant contralateral alteration of P1 over occipital area for the happy facial expression stimuli. The significant effect of gazed expression and its interaction with the emotional stimuli was indicated by the amplitude of N1. Furthermore, the findings of the study approved the validity effect as a congruency between gaze and target position, as indicated by the increment of P3 amplitude over centroparietal area as well as slower reaction time from behavioral response data during incongruency or invalid condition between gaze and target position compared with those during valid condition. Results of this study proved that attention to facial expression stimuli as a kind of communicative social signal was affected by fasting. Also, fasting improved the mood of practitioners. Moreover, findings from the behavioral and event-related potential data analyses indicated that the neural dynamics of facial emotion are processed faster than that of gazing, as the participants tended to react faster and prefer to relay on the type of facial emotions than to gaze direction while doing the task. Because of happy facial expression stimuli, right hemisphere activation was more than that of the left hemisphere. It indicated the consistency of the emotional lateralization concept rather than the valence concept of emotional processing.

Auditory event-related potentials are related to cognition at preschool age after very preterm birth

BACKGROUND

Auditory event-related potentials (AERP) are neurophysiological correlates of sound perception and cognitive processes. Our aim was to study in very preterm born children at preschool age if AERP correlate with cognitive outcome.

METHODS

Seventy children (mean ± SD gestational age 27.4 ± 1.9 wk, birth weight 996 ± 288 g) were investigated at age 4.3-5.3 y with psychological testing (WPPSI-R, four subtests of NEPSY). Electroencephalogram was recorded while they listened to a repeated standard tone, randomly replaced by one of three deviants. Latencies and amplitudes for AERP components and mean amplitudes in successive 50-ms AERP time windows were measured.

RESULTS

Better cognitive test results and higher gestational age correlated with shorter P1 latencies and more positive mean amplitudes 150-500 ms after stimulus change onset. Neonatal brain damage was associated with a negative displacement of AERP curves. Neonatal morbidity had an impact on earlier time windows while gestational age and brain damage on both early and later time windows.

CONCLUSION

AERP measures were associated with cognitive outcome. Neonatal morbidity mainly affects early cortical auditory encoding, while immaturity and brain damage additionally influence higher cortical functions of auditory perception and distraction. Perinatal auditory environment might play a role in development of auditory processing.

The use of EEG parameters as predictors of drug effects on cognition

It has been shown to be difficult to predict whether cognition-enhancing effects of drugs in animal studies have the same effect in humans. Various issues in translating findings from animal to human studies can be identified. Here we discuss whether EEG could be considered as a possible tool to translate the effects of cognition enhancers across species. Three different aspects of EEG measures are evaluated: frequency bands, event-related potentials, and coherence analysis. On basis of the comparison of these measures between species, and effects of drugs that improve or impair memory performance (mainly cholinergic drugs), it appears that event-related potentials and coherence analyses could be considered as potential translational tools to study cognition-enhancing drug effects in rodents and animals.

Trial effects in single-trial ERP components and autonomic responses at very long ISIs

Single-trial data from autonomic and ERP measures were used to capture the rapidly decreasing initial responses characteristic of the orienting reflex (OR) to repeated stimuli. Stimulus-response patterns were compared to determine central analogues of autonomic indices of processes leading to the OR, and the OR itself. Participants were presented with 12 indifferent tones in an auditory dishabituation paradigm. Temporal principal component analysis (PCA) decomposed EOG-corrected ERP data for 16 subjects. Response patterns of ERPs, cardiac, and respiratory responses were compared to the phasic skin conductance response (SCR). SCR decremented over trials, recovered on the change trial, and dishabituated to the representation of the standard, meeting the formal definition of habituation required of the OR. The evoked cardiac response showed no trial effects. Respiratory pause (RP) decreased linearly over trials, recovering marginally on the change trial. Nine identifiable ERP components were extracted: P1, N1-3, N1-1, processing negativity (PN), P2, P3a, P3b, a novelty-sensitive P3 component (labelled HabP3), and the slow wave (SW). P3b and SW showed decrement over trials, but with no recovery, HabP3 showed decrement and increased response on the change trial, while the P1, N1 subcomponents, P2 and P3a were insensitive to novelty. Stimulus-response patterns of the RP and HabP3 suggest sensitivity to novelty processing, while the P1, N1-3, N-1, PN, P2, P3a and cardiac deceleration appear to mark processing prior to novelty, such as stimulus transient detection (cardiac deceleration) and/or intensity processing. This study supports predictions of preliminary process theory, demonstrating fractionation of 3 autonomic and 9 ERP components to novelty, and disconfirming the unitary nature of the OR.

Repeated stimuli elicit diminished high-gamma electrocorticographic responses

In the phenomenon of repetition suppression (RS), when a person views a stimulus, the neural activity involved in processing that item is relatively diminished if that stimulus had been previously viewed. Previous noninvasive imaging studies mapped the prevalence of RS for different stimulus types to identify brain regions involved in representing a range of cognitive information. However, these noninvasive findings are challenging to interpret because they do not provide information on how RS relates to the brain's electrophysiological activity. We examined the electrophysiological basis of RS directly using brain recordings from implanted electrocorticographic (ECoG) electrodes in neurosurgical patients. Patients performed a memory task during ECoG recording and we identified high-gamma signals (65-128 Hz) that distinguished the neuronal representation of specific memory items. We then compared the neural representation of each item between novel and repeated viewings. This revealed the presence of RS, in which the neuronal representation of a repeated item had a significantly decreased amplitude and duration compared with novel stimuli. Furthermore, the magnitude of RS was greatest for the stimuli that initially elicited the largest activation at each site. These results have implications for understanding the neural basis of RS and human memory by showing that individual cortical sites exhibit the largest RS for the stimuli that they most actively represent.

Stimulus-specific adaptation in the auditory thalamus of the anesthetized rat

The specific adaptation of neuronal responses to a repeated stimulus (Stimulus-specific adaptation, SSA), which does not fully generalize to other stimuli, provides a mechanism for emphasizing rare and potentially interesting sensory events. Previous studies have demonstrated that neurons in the auditory cortex and inferior colliculus show SSA. However, the contribution of the medial geniculate body (MGB) and its main subdivisions to SSA and detection of rare sounds remains poorly characterized. We recorded from single neurons in the MGB of anaesthetized rats while presenting a sequence composed of a rare tone presented in the context of a common tone (oddball sequences). We demonstrate that a significant percentage of neurons in MGB adapt in a stimulus-specific manner. Neurons in the medial and dorsal subdivisions showed the strongest SSA, linking this property to the non-lemniscal pathway. Some neurons in the non-lemniscal regions showed strong SSA even under extreme testing conditions (e.g., a frequency interval of 0.14 octaves combined with a stimulus onset asynchrony of 2000 ms). Some of these neurons were able to discriminate between two very close frequencies (frequency interval of 0.057 octaves), revealing evidence of hyperacuity in neurons at a subcortical level. Thus, SSA is expressed strongly in the rat auditory thalamus and contribute significantly to auditory change detection.

The effects of immediate and short-term retest on the latencies and amplitudes of the auditory event-related potentials in healthy adults

OBJECTIVE

To evaluate the immediate and short-term effects of repeated within session trials on N1, P2, N2 and P3 latencies and P2, N2 and P3 amplitudes in healthy adults.

MATERIALS AND METHODS

ERPs were elicited by the auditory oddball paradigm and recorded over Fz, Cz and Pz in 18 healthy adults over two sessions, one to three days apart, and two within session trials with one to three minutes trial-retrial interval. The ERPs' latencies and amplitudes were blindly calculated and were analyzed by Analysis of Variance (ANOVA) with repeated measures.

RESULTS

Significant decreases of N2 amplitude at Fz, P3 amplitude at Cz and P3 latency at Pz were recorded in the second-compared to the first within session trial (p=0.034, 0.041, 0.046, respectively). There were no significant inter-session differences regarding N1, P2, N2 and P3 latencies or amplitudes. There was no significant interaction between session and trial. A statistically significant difference was found between the first session's mental count errors (p=0.039) but there were no significant differences between the second session's trials (p=0.581) or between sessions (p=0.328).

CONCLUSIONS

N1, P2, N2 and P3 latencies and amplitudes are stable at short-term intervals of one to three days, but one to three minutes' retrial interval may affect P3 latency and N2 and P3 amplitudes. We suggest that when primary novelty-induced cognitive processes are evaluated, single trial sessions or more than three-minute inter-trials interval should be employed in order to mitigate habituation.

Attention bias and sensitization in chemical sensitivity

OBJECTIVE

We investigated whether persons with self-reported chemical sensitivity (CS) have an attention bias and enhanced sensitization to chemical exposure.

METHODS

Chemosomatosensory, olfactory, and auditory event-related potentials (ERPs) were recorded from 21 CS subjects and 17 controls in attend and ignore conditions. Reaction times (RTs) and magnitude estimations of perceived intensity were collected in the attend condition. ERPs were averaged over attention conditions and during the first/second part of the testing.

RESULTS

ERP patterns indicated that CS subjects did not habituate to the same extent as the controls and had difficulties ignoring the chemical exposure. CS subjects had faster overall RT, and the perceived intensities for the chemosomatosensory stimuli did not decrease with time in the CS group, which was the case for the controls.

CONCLUSIONS

These results indicating attention bias and enhanced sensitization in CS suggest alterations in central, cognitive responses to chemical exposure.

Comparing the habituation of late auditory evoked potentials to loud and soft sound

The objective fitting of hearing aids and cochlear implants remains a challenge. In particular, the determination of whether sound is perceived as too loud or comfortable represents an unsolved problem in noncooperative patients. In a first step of an ongoing study, we assess the feasibility of habituation correlates in late auditory evoked potentials (LAEPs) to discriminate between a soft sound (SS) of 50 dB SPL and a loud sound (LS) of 100 dB SPL. We applied a new sweep-to-sweep time-scale coherence measure to analyse the habituation in LAEPs, i.e., relative changes within sweep sequences. From the comparison between both stimulation levels, a total discrimination of responses to SS and LS in the individual normal hearing subject was possible. As just relative changes in SS and LS sweep sequences were considered, purely exogenously driven morphological alternations in the responses such as intensity related amplitude and latency changes were excluded from the analysis. It is concluded that the proposed method allows for the reliable detection of auditory habituation and differentiation of SS from LS. The proposed scheme might provide an electrophysiological measurement and signal processing framework for the objective detection of the most comfortable loudness level and can be used in further, more clinically oriented studies.

Loudness dependence of auditory evoked potentials as indicator of central serotonergic neurotransmission: simultaneous electrophysiological recordings and in vivo microdialysis in the rat primary auditory cortex

Serotonin released in synapsis is one of the key neurotransmitters in psychiatry and psychopharmacology. The loudness dependence of auditory evoked potentials (LDAEP) has been proposed as a marker for central serotonergic neurotransmission. Several findings in animals and humans support this hypothesis. However, the in vivo measurement of cortical extracellular serotonin levels has never been performed simultaneously with the recording of auditory evoked potentials. The interrelationship between low cortical serotonergic activity and strong LDAEP is yet to be proven. The auditory evoked potentials were recorded in the epidura above the primary auditory cortex of male Wistar rats whereas extracellular serotonin levels in the primary auditory cortex were measured by in vivo microdialysis before and after i.p. application of the selective serotonin reuptake inhibitor citalopram. At baseline, the correlation of coefficients between the LDAEP, especially of the N1 component, and extracellular serotonin levels in the primary auditory cortex was negative. The increase of serotonin levels after citalopram application was significantly related to a decrease of LDAEP of the N1 component (r=-0.86, p=0.003). These data support the view that the LDAEP is closely modulated by cortical serotonergic activity. Thus, the LDAEP might serve as an inversely related marker of synaptically released serotonin in the CNS.

Processing of melodic contours in urethane-anaesthetized rats

The human brain can automatically detect changes even in repeated melodic contours of spectrally varying sounds. However, it is unclear whether this ability is specific to humans. We recorded event-related potentials (ERPs) in urethane-anaesthetized Wistar rats presented with rare pairs of tones ('deviants') interspersed with frequently repeated ones ('standards'). The frequency of the tones varied nonsystematically across their pairs so that deviants stood out from standards only in the melodic ordering (ascending or descending) of the tones of a pair. We found that the absolute amplitude of the ERP was significantly higher to deviants than standards between 106 and 136 ms from the onset of the deviance, suggesting that the ability to automatically detect changes in higher-order invariant attributes that emerge from consecutive sounds is not specific to humans.

Cholinergic drugs affect novel object recognition in rats: relation with hippocampal EEG?

This study examined the role of cognitively enhancing cholinergic drugs on both object memory and brain activity in rats, as well as the possible relation between the two measures. A group of twenty-four animals was used for assessing object recognition. In another group of eight rats, an electrode was implanted into the dorsal hippocampus to record an electroencephalogram (EEG) and auditory evoked potentials (AEP). In both groups, animals were treated with saline, 0.1 mg/kg scopolamine, 0.1 mg/kg methylscopolamine, 3 mg/kg donepezil, donepezil combined with scopolamine, 0.1 mg/kg nicotine, and nicotine combined with scopolamine. Scopolamine, but not methylscopolamine, impaired object recognition. Both donepezil and nicotine reversed this impairment. The N1 and N2 components of the AEP became closer to baseline after scopolamine, which was not reversed by donepezil or nicotine. Scopolamine increased the theta frequency in the EEG. When combined with donepezil, theta increased even more. Conversely, nicotine reversed the theta increment to control level. It is suggested that scopolamine caused a decrement in arousal in this study. Furthermore, the current results suggest a relation between EEG and object memory after cholinergic drug treatment. However, there was a clear dissociation between memory performance and EEG after combined treatment with drugs, which makes additional research where EEG and performance measures are co-registered imperative.