Post-stimulus Time Research Articles

Healthy aging delays scalp EEG sensitivity to noise in a face discrimination task

We used a single-trial ERP approach to quantify age-related changes in the time-course of noise sensitivity. A total of 62 healthy adults, aged between 19 and 98, performed a non-speeded discrimination task between two faces. Stimulus information was controlled by parametrically manipulating the phase spectrum of these faces. Behavioral 75% correct thresholds increased with age. This result may be explained by lower signal-to-noise ratios in older brains. ERP from each subject were entered into a single-trial general linear regression model to identify variations in neural activity statistically associated with changes in image structure. The fit of the model, indexed by R2, was computed at multiple post-stimulus time points. The time-course of the R2 function showed significantly delayed noise sensitivity in older observers. This age effect is reliable, as demonstrated by test–retest in 24 subjects, and started about 120 ms after stimulus onset. Our analyses suggest also a qualitative change from a young to an older pattern of brain activity at around 47 ± 4 years old.

Continuous time wavelet entropy of auditory evoked potentials

In this paper, the continuous time wavelet entropy (CTWE) of auditory evoked potentials (AEP) has been characterized by evaluating the relative wavelet energies (RWE) in specified EEG frequency bands. Thus, the rapid variations of CTWE due to the auditory stimulation could be detected in post-stimulus time interval. This approach removes the probability of missing the information hidden in short time intervals. The discrete time and continuous time wavelet based wavelet entropy variations were compared on non-target and target AEP data. It was observed that CTWE can also be an alternative method to analyze entropy as a function of time.

Neural activity-induced modulation of BOLD poststimulus undershoot independent of the positive signal

Despite intense research on the blood oxygenation level-dependent (BOLD) signal underlying functional magnetic resonance imaging, our understanding of its physiological basis is far from complete. In this study, it was investigated whether the so-called poststimulus BOLD signal undershoot is solely a passive vascular effect or actively induced by neural responses. Prolonged static and flickering black-white checkerboard stimulation with isoluminant grey screen as baseline condition were employed on eight human subjects. Within the same region of interest, the positive BOLD time courses for static and flickering stimuli were identical over the entire stimulus duration. In contrast, the static stimuli exhibited no poststimulus BOLD signal undershoot, whereas the flickering stimuli caused a strong BOLD poststimulus undershoot. To ease the interpretation, we performed an additional study measuring both BOLD signal and cerebral blood flow (CBF) using arterial spin labeling. Also for CBF, a difference in the poststimulus period was found for the two stimuli. Thus, a passive blood volume effect as the only contributor to the poststimulus undershoot comes short in explaining the BOLD poststimulus undershoot phenomenon for this particular experiment. Rather, an additional active neuronal activation or deactivation can strongly modulate the BOLD poststimulus behavior. In summary, the poststimulus time course of BOLD signal could potentially be used to differentiate neuronal activity patterns that are otherwise indistinguishable using the positive evoked response.

Age-related delay in information accrual for faces: Evidence from a parametric, single-trial EEG approach

BackgroundIn this study, we quantified age-related changes in the time-course of face processing by means of an innovative single-trial ERP approach. Unlike analyses used in previous studies, our approach does not rely on peak measurements and can provide a more sensitive measure of processing delays. Young and old adults (mean ages 22 and 70 years) performed a non-speeded discrimination task between two faces. The phase spectrum of these faces was manipulated parametrically to create pictures that ranged between pure noise (0% phase information) and the undistorted signal (100% phase information), with five intermediate steps.ResultsBehavioural 75% correct thresholds were on average lower, and maximum accuracy was higher, in younger than older observers. ERPs from each subject were entered into a single-trial general linear regression model to identify variations in neural activity statistically associated with changes in image structure. The earliest age-related ERP differences occurred in the time window of the N170. Older observers had a significantly stronger N170 in response to noise, but this age difference decreased with increasing phase information. Overall, manipulating image phase information had a greater effect on ERPs from younger observers, which was quantified using a hierarchical modelling approach. Importantly, visual activity was modulated by the same stimulus parameters in younger and older subjects. The fit of the model, indexed by R2, was computed at multiple post-stimulus time points. The time-course of the R2 function showed a significantly slower processing in older observers starting around 120 ms after stimulus onset. This age-related delay increased over time to reach a maximum around 190 ms, at which latency younger observers had around 50 ms time lead over older observers.ConclusionUsing a component-free ERP analysis that provides a precise timing of the visual system sensitivity to image structure, the current study demonstrates that older observers accumulate face information more slowly than younger subjects. Additionally, the N170 appears to be less face-sensitive in older observers.

Wavelet Analysis of the Frontal Auditory Evoked Potentials Obtained in the Passive Oddball Paradigm in Healthy Subjects and Schizophrenics

The goal of the present study was to apply the oscillatory brain dynamics model to the structural and quantitative analysis of neurocognitive functions considered as a potential marker of schizophrenia. This was achieved in tests of the detection of auditory events deviating in the regular auditory stream (oddball paradigm, MMN effect). It was hypothesized that the post-stimulus peaks of the oscillation power localized in post-stimulus time in the definite EEG oscillators represented neuro-electrical 'events' evoked in the specific neuronal nets characterized by this oscillation frequency band. We suggest that the time-frequency destination of these events related to the activation of the functional neuronal nets could be used for the determination of specific neurocognitive functions. Thus it was an attempt to distinguish the different neuro-functional parts of auditory processing and to compare these results between healthy subjects and patients with schizophrenia. The present results demonstrate the significant difference between the frontal averaged EEG oscillatory dynamics in healthy subjects and patients with schizophrenia related to neurocognitive function marked by the MMN and orienting response N200/P300a.

Response properties of cochlear nucleus neurons in monkeys

Much of what is known about how the cochlear nuclei participate in mammalian hearing comes from studies of non-primate mammalian species. To determine to what extent the cochlear nuclei of primates resemble those of other mammalian orders, we have recorded responses to sound in three primate species: marmosets, cynomolgus macaques, and squirrel monkeys. These recordings show that the same types of temporal firing patterns are found in primates that have been described in other mammals. Responses to tones of neurons in the ventral cochlear nucleus have similar tuning, latencies, post-stimulus time and interspike interval histograms as those recorded in non-primate cochlear nucleus neurons. In the dorsal cochlear nucleus, too, responses were similar. From these results it is evident that insights gained from non-primate studies can be applied to the peripheral auditory system of primates.

Biophysical Model of an Auditory Nerve Fiber With a Novel Adaptation Component

Recent data from feline auditory nerve fibers (ANFs) indicate that electrically stimulated fibers can undergo large degrees of rate adaptation to pulse-train stimuli using pulse rates within the range used by clinical auditory prostheses. However, the application of Hodgkin-Huxley-type models does not produce such adaptation, which occurs over time periods on the order of 100 ms. We describe our development of a computational ANF axon model that incorporates a time-changing external potassium concentration ( [K(+)](ext)) that depends on potassium currents produced by active nodal channel activity. This relatively simple and computationally tractable approach produces poststimulus time histograms that are similar to experimental (cat) data. Furthermore, this mechanism could be easily incorporated into other models to produce much more realistic estimates of the neural coding produced by repeated electric stimulation.

Single neuron recordings in dorsal cochlear nucleus (DCN) of awake gerbil

The auditory responses of neurons in the dorsal cochlear nucleus (DCN) are known to be sensitive to anesthesia, and consequently many studies have used an unanesthetized, decerebrate preparation. Decerebration, however, severs multiple descending pathways to the DCN and is traumatic to the brain, and so sensory responses may be influenced. It was possible, by combining sterile surgery, animal conditioning, and mild restraint to allow recordings in awake gerbils ( Merionus unguiculatus) . Response maps (RMs), post-stimulus time histograms (PSTHs), and responses to notch noise stimuli were recorded in awake gerbils. Some units’ responses were compared to those from previous experiments in anesthetized and decerebrate gerbil preparations. All RM types observed in decerebrate gerbils were also observed in the awake gerbil, with the notable exception of type IV units. Type III units were still the most commonly recorded units. No significant changes were observed in either RM shape or notch noise responses. Although most awake DCN units had spontaneous activity (typically <20 spikes/s), these rates were lower than those in decerebrate gerbils. Type III units’ spontaneous activity rates were significantly different from those in the decerebrate prep ( P < 0.05). DCN neurons may be more inhibited in awake gerbils. Although type III units were most sensitive to spectral notches, different unit types seem to respond slightly differently to notch noise. This study indicates that responses of DCN units in awake and decerebrate gerbils are different, though the shapes of RMs are the same.

Detecting alpha rhythm phase reset by phase sorting: Caveats to consider

One method being proposed for disentangling whether the ERP is generated by an evoked potential that is independent of and adds linearly to the ongoing rhythm or by (partial) phase resetting of the ongoing rhythm is sorting trials according to the pre-stimulus phase of the ongoing rhythm. The subsequent subtraction of phase-sorted resting-state trials is supposed to correct for phase-sorting effects otherwise maintained in the post stimulus time. Consequently, a true phase dependency of the ERP, considered to be indicative for a phase-reset (Risner, M.L., Aura, C.J., Black, J.E., Gawne, T.J., 2009. The visual evoked potential is independent of surface alpha rhythm phase. NeuroImage), would then become apparent. Here we disclose possible pitfalls of this approach causing invalid inferences. We also highlight that inferences based on this approach first depend on the assumed concept of a phase reset, and second can provide a hint at but are not sufficient for disentangling different mechanisms of ERP generation. We will discuss other criteria necessary for further confirmation of the phase reset theory.

Arousal effect on emotional face comprehension: Frequency band changes in different time intervals

The study aimed to explore the modulation of frequency bands (delta, theta, alpha-1 and alpha-2, and gamma) in response to emotional face within different post-stimulus time intervals (50–450 ms). Twenty adults looked at emotional (happy, sad, angry, fearful) or neutral faces. EEG results showed that motivational significance of face can modulate frequency bands, specifically for theta and gamma. Moreover, gamma can be varied related to degree of arousing feature (high or low) of facial expression. As a function of time, ANOVA and regression analysis revealed that emotional discrimination by gamma and theta is observable mainly within 150–250 time interval and, as revealed also by coherence analysis, that it is more distributed on the anterior-right (theta) or right (gamma) side of the scalp for the emotional stimuli, whereas delta is maximally increased within 250–350 interval and it is more posteriorly (parietal site) distributed for all the stimulus type. We proposed that band modulations respond to variations in processing emotional face, and, whereas delta reflects updating of the stimulus, and theta responds to the emotional significance of face, gamma reflects differences in the arousing power of facial expression.

The Sparseness of Neuronal Responses in Ferret Primary Visual Cortex

Various arguments suggest that neuronal coding of natural sensory stimuli should be sparse (i.e., individual neurons should respond rarely but should respond reliably). We examined sparseness of visual cortical neurons in anesthetized ferret to flashed natural scenes. Response behavior differed widely between neurons. The median firing rate of 4.1 impulses per second was slightly higher than predicted from consideration of metabolic load. Thirteen percent of neurons (12 of 89) responded to <5% of the images, but one-half responded to >25% of images. Multivariate analysis of the range of sparseness values showed that 67% of the variance was accounted for by differing response patterns to moving gratings. Repeat presentation of images showed that response variance for natural images exaggerated sparseness measures; variance was scaled with mean response, but with a lower Fano factor than for the responses to moving gratings. This response variability and the "soft" sparse responses (Rehn and Sommer, 2007) raise the question of what constitutes a reliable neuronal response and imply parallel signaling by multiple neurons. We investigated whether the temporal structure of responses might be reliable enough to give additional information about natural scenes. Poststimulus time histogram shape was similar for "strong" and "weak" stimuli, with no systematic change in first-spike latency with stimulus strength. The variance of first-spike latency for repeat presentations of the same image was greater than the latency variance between images. In general, responses to flashed natural scenes do not seem compatible with a sparse encoding in which neurons fire rarely but reliably.

The experience of intrusions scale: A preliminary examination

Intrusive thoughts (i.e., unwelcome, distressing, involuntary thoughts) are prevalent in a variety of clinical conditions and are increasingly a focus of translational research. The goal of this study was to develop and preliminarily examine a brief self-report measure designed to assess clinically relevant aspects of the experience of intrusive thoughts related to a particular target. The Experience of Intrusions Scale (EIS) is a five-item measure that assesses the frequency, unpredictability, and unwantedness of intrusive thoughts, as well as the interference and distress caused by the intrusions, each on a five-point Likert-type scale. Five times over a four-] period, female undergraduates (N=160) completed the EIS in response to intrusive thoughts regarding a film clip depicting a sexual assault. On the first and last days, participants completed the EIS five minutes after watching the clip. In between film clip viewings, participants completed the EIS once per day. The EIS demonstrated good internal consistency, good to excellent test–retest reliability using both immediate post-stimulus and 24-hour time intervals, and convergent validity with two existing measures of intrusive phenomena: the White Bear Suppression Inventory (Wegner & Zanakos, 1994) and the Post-traumatic Stress Disorder Checklist-Civilian Version (Weathers, Litz, Herman, Huska, & Keane, 1993).

Determining the time course of lexical frequency and age of acquisition using ERP

The main goal of the present study was to dissociate the effects on reading of frequency, age of acquisition (AoA) and imageability using the evoked response potential paradigm. Twenty participants read words from three experimental conditions: high and low frequency, late and early age of acquisition and high and low imageability. High frequency words produced more positive mean amplitude than low frequency words in the 175-360 ms post-stimulus onset time window and late AoA produced more negative amplitudes than early AoA in the 400-610 ms window. Imageability did not produce any effect in any time window tested. Brain electromagnetic tomography showed the most activated cortical areas for each category of stimuli. The lexical frequency of words seems to affect an early phase in the recognition process, perhaps at the level of the orthographic input lexicon, while AoA was observed at a later stage, indicating that this variable influence processing at a semantic level or at the links between semantics and phonology. EEG permits the researcher to investigate the time course, and approximate location in the brain, of psycholinguistic variables.

Neural Signal Manager: a collection of classical and innovative tools for multi‐channel spike train analysis

AbstractRecent developments in the neuroengineering field and the widespread use of the micro electrode arrays (MEAs) for electrophysiological investigations made available new approaches for studying the dynamics of dissociated neuronal networks as well as acute/organotypic slices maintained ex vivo. Importantly, the extraction of relevant parameters from these neural populations is likely to involve long‐term measurements, lasting from a few hours to entire days. The processing of huge amounts of electrophysiological data, in terms of computational time and automation of the procedures, is actually one of the major bottlenecks for both in vivo and in vitro recordings. In this paper we present a collection of algorithms implemented within a new software package, named the Neural Signal Manager (NSM), aimed at analyzing a huge quantity of data recorded by means of MEAs in a fast and efficient way. The NSM offers different approaches for both spike and burst analysis, and integrates state‐of‐the‐art statistical algorithms, such as the inter‐spike interval histogram or the post stimulus time histogram, with some recent ones, such as the burst detection and its related statistics. In order to show the potentialities of the software, the application of the developed algorithms to a set of spontaneous activity recordings from dissociated cultures at different ages is presented in the Results section. Copyright © 2008 John Wiley &amp; Sons, Ltd.

Searching for semantics in the retinal code

This paper deals with retinal code semantics by introducing a new approach for the functional classification of retinal ganglion cells based on their firing patterns and coding capabilities. Multielectrode extracellular recordings were obtained from ganglion cell populations in isolated superfused albino rabbit retina using a rectangular array of 100 microelectrodes (Utah's array). To identify classes or groups of neurons that behave similarly two spike train analysis methods including autocorrelations and post-stimulus time histograms (PSTHs) have been used. Information theory (IT) permits to assess the quality and reliability of the subpopulations obtained. Furthermore correlations between the cells of each class have been studied. The most effective clustering strategy was achieved by using the autocorrelations of the recorded cells. The method was useful for defining subsets of retinal ganglion cells, which share similar temporal responses, identifying the best subset coder for different visual parameters.

Improving the diagnostic potential of the photopic electroretinogram (ERG) with refined mathematical tools

Abstract Purpose Examine if the use of refined mathematical tools increases the understanding and diagnostic potential of the photopic ERGs. Methods Analyses were performed on normal and pathological photopic ERGs (background of 30 cd.m‐2; flash intensities ranging from ‐0.8 to 2.84 log cd.sec.m‐2 in 15 steps; n&gt;100). Mathematical instruments included: 1‐ the Continuous Wavelet Transform (CWT)2‐ The Reiman Integrator and 3‐ the Photopic Hill Gaussian:Logistic ratio [GL=Gb/(Gb+Vbmax)]claimed to weight the contribution of the OFF and ON pathways to the photopic ERG. Results 1‐ CWT revealed 3 distinct frequency domains within the 10‐50 msec poststimulus time interval, namely: 20‐40 Hz, 90‐150 Hz and 200‐300 Hz that appeared to be independently modulated by flash intensity and/or pathology. 2‐ Integration of the OPs accurately reconstructed the broadband ERGs (r&gt;0.90), irrespective of CWT. 3‐ In our cohort of RP patients (n=50), the GL ratio was 0.43 ±0.23, compared to 0.60±0.08 in normal (p&lt;.05). Conclusion 1‐CWT dissects the ERG into its primary components, a method that should allow a more accurate quantification of ERG responses. 2‐ Strong evidence supports the concept that the b‐wave results from the integration of the OPs. The value of the integration constant (1 sec) is close to the time constant of the Muller cell membrane suggesting that the latter could be at the origin of this integration. 3‐The broader GL distribution obtained from our RP cohort suggests that in some the OFF retinal pathway is primarily affected while in others the ON is. It remains to be determined if the latter represents two stages of the same disease process or two different disease paths. Supported by CIHR and Réseau Vision.

Increased absolute magnitude of gamma synchrony in first-episode psychosis

Objectives Recent studies have explored a model of the disconnection hypothesis of schizophrenia through the demonstration of abnormal stimulus induced gamma phase synchrony (GPS). These studies have principally examined synchrony in the 40 Hz band elicited in post-stimulus time periods, relative to a pre-stimulus baseline. In this study we examined the absolute magnitude of GPS elicited by a selective attention task, in first-episode psychosis (FEP). We hypothesized that FEP would be associated with abnormalities in absolute GPS, particularly when required to selectively attend to task-relevant stimuli. Methods Fifty-five first-episode psychosis (FEP) subjects and one hundred and ten matched healthy control subjects underwent an auditory oddball selective attention task during EEG recording. The absolute magnitude of GPS was extracted for the range 35–45 Hz, and time-locked to stimulus onset. GPS averaged were computed for oddball ‘target’ (task-relevant) and ‘non-target’ (task-irrelevant) stimuli, for each subject. Results FEP subjects showed a significant elevation in absolute GPS relative to controls, apparent across the 35–45 Hz range. This elevation was most marked in the left centro-temporal region, across the 800 ms post-stimulus period. In FEP subjects, the elevation in GPS was also greater for target compared to non-target stimuli, while healthy controls did not show a stimulus effect. Conclusion These findings complement previous evidence for reductions in peak gamma synchrony, calculated relative to a pre-stimulus baseline, in schizophrenia. The results an excess of absolute GPS in schizophrenia may contribute to an inability to effectively integrate task-relevant information, which underlie psychotic symptoms.

The medial nucleus of the trapezoid body in rat: spectral and temporal properties vary with anatomical location of the units

The medial nucleus of the trapezoid body (MNTB) is a distinct nucleus in the superior olivary complex that transforms excitatory input from the cochlear nucleus into a widespread inhibitory output to distinct auditory brainstem nuclei. Few studies have dealt with the response properties of MNTB neurons to sound stimulation using in vivo preparations. In order to have a better understanding of the functional significance of the MNTB in auditory processing we report the basic temporal and spectral response properties of its principal cells using single-unit extracellular recordings to acoustic stimulation with pure tones and amplitude-modulated stimuli in the rat. Ninety-seven per cent of units showed V-shaped frequency response areas. Rate level functions were mainly saturating (51%) or monotonic (45%) at high intensities. Post-stimulus time histograms typically were characterised as primary-like with notch (59%) or primary-like (33%). Units showed good phase-locking to sinusoidally amplitude-modulated signals with vector strength VS values up to 0.87. Modulation transfer functions had low-pass shapes at near-threshold levels, with cut-off frequencies ranging from 370 to 1270 Hz. Exploration of the relationship between the temporal and spectral properties and the location of the units in the MNTB yielded characteristic frequency (CF)-dependent response properties (latency, Q(10) and cut-off frequency) following a medio-lateral gradient, and CF-independent response features (maximum firing rate) following a dorso-ventral gradient.

Temporary hearing loss influences post-stimulus time histogram and single neuron action potential estimates from human compound action potentials

An analytic compound action potential (CAP) obtained by convolving functional representations of the post-stimulus time histogram summed across auditory nerve neurons [P(t)] and a single neuron action potential [U(t)] was fit to human CAPs. The analytic CAP fit to pre- and postnoise-induced temporary hearing threshold shift (TTS) estimated in vivo P(t) and U(t) and the number of neurons contributing to the CAPs (N). The width of P(t) decreased with increasing signal level and was wider at the lowest signal level following noise exposure. P(t) latency decreased with increasing signal level and was shorter at all signal levels following noise exposure. The damping and oscillatory frequency of U(t) increased with signal level. For subjects with large amounts of TTS, U(t) had greater damping than before noise exposure particularly at low signal levels. Additionally, U(t) oscillation was lower in frequency at all click intensities following noise exposure. N increased with signal level and was smaller after noise exposure at the lowest signal level. Collectively these findings indicate that neurons contributing to the CAP during TTS are fewer in number, shorter in latency, and poorer in synchrony than before noise exposure. Moreover, estimates of single neuron action potentials may decay more rapidly and have a lower oscillatory frequency during TTS.

Event-Related Oscillations (ERO) and Event-Related Potentials (ERP) in Emotional Face Recognition

The study aims to explore the significance of event-related potentials (ERPs) and event-related brain oscillations (EROs) (delta, theta, and alpha power) in response to emotional face during 180–250 poststimulus time interval. Twenty-one adults looked at emotional (sad, happy, fearful) or neutral faces. The results demonstrated that the emotional face elicited a negative peak at approximately 230 ms (N230). Moreover EEG measures showed that motivational significance of face (stimulus type) can modulate the amplitude of EEG, especially for theta and delta. Regression analysis showed that theta oscillations mainly effect as oscillation activity at the N2 latency. Thus, this frequency band variation could represent a complex set of cognitive processes, whereby selective attention becomes focused on an emotional relevant stimulus.