Abstract: Semantic processing of verbal and visual stimuli has been investigated in semantic violation or semantic priming paradigms in which a stimulus is either related or unrelated to a previously established semantic context. A hallmark of semantic priming is the N400 event-related potential (ERP)-a deflection of the ERP that is more negative for semantically unrelated target stimuli. The majority of studies investigating the N400 and semantic integration have used verbal material (words or sentences), and standardized stimulus sets with norms for semantic relatedness have been published for verbal but not for visual material. However, semantic processing of visual objects (as opposed to words) is an important issue in research on visual cognition. In this study, we present a set of 800 pairs of semantically related and unrelated visual objects. The images were rated for semantic relatedness by a sample of 132 participants. Furthermore, we analyzed low-level image properties and matched the two semantic categories according to these features. An ERP study confirmed the suitability of this image set for evoking a robust N400 effect of semantic integration. Additionally, using a general linear modeling approach of single-trial data, we also demonstrate that low-level visual image properties and semantic relatedness are in fact only minimally overlapping. The image set is available for download from the authors' website. We expect that the image set will facilitate studies investigating mechanisms of semantic and contextual processing of visual stimuli.
Abstract: Even in well-controlled laboratory environments, apparently identical repetitions of an experimental trial can give rise to highly variable perceptual outcomes and behavioral responses. This variability is generally discarded as a reflection of intrinsic noise in neuronal systems. However, part of this variability may be accounted for by trial-by-trial fluctuations of the phase of ongoing oscillations at the moment of stimulus presentation. For example, the phase of an electro-encephalogram (EEG) oscillation reflecting the rapid waxing and waning of sustained attention can predict the perception of a subsequent visual stimulus at threshold. Similar ongoing periodicities account for a portion of the trial-by-trial variability of visual reaction times. We review the available experimental evidence linking ongoing EEG phase to perceptual and attentional variability, and the corresponding methodology. We propose future tests of this relation, and discuss the theoretical implications for understanding the neuronal dynamics of sensory perception.
Abstract: Cortical oscillatory activity in the gamma-band range (>30Hz) is a fundamental mechanism of neural coding that arises during a range of cognitive processes in both animals and humans. Since the first report on high frequency oscillatory synchrony between V1 neurons belonging to the same orientation column (Gray and Singer, 1989, PNAS, 86, 1698-1702), the role of such oscillations in visual perception has been extensively researched. Visual stimuli elicit an early, evoked gamma-band response and a later, induced (neither time nor phase-locked) response. An abundance of experimental evidence now links both evoked and induced high frequency oscillations to a range of visual stimulus properties. On the basis of early studies into gamma-band oscillations in vision, induced high frequency oscillatory activity has been proposed as a putative cortical mechanism of coherent percept formation and object representation while evoked high frequency activity was related to the processing of image features. Recent studies demonstrate that both evoked and induced gamma-band activity are correlated with the speed and accuracy of visual detection and discrimination. Furthermore, induced gamma-band oscillations in the visual cortex are also correlated with fixational eye movement patterns. These direct relations between gamma-band activity and the efficacy of visual perception strongly suggest that cortical high frequency synchronisations constitute a neural mechanism that subserves processes essential for the organised intake and analysis of visual information.
Abstract: An important effect of sustained attention is the facilitation of perception. Although the term "sustained" suggests that this beneficial effect endures continuously as long as something is attended, we present electrophysiological evidence that perception at attended locations is actually modulated periodically. Subjects detected brief light flashes that were presented peripherally at locations that were either attended or unattended. We analyzed the correlation between detection performance for attended and unattended stimuli and the phase of ongoing EEG oscillations, which relate to subsecond fluctuations of neuronal excitability. Although on average, detection performance was improved by attention--indicated by reduced detection thresholds at attended locations--we found that detection performance for attended stimuli actually fluctuated over time along with the phase of spontaneous oscillations in the (≈7 Hz) frequency band just before stimulus onset. This fluctuation was absent for unattended stimuli. This pattern of results suggests that "sustained" attention in fact exerts its facilitative effect on perception in a periodic fashion.
Abstract: Change blindness is the failure to detect changes in visual scenes. Changes can elicit phenomenologically different perceptual experiences, possibly relating to different mechanisms: changes may be entirely missed, merely detected, located, or identified. We presented sequences of meaningful objects, one of which could change between the presentations. Changes had to be located and identified. Observers sometimes located the change without knowing which object had changed. However, effects of localization with and without identification were remarkably similar on a sequence of event-related potential components (including change-related positivity and N2pc). Only a late contralateral positivity was found exclusively for identified changes, indicating that change localization and change identification initially rely on a common processing sequence and differ only at later stages.
Abstract: What is the observer's conscious experience in a change blindness task? Overgaard, Jensen, and Sandberg argue that subjective measures are required for any conclusions about conscious experience. We will lay out how the choice of subjective or objective measures depends on the given research question and that objective measures allow inferences on experience given plausible assumptions regarding the relation between task performance and experience.
Abstract: Numerous studies have demonstrated that observers often fail to notice large changes in visual scenes, a phenomenon known as change blindness. Some experiments have suggested that phenomenological experience in change blindness experiments is more diverse than the common distinction between change detection and change blindness allows to resolve. Recently, it has been debated whether changes in visual scenes can be detected ("sensed") without a corresponding perception of the changing object ("seeing") and whether these phenomena build on fundamentally different perceptual processes. The present study investigated whether phenomenologically different perceptual processes such as sensing and seeing rely on different or similar neural processes. We studied ERP effects of visual change processing (as compared to change blindness) when observers merely detected the presence of a change ("sensing") and when they identified the changing object in addition to detection ("seeing"). Although the visual awareness negativity (VAN)/selection negativity was similar for detection with and without identification, a change-related positivity and the N2pc contralateral to changes were found exclusively when the change was fully identified. This finding indicates that change identification requires perceptual and neural processes that are not involved in mere detection. In a second experiment, we demonstrated that the VAN and N2pc effects are similar to effects of selective attention in a visual search task. By contrast, the change-related positivity was specific for conscious processing of visual changes. The results suggest that changes can be detected ("sensed") without perception of the changing object. Furthermore, sensing and seeing seem to rely on different neural processes and seem to constitute different types of visual perception. These findings bear implications for how different categories of visual awareness are related to different stages in visual processing.
Abstract: Oscillations are ubiquitous in electrical recordings of brain activity. While the amplitude of ongoing oscillatory activity is known to correlate with various aspects of perception, the influence of oscillatory phase on perception remains unknown. In particular, since phase varies on a much faster timescale than the more sluggish amplitude fluctuations, phase effects could reveal the fine-grained neural mechanisms underlying perception. We presented brief flashes of light at the individual luminance threshold while EEG was recorded. Although the stimulus on each trial was identical, subjects detected approximately half of the flashes (hits) and entirely missed the other half (misses). Phase distributions across trials were compared between hits and misses. We found that shortly before stimulus onset, each of the two distributions exhibited significant phase concentration, but at different phase angles. This effect was strongest in the theta and alpha frequency bands. In this time-frequency range, oscillatory phase accounted for at least 16% of variability in detection performance and allowed the prediction of performance on the single-trial level. This finding indicates that the visual detection threshold fluctuates over time along with the phase of ongoing EEG activity. The results support the notion that ongoing oscillations shape our perception, possibly by providing a temporal reference frame for neural codes that rely on precise spike timing.
Notes: New Scientist: Timewarp: How your brain creates the fourth dimension
Abstract: An optimal correspondence of temporal information between the physical world and our perceptual world is important for survival. In the current study, we demonstrate a novel temporal illusion in which the cause of a perceptual event is perceived after the event itself. We used a paradigm referred to as motion-induced blindness (MIB), in which a static visual target presented on a constantly rotating background disappears and reappears from awareness periodically, with the dynamic characteristics of bistable perception. A sudden stimulus onset (e.g., a flash) presented during a period of perceptual suppression (i.e., during MIB) is known to trigger the almost instantaneous reappearance of the suppressed target. Surprisingly, however, we report here that although the sudden flash is the cause of the static target's reappearance (the corresponding effect), it is systematically perceived as occurring after this reappearance. Further investigation revealed that this illusory temporal reversal is caused by an approximately 100 ms advantage for the unconscious representation of the perceptually suppressed target to access consciousness, as compared to the newly presented flash. This new temporal illusion therefore reveals the normally hidden delays in bringing new visual events to awareness.
Abstract: BACKGROUND: Reactions to sensory events sometimes require quick responses whereas at other times they require a high degree of accuracy-usually resulting in slower responses. It is important to understand whether visual processing under different response speed requirements employs different neural mechanisms. METHODOLOGY/PRINCIPAL FINDINGS: We asked participants to classify visual patterns with different levels of detail as real-world or non-sense objects. In one condition, participants were to respond immediately, whereas in the other they responded after a delay of 1 second. As expected, participants performed more accurately in delayed response trials. This effect was pronounced for stimuli with a high level of detail. These behavioral effects were accompanied by modulations of stimulus related EEG gamma oscillations which are an electrophysiological correlate of early visual processing. In trials requiring speeded responses, early stimulus-locked oscillations discriminated real-world and non-sense objects irrespective of the level of detail. For stimuli with a higher level of detail, oscillatory power in a later time window discriminated real-world and non-sense objects irrespective of response speed requirements. CONCLUSIONS/SIGNIFICANCE: Thus, it seems plausible to assume that different response speed requirements trigger different dynamics of processing.
Abstract: In everyday life we often encounter situations in which we can expect a visual stimulus before we actually see it. Here, we study the impact of such stimulus anticipation on the actual response to a visual stimulus. Participants were to indicate the sex of deer and cattle on photographs of the respective animals. On some trials, participants were cued on the species of the upcoming animal whereas on other trials this was not the case. Time frequency analysis of the simultaneously recorded EEG revealed modulations by this cue stimulus in two time windows. Early [Formula: see text] spectral responses [Formula: see text] displayed strongest stimulus-locking for stimuli that were preceded by a cue if they were sufficiently large. Late [Formula: see text] responses displayed enhanced amplitudes in response to large stimuli and to stimuli that were preceded by a cue. For late responses, however, no interaction between cue and stimulus size was observed. We were able to explain these results in a simulation by prestimulus gain modulations (early response) and by decreased response thresholds (late response). Thus, it seems plausible, that stimulus anticipation results in a pretuning of local neural populations.
Abstract: Different forms of perceptual memory have opposite physiological effects. Whereas repetition priming often leads to suppression of brain responses, explicit recognition has been found to enhance brain activity. We investigated effects of repetition priming and recognition memory on early gamma-band responses. In a study phase, participants performed a visual discrimination task with task-irrelevant item repetitions. Stimulus repetition suppressed early evoked gamma responses in participants with strong behavioral repetition effects. In a test phase, participants discriminated old from new items. Evoked and induced gamma activity was enhanced for old items. Effects were stronger in participants with better recognition performance. The results demonstrate a modulation of earliest stages of visual information processing by different memory systems, which is dependent on retrieval intention and predicts individual behavioral performance.
Abstract: Benjamin Libet has demonstrated that the readiness potential precedes the time at which participants consciously decide to perform an intentional motor act, and suggested that free will is an illusion. We performed an experiment where participants observed a stimulus on a computer monitor and were instructed to press one of two buttons, depending on the presented stimulus. We found neural activity preceding the motor response, similar to Libet's experiments. However, this activity was already present prior to stimulus presentation, and thus before participants could decide which button to press. Therefore, we argue that this activity does not specifically determine behaviour. Instead, it may reflect a general expectation. This interpretation would not interfere with the notion of free will.
Abstract: BACKGROUND: Phase-locked gamma oscillations have so far mainly been described in relation to perceptual processes such as sensation, attention or memory matching. Due to its very short latency ( approximately 90 ms) such oscillations are a plausible candidate for very rapid integration of sensory and motor processes. RESULTS: We measured EEG in 13 healthy participants in a speeded reaction task. Participants had to press a button as fast as possible whenever a visual stimulus was presented. The stimulus was always identical and did not have to be discriminated from other possible stimuli. In trials in which the participants showed a fast response, a slow negative potential over central electrodes starting approximately 800 ms before the response and highly phase-locked gamma oscillations over central and posterior electrodes between 90 and 140 ms after the stimulus were observed. In trials in which the participants showed a slow response, no slow negative potential was observed and phase-locked gamma oscillations were significantly reduced. Furthermore, for slow response trials the phase-locked gamma oscillations were significantly delayed with respect to fast response trials. CONCLUSION: These results indicate the relevance of phase-locked gamma oscillations for very fast (not necessarily detailed) integration processes.
Abstract: Some authors have proposed that event-related potentials (ERPs) are generated by a neuronal response which is additive to and independent of ongoing activity, others demonstrated that they are generated by partial phase-resetting of ongoing activity. We investigated the relationship between event-related oscillatory activity in the alpha band and prestimulus levels of ongoing alpha activity on ERPs. EEG was recorded from 23 participants performing a visual discrimination task. Individuals were assigned to one of three groups according to the amount of prestimulus total alpha activity, and distinct differences of the event-related EEG dynamics between groups were observed. While all groups exhibited an event-related increase in phase-locked (evoked) alpha activity, only individuals with sustained prestimulus alpha activity showed alpha-blocking, that is, a considerable decrease of poststimulus non-phase-locked alpha activity. In contrast, individuals without observable prestimulus total alpha activity showed a concurrent increase of phase-locked and non-phase-locked alpha activity after stimulation. Data from this group seems to be in favor of an additive event-related neuronal response without alpha-blocking. However, the dissociable EEG dynamics of total and evoked alpha activities together with a complementary simulation analysis indicated a partial event-related reorganization of ongoing brain activity. We conclude that both partial phase-resetting and partial additive power contribute dynamically to the generation of ERPs. The prestimulus brain state exerts a prominent influence on event-related brain responses.
Abstract: The present study used multiple repetitions of meaningless pictorial stimuli to examine the electrophysiological correlates of the creation of a new stimulus representation. Study participants judged whether preexperimentally unfamiliar figures (meaningless line drawings) that were repeated up to four times contained a crossover in their contour. Stimulus repetition thereby led to a reduction of the visual N1 component in event-related potentials as well as to a late (430-600 ms), successively increasing positivity over posterior electrodes. In particular, the size of this latter event-related potential effect highly correlated with and thus predicted participants' performance in a subsequent recognition memory test. It can therefore be interpreted as neural correlate of the creation of a new memory-effective stimulus representation.
Abstract: We studied the effect of different sound intensities on the auditory evoked gamma-band response (GBR). Previous studies observed oscillatory gamma activity in the auditory cortex of animals and humans. For the visual modality, it has been demonstrated that the GBR can be modulated by top-down (attention, memory) as well as bottom-up factors (stimulus properties). Therefore, we expected to find a sound intensity modulation for the auditory GBR. 21 healthy participants without hearing deficits were investigated in a forced-choice discrimination task. Sinusoidal tones were presented at three systematically varied sound intensities (30, 45, 60 dB hearing level). The results of the auditory evoked potentials were predominantly consistent with previous studies. Furthermore, we observed an augmentation of the evoked GBR with increasing sound intensity. The analysis indicated that this intensity difference in the GBR amplitude most likely arises from increased phase-locking. The results demonstrate a distinct dependency between sound intensity and gamma-band oscillations. Future experiments that investigate the relationship between auditory evoked GBRs and higher cognitive processes should therefore select stimuli with an adequate sound intensity and control this variable to avoid confounding effects. In addition, it seems that gamma-band activity is more sensitive to exogenous stimulus parameters than evoked potentials.
Abstract: We studied the effect of different contrast levels on the visual evoked gamma-band response (GBR) in order to investigate whether the GBR is modulated in a similar manner as previously reported for visual evoked potentials. Previous studies showed that the GBR can be modulated by individual characteristics (age) and experimental conditions (task difficulty, attention). However, stimulus properties, such as size and spatial frequency, also have a large impact on the GBR, which necessitates identification and control of relevant stimulus properties for optimal experimental setups. Twenty-one healthy participants were investigated during a forced-choice discrimination task. Sinusoidal gratings were presented at three contrast levels with a constant spatial frequency of 5 cycles per degree visual arc (cpd). The present data replicate the results reported for visual evoked potentials and exhibit a contrast dependent modulation of the GBR. Gamma activity is increased for higher contrast levels. These results demonstrate the importance of stimulus contrast for evoked gamma activity. Thus, it appears meaningful to control the contrast of stimuli in experiments investigating the role of gamma activity in perception and information processing.
Abstract: OBJECTIVE: Evoked, phase-locked gamma oscillations of the electroencephalogram (EEG) have been demonstrated to be modulated by both bottom-up as well as top-down factors. However, to date the test-retest reliability of these oscillations has not been studied systematically. METHODS: We recorded EEG activity of 12 healthy volunteers in response to stimuli of different sizes. Each participant took part in two sessions separated by two weeks in time. To obtain an estimate of the reliability of evoked gamma band responses (GBRs), we compared frequency and magnitude of phase-locked EEG oscillations between sessions. RESULTS: In response to large stimuli magnitude and frequency of the evoked GBR yielded significant reliability. However, this was not the case for stimuli which were too small to evoke detectable GBRs. CONCLUSIONS: The results are in accordance with studies demonstrating a dependence of gamma oscillations on stimulus parameters. SIGNIFICANCE: The current findings suggest that using appropriate stimulation, the evoked gamma response has sufficient test-retest reliability for use in assessing clinical changes in neurophysiological status.
Abstract: Human subjects typically keep about seven items (plus or minus two) in short-term memory (STM). A theoretical neuronal model has been proposed to explain this phenomenon with physiological parameters of brain oscillations in the gamma and theta frequency range, i.e., roughly 30-80 and 4-8 Hz, respectively. In that model, STM capacity equals the number of gamma cycles (e.g., 25 ms for 40 Hz), which fit into one theta cycle (e.g., 166 ms for 6 Hz). The model is based on two assumptions: (1) theta activity should modulate gamma activity; and (2) the theta/gamma ratio should correlate with human STM capacity. The first assumption is supported by electrophysiological data showing that the amplitude of gamma oscillations is modulated by the phase of theta activity. However, so far, this has only been demonstrated for intracranial recordings. We analyzed human event-related EEG oscillations recorded in a memory experiment in which 13 subjects perceived known and unknown visual stimuli. The paradigm revealed event-related oscillations in the gamma range, which depended significantly on the phase of simultaneous theta activity. Our data are the first scalp-recorded human EEG recordings revealing a relationship between the gamma amplitude and the phase of theta oscillations, supporting the first assumption of the above-mentioned theory. Interestingly, the involved frequencies revealed a 7:1 ratio. However, this ratio does not necessarily determine human STM capacity. Since such a correlation was not explicitly tested in our paradigm, our data are not conclusive about the second assumption. Instead of theta phase modulating gamma amplitude, it is also conceivable that focal gamma activity needs to be downsampled to theta activity, before it can interact with more distant brain regions.
Abstract: In recent years the cognitive functions of human gamma-band activity (30-100 Hz) advanced continuously into scientific focus. Not only bottom-up driven influences on 40 Hz activity have been observed, but also top-down processes seem to modulate responses in this frequency band. Among the various functions that have been related to gamma activity a pivotal role has been assigned to memory processes. Visual experiments suggested that gamma activity is involved in matching visual input to memory representations. Based on these findings we hypothesized that such memory related modulations of gamma activity exist in the auditory modality, as well. Thus, we chose environmental sounds for which subjects already had a long-term memory (LTM) representation and compared them to unknown, but physically similar sounds. 21 subjects had to classify sounds as 'recognized' or 'unrecognized', while EEG was recorded. Our data show significantly stronger activity in the induced gamma-band for recognized sounds in the time window between 300 and 500 ms after stimulus onset with a central topography. The results suggest that induced gamma-band activity reflects the matches between sounds and their representations in auditory LTM.
Abstract: The present study investigated neuronal correlates of stimulus processing leading to conscious perception of a task irrelevant global structure in a visual display. To study the underlying neuronal processes, participants were presented different types of dot patterns (Glass patterns) either forming a global structure or forming no global structure while EEG was recorded. Participants were naive about the pattern types and performed a demanding colour discrimination task. Following the experiment, the degree to which participants acquired awareness of the global visual structure was assessed. Early gamma-frequency band responses (gamma, 25-100 Hz) over occipital, parietal, and central areas were enhanced to circular Glass patterns as compared to random dot patterns at 90 ms post-stimulus. This effect was observed exclusively in participants who were subjectively aware of the global pattern structure. In this group of observers, the pattern effect built up gradually during the course of the experiment. The significance of enhanced early gamma responses to global patterns for the production of awareness of the pattern might lie in the increased impact of information conveyed by well synchronised neuronal assemblies to upstream cortical areas.
Abstract: Physical properties of visual stimuli affect electrophysiological markers of perception. One important stimulus property is spatial frequency (SF). Therefore, we studied the influence of SF on human alpha (8-13 Hz) and gamma (>30 Hz) electroencephalographic (EEG) responses in a choice reaction task. Since real world images contain multiple SFs, an SF mixture was also examined. Event related potentials were modulated by SF around 80 and 300 ms. Evoked gamma responses were strongest for the low SF and the mixture stimulus; alpha responses were strongest for high SFs. The results link evoked and induced alpha and evoked gamma responses in human EEG to different modes of stimulus processing.
Abstract: The early visual gamma-band response is an oscillatory signal evoked approximately 100 ms after stimulation. While some studies have found effects of various cognitive processes on this signal, such effects could not be replicated in other studies. Accordingly, some authors have claimed that evoked gamma-band activity reflects merely sensory functions. To resolve these conflicting positions, we conducted a target detection experiment in which the feature that defined the target could be distributed over a large or a small part of the entire stimulus. Only targets covering a larger area of the entire stimulus evoked stronger gamma-band activity than standards although the over-all stimulus size was identical for all stimuli. This increase in evoked activity resulted from stronger oscillatory power and not exclusively from stronger phase-locking. In contrast, N1 and P3 amplitudes were larger for target stimuli irrespective of the distribution of the relevant stimulus feature. These results are consistent with the notion that early gamma-band activity is generated by feature-selective neural assemblies the activity of which can in fact be modulated by top-down processes. This interaction, however, may be only detectable in scalp-recorded EEG if it affects a sufficient number of neural assemblies.
Abstract: This study proposes a standard paradigm for the investigation of visual information processing by means of gamma activity and presents a novel set of stimuli with a broad range of complex, coloured familiar real world and unfamiliar nonsense objects which are well matched with respect to physical stimulus properties. In order to demonstrate that the paradigm and stimulus set yield reliable results both were employed in two electrophysiological investigations in two independent laboratories. Participants were required to discriminate familiar from unfamiliar stimuli. The pattern of results was very consistent across laboratories. Early event-related potentials were not influenced by the stimulus type suggesting that physical stimulus properties did not confound object familiarity. Induced gamma band activity was stronger for familiar than for unfamiliar objects, supporting the notion of gamma activity as a signature of cortical networks underlying object representations.
Abstract: Glass patterns are randomized dot arrays that generate the perception of a global structure. They consist of correlated dot pairs which are generated by geometric transformations. The present study employed behavioral and event-related brain potential (ERP) measures to characterize the underlying neuronal processing when such patterns are perceived. Stimuli were circular, parallel, and randomized Glass patterns presented in two isoluminant colors using a choice reaction paradigm. Sixteen subjects were instructed to differentiate between colors with a button-press response. The N170 component increased in amplitude for circular patterns, and this effect was most pronounced bilaterally over occipito-temporal areas. The results suggest that the global perception of form generated by Glass patterns occurs at a stage of visual processing past area V1.
Abstract: OBJECTIVE: The effects of stimulus size, duration and eccentricity on the visual gamma-band response (GBR) in human EEG were investigated and compared to visual evoked potentials (VEPs) in order to differentiate in future (and past) experiments whether changes in GBRs are due to stimulus-related (exogenous) or cognitive effects. METHODS: EEG was recorded from 23 subjects while they performed a simple choice reaction time task requiring discrimination of squares and circles. In separate blocks stimulus size, duration, and eccentricity were manipulated. EEG was recorded from 64 electrodes. A wavelet transform based on Morlet wavelets was employed for the analysis of gamma-band activity. RESULTS: Amplitude of the GBR was diminished for small and peripheral stimuli. With short stimulus durations ON and OFF responses of the GBR merged into one peak. In comparison, VEP amplitudes were less susceptible to stimulus features. In contrast to VEP latencies, however, GBR latency did not show a lateralization for eccentric stimuli. CONCLUSIONS: In addition to previous experiments which have shown a modulation of the GBR by various cognitive processes, the present results demonstrate the susceptibility of the GBR in human EEG to exogenous factors, as numerous intracortical recordings in non-human primates have shown before. The results suggest that the human GBR resides in early visual areas. SIGNIFICANCE: The demonstration of the susceptibility of the GBR to stimulus properties implies that studies aimed at exploring the involvement of the GBR in information processing have to be designed carefully. It also constrains the localization of the human GBR.
Abstract: BACKGROUND: Human brain activity in the gamma frequency range has been shown to be a correlate of numerous cognitive functions like attention, perception and memory access. More specifically, gamma activity has been found to be enhanced when stimuli are stored in or match with short-term memory (STM). We tested the hypothesis that gamma activity is also evoked when stimuli match representations in long-term-memory (LTM). EEG was recorded from 13 subjects performing a choice reaction task. Visual stimuli were either known real-world objects with a memory representation or novel configurations never seen before. RESULTS: All stimuli evoked an early gamma response which was maximal over occipital electrodes. This evoked gamma activity was significantly larger for items that matched memory templates. CONCLUSIONS: Therefore, we argue that gamma activity results from the feedback from memory into perception systems. This assumption seems to be true for STM as well as LTM.
Abstract: Behavioral studies have indicated that multiple features of one object can be stored in working memory without additional costs. In contrast, visual search experiments revealed that search for a multi-featured object takes more time than for a single-featured object. We used EEG to differentiate the effect of object-load and feature-load in a short-term memory task. We independently varied the amount of objects and features that had to be memorized. Object-load modulated P3 amplitude during encoding and induced 10 Hz oscillations during the retention interval. Feature-load modulated the P3 during retrieval. Thus, only object-load seemed to influence encoding and retention while feature-load played a crucial role during retrieval. Our results demonstrate that object-load and feature-load influence short-term memory at different stages.
Abstract: Event related field responses to line drawings of novel visual objects were recorded in a simple target detection task using whole-head magnetoencephalography. Brain-current-source-density reconstruction indicated that, relative to initial presentations, immediately repeated nontarget stimuli elicited reduced neural activity in a region of cortex extending from the parietal lobe into the superior frontal lobe in a time window from 250 ms and 450 ms. There are at least two plausible accounts of this neural activity reduction between conditions. It may reflect facilitated stimulus processing due to the existence of a representation of the repeated stimulus or it may reflect differential levels of attentional allocation to initial and repeated stimulus presentations.
