Honors & Awards

  • Postgraduate Research Award, Swinburne University of Technology (2005 - 2009)
  • Dean's Postdoctoral Fellowship, School of Medicine, Stanford University (2010 - 2011)
  • Pathway to Independence (K99/R00), NIH (2014 - 2019)

Professional Education

  • Bachelor of Science, Swinburne University of Technology (2005)
  • Doctor of Philosophy, Swinburne University of Technology (2010)

Stanford Advisors

Current Research and Scholarly Interests

My research seeks to connect basic physiological mechanisms of neural coordination with functional brain networks that are essential for constructing coherent conscious states and their future recall. As a confluence of these interests, I'm currently studying the neural population electrophysiology of human posteromedial cortex (PMC), a unique brain region with pronounced resting-state activity that has been linked to levels of conscious state (e.g. during anesthesia) and more cognitively plays an integral role in retrieving episodic/autobiographical memories as part of the default-mode network. I explore these questions using intracranial electrophysiology, functional brain imaging and electrical brain stimulation techniques. To enhance these empirical studies, I also work on the modeling, analysis, and interpretation of neural population dynamics in cerebral cortex.

All Publications

  • Intrinsic and Task-Dependent Coupling of Neuronal Population Activity in Human Parietal Cortex NEURON Foster, B. L., Rangarajan, V., Shirer, W. R., Parvizi, J. 2015; 86 (2): 578-590


    Human neuroimaging studies have suggested that subregions of the medial and lateral parietal cortex form key nodes of a larger brain network supporting episodic memory retrieval. To explore the electrophysiological correlates of functional connectivity between these subregions, we recorded simultaneously from medial and lateral parietal cortex using intracranial electrodes in three human subjects. We observed electrophysiological co-activation of retrosplenial/posterior cingulate cortex (RSC/PCC) and angular gyrus (AG) in the high-frequency broadband (HFB, or high-gamma) range, for conditions that required episodic retrieval. During resting and sleeping states, slow fluctuations (<1 Hz) of HFB activity were highly correlated between these task-co-activated neuronal populations. Furthermore, intrinsic electrophysiological connectivity patterns matched those obtained with resting-state fMRI from the same subjects. Our findings quantify the spatiotemporal dynamics of parietal cortex during episodic memory retrieval and provide clear neurophysiological correlates of intrinsic and task-dependent functional connectivity in the human brain.

    View details for DOI 10.1016/j.neuron.2015.03.018

    View details for Web of Science ID 000353410000023

    View details for PubMedID 25863718

  • Human Retrosplenial Cortex Displays Transient Theta Phase Locking with Medial Temporal Cortex Prior to Activation during Autobiographical Memory Retrieval JOURNAL OF NEUROSCIENCE Foster, B. L., Kaveh, A., Dastjerdi, M., Miller, K. J., Parvizi, J. 2013; 33 (25): 10439-10446


    The involvement of retrosplenial cortex (RSC) in human autobiographical memory retrieval has been confirmed by functional brain imaging studies, and is supported by anatomical evidence of strong connectivity between the RSC and memory structures within the medial temporal lobe (MTL). However, electrophysiological investigations of the RSC and its interaction with the MTL have mostly remained limited to the rodent brain. Recently, we reported a selective increase of high-frequency broadband (HFB; 70-180 Hz) power within the human RSC during autobiographical retrieval, and a predominance of 3-5 Hz theta band oscillations within the RSC during the resting state. In the current study, we aimed to explore the temporal dynamics of theta band interaction between human RSC and MTL during autobiographical retrieval. Toward this aim, we obtained simultaneous recordings from the RSC and MTL in human subjects undergoing invasive electrophysiological monitoring, and quantified the strength of RSC-MTL theta band phase locking. We observed significant phase locking in the 3-4 Hz theta range between the RSC and the MTL during autobiographical retrieval. This theta band phase coupling was transient and peaked at a consistent latency before the peak of RSC HFB power across subjects. Control analyses confirmed that theta phase coupling between the RSC and MTL was not seen for other conditions studied, other sites of recording, or other frequency ranges of interest (1-20 Hz). Our findings provide the first evidence of theta band interaction between the human RSC and MTL during conditions of autobiographical retrieval.

    View details for DOI 10.1523/JNEUROSCI.0513-13.2013

    View details for Web of Science ID 000320596400022

    View details for PubMedID 23785155

  • Neural populations in human posteromedial cortex display opposing responses during memory and numerical processing PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA Foster, B. L., Dastjerdi, M., Parvizi, J. 2012; 109 (38): 15514-15519


    Our understanding of the human default mode network derives primarily from neuroimaging data but its electrophysiological correlates remain largely unexplored. To address this limitation, we recorded intracranially from the human posteromedial cortex (PMC), a core structure of the default mode network, during various conditions of internally directed (e.g., autobiographical memory) as opposed to externally directed focus (e.g., arithmetic calculation). We observed late-onset (>400 ms) increases in broad high ?-power (70-180 Hz) within PMC subregions during memory retrieval. High ?-power was significantly reduced or absent when subjects retrieved self-referential semantic memories or responded to self-judgment statements, respectively. Conversely, a significant deactivation of high ?-power was observed during arithmetic calculation, the duration of which correlated with reaction time at the signal-trial level. Strikingly, at each recording site, the magnitude of activation during episodic autobiographical memory retrieval predicted the degree of suppression during arithmetic calculation. These findings provide important anatomical and temporal details-at the neural population level-of PMC engagement during autobiographical memory retrieval and address how the same populations are actively suppressed during tasks, such as numerical processing, which require externally directed attention.

    View details for DOI 10.1073/pnas.1206580109

    View details for Web of Science ID 000309211000087

    View details for PubMedID 22949666

  • Resting oscillations and cross-frequency coupling in the human posteromedial cortex NEUROIMAGE Foster, B. L., Parvizi, J. 2012; 60 (1): 384-391


    Using rare intracranial recordings from the posterior interhemispheric region of the human brain, we explored the oscillatory properties of the posteromedial cortex (PMC) during rest. The PMC is a core structure of the default mode network, which is known for its higher activity during the resting state. We found that resting PMC spectral power peaked in the theta band range (4-7 Hz) and was clearly distinguishable from adjacent cortical sites in the occipital lobe displaying peaks in the alpha band range (8-12 Hz). Additionally, the phase of PMC theta oscillations modulated the amplitude of ongoing high gamma (70-180 Hz) activity during the resting state. The magnitude of this cross-frequency modulation was shown to fluctuate at time scales comparable to those observed in functional neuroimaging studies of intrinsic functional connectivity networks (~0.1 Hz). The difference of canonical oscillations in the PMC compared to its adjacent cortical sites conforms to functional specialization across anatomical boundaries. Such differences may reflect separate oscillatory preferences between networks that are functionally connected.

    View details for DOI 10.1016/j.neuroimage.2011.12.019

    View details for Web of Science ID 000301218700039

    View details for PubMedID 22227048

  • Differential electrophysiological response during rest, self-referential, and non-self-referential tasks in human posteromedial cortex PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA Dastjerdi, M., Foster, B. L., Nasrullah, S., Rauschecker, A. M., Dougherty, R. F., Townsend, J. D., Chang, C., Greicius, M. D., Menon, V., Kennedy, D. P., Parvizi, J. 2011; 108 (7): 3023-3028


    The electrophysiological basis for higher brain activity during rest and internally directed cognition within the human default mode network (DMN) remains largely unknown. Here we use intracranial recordings in the human posteromedial cortex (PMC), a core node within the DMN, during conditions of cued rest, autobiographical judgments, and arithmetic processing. We found a heterogeneous profile of PMC responses in functional, spatial, and temporal domains. Although the majority of PMC sites showed increased broad gamma band activity (30-180 Hz) during rest, some PMC sites, proximal to the retrosplenial cortex, responded selectively to autobiographical stimuli. However, no site responded to both conditions, even though they were located within the boundaries of the DMN identified with resting-state functional imaging and similarly deactivated during arithmetic processing. These findings, which provide electrophysiological evidence for heterogeneity within the core of the DMN, will have important implications for neuroimaging studies of the DMN.

    View details for DOI 10.1073/pnas.1017098108

    View details for Web of Science ID 000287377000073

    View details for PubMedID 21282630

  • Electrocorticography reveals the temporal dynamics of posterior parietal cortical activity during recognition memory decisions PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA Gonzalez, A., Hutchinson, J. B., Uncapher, M. R., Chen, J., LaRocque, K. F., Foster, B. L., Rangarajan, V., Parvizi, J., Wagner, A. D. 2015; 112 (35): 11066-11071
  • Corresponding ECoG and fMRI category-selective signals in Human ventral temporal cortex. Neuropsychologia Jacques, C., Witthoft, N., Weiner, K. S., Foster, B. L., Rangarajan, V., Hermes, D., Miller, K. J., Parvizi, J., Grill-Spector, K. 2015


    Functional magnetic resonance imaging (fMRI) and electrocorticography (ECoG) research have been influential in revealing the functional characteristics of category-selective responses in human ventral temporal cortex (VTC). One important, but unanswered, question is how these two types of measurements might be related with respect to the VTC. Here we examined which components of the ECoG signal correspond to the fMRI response, by using a rare opportunity to measure both fMRI and ECoG responses from the same individuals to images of exemplars of various categories including faces, limbs, cars and houses. Our data reveal three key findings. First, we discovered that the coupling between fMRI and ECoG responses is frequency and time dependent. The strongest and most sustained correlation is observed between fMRI and high frequency broadband (HFB) ECoG responses (30-160hz). In contrast, the correlation between fMRI and ECoG signals in lower frequency bands is temporally transient, where the correlation is initially positive, but then tapers off or becomes negative. Second, we find that the strong and positive correlation between fMRI and ECoG signals in all frequency bands emerges rapidly around 100ms after stimulus onset, together with the onset of the first stimulus-driven neural signals in VTC. Third, we find that the spatial topology and representational structure of category-selectivity in VTC reflected in ECoG HFB responses mirrors the topology and structure observed with fMRI. These findings of a strong and rapid coupling between fMRI and HFB responses validate fMRI measurements of functional selectivity with recordings of direct neural activity and suggest that fMRI category-selective signals in VTC are associated with feed-forward neural processing.

    View details for DOI 10.1016/j.neuropsychologia.2015.07.024

    View details for PubMedID 26212070

  • Electrophysiological Responses in the Ventral Temporal Cortex During Reading of Numerals and Calculation. Cerebral cortex (New York, N.Y. : 1991) Hermes, D., Rangarajan, V., Foster, B. L., King, J. R., Kasikci, I., Miller, K. J., Parvizi, J. 2015


    Recent evidence suggests that specific neuronal populations in the ventral temporal cortex show larger electrophysiological responses to visual numerals compared with morphologically similar stimuli. This study investigates how these responses change from simple reading of numerals to the active use of numerals in an arithmetic context. We recorded high-frequency broadband (HFB) signals, a reliable measure for local neuronal population activity, while 10 epilepsy patients implanted with subdural electrodes performed separate numeral reading and calculation tasks. We found that calculation increased activity in the posterior inferior temporal gyrus (ITG) with a factor of approximately 1.5 over the first 500 ms of calculation, whereas no such increase was noted for reading numerals without calculation or reading and judging memory statements. In a second experiment conducted in 2 of the same subjects, we show that HFB responses increase in a systematic manner when the single numerals were presented successively in a calculation context: The HFB response in the ITG, to the second and third numerals (i.e., b and c in a + b = c), was approximately 1.5 times larger than the responses to the first numeral (a). These results provide electrophysiological evidence for modulation of local neuronal population responses to visual stimuli based on increasing task demands.

    View details for DOI 10.1093/cercor/bhv250

    View details for PubMedID 26503267

  • Electrical Stimulation of the Left and Right Human Fusiform Gyrus Causes Different Effects in Conscious Face Perception JOURNAL OF NEUROSCIENCE Rangarajan, V., Hermes, D., Foster, B. L., Weiner, K. S., Jacques, C., Grill-Spector, K., Parvizi, J. 2014; 34 (38): 12828-12836
  • Human hippocampal increases in low-frequency power during associative prediction violations NEUROPSYCHOLOGIA Chen, J., Dastjerdi, M., Foster, B. L., LaRocque, K. F., Rauschecker, A. M., Parvizi, J., Wagner, A. D. 2013; 51 (12): 2344-2351


    Environmental cues often trigger memories of past events (associative retrieval), and these memories are a form of prediction about imminent experience. Learning is driven by the detection of prediction violations, when the past and present diverge. Using intracranial electroencephalography (iEEG), we show that associative prediction violations elicit increased low-frequency power (in the slow-theta range) in human hippocampus, that this low-frequency power increase is modulated by whether conditions allow predictions to be generated, that the increase rapidly onsets after the moment of violation, and that changes in low-frequency power are not present in adjacent perirhinal cortex. These data suggest that associative mismatch is computed within hippocampus when cues trigger predictions that are violated by imminent experience.

    View details for DOI 10.1016/j.neuropsychologia.2013.03.019

    View details for Web of Science ID 000328526800004

    View details for PubMedID 23571081

  • Numerical processing in the human parietal cortex during experimental and natural conditions NATURE COMMUNICATIONS Dastjerdi, M., Ozker, M., Foster, B. L., Rangarajan, V., Parvizi, J. 2013; 4

    View details for DOI 10.1038/ncomms3528

    View details for Web of Science ID 000326467900002

  • Asynchronous Broadband Signals Are the Principal Source of the BOLD Response in Human Visual Cortex CURRENT BIOLOGY Winawer, J., Kay, K. N., Foster, B. L., Rauschecker, A. M., Parvizi, J., Wandell, B. A. 2013; 23 (13): 1145-1153


    Activity in the living human brain can be studied using multiple methods, spanning a wide range of spatial and temporal resolutions. We investigated the relationship between electric field potentials measured with electrocorticography (ECoG) and the blood oxygen level-dependent (BOLD) response measured with functional magnetic resonance imaging (fMRI). We set out to explain the full set of measurements by modeling the underlying neural circuits.ECoG responses in visual cortex can be separated into two visually driven components. One component is a specific temporal response that follows each stimulus contrast reversal ("stimulus locked"); the other component is an increase in the response variance ("asynchronous"). For electrodes in visual cortex (V1, V2, V3), the two measures respond to stimuli in the same region of visual space, but they have different spatial summation properties. The stimulus-locked ECoG component sums contrast approximately linearly across space; spatial summation in the asynchronous ECoG component is subadditive. Spatial summation measured using BOLD closely matches the asynchronous component. We created a neural simulation that accurately captures the main features of the ECoG time series; in the simulation, the stimulus-locked and asynchronous components arise from different neural circuits.These observations suggest that the two ECoG components arise from different neural sources within the same cortical region. The spatial summation measurements and simulations suggest that the BOLD response arises primarily from neural sources that generate the asynchronous broadband ECoG component.

    View details for DOI 10.1016/j.cub.2013.05.001

    View details for Web of Science ID 000321605600015

    View details for PubMedID 23770184

  • A Brain Area for Visual Numerals JOURNAL OF NEUROSCIENCE Shum, J., Hermes, D., Foster, B. L., Dastjerdi, M., Rangarajan, V., Winawer, J., Miller, K. J., Parvizi, J. 2013; 33 (16): 6709-6715


    Is there a distinct area within the human visual system that has a preferential response to numerals, as there is for faces, words, or scenes? We addressed this question using intracranial electrophysiological recordings and observed a significantly higher response in the high-frequency broadband range (high γ, 65-150 Hz) to visually presented numerals, compared with morphologically similar (i.e., letters and false fonts) or semantically and phonologically similar stimuli (i.e., number words and non-number words). Anatomically, this preferential response was consistently localized in the inferior temporal gyrus and anterior to the temporo-occipital incisure. This region lies within or close to the fMRI signal-dropout zone produced by the nearby auditory canal and venous sinus artifacts, an observation that may account for negative findings in previous fMRI studies of preferential response to numerals. Because visual numerals are culturally dependent symbols that are only learned through education, our novel finding of anatomically localized preferential response to such symbols provides a new example of acquired category-specific responses in the human visual system.

    View details for DOI 10.1523/JNEUROSCI.4558-12.2013

    View details for Web of Science ID 000317723000002

  • Hand posture classification using electrocorticography signals in the gamma band over human sensorimotor brain areas. Journal of neural engineering Chestek, C. A., Gilja, V., Blabe, C. H., Foster, B. L., Shenoy, K. V., Parvizi, J., Henderson, J. M. 2013; 10 (2): 026002-?


    Brain-machine interface systems translate recorded neural signals into command signals for assistive technology. In individuals with upper limb amputation or cervical spinal cord injury, the restoration of a useful hand grasp could significantly improve daily function. We sought to determine if electrocorticographic (ECoG) signals contain sufficient information to select among multiple hand postures for a prosthetic hand, orthotic, or functional electrical stimulation system.We recorded ECoG signals from subdural macro- and microelectrodes implanted in motor areas of three participants who were undergoing inpatient monitoring for diagnosis and treatment of intractable epilepsy. Participants performed five distinct isometric hand postures, as well as four distinct finger movements. Several control experiments were attempted in order to remove sensory information from the classification results. Online experiments were performed with two participants.Classification rates were 68%, 84% and 81% for correct identification of 5 isometric hand postures offline. Using 3 potential controls for removing sensory signals, error rates were approximately doubled on average (2.1×). A similar increase in errors (2.6×) was noted when the participant was asked to make simultaneous wrist movements along with the hand postures. In online experiments, fist versus rest was successfully classified on 97% of trials; the classification output drove a prosthetic hand. Online classification performance for a larger number of hand postures remained above chance, but substantially below offline performance. In addition, the long integration windows used would preclude the use of decoded signals for control of a BCI system.These results suggest that ECoG is a plausible source of command signals for prosthetic grasp selection. Overall, avenues remain for improvement through better electrode designs and placement, better participant training, and characterization of non-stationarities such that ECoG could be a viable signal source for grasp control for amputees or individuals with paralysis.

    View details for DOI 10.1088/1741-2560/10/2/026002

    View details for PubMedID 23369953

  • Hand posture classification using electrocorticography signals in the gamma band over human sensorimotor brain areas JOURNAL OF NEURAL ENGINEERING Chestek, C. A., Gilja, V., Blabe, C. H., Foster, B. L., Shenoy, K. V., Parvizi, J., Henderson, J. M. 2013; 10 (2)
  • Modulation of Functional EEG Networks by the NMDA Antagonist Nitrous Oxide PLOS ONE Kuhlmann, L., Foster, B. L., Liley, D. T. 2013; 8 (2)


    Parietal networks are hypothesised to play a central role in the cortical information synthesis that supports conscious experience and behavior. Significant reductions in parietal level functional connectivity have been shown to occur during general anesthesia with propofol and a range of other GABAergic general anesthetic agents. Using two analysis approaches (1) a graph theoretic analysis based on surrogate-corrected zero-lag correlations of scalp EEG, and (2) a global coherence analysis based on the EEG cross-spectrum, we reveal that sedation with the NMDA receptor antagonist nitrous oxide (N2O), an agent that has quite different electroencephalographic effects compared to the inductive general anesthetics, also causes significant alterations in parietal level functional networks, as well as changes in full brain and frontal level networks. A total of 20 subjects underwent N2O inhalation at either 20%, 40% or 60% peak N2O/O2 gas concentration levels. N2O-induced reductions in parietal network level functional connectivity (on the order of 50%) were exclusively detected by utilising a surface Laplacian derivation, suggesting that superficial, smaller spatial scale, cortical networks were most affected. In contrast reductions in frontal network functional connectivity were optimally discriminated using a common-reference derivation (reductions on the order of 10%), indicating that the NMDA antagonist N2O induces spatially coherent and widespread perturbations in frontal activity. Our findings not only give important weight to the idea of agent invariant final network changes underlying drug-induced reductions in consciousness, but also provide significant impetus for the application and development of multiscale functional analyses to systematically characterise the network level cortical effects of NMDA receptor related hypofunction. Future work at the source space level will be needed to verify the consistency between cortical network changes seen at the source level and those presented here at the EEG sensor space level.

    View details for DOI 10.1371/journal.pone.0056434

    View details for Web of Science ID 000315602700073

    View details for PubMedID 23457568

  • Effects of nitrous oxide sedation on resting electroencephalogram topography CLINICAL NEUROPHYSIOLOGY Foster, B. L., Liley, D. T. 2013; 124 (2): 417-423


    To quantify the effects of nitrous oxide (N(2)O) gas on electroencephalogram (EEG) topography in healthy male participants.Healthy male participants were administered 20% (n=8) or 40% (n=8) N(2)O while having high-density (modified 10-20) noise minimized EEG recordings.Nitrous oxide was found to produce clear reductions in resting total power, particularly at frontal-vertex sites. These reductions were found to principally reflect reductions in band-limited delta power. Following the termination of N(2)O inhalation, during N(2)O washout, selective increases in frontal theta power were observed that increased above baseline values.Nitrous oxide does not produce the classical anteriorization of slow wave activity typically seen during anesthetic induction. Instead N(2)O reduces frontal slow wave (delta) activity, which during gas washout produces a withdrawal response of enhanced frontal slow wave (theta) activity.Attempts to characterize a unitary mechanism of loss of consciousness during anesthesia on the basis of the topographic electroencephalographic changes is challenged by the distinct EEG effects that N(2)O has when compared to other well known anesthetic agents that include propofol and sevoflurane.

    View details for DOI 10.1016/j.clinph.2012.08.007

    View details for Web of Science ID 000313592400025

    View details for PubMedID 22968005

  • Numerical processing in the human parietal cortex during experimental and natural conditions. Nature communications Dastjerdi, M., Ozker, M., Foster, B. L., Rangarajan, V., Parvizi, J. 2013; 4: 2528-?


    Human cognition is traditionally studied in experimental conditions wherein confounding complexities of the natural environment are intentionally eliminated. Thus, it remains unknown how a brain region involved in a particular experimental condition is engaged in natural conditions. Here we use electrocorticography to address this uncertainty in three participants implanted with intracranial electrodes and identify activations of neuronal populations within the intraparietal sulcus region during an experimental arithmetic condition. In a subsequent analysis, we report that the same intraparietal sulcus neural populations are activated when participants, engaged in social conversations, refer to objects with numerical content. Our prototype approach provides a means for both exploring human brain dynamics as they unfold in complex social settings and reconstructing natural experiences from recorded brain signals.

    View details for DOI 10.1038/ncomms3528

    View details for PubMedID 24129341

  • Does rhythmic entrainment represent a generalized mechanism for organizing computation in the brain? FRONTIERS IN COMPUTATIONAL NEUROSCIENCE Miller, K. J., Foster, B. L., Honey, C. J. 2012; 6

    View details for DOI 10.3389/fncom.2012.00085

    View details for Web of Science ID 000310147800001

    View details for PubMedID 23112771

  • Electrical Stimulation of Human Fusiform Face-Selective Regions Distorts Face Perception JOURNAL OF NEUROSCIENCE Parvizi, J., Jacques, C., Foster, B. L., Withoft, N., Rangarajan, V., Weiner, K. S., Grill-Spector, K. 2012; 32 (43): 14915-14920


    Face-selective neural responses in the human fusiform gyrus have been widely examined. However, their causal role in human face perception is largely unknown. Here, we used a multimodal approach of electrocorticography (ECoG), high-resolution functional magnetic resonance imaging (fMRI), and electrical brain stimulation (EBS) to directly investigate the causal role of face-selective neural responses of the fusiform gyrus (FG) in face perception in a patient implanted with subdural electrodes in the right inferior temporal lobe. High-resolution fMRI identified two distinct FG face-selective regions [mFus-faces and pFus-faces (mid and posterior fusiform, respectively)]. ECoG revealed a striking anatomical and functional correspondence with fMRI data where a pair of face-selective electrodes, positioned 1 cm apart, overlapped mFus-faces and pFus-faces, respectively. Moreover, electrical charge delivered to this pair of electrodes induced a profound face-specific perceptual distortion during viewing of real faces. Specifically, the subject reported a "metamorphosed" appearance of faces of people in the room. Several controls illustrate the specificity of the effect to the perception of faces. EBS of mFus-faces and pFus-faces neither produced a significant deficit in naming pictures of famous faces on the computer, nor did it affect the appearance of nonface objects. Further, the appearance of faces remained unaffected during both sham stimulation and stimulation of a pair of nearby electrodes that were not face-selective. Overall, our findings reveal a striking convergence of fMRI, ECoG, and EBS, which together offer a rare causal link between functional subsets of the human FG network and face perception.

    View details for DOI 10.1523/JNEUROSCI.2609-12.2012

    View details for Web of Science ID 000310523900008

    View details for PubMedID 23100414

  • Problem of signal contamination in interhemispheric dual-sided subdural electrodes EPILEPSIA Nune, G., Winawer, J., Rauschecker, A. M., Dastjerdi, M., Foster, B. L., Wandell, B., Parvizi, J. 2011; 52 (11): E176-E180


    Dual-sided subdural electrodes are used in the localization and lateralization of seizure-onset zones when the area of interest is within the interhemispheric fissure. We designed the current study to test the validity of the assumption that each side of the dual-sided electrodes records from the hemisphere it faces. We recorded with dual-sided strip and grid electrodes implanted in the occipital interhemispheric space in two patients with nonoccipital epilepsy during two visual stimulation tasks in which subjects were presented with visual stimuli in the ipsilateral or contralateral visual hemifields. Our findings show substantial contamination of recordings from the opposite hemisphere. Although, as expected, electrodes recording through the falx record faintly from the contralateral cortical surface, they unexpectedly pick up strong signals from the cortex behind them. Therefore, we conclude that these electrodes should not be used for lateralization of the origin of epileptic activity or evoked responses.

    View details for DOI 10.1111/j.1528-1167.2011.03284.x

    View details for Web of Science ID 000297049700004

    View details for PubMedID 21973215

  • Nitrous Oxide Paradoxically Modulates Slow Electroencephalogram Oscillations: Implications for Anesthesia Monitoring ANESTHESIA AND ANALGESIA Foster, B. L., Liley, D. T. 2011; 113 (4): 758-765


    Nitrous oxide (N(2)O) is one of the oldest analgesics/adjuvant agents still in use today; however, its effects on the human electroencephalogram (EEG) remain unclear. It has been proposed that N(2)O may enhance higher-frequency EEG activity (often indicative of alert states and cognition) duration sedation. This possibly paradoxical effect has been used to explain the failure of many EEG monitors to capture the effects of N(2)O on patient state during anesthesia. To better understand the poor efficacy of current EEG approaches to monitoring N(2)O action, we quantitatively studied the sole effect of N(2)O on the resting EEG in healthy volunteers using multichannel EEG recordings under noise-minimized laboratory conditions.Healthy male volunteers were administered 20% (n = 10), 40% (n = 10), or 60% (n = 5) inspired N(2)O mixed with oxygen during noise-shielded EEG recordings. N(2)O was administered over a 20-minute period involving a 5-minute equilibration period and 5-minute washout. EEG spectral edge frequency (95%), median power frequency, total power, and band-limited power (?, , ?, ?, and ?) were used as quantitative EEG parameters. The changes in these EEG parameters were quantified throughout N(2)O inhalation and compared between predrug baseline, peak drug effect, and washout.Quantification of changes in spectral power during N(2)O inhalation showed only minor changes in estimates of spectral edge and median power frequency, whereas significant reductions in total power were observed at frontal sites during peak gas effect (P = 0.001; mean reduction [95% confidence interval]: 41.90 ?V(2) [18.19-65.61 ?V(2)]) that rebounded during N(2)O washout. Such changes in total power were driven by shifts in low-frequency power (?/), which were most elevated at frontal sites.Rather than directly enhancing high-frequency EEG power (? or ? bands), N(2)O seems to preserve the awake features of resting EEG (? band) and suppress power in those bands in which increases are typically associated with sedation/hypnosis (? and ). These data suggest that N(2)O's suppression of low-frequency EEG power may help to explain previously reported difficulties in attempting to monitor patient state with the EEG during anesthesia involving N(2)O. Because increases in low-frequency power typically indicate increasing anesthesia, N(2)O's suppression of such activity and its rebound during washout would paradoxically influence EEG monitoring parameters. Therefore, correcting for such effects is expected to improve future monitoring methods.

    View details for DOI 10.1213/ANE.0b013e318227b688

    View details for Web of Science ID 000295215100017

    View details for PubMedID 21788312

  • Gelastic epilepsy and hypothalamic hamartomas: neuroanatomical analysis of brain lesions in 100 patients BRAIN Parvizi, J., Le, S., Foster, B. L., Bourgeois, B., Riviello, J. J., Prenger, E., Saper, C., Kerrigan, J. F. 2011; 134: 2960-2968


    Hypothalamic hamartomas present with isolated fits of ictal laughter (gelastic epilepsy) or a combination of gelastic and other types of seizures. Many of these patients also suffer from cognitive decline, neuropsychiatric comorbidities and precocious puberty. Although there is a large body of anecdotal evidence about hypothalamic hamartomas and gelastic seizures, many questions still remain to be answered. For instance, which specific hypothalamic regions are most affected by the location of hamartomas causing laughing versus other types of seizures? Does the neuroanatomical localization of the lesions differ in cases with only gelastic seizures or a combination of gelastic and other types of seizures? Does the location of the lesions correlate with the presence of precocious puberty, and does the type of lesion influence the severity or the type of seizures? In a retrospective review of clinical and structural neuroimaging data from 100 cases of gelastic epilepsy and hypothalamic hamartoma, we aimed to address these questions by analysing the clinical presentation and the neuroanatomical features of the hypothalamic lesions in these patients. Our findings suggest that in all 100 cases, lesions were centred at the level of the mammillary bodies in the posterior hypothalamus. Compared with the patients with pure gelastic seizures (n = 32), those with gelastic and other types of seizures (n = 68) had significantly longer duration of epilepsy (P < 0.001), whereas age of seizure onset, the volume of lesions and the proximity to the mammillary bodies were not different between the two groups. In contrast, patients with cognitive or developmental impairment and those with precocious puberty had significantly larger lesions involving the anterior and posterior hypothalamus.

    View details for DOI 10.1093/brain/awr235

    View details for Web of Science ID 000295681400015

    View details for PubMedID 21975589

  • Understanding the Effects of Anesthetic Agents on the EEG through Neural Field Theory. Foster, B. L., Bojak, I., Liley, D. T. IEEE. 2011: 4709-4712


    Anesthetic and analgesic agents act through a diverse range of pharmacological mechanisms. Existing empirical data clearly shows that such "microscopic" pharmacological diversity is reflected in their "macroscopic" effects on the human electroencephalogram (EEG). Based on a detailed mesoscopic neural field model we theoretically posit that anesthetic induced EEG activity is due to selective parametric changes in synaptic efficacy and dynamics. Specifically, on the basis of physiologically constrained modeling, it is speculated that the selective modification of inhibitory or excitatory synaptic activity may differentially effect the EEG spectrum. Such results emphasize the importance of neural field theories of brain electrical activity for elucidating the principles whereby pharmacological agents effect the EEG. Such insights will contribute to improved methods for monitoring depth of anesthesia using the EEG.

    View details for Web of Science ID 000298810003257

    View details for PubMedID 22255389

  • Population based models of cortical drug response: insights from anaesthesia COGNITIVE NEURODYNAMICS Foster, B. L., Bojak, I., Liley, D. T. 2008; 2 (4): 283-296


    A great explanatory gap lies between the molecular pharmacology of psychoactive agents and the neurophysiological changes they induce, as recorded by neuroimaging modalities. Causally relating the cellular actions of psychoactive compounds to their influence on population activity is experimentally challenging. Recent developments in the dynamical modelling of neural tissue have attempted to span this explanatory gap between microscopic targets and their macroscopic neurophysiological effects via a range of biologically plausible dynamical models of cortical tissue. Such theoretical models allow exploration of neural dynamics, in particular their modification by drug action. The ability to theoretically bridge scales is due to a biologically plausible averaging of cortical tissue properties. In the resulting macroscopic neural field, individual neurons need not be explicitly represented (as in neural networks). The following paper aims to provide a non-technical introduction to the mean field population modelling of drug action and its recent successes in modelling anaesthesia.

    View details for DOI 10.1007/s11571-008-9063-z

    View details for Web of Science ID 000266413100001

    View details for PubMedID 19003456