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ABSTRACT: Here we summarize the points raised in our dialogue with Ales and colleagues on the cortical generators of the early visual evoked potential (VEP), and offer observations on the results of additional simulations that were run in response to our original comment. For small stimuli placed at locations in the upper and lower visual field for which the human VEP has been well characterized, simulated scalp projections of each of the visual areas V1, V2 and V3 invert in polarity. However, the empirically measured, earliest VEP component, "C1," matches the simulated V1 generators in terms of polarity and topography, but not the simulated V2 and V3 generators. We thus conclude that, 1) consistent with the title of Ales et al (2010a), polarity inversion on its own is not a sufficient criterion for inferring neuroelectric sources in primary visual cortex; but 2) inconsistent with additional claims made in Ales et al (2010a), the simulated topographies provide additional evidence for - not against - the tenet that the C1 component is generated in V1.
NeuroImage 06/2013; · 5.89 Impact Factor
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Elana M Zion Golumbic,
Nai Ding,
Stephan Bickel,
Peter Lakatos,
Catherine A Schevon,
Guy M McKhann,
Robert R Goodman,
Ronald Emerson,
Ashesh D Mehta,
Jonathan Z Simon,
David Poeppel, Charles E Schroeder
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ABSTRACT: The ability to focus on and understand one talker in a noisy social environment is a critical social-cognitive capacity, whose underlying neuronal mechanisms are unclear. We investigated the manner in which speech streams are represented in brain activity and the way that selective attention governs the brain's representation of speech using a "Cocktail Party" paradigm, coupled with direct recordings from the cortical surface in surgical epilepsy patients. We find that brain activity dynamically tracks speech streams using both low-frequency phase and high-frequency amplitude fluctuations and that optimal encoding likely combines the two. In and near low-level auditory cortices, attention "modulates" the representation by enhancing cortical tracking of attended speech streams, but ignored speech remains represented. In higher-order regions, the representation appears to become more "selective," in that there is no detectable tracking of ignored speech. This selectivity itself seems to sharpen as a sentence unfolds.
Neuron 03/2013; 77(5):980-91. · 14.74 Impact Factor
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Ido Davidesco,
Elana Zion-Golumbic,
Stephan Bickel,
Michal Harel,
David M Groppe,
Corey J Keller,
Catherine A Schevon,
Guy M McKhann,
Robert R Goodman,
Gadi Goelman, Charles E Schroeder,
Ashesh D Mehta,
Rafael Malach
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ABSTRACT: While brain imaging studies emphasized the category selectivity of face-related areas, the underlying mechanisms of our remarkable ability to discriminate between different faces are less understood. Here, we recorded intracranial local field potentials from face-related areas in patients presented with images of faces and objects. A highly significant exemplar tuning within the category of faces was observed in high-Gamma (80-150 Hz) responses. The robustness of this effect was supported by single-trial decoding of face exemplars using a minimal (n = 5) training set. Importantly, exemplar tuning reflected the psychophysical distance between faces but not their low-level features. Our results reveal a neuronal substrate for the establishment of perceptual distance among faces in the human brain. They further imply that face neurons are anatomically grouped according to well-defined functional principles, such as perceptual similarity.
Cerebral Cortex 02/2013; · 6.54 Impact Factor
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ABSTRACT: Although we have convincing evidence that attention to auditory stimuli modulates neuronal responses at or before the level of primary auditory cortex (A1), the underlying physiological mechanisms are unknown. We found that attending to rhythmic auditory streams resulted in the entrainment of ongoing oscillatory activity reflecting rhythmic excitability fluctuations in A1. Strikingly, although the rhythm of the entrained oscillations in A1 neuronal ensembles reflected the temporal structure of the attended stream, the phase depended on the attended frequency content. Counter-phase entrainment across differently tuned A1 regions resulted in both the amplification and sharpening of responses at attended time points, in essence acting as a spectrotemporal filter mechanism. Our data suggest that selective attention generates a dynamically evolving model of attended auditory stimulus streams in the form of modulatory subthreshold oscillations across tonotopically organized neuronal ensembles in A1 that enhances the representation of attended stimuli.
Neuron 02/2013; 77(4):750-61. · 14.74 Impact Factor
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ABSTRACT: How does auditory cortex respond to silence? In this issue of Neuron, show that activity in macaque auditory cortex is highly structured even in the absence of sensory stimuli. These data reveal a close link between spontaneous neural activity and the functional organization of auditory cortex.
Neuron 06/2012; 74(5):770-2. · 14.74 Impact Factor
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ABSTRACT: Local field potentials (LFPs) are of growing importance in neurophysiological investigations. LFPs supplement action potential recordings by indexing activity relevant to EEG, magnetoencephalographic, and hemodynamic (fMRI) signals. Recent reports suggest that LFPs reflect activity within very small domains of several hundred micrometers. We examined this conclusion by comparing LFP, current source density (CSD), and multiunit activity (MUA) signals in macaque auditory cortex. Estimated by frequency tuning bandwidths, these signals' "listening areas" differ systematically with an order of MUA < CSD < LFP. Computational analyses confirm that observed LFPs receive local contributions. Direct measurements indicate passive spread of LFPs to sites more than a centimeter from their origins. These findings appear to be independent of the frequency content of the LFP. Our results challenge the idea that LFP recordings typically integrate over extremely circumscribed local domains. Rather, LFPs appear as a mixture of local potentials with "volume conducted" potentials from distant sites.
Neuron 12/2011; 72(5):847-58. · 14.74 Impact Factor
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ABSTRACT: Previous studies raise the hypothesis that attentional bias in the phase of neocortical excitability fluctuations (oscillations) represents a fundamental mechanism for tuning the brain to the temporal dynamics of task-relevant event patterns. To evaluate this hypothesis, we recorded intracranial electrocortical activity in human epilepsy patients while they performed an audiovisual stream selection task. Consistent with our hypothesis, (1) attentional modulation of oscillatory entrainment operates in a distinct network of areas including auditory, visual, posterior parietal, inferior motor, inferior frontal and superior midline frontal cortex, (2) the degree of oscillatory entrainment depends on the predictability of the stimulus stream, and (3) the attentional phase shift of entrained oscillation cooccurs with classical attentional effects observed on phase-locked evoked activity in sensory-specific areas but seems to operate on entrained low-frequency oscillations that cannot be explained by sensory activity evoked at the rate of stimulation. Thus, attentional entrainment appears to tune a network of brain areas to the temporal dynamics of behaviorally relevant event streams, contributing to its perceptual and behavioral selection.
Journal of Neuroscience 03/2011; 31(9):3176-85. · 7.11 Impact Factor
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ABSTRACT: Field potential oscillations in the ∼10 Hz range are known as the alpha rhythm. The genesis and function of alpha has been the subject of intense investigation for the past 80 years. Whereas early work focused on the thalamus as the pacemaker of alpha rhythm, subsequent slice studies revealed that pyramidal neurons in the deep layers of sensory cortices are capable of oscillating in the alpha frequency range independently. How thalamic and cortical generating mechanisms in the intact brain might interact to shape the organization and function of alpha oscillations remains unclear. We addressed this problem by analyzing laminar profiles of local field potential and multiunit activity (MUA) recorded with linear array multielectrodes from the striate cortex of two macaque monkeys performing an intermodal selective attention task. Current source density (CSD) analysis was combined with CSD-MUA coherence to identify intracortical alpha current generators and assess their potential for pacemaking. Coherence and Granger causality analysis was applied to delineate the patterns of interaction among different alpha current generators. We found that (1) separable alpha current generators are located in superficial, granular, and deep layers, with both layer 4C and deep layers containing primary local pacemaking generators, suggesting the involvement of the thalamocortical network, and (2) visual attention reduces the magnitude of alpha oscillations as well as the level of alpha interactions, consistent with numerous reports of occipital alpha reduction with visual attention in human EEG. There is also indication that alpha oscillations in the lateral geniculate cohere with those in V1.
Journal of Neuroscience 03/2011; 31(13):4935-43. · 7.11 Impact Factor
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ABSTRACT: Inhibition plays an essential role in shaping and refining the brain's representation of sensory stimulus attributes. In primary auditory cortex (A1), so-called "sideband" inhibition helps to sharpen the tuning of local neuronal responses. Several distinct types of anatomical circuitry could underlie sideband inhibition, including direct thalamocortical (TC) afferents, as well as indirect intracortical mechanisms. The goal of the present study was to characterize sideband inhibition in A1 and to determine its mechanism by analyzing laminar profiles of neuronal ensemble activity. Our results indicate that both lemniscal and nonlemniscal TC afferents play a role in inhibitory responses via feedforward inhibition and oscillatory phase reset, respectively. We propose that the dynamic modulation of excitability in A1 due to the phase reset of ongoing oscillations may alter the tuning of local neuronal ensembles and can be regarded as a flexible overlay on the more obligatory system of lemniscal feedforward type responses.
Neuron 02/2011; 69(4):805-17. · 14.74 Impact Factor
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ABSTRACT: Recent work reported the observation of alpha frequency oscillations (8-12 Hz) in several regions of macaque visual cortex, including V2, V4, and inferotemporal cortex (IT). While alpha-related physiology in V2 and V4 appears consistent with a role in attention-related suppression, in IT, alpha reactivity appears conflicted with such a role. We addressed this issue directly by analyzing laminar profiles of local field potentials and multiunit activities from the IT of macaque monkeys during performance of an intermodal selective attention task (visual versus auditory). We found that (1) before visual stimulus onset (-200 to 0 ms), attention to visual input increased ongoing alpha power in IT relative to attention to auditory input, and (2) in contrast to the prevailing view of alpha inhibition, the increased ongoing alpha activity is accompanied by increased concurrent multiunit firing and facilitates visual stimulus processing. These results suggest that ongoing alpha oscillations in IT play a different functional role than that in the occipital cortex and may be part of the neuronal mechanism representing task-relevant information.
Journal of Neuroscience 01/2011; 31(3):878-82. · 7.11 Impact Factor
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ABSTRACT: Current source density (CSD) is the second spatial derivative of the local field potential (LFP). CSD analysis has been used
extensively to localize the pattern of transmembrane current flow in neuronal ensembles. For brain responses to repeated external
stimulation, the LFP data are epoched and averaged across an ensemble of trials, from which the CSD profile is then derived.
For spontaneous brain activity, however, the lack of an external triggering event makes ensemble average difficult, hampering
the investigation of such important cognitive functions as anticipatory attention and working memory. In this chapter, we
describe a new method called phase realigned averaging technique (PRAT), which can overcome this difficulty and achieve CSD
profiles on a frequency-by-frequency basis. The method is first validated on simulation examples and then applied to LFP recordings
from a monkey performing an intermodal selective attention task.
Key wordsLocal field potential–Current source density analysis–Alpha oscillations–Cortical column
11/2010: pages 27-40;
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ABSTRACT: Previous studies indicate that primary somatosensory cortical area 3b in macaques contains a somatotopic map of the hand, encompassing representations of each digit. However, numerous observations including recent findings in anesthetized New World monkeys indicate that that the digit representations within the map are not discrete. We assessed the generality and spatial extent of these effects in awake macaques. We show that, within a given digit representation, (1) there is response to stimulation of all other digits tested, extending across most or all of the digit map, and (2) response to stimulation of the locally preferred digit is modulated by concurrent stimulation of each of the other digits. Control experiments rule out effects of attention and mechanical spread of stimulation. We thus confirm that, even at the first level of somatosensory cortical processing, inputs from potentially all of the digits frame the context within which the input to a single digit is represented.
Journal of Neuroscience 11/2010; 30(47):15895-903. · 7.11 Impact Factor
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ABSTRACT: Sensory processing is often regarded as a passive process in which biological receptors like photoreceptors and mechanoreceptors transduce physical energy into a neural code. Recent findings, however, suggest that: first, most sensory processing is active, and largely determined by motor/attentional sampling routines; second, owing to rhythmicity in the motor routine, as well as to its entrainment of ambient rhythms in sensory regions, sensory inflow tends to be rhythmic; third, attentional manipulation of rhythms in sensory pathways is instrumental to perceptual selection. These observations outline the essentials of an Active Sensing paradigm, and argue for increased emphasis on the study of sensory processes as specific to the dynamic motor/attentional context in which inputs are acquired.
Current opinion in neurobiology 03/2010; 20(2):172-6. · 7.21 Impact Factor
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Hearing research 12/2009; 258(1-2):1-3. · 2.18 Impact Factor
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ABSTRACT: Attending to a stimulus enhances its neuronal representation, even at the level of primary sensory cortex. Cross-modal modulation can similarly enhance a neuronal representation, and this process can also operate at the primary cortical level. Phase reset of ongoing neuronal oscillatory activity has been shown to be an important element of the underlying modulation of local cortical excitability in both cases. We investigated the influence of attention on oscillatory phase reset in primary auditory and visual cortices of macaques performing an intermodal selective attention task. In addition to responses "driven" by preferred modality stimuli, we noted that both preferred and nonpreferred modality stimuli could "modulate" local cortical excitability by phase reset of ongoing oscillatory activity, and that this effect was linked to their being attended. These findings outline a supramodal mechanism by which attention can control neurophysiological context, thus determining the representation of specific sensory content in primary sensory cortex.
Neuron 11/2009; 64(3):419-30. · 14.74 Impact Factor
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ABSTRACT: Studies in humans and monkeys report widespread multisensory interactions at or near primary visual and auditory areas of neocortex. The range and scale of these effects has prompted increased interest in interconnectivity between the putatively "unisensory" cortices at lower hierarchical levels. Recent anatomical tract-tracing studies have revealed direct projections from auditory cortex to primary visual area (V1) and secondary visual area (V2) that could serve as a substrate for auditory influences over low-level visual processing. To better understand the significance of these connections, we looked for reciprocal projections from visual cortex to caudal auditory cortical areas in macaque monkeys. We found direct projections from area prostriata and the peripheral visual representations of area V2. Projections were more abundant after injections of temporoparietal area and caudal parabelt than after injections of caudal medial belt and the contiguous areas near the fundus of the lateral sulcus. Only one injection was confined to primary auditory cortex (area A1) and did not demonstrate visual connections. The projections from visual areas originated mainly from infragranular layers, suggestive of a "feedback"-type projection. The selective localization of these connections to peripheral visual areas and caudal auditory cortex suggests that they are involved in spatial localization.
Cerebral Cortex 10/2009; 20(7):1529-38. · 6.54 Impact Factor
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ABSTRACT: Multivariate neural recordings are becoming commonplace. Statistical techniques such as Granger causality promise to reveal
the patterns of neural interactions and their functional significance in these data. In this chapter, we start by reviewing
the essential mathematical elements of Granger causality with special emphasis on its spectral representation. Practical issues
concerning the estimation of such measures from time series data via autoregressive models are discussed. Simulation examples
are used to illustrate the technique. Finally, we analyze local field potential recordings from the visual cortex of behaving
monkeys to address the neuronal mechanisms of the alpha oscillation.
08/2009: pages 169-189;
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ABSTRACT: Most auditory events in nature are accompanied by non-auditory signals, such as a view of the speaker's face during face-to-face communication or the vibration of a string during a musical performance. While it is known that accompanying visual and somatosensory signals can benefit auditory perception, often by making the sound seem louder, the specific neural bases for sensory amplification are still debated. In this review, we want to deal with what we regard as confusion on two topics that are crucial to our understanding of multisensory integration mechanisms in auditory cortex: (1) Anatomical Underpinnings (e.g., what circuits underlie multisensory convergence), and (2) Temporal Dynamics (e.g., what time windows of integration are physiologically feasible). The combined evidence on multisensory structure and function in auditory cortex advances the emerging view of the relationship between perception and low level multisensory integration. In fact, it seems that the question is no longer whether low level, putatively unisensory cortex is accessible to multisensory influences, but how.
Hearing research 08/2009; 258(1-2):72-9. · 2.18 Impact Factor
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ABSTRACT: Neuroelectric oscillations reflect rhythmic shifting of neuronal ensembles between high and low excitability states. In natural settings, important stimuli often occur in rhythmic streams, and when oscillations entrain to an input rhythm their high excitability phases coincide with events in the stream, effectively amplifying neuronal input responses. When operating in a 'rhythmic mode', attention can use these differential excitability states as a mechanism of selection by simply enforcing oscillatory entrainment to a task-relevant input stream. When there is no low-frequency rhythm that oscillations can entrain to, attention operates in a 'continuous mode', characterized by extended increase in gamma synchrony. We review the evidence for early sensory selection by oscillatory phase-amplitude modulations, its mechanisms and its perceptual and behavioral consequences.
Trends in Neurosciences 12/2008; 32(1):9-18. · 14.23 Impact Factor
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ABSTRACT: Field potential oscillations at approximately 10 Hz (alpha rhythm) are widely noted in the visual cortices, but their physiological mechanisms and significance are poorly understood. In vitro studies have implicated pyramidal neurons in both infragranular and supragranular layers as pacemakers. The generality of these observations for the intact brain in the behaving subject is unknown. We analyzed laminar profiles of spontaneous local field potentials and multiunit activity (MUA) recorded with linear array multielectrodes from visual areas V2, V4, and inferotemporal (IT) cortex of two macaque monkeys during performance of a sensory discrimination task. Current source density (CSD) analysis was combined with CSD-MUA coherence to identify intracortical alpha current generators and their potential for alpha pacemaking. The role of each alpha current generator was further delineated by Granger causality analyses. In V2 and V4, alpha current generators were found in all layers, with the infragranular generator acting as primary local pacemaking generator. In contrast, in IT, alpha current generators were found only in supragranular and infragranular layers, with the supragranular generator acting as primary local pacemaking generator. The amplitude of alpha activity in V2 and V4 was negatively correlated with behavioral performance, whereas the opposite was true in IT. The alpha rhythm in IT thus appears to differ from that in the lower-order cortices, both in terms of its underlying physiological mechanism and its behavioral correlates. This work may help to reconcile some of the diverse findings and conclusions on the functional significance of alpha band oscillations in the visual system.
Journal of Neuroscience 11/2008; 28(40):9976-88. · 7.11 Impact Factor
