Sub-centimeter language organization in the human temporal lobe

Authors:

• Adeen Flinker

• Edward F. Chang

• Nicholas M. Barbaro

• Mitchel S. Berger

• Robert T. Knight

Date: 2010

DOI: 10.1016/j.bandl.2010.09.009

PubMed: 20961611

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Abstract:

The human temporal lobe is well known to be critical for language comprehension. Previous physiological research has focused mainly on non-invasive neuroimaging and electrophysiological techniques with each approach requiring averaging across many trials and subjects. The results of these studies have implicated extended anatomical regions in peri-sylvian cortex in speech perception. These non-invasive studies typically report a spatially homogenous functional pattern of activity across several centimeters of cortex. We examined the spatiotemporal dynamics of word processing using electrophysiological signals acquired from high-density electrode arrays (4mm spacing) placed directly on the human temporal lobe. Electrocorticographic (ECoG) activity revealed a rich mosaic of language activity, which was functionally distinct at four mm separation. Cortical sites responding specifically to word and not phoneme stimuli were surrounded by sites that responded to both words and phonemes. Other sub-regions of the temporal lobe responded robustly to self-produced speech and minimally to external stimuli while surrounding sites at 4mm distance exhibited an inverse pattern of activation. These data provide evidence for temporal lobe specificity to words as well as self-produced speech. Furthermore, the results provide evidence that cortical processing in the temporal lobe is not spatially homogenous over centimeters of cortex. Rather, language processing is supported by independent and spatially distinct functional sub-regions of cortex at a resolution of at least 4mm.

Prefrontal and basal ganglia contributions to visual working memory

Authors:

• Bradley Voytek

• Robert T. Knight

Date: 2010

DOI: 10.1073/pnas.1007277107

PubMed: 20921401

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Abstract:

Visual working memory (VWM) is a remarkable skill dependent on the brain’s ability to construct and hold an internal representation of the world for later comparison with an external stimulus. The prefrontal cortex (PFC) and basal ganglia (BG) interact within a cortical and subcortical network supporting VWM. We used scalp electroencephalography in groups of patients with unilateral PFC or BG lesions to provide evidence that these regions play complementary but dissociable roles in VWM. PFC patients show behavioral and electrophysiological deficits manifested by attenuation of extrastriate attention and VWM-related neural activity only for stimuli presented to the contralesional visual field. In contrast, patients with BG lesions show behavioral and electrophysiological VWM deficits independent of the hemifield of stimulus presentation but have intact extrastriate attention activity. The results support a model wherein the PFC is critical for top-down intrahemispheric modulation of attention and VWM with the BG involved in global support of VWM processes.

The orbitofrontal cortex is necessary for temporal context memory

Authors:

• Audrey Duarte

• Richard N. Henson

• Robert T. Knight

• Tina Emery

• Kim S. Graham

Date: 2010

DOI: 10.1162/jocn.2009.21316

PubMed: 19642880

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Abstract:

Lesion and neuroimaging studies suggest that the orbitofrontal cortex (OFC) supports temporal aspects of episodic memory. However, it is unclear whether OFC contributes to the encoding and/ or retrieval of temporal context and whether it is selective for temporal relative to non-temporal (spatial) context memory. We addressed this issue with two complimentary studies: functional magnetic resonance imaging (fMRI) to measure OFC activity associated with successful temporal and spatial context memory during encoding and retrieval in healthy young participants and a neuropsychological investigation to measure changes in spatial and temporal context memory in OFC lesion patients. Imaging results revealed that OFC contributed to encoding and retrieval of associations between objects and their temporal but not their spatial contexts. Consistent with this, OFC patients exhibited impairments in temporal but not spatial source memory accuracy. These results suggest that the OFC plays a critical role in the formation and subsequent retrieval of temporal context.

Role of frontal and parietal cortices in the control of bottom-up and top-down attention in humans

Authors:

• Li Ling

• Caterina Gratton

• Dezhong Yao

• Robert T. Knight

Date: 2010

DOI: 10.1016/j.brainres.2010.05.016

PubMed: 20470762

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Abstract:

We investigated the contribution of frontal and parietal cortices to bottom-up and top-down visual attention using electrophysiological measures in humans. Stimuli consisted of triangles, each with a different color and orientation. Subjects were presented with a sample triangle which served as the target for that trial. An array was subsequently presented with the target and three additional distractor stimuli, which were constructed to induce either automatic "pop-out" (50%) or effortful "search" (50%) behavior. For pop-out, both the color and orientation of the distractors differed from the target, which attracted attention automatically. For search, only the orientation of the distractors differed from the target, so effortful attention was required. Pop-out target detection generated a P300 event-related potential (ERP) with a peak amplitude over parietal sites whereas the search condition generated a fronto-centrally distributed P300. Reaction times and associated P300 latency in frontal areas were shorter for pop-out targets than for search targets. We used time-frequency analysis to compare pop-out and search conditions, within a 200-650 ms time-window and a 4-55 Hz frequency band. There was a double dissociation, with significantly increased power from 4 to 24 Hz in parietal areas for pop-out targets and increased power from 4 to 24 Hz in frontal regions for search targets. Taken together the ERP and time-frequency results provide evidence that the control of bottom-up and top-down attention depend on differential contributions from parietal and frontal cortices.

Electrophysiological evidence for aging effects on local contextual processing

Authors:

• Noa Fogelson

• Mona Shah

• Frederique Bonnet-Brilhault

• Robert T. Knight

Date: 2010

DOI: 10.1016/j.cortex.2009.05.007

PubMed: 19559410

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Abstract:

We used event-related potentials to investigate how aging affects local contextual processing. Local context was defined as the occurrence of a short predictive series of visual stimuli before delivery of a target event. Stimuli were presented to either the left or right visual field and consisted of 15% targets (downward facing triangle) and 85% of equal numbers of three types of standards (triangles facing left, upwards and right). Recording blocks consisted of targets preceded by either randomized sequences of standards or by sequences including a three-standard predictive sequence signaling the occurrence of a subsequent target event. Subjects pressed a button in response to targets. Predictive local context affected target detection by reducing the duration of stimulus evaluation compared to detection of non-predictive random targets comparably for both young and older adults, as shown by a P3b latency shift. The peak of an earlier latency context positivity, which was seen only in the predicted compared to the random target condition, was prolonged in the older population compared to young adults. Finally, older subjects elicited a late sustained positivity in the predictive condition, not seen in the younger subjects. Taken together, theses findings suggest that local contextual effects on target detection processes are altered with age.

Trees over forests: Unpleasant stimuli compete for attention with global stimuli

Authors:

• Kaisa M. Hartikainen

• Keith H. Ogawa

• Robert T. Knight

Date: 2010

DOI: 10.1097/WNR.0b013e328336eeb3

PubMed: 20168261

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Abstract:

Whether emotional distracters call for attentional resources has been discussed in several studies. We have earlier shown that brief unpleasant distracters captured right hemisphere (RH) attentional resources as evidenced with reduced event-related potential responses and increased reaction times to nonemotional left visual field/RH targets. The aim of this study was to investigate whether emotional distracters selectively interfere with processes predominantly relying on the RH such as processing global visual features. Evoked potentials were recorded from 18 participants carrying out a visual discrimination task engaging global RH and local left hemisphere-dependent processes. Unpleasant distracters reduced global target detection-related right parietal activity. We conclude that brief unpleasant distracters compete for RH attentional resources with global level processing.

Load effects in auditory selective attention: Evidence for distinct facilitation and inhibition mechanisms

Authors:

• Aurélie Bidet-Caulet

• Constanze Mikyska

• Robert T. Knight

Date: 2010

DOI: 10.1016/j.neuroimage.2009.12.039

PubMed: 20026231

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Abstract:

It is unknown whether facilitation and inhibition of stimulus processing represent one or two mechanisms in auditory attention. We performed electrophysiological experiments in humans to address these two competing hypothesis. Participants performed an attention task under low or high memory load. Facilitation and inhibition were measured by recording electrophysiological responses to attended and ignored sounds and comparing them to responses to these same sounds when attention was considered to be equally distributed towards all sounds. We observed two late frontally distributed components: a negative one in response to attended sounds, and a positive one to ignored sounds. These two frontally distributed responses had distinct timing and scalp topographies and were differentially affected by memory load. Taken together these results provide evidence that attention-mediated top-down control reflects the activity of distinct facilitation and inhibition mechanisms.

Spatiotemporal imaging of cortical activation during verb generation and picture naming

Authors:

• Erik Edwards

• Srikantan S. Nagarajan

• Sarang S. Dalal

• Ryan T. Canolty

• Heidi E. Kirsch

• Nicholas M. Barbaro

• Robert T. Knight

Date: 2010

DOI: 10.1016/j.neuroimage.2009.12.035.

PubMed: 20026224

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Abstract:

One hundred and fifty years of neurolinguistic research has identified the key structures in the human brain that support language. However, neither the classic neuropsychological approaches introduced by Broca (1861) and Wernicke (1874), nor modern neuroimaging employing PET and fMRI has been able to delineate the temporal flow of language processing in the human brain. We recorded the electrocorticogram (ECoG) from indwelling electrodes over left hemisphere language cortices during two common language tasks, verb generation and picture naming. We observed that the very high frequencies of the ECoG (high-gamma, 70–160 Hz) track language processing with spatial and temporal precision. Serial progression of activations is seen at a larger timescale, showing distinct stages of perception, semantic association/selection, and speech production. Within the areas supporting each of these larger processing stages, parallel (or “incremental”) processing is observed. In addition to the traditional posterior vs. anterior localization for speech perception vs. production, we provide novel evidence for the role of premotor cortex in speech perception and of Wernicke’s and surrounding cortex in speech production. The data are discussed with regards to current leading models of speech perception and production, and a “dual ventral stream” hybrid of leading speech perception models is given.

Neural activity during social signal perception correlates with selfreported empathy

Authors:

• Christine I. Hooker

• Sara C. Verosky

• Laura T. Germine

• Robert T. Knight

• Mark D'Esposito

Date: 2010

DOI: 10.1016/j.brainres.2009.10.006

PubMed: 19836364

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Abstract:

Empathy is an important component of human relationships, yet the neural mechanisms that facilitate empathy are unclear. The broad construct of empathy incorporates both cognitive and affective components. Cognitive empathy includes mentalizing skills such as perspective-taking. Affective empathy consists of the affect produced in response to someone else's emotional state, a process which is facilitated by simulation or "mirroring." Prior evidence shows that mentalizing tasks engage a neural network which includes the temporoparietal junction, superior temporal sulcus, and medial prefrontal cortex. On the other hand, simulation tasks engage the fronto-parietal mirror neuron system (MNS) which includes the inferior frontal gyrus (IFG) and the somotosensory related cortex (SRC). Here, we tested whether neural activity in these two neural networks was related to self-reports of cognitive and affective empathy in daily life. Participants viewed social scenes in which the shift of direction of attention of a character did or did not change the character's mental and emotional state. As expected, the task robustly activated both mentalizing and MNS networks. We found that when detecting the character's change in mental and emotional state, neural activity in both networks is strongly related to cognitive empathy. Specifically, neural activity in the IFG, SRC, and STS were related to cognitive empathy. Activity in the precentral gyrus was related to affective empathy. The findings suggest that both simulation and mentalizing networks contribute to multiple components of empathy.

Effective Connectivity of the Fronto-parietal Network during Attentional Control

Authors:

• Liang Wang

• Xun Liu

• Kevin Guise

• Robert T. Knight

• Jamshid Ghajar

• Jin Fan

Date: 2010

DOI: 10.1162/jocn.2009.21210

PubMed: 19301995

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Abstract:

The ACC, the dorsolateral prefrontal cortex (DLPFC), and the parietal cortex near/along the intraparietal sulcus (IPS) are members of a network subserving attentional control. Our recent study revealed that these regions participate in both response anticipation and conflict processing. However, little is known about the relative contribution of these regions in attentional control and how the dynamic interactions among these regions are modulated by detection of predicted versus unpredicted targets and conflict processing. Here, we examined effective connectivity using dynamic causal modeling among these three regions during a flanker task with or without a target onset cue. We compared various models in which different connections among ACC, DLPFC, and IPS were modulated by bottom-up stimulus-driven surprise and top-down conflict processing using Bayesian model selection procedures. The most optimal of these models incorporated contextual modulation that allowed processing of unexpected (surprising) targets to mediate the influence of the IPS over ACC and DLPFC and conflict processing to mediate the influence of ACC and DLPFC over the IPS. This result suggests that the IPS plays an initiative role in this network in the processing of surprise targets, whereas ACC and DLPFC interact with each other to resolve conflict through attentional modulation implemented via the IPS.

Cortical spatiotemporal dynamics underlying phonological target detection in humans

Authors:

• Edward F. Chang

• Erik Edwards

• Srikantan S. Nagarajan

• Noa Fogelson

• Sarang S. Dalal

• Ryan T. Canolty

• Heidi E. Kirsch

• Nicholas M. Barbaro

• Robert T. Knight

Date: 2010

DOI: 10.1162/jocn.2010.21466

PubMed: 20465359

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Abstract:

Selective processing of task-relevant stimuli is critical for goal-directed behavior. We used electrocorticography to assess the spatio-temporal dynamics of cortical activation during a simple phonological target detection task, in which subjects press a button when a prespecified target syllable sound is heard. Simultaneous surface potential recordings during this task revealed a highly ordered temporal progression of high gamma (HG, 70–200 Hz) activity across the lateral hemisphere in less than 1 sec. The sequence demonstrated concurrent regional sensory processing of speech syllables in the posterior superior temporal gyrus (STG) and speech motor cortex, and then transitioned to sequential task-dependent processing from prefrontal cortex (PFC), to the final motor response in the hand sensorimotor cortex. STG activation was modestly enhanced for target over nontarget sounds, supporting a selective gain mechanism in early sensory processing, whereas PFC was entirely selective to targets, supporting its role in guiding response behavior. These results reveal that target detection is not a single cognitive event, but rather a process of progressive target selectivity that involves large-scale rapid parallel and serial processing in sensory, cognitive, and motor structures to support goal-directed human behavior.

Hemicraniectomy: A New Model for Human Electrophysiology with High Spatio-temporal Resolution

Authors:

• Bradley Voytek

• Lavi Secundo

• Aurélie Bidet-Caulet

• Shirley I. Stiver

• Alisa D. Gean

• Geoffrey T. Manley

• Robert T. Knight

Date: 2010

DOI: 10.1162/jocn.2009.21384

PubMed: 19925193

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Abstract:

Human electrophysiological research is generally restricted to scalp EEG, magneto-encephalography, and intracranial electrophysiology. Here we examine a unique patient cohort that has undergone decompressive hemicraniectomy, a surgical procedure wherein a portion of the calvaria is removed for several months during which time the scalp overlies the brain without intervening bone. We quantify the differences in signals between electrodes over areas with no underlying skull and scalp EEG electrodes over the intact skull in the same subjects. Signals over the hemicraniectomy have enhanced amplitude and greater task-related power at higher frequencies (60–115 Hz) compared with signals over skull. We also provide evidence of a metric for trial-by-trial EMG/EEG coupling that is effective over the hemicraniectomy but not intact skull at frequencies >60 Hz. Taken together, these results provide evidence that the hemicraniectomy model provides a means for studying neural dynamics in humans with enhanced spatial and temporal resolution.

Cortical representation of ipsilateral arm movements in monkey and man

Authors:

• Karunesh Ganguly

• Lavi Secundo

• Gireeja Ranade

• Amy Orsborn

• Edward F. Chang

• Dragan F. Dimitrov

• Johnathan D. Wallis

• Nicholas M. Barbaro

• Robert T. Knight

• Jose M. Carmena

Date: 2009

DOI: 10.1523/JNEUROSCI.2471-09.2009

PubMed: 19828809

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Abstract:

A fundamental organizational principle of the primate motor system is cortical control of contralateral limb movements. Motor areas also appear to play a role in the control of ipsilateral limb movements. Several studies in monkeys have shown that individual neurons in primary motor cortex (M1) may represent, on average, the direction of movements of the ipsilateral arm. Given the increasing body of evidence demonstrating that neural ensembles can reliably represent information with a high temporal resolution, here we characterize the distributed neural representation of ipsilateral upper limb kinematics in both monkey and man. In two macaque monkeys trained to perform center-outreaching movements, we found thatthe ensemble spiking activity in M1 could continuously representipsilateral limb position. Interestingly, this representation was more correlated with joint angles than hand position. Using bilateral electromyography recordings, we excluded the possibility that postural or mirror movements could exclusively account for these findings. In addition, linear methods could decode limb position from cortical field potentials in both monkeys. We also found that M1 spiking activity could control a biomimetic brain–machine interface reflecting ipsilateral kinematics. Finally, we recorded cortical field potentials from three human subjects and also consistently found evidence of a neural representation for ipsilateral movement parameters. Together, our results demonstrate the presence of a high-fidelity neural representation for ipsilateral movement and illustrates that it can be successfully incorporated into a brain–machine interface.

Multimodal effects of local context on target detection: evidence from P3b

Authors:

• Noa Fogelson

• Xue Wang

• Jeffrey B. Lewis

• Mark M. Kishiyama

• Mingzhou Ding

• Robert T. Knight

Date: 2009

DOI: 10.1162/jocn.2009.21071

PubMed: 18702574

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Abstract:

We used the P300 component to investigate how changes in local context influenced the ability to detect target stimuli. Local context was defined as the occurrence of a short predictive series of stimuli before delivery of a target event. EEG was recorded in 12 subjects during auditory and visual sessions. Stimuli were presented in the center of the auditory and visual field and consisted of 15% targets (1000 Hz tone or downward facing triangle) and 85% of equal amounts of three types of standards (1500, 2000, and 2500 Hz tones or triangles facing left, upward, and right). Recording blocks consisted of targets preceded by either randomized sequences of standards or by sequences including a three-standard predictive sequence signaling the occurrence of a subsequent target event. Subjects pressed a button in response to targets. Peak target P300 (P3b) amplitude and latency were evaluated for targets after predictive and nonpredictive sequences using conventional averaging and a novel single-trial analysis procedure. Reaction times were shorter for predictable targets than for nonpredicted targets. P3b latency was shorter for predicted targets than for nonpredictive targets, and there were no significant P3b amplitude differences between predicted and random targets, as determined by both conventional averaging and single-trial analysis. Comparable effects on amplitude and latency were observed in both the auditory and visual modalities. The results indicate that local context has differential effects on P3b amplitude and latency, and exerts modality-independent effects on cognitive processing.

Consciousness Unchained: Ethical Issues and the Vegetative and Minimally Conscious State

Authors:

• Robert T. Knight

Date: 2008

DOI: 10.1080/15265160802414524

PubMed: 18853369

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Prefrontal cortex is critical for contextual processing: evidence from brain lesions

Authors:

• Noa Fogelson

• Mona Shah

• Robert T. Knight

Date: 2009

DOI: 10.1093/brain/awp230

PubMed: 19713281

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Abstract:

We investigated the role of prefrontal cortex (PFC) in local contextual processing using a combined event-related potentials and lesion approach. Local context was defined as the occurrence of a short predictive series of visual stimuli occurring before delivery of a target event. Targets were preceded by either randomized sequences of standards or by sequences including a three-stimulus predictive sequence signalling the occurrence of a subsequent target event. PFC lesioned patients were impaired in their ability to use local contextual information. The response time for controls revealed a larger benefit for predictable targets than for random targets relative to PFC patients. PFC patients had reduced amplitude of a context-dependent positivity and failed to generate the expected P3b latency shift between predictive and non-predictive targets. These findings show that PFC patients are unable to utilize predictive local context to guide behaviour, providing evidence for a critical role of PFC in local contextual processing.

Prefrontal cortex is critical for contextual processing: evidence from brain lesions

Authors:

• Noa Fogelson

• Mona Shah

• Robert T. Knight

Date: 2009

DOI: 10.1093/brain/awp230

PubMed: 19713281

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Abstract:

We investigated the role of prefrontal cortex (PFC) in local contextual processing using a combined event-related potentials and lesion approach. Local context was defined as the occurrence of a short predictive series of visual stimuli occurring before delivery of a target event. Targets were preceded by either randomized sequences of standards or by sequences including a three-stimulus predictive sequence signalling the occurrence of a subsequent target event. PFC lesioned patients were impaired in their ability to use local contextual information. The response time for controls revealed a larger benefit for predictable targets than for random targets relative to PFC patients. PFC patients had reduced amplitude of a context-dependent positivity and failed to generate the expected P3b latency shift between predictive and non-predictive targets. These findings show that PFC patients are unable to utilize predictive local context to guide behaviour, providing evidence for a critical role of PFC in local contextual processing.

Ipsilateral trajectory control is related to contralesional paralysis after left hemisphere damage

Authors:

• Kathleen Y. Haaland

• Sydney Y. Schaefer

• Robert T. Knight

• John Adair

• Alvaro Magalhaes

• Joseph Sadek

• Robert L. Sainburg

Date: 2009

DOI: 10.1007/s00221-009-1836-z

PubMed: 19479246

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Abstract:

We have recently shown ipsilateral dynamic deficits in trajectory control are present in left hemisphere damaged (LHD) patients with paresis, as evidenced by impaired modulation of torque amplitude as response amplitude increases. The purpose of the current study is to determine if these ipsilateral deficits are more common with contralateral hemiparesis and greater damage to the motor system, as evidenced by structural imaging. Three groups of right-handed subjects (healthy controls, LHD stroke patients with and without upper extremity paresis) performed single-joint elbow movements of varying amplitudes with their left arm in the left hemispace. Only the paretic group demonstrated dynamic deficits characterized by decreased modulation of peak torque (reflected by peak acceleration changes) as response amplitude increased. These results could not be attributed to lesion volume or peak velocity as neither variable differed across the groups. However, the paretic group had damage to a larger number of areas within the motor system than the non-paretic group suggesting that such damage increases the probability of ipsilesional deficits in dynamic control for modulating torque amplitude after left hemisphere damage.

Temporal characteristics of audiovisual information processing

Authors:

• Galit Fuhrmann Alpert

• Grit Hein

• Nancy Tsai

• Marcus J. Naumer

• Robert T. Knight

Date: 2008

DOI: 10.1523/JNEUROSCI.5039-07.2008

PubMed: 18480290

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Abstract:

In complex natural environments, auditory and visual information often have to be processed simultaneously. Previous functional magnetic resonance imaging (fMRI) studies focused on the spatial localization of brain areas involved in audiovisual (AV) information processing, but the temporal characteristics of AV information flow in these regions remained unclear. In this study, we used fMRI and a novel information–theoretic approach to study the flow of AV sensory information. Subjects passively perceived sounds and images of objects presented either alone or simultaneously. Applying the measure of mutual information, we computed for each voxel the latency in which the blood oxygenation level-dependent signal had the highest information content about the preceding stimulus. The results indicate that, after AV stimulation, the earliest informative activity occurs in right Heschl’s gyrus, left primary visual cortex, and the posterior portion of the superior temporal gyrus, which is known as a region involved in object-related AV integration. Informative activity in the anterior portion of superior temporal gyrus, middle temporal gyrus, right occipital cortex, and inferior frontal cortex was found at a later latency. Moreover, AV presentation resulted in shorter latencies in multiple cortical areas compared with isolated auditory or visual presentation. The results provide evidence for bottom-up processing from primary sensory areas into higher associ- ation areas during AV integration in humans and suggest that AV presentation shortens processing time in early sensory cortices.

Novelty Enhancements in Memory Are Dependent on Lateral Prefrontal Cortex

Authors:

• Mark M. Kishiyama

• Andrew P. Yonelinas

• Robert T. Knight

Date: 2009

DOI: 10.1523/JNEUROSCI.5507-08.2009

PubMed: 19553451

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Abstract:

Physiological evidence indicates thatseveral brain regions, including the medial temporal lobes and prefrontal cortex (PFC), are involved in processing events that are novel or distinctive in their immediate context. However, behavioral studies that investigate whether these regions are critical for producing stimulus novelty advantages in memory are limited. For example, evidence from an animal lesion study indicated that the PFC is involved in stimulus novelty effects, but this has not been examined in humans. In the current study, we used a von Restorff novelty paradigm to testa large cohortof lateral PFC patients (n
16). We found thatpatients with lateral PFC damage were impaired in recollection- and familiarity-based recognition, and they did not exhibit a normal memory advantage for novel compared with non-novel items. These results provide neuropsychological evidence supporting a key role for the lateral PFC in producing stimulus novelty advantages in memory.