The medial temporal lobe supports conceptual implicit memory

Authors:

• Wei-Chun Wang

• Michele Lazzara

• Charan Ranganath

• Robert T. Knight

• Andrew P. Yonelinas

Date: 2010

DOI: 10.1016/j.neuron.2010.11.009

PubMed: 21144998

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Abstract:

The medial temporal lobe (MTL) is generally thought to be critical for explicit, but not implicit, memory. Here, we demonstrate that the perirhinal cortex (PRc), within the MTL, plays a role in conceptuallydriven implicit memory. Amnesic patients with MTL lesions that converged on the left PRc exhibited defi- cits on two conceptual implicit tasks (i.e., exemplar generation and semantic decision). A separate functional magnetic resonance imaging (fMRI) study in healthy subjects indicated that PRc activation during encoding of words was predictive of subsequent exemplar generation. Moreover, across subjects, the magnitude of the fMRI and behavioral conceptual priming effects were directly related. Additionally, the PRc region implicated in the fMRI study was the same region of maximal lesion overlap in the patients with impaired conceptual priming. These patient and imaging results converge to suggest that the PRc plays a critical role in conceptual implicit memory, and possibly conceptual processing in general.

Single-trial speech suppression of auditory cortex activity in humans.

Authors:

• Adeen Flinker

• Edward F. Chang

• Heidi E. Kirsch

• Nicholas M. Barbaro

• Nathan E. Crone

• Robert T. Knight

Date: 2010

PubMed: 21148003

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Abstract:

The human auditory cortex is engaged in monitoring the speech of interlocutors as well as self-generated speech. During vocalization, auditory cortex activity is reported to be suppressed, an effect often attributed to the influence of an efference copy from motor cortex. Single-unit studies in non-human primates have demonstrated a rich dynamic range of single-trial auditory responses to self-speech consisting of suppressed, nonsuppressed and excited auditory neurons. However, human research using noninvasive methods has only reported suppression of averaged auditory cortex responses to self-generated speech. We addressed this discrepancy by recording electrocorticographic activity from neurosurgical subjects performing auditory repetition tasks. We observed that the degree of suppression varied across different regions of auditory cortex, revealing a variety of suppressed and nonsuppressed responses during vocalization. Importantly, single-trial high-gamma power (γ(High), 70-150 Hz) robustly tracked individual auditory events and exhibited stable responses across trials for suppressed and nonsuppressed regions.

The functional role of cross-frequency coupling

Authors:

• Ryan T. Canolty

• Robert T. Knight

Date: 2010

DOI: 10.1016/j.tics.2010.09.00

PubMed: 20932795

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Abstract:

Recent studies suggest that cross-frequency coupling (CFC) might play a functional role in neuronal computation, communication and learning. In particular, the strength of phase-amplitude CFC differs across brain areas in a task-relevant manner, changes quickly in response to sensory, motor and cognitive events, and correlates with performance in learning tasks. Importantly, whereas high-frequency brain activity reflects local domains of cortical processing, low-frequency brain rhythms are dynamically entrained across distributed brain regions by both external sensory input and internal cognitive events. CFC might thus serve as a mechanism to transfer information from large-scale brain networks operating at behavioral timescales to the fast, local cortical processing required for effective computation and synaptic modification, thus integrating functional systems across multiple spatiotemporal scales

Shifts in gamma phase-amplitude coupling frequency from theta to alpha over posterior cortex during visual tasks

Authors:

• Bradley Voytek

• Ryan T. Canolty

• Avgusta Shestyuk

• Nathan E. Crone

• Josef Parvizi

• Robert T. Knight

Date: 2010

DOI: 10.3389/fnhum.2010.00191

PubMed: 21060716

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Abstract:

The phase of ongoing theta (4–8 Hz) and alpha (8–12 Hz) electrophysiological oscillations is coupled to high gamma (80–150 Hz) amplitude, which suggests that low-frequency oscillations modulate local cortical activity. While this phase–amplitude coupling (PAC) has been demonstrated in a variety of tasks and cortical regions, it has not been shown whether task demands differentially affect the regional distribution of the preferred low-frequency coupling to high gamma. To address this issue we investigated multiple-rhythm theta/alpha to high gamma PAC in two subjects with implanted subdural electrocorticographic grids. We show that high gamma amplitude couples to the theta and alpha troughs and demonstrate that, during visual tasks, alpha/high gamma coupling preferentially increases in visual cortical regions. These results suggest that low-frequency phase to high-frequency amplitude coupling is modulated by behavioral task and may reflect a mechanism for selection between communicating neuronal networks.

Categorical speech representation in the human superior temporal gyrus

Authors:

• Edward F. Chang

• Jochem W. Rieger

• Keith Johnson

• Mitchel S. Berger

• Nicholas M. Barbaro

• Robert T. Knight

Date: 2010

DOI: 10.1038/nn.264

PubMed: 20890293

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Abstract:

Speech perception requires the rapid and effortless extraction of meaningful phonetic information from a highly variable acoustic signal. A powerful example of this phenomenon is categorical speech perception, in which a continuum of acoustically varying sounds is transformed into perceptually distinct phoneme categories. We found that the neural representation of speech sounds is categorically organized in the human posterior superior temporal gyrus. Using intracranial high-density cortical surface arrays, we found that listening to synthesized speech stimuli varying in small and acoustically equal steps evoked distinct and invariant cortical population response patterns that were organized by their sensitivities to critical acoustic features. Phonetic category boundaries were similar between neurometric and psychometric functions. Although speech-sound responses were distributed, spatially discrete cortical loci were found to underlie specific phonetic discrimination. Our results provide direct evidence for acoustic-to–higher order phonetic level encoding of speech sounds in human language receptive cortex.

Categorical speech representation in human superior temporal gyrus

Authors:

• Edward F. Chang

• Jochem W. Rieger

• Keith Johnson

• Mitchel S. Berger

• Nicholas M. Barbaro

• Robert T. Knight

Date: 2010

PubMed: 20890293

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Abstract:

Speech perception requires the rapid and effortless extraction of meaningful phonetic information from a highly variable acoustic signal. A powerful example of this phenomenon is categorical speech perception, in which a continuum of acoustically varying sounds is transformed into perceptually distinct phoneme categories. We found that the neural representation of speech sounds is categorically organized in the human posterior superior temporal gyrus. Using intracranial high-density cortical surface arrays, we found that listening to synthesized speech stimuli varying in small and acoustically equal steps evoked distinct and invariant cortical population response patterns that were organized by their sensitivities to critical acoustic features. Phonetic category boundaries were similar between neurometric and psychometric functions. Although speech-sound responses were distributed, spatially discrete cortical loci were found to underlie specific phonetic discrimination. Our results provide direct evidence for acoustic-to–higher order phonetic level encoding of speech sounds in human language receptive cortex.

Categorical speech representation in human superior temporal gyrus

Authors:

• Edward F. Chang

• Jochem W. Rieger

• Keith Johnson

• Mitchel S. Berger

• Nicholas M. Barbaro

• Robert T. Knight

Date: 2010

PubMed: 20890293

View PDF

Abstract:

Speech perception requires the rapid and effortless extraction of meaningful phonetic information from a highly variable acoustic signal. A powerful example of this phenomenon is categorical speech perception, in which a continuum of acoustically varying sounds is transformed into perceptually distinct phoneme categories. We found that the neural representation of speech sounds is categorically organized in the human posterior superior temporal gyrus. Using intracranial high-density cortical surface arrays, we found that listening to synthesized speech stimuli varying in small and acoustically equal steps evoked distinct and invariant cortical population response patterns that were organized by their sensitivities to critical acoustic features. Phonetic category boundaries were similar between neurometric and psychometric functions. Although speech-sound responses were distributed, spatially discrete cortical loci were found to underlie specific phonetic discrimination. Our results provide direct evidence for acoustic-to–higher order phonetic level encoding of speech sounds in human language receptive cortex.

Dynamic neuroplacticity after human prefrontal cortex damage

Authors:

• Bradley Voytek

• Matar Davis

• Elena Yago

• Francisco Barceló

• Edward K. Vogel

• Robert T. Knight

Date: 2010

DOI: 10.1016/j.neuron.2010.09.018

PubMed: 21040843

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Abstract:

Memory and attention deficits are common after prefrontal cortex (PFC) damage, yet people generally recover some function over time. Recovery is thought to be dependent upon undamaged brain regions, but the temporal dynamics underlying cognitive recovery are poorly understood. Here, we provide evidence that the intact PFC compensates for damage in the lesioned PFC on a trial-by-trial basis dependent on cognitive load. The extent of this rapid functional compensation is indexed by transient increases in electrophysiological measures of attention and memory in the intact PFC, detectable within a second after stimulus presentation and only when the lesioned hemisphere is challenged. These observations provide evidence supporting a dynamic and flexible model of compensatory neural plasticity.

Poverty, stress and cognitive functioning

Authors:

• Mark M. Kishiyama

• Robert T. Knight

Date: 2010

Abstract:

Social inequalities in income and wealth have a profound effect on the physical and mental health of children. For example, children from low-socioeconomic-status (SES) backgrounds are at greater risk for most forms of childhood morbidities compared to children from higher-SES backgrounds. [Note that the determination of SES is based on varying indices of family income (in relation to federally determined poverty thresholds), parental education (for example, those with college degrees versus those without), and parental occupation (for example, professional versus unskilled labor)]. Impoverished conditions during childhood are associated with poorer adult health, and low childhood SES may be the single most powerful contributor to premature mortality and morbidity worldwide. Significant relationships have been observed between SES and cognitive ability and between SES and academic achievement in childhood. In fact, SES has a stronger relationship with cognitive performance than does physical health. Children from low-SES backgrounds perform below children from higher-SES backgrounds on tests of intelligence and academic achievement. In addition, SES has been found to have a major effect on language development. For example, one study found that the average vocabulary size of 3-year-old children from families receiving welfare was less than half the size of the average vocabulary of children from higher-SES (professional) families. Low-SES children are also more likely to fail courses, be placed in special education, and drop out of high school compared to high-SES children.

Faces are special but not too special: spared face recognition in amnesia is based on familiarity

Authors:

• Mariam Aly

• Robert T. Knight

• Andrew P. Yonelinas

Date: 2010

DOI: 10.1016/j.neuropsychologia.2010.09.005

PubMed: 20833190

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Abstract:

Most current theories of human memory are material-general in the sense that they assume that the medial temporal lobe (MTL) is important for retrieving the details of prior events, regardless of the specific type of materials. Recent studies of amnesia have challenged the material-general assumption by suggesting that the MTL may be necessary for remembering words, but is not involved in remembering faces. We examined recognition memory for faces and words in a group of amnesic patients, which included hypoxic patients and patients with extensive left or right MTL lesions. Recognition confidence judgments were used to plot receiver operating characteristics (ROCs) in order to more fully quantify recognition performance and to estimate the contributions of recollection and familiarity. Consistent with the extant literature, an analysis of overall recognition accuracy showed that the patients were impaired at word memory but had spared face memory. However, the ROC analysis indicated that the patients were generally impaired at high confidence recognition responses for faces and words, and they exhibited significant recollection impairments for both types of materials. Familiarity for faces was preserved in all patients, but extensive left MTL damage impaired familiarity for words. These results show that face recognition may appear to be spared because performance tends to rely heavily on familiarity, a process that is relatively well preserved in amnesia. In addition, the findings challenge material-general theories of memory, and suggest that both material and process are important determinants of memory performance in amnesia.

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.