Biomedical subjects

Christopher Summerfield

Publications and source records attributed to Christopher Summerfield.

11 recordsLinked to original sources

PubMed2006

Predictive codes for forthcoming perception in the frontal cortex.

Incoming sensory information is often ambiguous, and the brain has to make decisions during perception. "Predictive coding" proposes that the brain resolves perceptual ambiguity by anticipating the forthcoming sensory environment, generating a template against which to match observed sensory evidence. We observed a neural representation of predicted perception in the medial frontal cortex, while human subjects decided whether visual objects were faces or not. Moreover, perceptual decisions about faces were associated with an increase in top-down connectivity from the frontal cortex to face-sensitive visual areas, consistent with the matching of predicted and observed evidence for the presence of faces.

Amygdala↗

PubMed2006

Neocortical connectivity during episodic memory formation.

During the formation of new episodic memories, a rich array of perceptual information is bound together for long-term storage. However, the brain mechanisms by which sensory representations (such as colors, objects, or individuals) are selected for episodic encoding are currently unknown. We describe a functional magnetic resonance imaging experiment in which participants encoded the association between two classes of visual stimuli that elicit selective responses in the extrastriate visual cortex (faces and houses). Using connectivity analyses, we show that correlation in the hemodynamic signal between face- and place-sensitive voxels and the left dorsolateral prefrontal cortex is a reliable predictor of successful face-house binding. These data support the view that during episodic encoding, "top-down" control signals originating in the prefrontal cortex help determine which perceptual information is fated to be bound into the new episodic memory trace.

Adult↗

PubMed2006

Dissociable neural mechanisms for encoding predictable and unpredictable events.

Attention is a necessary condition for the formation of new episodic memories, yet little is known about how dissociable attentional mechanisms for "top-down" and "bottom-up" orienting contribute to encoding. Here, subjects performed an intentional encoding task in which to-be-learned items were interspersed with irrelevant stimuli such that subjects could anticipate the appearance of some study items but not others. Subjects were more likely to later remember stimuli whose appearance was predictable at encoding. Electroencephalographic data were acquired during the study phase of the experiment to assess how synchronous neural activity related to later memory for predictable stimuli (to which attention could be oriented in a top-down fashion) and unpredictable stimuli (which rely to a greater extent on bottom-up attentional orienting). Over left frontal regions, gamma-band activity (25-55 Hz) early (approximately 150 msec) in the epoch was a robust predictor of later memory for predictable items, consistent with an emerging view that links high-frequency neural synchrony to top-down attention. By contrast, later (approximately 400 msec) theta-band activity (4-8 Hz) over the left and midline frontal cortex predicted subsequent memory for unpredictable items, suggesting a role in bottom-up attentional orienting. These results reveal for the first time the contribution of dissociable attentional mechanisms to successful encoding and contribute to a growing literature dedicated to understanding the role of neural synchrony in cognition.

Adolescent↗

PubMed2005

Mistaking a house for a face: neural correlates of misperception in healthy humans.

Individuals with normal vision can sometimes momentarily mistake one object for another. In this functional magnetic resonance imaging study, we investigated how extrastriate visual regions respond during these erroneous perceptual judgements. Subjects were asked to discriminate images of houses and faces that were degraded such that they were close to an individually defined threshold for perception. On correct trials, voxels localized on the inferior occipital (OFA), fusiform (FFA) and parahippocampal (PPA) gyri exhibited selectivity for face and house images as expected. On incorrect trials, no face- or place-selectivity was observed for OFA or PPA. However, consistent with 'predictive coding' accounts of perception, we observed that the FFA also responded robustly on trials where a house was misperceived as a face, and concurrent activation was observed in medio-frontal and right parietal regions previously implicated in decision making under uncertainty. We suggest that FFA responses during misperception may be driven by a predictive top-down signal from these regions.

Adult↗

PubMed2005

Functional coupling between frontal and parietal lobes during recognition memory.

Neuroimaging studies have suggested that the frontal and parietal lobes may be important for the process by which we remember information. However, little is known about how these regions exchange information during memory retrieval. We measured EEG synchronisation in the gamma-band (25-55 Hz), a putative measure of functional coupling between brain regions, while human subjects performed a recognition memory task. Fronto-parietal synchrony was increased for true old memories relative to false memories and new items. Our results suggest that synchronization of neuronal responses in the gamma-band may be an important mechanism by which frontal and parietal regions exchange information during the recognition of past events.

Adult↗

PubMed2005

Coherent theta-band EEG activity predicts item-context binding during encoding.

Episodic memories consist of semantic information coupled with a rich array of contextual detail. Here, we investigate the neural processes by which information about the sensory context of a learning event is "bound" to the semantic representation of the to-be-encoded item. We present evidence that item-context binding during encoding is mediated by frontoposterior electroencephalographic (EEG) phase locking within and between hemispheres in the theta (4-8 Hz) band. During a task in which subjects encoded words in different font colors, later memory for the word was associated with sustained frontal theta activity and frontoposterior theta-band coherence, primarily within the left hemisphere. When the word-color association was later successfully retrieved, however, neurons synchronized their theta-band responses bilaterally in a more sustained fashion, particularly during the latter part of the stimulus epoch (>800 ms). Our results confirm the importance of functional coupling between frontal and posterior regions for successful encoding. One interpretation of these data is hemispheric contributions to item and context encoding may be asymmetric, with left hemisphere coherence facilitating semantic processing of an item and right hemisphere coherence facilitating processing of sensory context. Theta-band coherence may be an important mechanism by which brain networks exchange information during learning.

Adult↗

PubMed2005

Structural brain changes in Parkinson disease with dementia: a voxel-based morphometry study.

BACKGROUND: Parkinson disease with dementia (PDD) results from neuropathological changes in cortical and subcortical brain regions. Voxel-based morphometric analysis of magnetic resonance images can contribute to in vivo identification of the cerebral regions predominantly involved in PDD. OBJECTIVE: To identify structural cerebral regions most closely related to the presence of PDD. DESIGN: Magnetic resonance images were obtained from 16 patients who had PDD, 13 patients with PD without dementia, and 13 age-matched healthy control subjects. Gray matter volumes were compared using optimized voxel-based morphometric analyses. RESULTS: Compared with healthy controls, patients with PDD showed gray matter volume decreases in several of the following regions: bilateral putamen, accumbens nuclei, left side of the thalamus, bilateral hippocampus, parahippocampal region, and anterior cingulate gyrus. Patients with PD also showed gray matter reductions compared with healthy controls in the right side of the hippocampus, left anterior cingulate gyrus, and left superior temporal gyrus. CONCLUSIONS: The hippocampus, thalamus, and anterior cingulate are the regions most affected in PDD. Our results agree with recent neuropathological findings suggesting the involvement of the limbic and cortical areas in PD.

Aged↗

PubMed2005

Amygdalar and hippocampal MRI volumetric reductions in Parkinson's disease with dementia.

Parkinson's disease (PD) involves neuropathological changes in the limbic system that lead to neuronal loss and volumetric reductions of several nuclei. We investigated possible volumetric reductions of the amygdala and hippocampus associated to PD. We carried out magnetic resonance imaging (MRI) volumetric studies in 16 patients with PD and dementia (PDD), 16 patients with PD without dementia (PD), and 16 healthy subjects. The general analysis of variance (ANOVA) showed a significant group effect (for the amygdala, P = 0.01; for the hippocampus, P = 0.005). A post-hoc test demonstrated that the differences were due to PDD and control group comparisons for the amygdala (P = 0.008) and for the hippocampus (P = 0.004). In nondemented PD subjects, we observed an 11% reduction in the amygdala and a 10% reduction in the hippocampus compared with that in controls. In summary, demented PD patients have clear amygdalar and hippocampal atrophy that remains statistically significant after controlling for global cerebral atrophy. Nondemented PD patients also showed a degree of volumetric reduction in these structures although the differences were not statistically significant.

Aged↗

PubMed2004

Effects of subthalamic nucleus stimulation on characteristics of EMG activity underlying reaction time in Parkinson's disease.

We examined the effects of high-frequency deep brain stimulation of the subthalamic nucleus (STN-DBS) on characteristics of electromyographic (EMG) activity of the agonist muscle in 8 patients with Parkinson's disease (PD). Patients were examined during STN-DBS (ON), and 30 minutes after switching off both stimulators (OFF). They were asked to make a ballistic movement in paradigms of simple reaction time (SRT) and choice reaction time (CRT) tasks. Onset of movement (MOVonset) was measured as the latency of the initial displacement from baseline of the signal from an accelerometer attached to the dorsum of the hand. In the associated EMG activity, recorded from wrist extensor muscles, we measured onset latency (EMGonset), size of the first EMG burst (EMGsize), and number of EMG bursts (EMGbursts) counted between EMGonset and task execution. MOVonset and EMGonset were significantly shorter in ON than in OFF conditions in CRT. EMGsize was larger, EMGbursts were reduced, and peak of the acceleration profile was larger in ON compared with OFF conditions in both SRT and CRT. Our results indicate that STN-DBS induces a significant improvement in motor performance of reaction time tasks in PD patients. Such improvement is associated with a change in features of the EMG activity suggesting an increase in the excitability of the motor pathways engaged in ballistic movements.

Aged↗

PubMed2002

Dementia in Parkinson disease: a proton magnetic resonance spectroscopy study.

BACKGROUND: Magnetic resonance spectroscopy has been shown to be useful in differentiating idiopathic Parkinson disease (PD) from atypical parkinsonian syndromes such as progressive supranuclear palsy, multiple system atrophy, and corticobasal degeneration. OBJECTIVE: To systematically investigate the utility of proton magnetic resonance spectroscopy in distinguishing between idiopathic PD with dementia (PDD) and without dementia. DESIGN: Group comparisons and correlations of brain metabolites with clinical and neuropsychological variables. PATIENTS AND METHODS: Metabolite concentrations were acquired from voxels localized to the basal ganglia and occipital cortex in 14 patients diagnosed as having idiopathic PDD, 12 patients with PD without dementia, and 13 matched control subjects. The 3 groups underwent clinical and neuropsychological assessment. RESULTS: In the occipital region, N-acetylaspartate levels were significantly reduced in the PDD group relative to the PD and control groups. N-acetylaspartate values correlated with neuropsychological performance but not with severity of motor impairment. CONCLUSIONS: N-acetylaspartate reduction in occipital lobes may be a marker for dementia in PD. The distribution of metabolite reduction differs from that reported in Alzheimer disease. These findings suggest that proton spectroscopy may serve as a metabolic marker of cognitive disturbance in patients with PD.

Aged↗

PubMed2002

Induced gamma activity is associated with conscious awareness of pattern masked nouns.

The aim of this study was to explore the relationship between electroencephalographic (EEG) activity in the gamma frequency range and conscious awareness of a visual stimulus. EEG was recorded from subjects while they were shown backward-masked words only some of which they were able to discriminate correctly. The results showed that activity in the gamma frequency range increased with reported awareness of a word independently of whether it was correctly discriminated or not. It is concluded that gamma power is associated with awareness-dependent visual processing but not with processing that occurs in the absence of awareness.

Adolescent↗