Skip Nav Destination
Close Modal
Update search
NARROW
Format
Journal
TocHeadingTitle
Date
Availability
1-5 of 5
Karl J. Friston
Close
Follow your search
Access your saved searches in your account
Would you like to receive an alert when new items match your search?
Sort by
Journal Articles
Publisher: Journals Gateway
Journal of Cognitive Neuroscience (2015) 27 (3): 573–582.
Published: 01 March 2015
FIGURES
| View All (4)
Abstract
View article
PDF
The rubber hand illusion (RHI) paradigm—in which illusory bodily ownership is induced by synchronous tactile stimulation of a participant's (hidden) hand and a (visible) surrogate—allows one to investigate how the brain resolves conflicting multisensory evidence during perceptual inference. To identify the functional anatomy of the RHI, we used multichannel EEG, acquired under three conditions of tactile stimulation. Evoked potentials were averaged from EEG signals registered to the timing of brushstrokes to the participant's hand. The participant's hand was stroked either in the absence of an artificial hand (REAL) or synchronously with an artificial hand, which either lay in an anatomically plausible (CONGRUENT) or impossible (INCONGRUENT) position. The illusion was reliably elicited in the CONGRUENT condition. For right-hand stimulation, significant differences between conditions emerged at the sensor level around 55 msec after the brushstroke at left frontal and right parietal electrodes. Response amplitudes were smaller for illusory (CONGRUENT) compared with nonillusory (INCONGRUENT and REAL) conditions in the contralateral perirolandic region (pre- and postcentral gyri), superior and inferior parietal lobule, whereas veridical perception of the artificial hand (INCONGRUENT) amplified responses at a scalp region overlying the contralateral postcentral gyrus and inferior parietal lobule compared with the remaining two conditions. Left-hand stimulation produced similar contralateral patterns. These results are consistent with predictive coding models of multisensory integration and may reflect the attenuation of somatosensory precision that is required to resolve perceptual hypotheses about conflicting multisensory input.
Journal Articles
Publisher: Journals Gateway
Journal of Cognitive Neuroscience (2011) 23 (10): 3084–3094.
Published: 01 October 2011
FIGURES
| View All (6)
Abstract
View article
PDF
In this work, we show that electrophysiological responses during pitch perception are best explained by distributed activity in a hierarchy of cortical sources and, crucially, that the effective connectivity between these sources is modulated with pitch strength. Local field potentials were recorded in two subjects from primary auditory cortex and adjacent auditory cortical areas along the axis of Heschl's gyrus (HG) while they listened to stimuli of varying pitch strength. Dynamic causal modeling was used to compare system architectures that might explain the recorded activity. The data show that representation of pitch requires an interaction between nonprimary and primary auditory cortex along HG that is consistent with the principle of predictive coding.
Journal Articles
Publisher: Journals Gateway
Journal of Cognitive Neuroscience (2005) 17 (11): 1753–1765.
Published: 01 November 2005
Abstract
View article
PDF
Previous studies of patients with phonological and surface alexia have demonstrated a double dissociation between the reading of pseudo words and words with atypical spelling-to-sound relationships. A corresponding double dissociation in the neuronal activation patterns for pseudo words and exception words has not, however, been consistently demonstrated in normal subjects. Motivated by the literature on acquired alexia, the present study contrasted pseudo words to exception words and explored how neuronal interactions within the reading system are influenced by word type. Functional magnetic resonance imaging was used to measure neuronal responses during reading in 22 healthy volunteers. The direct comparison of reading pseudo words and exception words revealed a double dissociation within the left frontal cortex. Pseudo words preferentially increased left dorsal premotor activation, whereas exception words preferentially increased left pars triangularis activation. Critically, these areas correspond to those previously associated with phonological and semantic processing, respectively. Word-type dependent interactions between brain areas were then investigated using dynamic causal modeling. This revealed that increased activation in the dorsal premotor cortex for pseudo words was associated with a selective increase in effective connectivity from the posterior fusiform gyrus. In contrast, increased activation in the pars triangularis for exception words was associated with a selective increase in effective connectivity from the anterior fusiform gyrus. The present investigation is the first to identify distinct neuronal mechanisms for semantic and phonological contributions to reading.
Journal Articles
Publisher: Journals Gateway
Journal of Cognitive Neuroscience (2003) 15 (7): 925–934.
Published: 01 October 2003
Abstract
View article
PDF
In this study, we combined functional magnetic resonance imaging (fMRI) and dynamic causal modeling (DCM) to investigate whether object category effects in the occipital and temporal cortex are mediated by inputs from early visual cortex or parietal regions. Resolving this issue may provide anatomical constraints on theories of category specificity— which make different assumptions about the underlying neurophysiology. The data were acquired by Ishai, Ungerleider, Martin, Schouten, and Haxby (1999, 2000) and provided by the National fMRI Data Center (http://www.fmridc.org). The original authors used a conventional analysis to estimate differential effects in the occipital and temporal cortex in response to pictures of chairs, faces, and houses. We extended this approach by estimating neuronal interactions that mediate category effects using DCM. DCM uses a Bayesian framework to estimate and make inferences about the influence that one region exerts over another and how this is affected by experimental changes. DCM differs from previous approaches to brain connectivity, such as multivariate autoregressive models and structural equation modeling, as it assumes that the observed hemodynamic responses are driven by experimental changes rather than endogenous noise. DCM therefore brings the analysis of brain connectivity much closer to the analysis of regionally specific effects usually applied to functional imaging data. We used DCM to estimate the influence that V3 and the superior/inferior parietal cortex exerted over category-responsive regions and how this was affected by the presentation of houses, faces, and chairs. We found that category effects in occipital and temporal cortex were mediated by inputs from early visual cortex. In contrast, the connectivity from the superior/inferior parietal area to the category-responsive areas was unaffected by the presentation of chairs, faces, or houses. These findings indicate that category effects in the occipital and temporal cortex can be mediated by bottom–up mechanisms—a finding that needs to be embraced by models of category specificity.
Journal Articles
Publisher: Journals Gateway
Journal of Cognitive Neuroscience (2000) 12 (Supplement 2): 145–156.
Published: 01 November 2000
Abstract
View article
PDF
The effect of stimulus rate and its interaction with stimulus type on brain activity during reading was investigated using functional magnetic resonance imaging (fMRI). This (i) enabled the segregation of brain regions showing differential responses, (ii) identified the optimum experimental design parameters for maximizing sensitivity, and (iii) allowed us to evaluate further the sources of discrepancy between positron emission tomography (PET) and fMRI signals. The effect of visual word rate has already been investigated in a previous PET study. However, rate effects can be very different in PET and fMRI, as seen in previous studies of auditory word processing. In this work, we attempt to replicate rate-sensitive activations observed with PET using fMRI. Our objective was to characterize the discrepancies in regionally specific rate-sensitive effects between the two imaging modalities. Subjects were presented with words and pseudowords at varying rates while performing a silent reading task. The analysis specifically identified regions showing (i) an effect of stimulus rate on brain activity during reading; (ii) modulation of this effect by word type; and (iii) increased activity during reading relative to rest, but with no dependence on stimulus rate. The results identified similar effects of rate for words and pseudowords (no interactions between rate and word type reached significance). Irrespective of word type, strong positive linear effects of rate (i.e., activity increasing with rate) were detected in visual areas, right superior temporal gyrus, and bilateral precentral gyrus. These findings replicate the results of the previous PET study, confirming that activation in regions associated with visual processing and response generation increases with the number of stimuli. Likewise, we detected rate-independent effects reported in the previous PET study in bilateral anterior middle temporal, inferior frontal, and superior parietal regions. These results differentiate the functionally specific responses in rate-dependent and rate-independent areas. However, for negative effects of rate, fMRI did not replicate the effects seen in PET, suggesting some form of hemodynamic “rectification.” The discussion focuses on differences between evoked rCBF and BOLD signals.