Biomedical subjects

Maurizio Corbetta

Publications and source records attributed to Maurizio Corbetta.

25 records · Page 2Linked to original sources

PubMed2003

Identification of cerebral networks by classification of the shape of BOLD responses.

Changes in regional blood oxygen level dependent (BOLD) signals in response to brief visual stimuli can exhibit a variety of time-courses. To demonstrate the anatomical distribution of BOLD response shapes during a match to sample task, a formal analysis of their time-courses is presented. An event-related design was used to estimate regional BOLD responses evoked by a cue word, which instructed the subject to attend to the motion or color of an upcoming target, and those evoked by a briefly presented moving target consisting of colored dots. Regional BOLD time-courses were adequately represented by the linear combination of three orthogonal waveforms. BOLD response shapes were then classified using a fuzzy clustering scheme. Three classes (sustained, phasic, and negative) best characterized cue responses. Four classes (sustained, sustained-phasic, phasic, and bi-phasic) best characterized target responses. In certain regions, the shape of the BOLD responses was modulated by the instruction to attend to the target's motion or color. A left frontal and a posterior parietal region showed sustained activity when motion was cued and transient activity when color was cued. A right thalamic and a left lateral occipital region showed sustained activity when color was cued and transient activity when motion was cued. Following the target several regions showed more sustained activity during motion than color trials. In summary, the effect of the task variable was focal following the cue and widespread following the target. We conclude that the temporal patterns of neural activity affected the shape of the BOLD signal.

Brain↗

PubMed2002

Functional reorganization and stability of somatosensory-motor cortical topography in a tetraplegic subject with late recovery.

The functional organization of somatosensory and motor cortex was investigated in an individual with a high cervical spinal cord injury, a 5-year absence of nearly all sensorymotor function at and below the shoulders, and rare recovery of some function in years 6-8 after intense and sustained rehabilitation therapies. We used functional magnetic resonance imaging to study brain activity to vibratory stimulation and voluntary movements of body parts above and below the lesion. No response to vibratory stimulation of the hand was observed in the primary somatosensory cortex (SI) hand area, which was conversely recruited during tongue movements that normally evoke responses only in the more lateral face area. This result suggests SI reorganization analogous to previously reported neuroplasticity changes after peripheral lesions in animals and humans. In striking contradistinction, vibratory stimulation of the foot evoked topographically appropriate responses in SI and second somatosensory cortex (SII). Motor cortex responses, tied to a visuomotor tracking task, displayed a near-typical topography, although they were more widespread in premotor regions. These findings suggest possible preservation of motor and some somatosensory cortical representations in the absence of overt movements or conscious sensations for several years after spinal cord injury and have implications for future rehabilitation and neural-repair therapies.

Adult↗

PubMed2002

Word retrieval learning modulates right frontal cortex in patients with left frontal damage.

Previous studies have suggested that recovery or compensation of language function after a lesion in the left hemisphere may depend on mechanisms in the right hemisphere. However, a direct relationship between performance and right hemisphere activity has not been established. Here, we show that patients with left frontal lesions and partially recovered aphasia learn, at a normal rate, a novel word retrieval task that requires the damaged cortex. Verbal learning is accompanied by specific response decrements in right frontal and right occipital cortex, strongly supporting the compensatory role of the right hemisphere. Furthermore, responses in left occipital cortex are abnormal and not modulated by practice. These findings indicate that frontal cortex is a source of top-down signals during learning.

Aphasia, Broca↗

PubMed2002

Neural systems for visual orienting and their relationships to spatial working memory.

We investigated neural correlates of human visual orienting using event-related functional magnetic resonance imaging (fMRI). When subjects voluntarily directed attention to a peripheral location, we recorded robust and sustained signals uniquely from the intraparietal sulcus (IPs) and superior frontal cortex (near the frontal eye field, FEF). In the ventral IPs and FEF only, the blood oxygen level dependent signal was modulated by the direction of attention. The IPs and FEF also maintained the most sustained level of activation during a 7-sec delay, when subjects maintained attention at the peripheral cued location (working memory). Therefore, the IPs and FEF form a dorsal network that controls the endogenous allocation and maintenance of visuospatial attention. A separate right hemisphere network was activated by the detection of targets at unattended locations. Activation was largely independent of the target's location (visual field). This network included among other regions the right temporo-parietal junction and the inferior frontal gyrus. We propose that this cortical network is important for reorienting to sensory events.

Adolescent↗

PubMed2002

Control of goal-directed and stimulus-driven attention in the brain.

We review evidence for partially segregated networks of brain areas that carry out different attentional functions. One system, which includes parts of the intraparietal cortex and superior frontal cortex, is involved in preparing and applying goal-directed (top-down) selection for stimuli and responses. This system is also modulated by the detection of stimuli. The other system, which includes the temporoparietal cortex and inferior frontal cortex, and is largely lateralized to the right hemisphere, is not involved in top-down selection. Instead, this system is specialized for the detection of behaviourally relevant stimuli, particularly when they are salient or unexpected. This ventral frontoparietal network works as a 'circuit breaker' for the dorsal system, directing attention to salient events. Both attentional systems interact during normal vision, and both are disrupted in unilateral spatial neglect.

Animals↗

PubMed2002

Two attentional processes in the parietal lobe.

We report fMRI evidence for two attentional processes in parietal cortex. Subjects matched a feature, cued by a word, to a test display of moving colored dots. Either color (red, green) or motion direction (left, right) was cued on mixed scans while only one dimension was cued on blocked scans. An event-related paradigm separated the preparatory activity generated by the cue from the subsequent activity related to the test display. One attentional process specified task information while a second process was motion selective. During the cue period, a pure effect of task specification was observed in left frontal cortex while combined effects of task specification and motion selectivity were observed in left posterior parietal cortex. The frontal task-specification signal may have been the source of the corresponding signal in parietal cortex. Effects of task specification generalized over cue dimension, indicating that the information was coded in a sufficiently abstract form to affect color and motion processing. During the subsequent test period, task-specification and motion-selective signals were again observed in left parietal cortex. Task specification did not significantly affect occipital motion-selective regions, such as MT+, however, indicating that this process did not influence the lower cortical tier of the motion processing stream. These results provide evidence for general and specialized task representations within left parietal cortex during task preparation and execution.

Analysis of Variance↗

PubMed2002

Reactivation of networks involved in preparatory states.

We report an endogenous signal that has a widespread cortical distribution and is time-locked to the termination of a sustained state of task-readiness. In three event-related functional magnetic resonance imaging (fMRI) experiments, subjects saw an arrow cue that predicted either the direction of motion or the location of a subsequent test stimulus. A reactivation of the BOLD (blood oxygenation level-dependent) signal occurred at the termination of the state of readiness in occipital regions that were transiently activated by the cue and in frontal-parietal regions that maintained an attentional set over the trial. Moreover, a delayed activation occurred in prefrontal and temporo-parietal regions that did not initially respond to the cue and that have been implicated in re-orienting attention to novel sensory events. These latter regions may have generated control signals that ended the state of readiness in regions active during the cue period. These results indicate that terminating a state of readiness produces a widely distributed cortical signal and suggest that areas involved in a preparatory state may be maintained as a network which can be modulated as a whole.

Attention↗