Biomedical subjects

A C Nobre

Publications and source records attributed to A C Nobre.

At least 19 recordsLinked to original sources

PubMed2007

Temporal order is coded temporally in the brain: early event-related potential latency shifts underlying prior entry in a cross-modal temporal order judgment task.

The speeding-up of neural processing associated with attended events (i.e., the prior-entry effect) has long been proposed as a viable mechanism by which attention can prioritize our perception and action. In the brain, this has been thought to be regulated through a sensory gating mechanism, increasing the amplitudes of early evoked potentials while leaving their latencies unaffected. However, the majority of previous research has emphasized speeded responding and has failed to emphasize fine temporal discrimination, thereby potentially lacking the sensitivity to reveal putative modulations in the timing of neural processing. In the present study, we used a cross-modal temporal order judgment task while shifting attention between the visual and tactile modalities to investigate the mechanisms underlying selective attention electrophysiologically. Our results indicate that attention can indeed speed up neural processes during visual perception, thereby providing the first electrophysiological support for the existence of prior entry.

Adolescent↗

PubMed2005

Modulation of brain activity by selective task sets observed using event-related potentials.

We investigated the ability of subjects to shift dynamically between selective task sets, using informative trial-by-trial cues. Two tasks were used which involved non-overlapping neural systems and different hemispheric specialization. In a verbal task, subjects decided whether a letter string was a real word or a non-word. In a spatial task, subjects decided whether an angle was acute or obtuse. A behavioural experiment showed that performance improved when cues predicted the upcoming task (80% validity), compared to when neutral cues did not afford selective task sets. Event-related potentials (ERPs) revealed brain activity related to forming selective task expectations, to switching tasks, and to the modulation of target processing as a function of such expectations and switches. Activity predicting the probable task started over parietal electrodes 160 ms after cue presentation, while activity related to task switching started at frontal electrodes around 280 ms. Both types of activities developed before target onset. Target processing was significantly influenced by the validity of the cue prediction, including strong modulation of language-related potentials. These results show that it is possible to switch dynamically between task sets involving distinct neural systems, even before the appearance of an imperative target stimulus, and that the nature of the task sets can influence neural activity related to task-set reconfiguration. Selective task sets can in turn modulate the processing of target stimuli. The effects also apply to the case of foveally presented words, whose processing has often been hypothesized to be automatic and outside the influence of selective attention.

Adult↗

PubMed2005

Components of attentional set-switching.

A series of distinct event-related potentials (ERPs) have been recorded from the scalp of human subjects as they switch from one task to another. It is possible that task switching may depend on different mechanisms depending on whether the switch requires a change in attentional set, in other words the redirecting of attention to different aspects of a sensory stimulus, or whether it requires a change in intentional set, in others words a change in the way that responses are selected. To address this issue, the current study recorded ERPs while subjects switched between attentional sets and the results were compared with those of a previous investigation in which subjects switched between intentional sets. Subjects selected stimuli according to two conflicting attentional sets, each emphasizing one visual stimulus dimension (colour, shape). Pairs of stimuli, only one of which was to be attended, were presented for between eight and seventeen trials then either a switch or a stay cue was shown. The switch cue instructed subjects to switch from the current attentional set to the other set, while the stay cue instructed subjects to maintain the current set. Comparing ERPs time-locked to the switch and stay cues revealed neural correlates of the initiation of a task switch. Comparing the ERPs time locked to the first stimuli after either stay or switch cues identified neural correlates of the implementation of a task switch. A similar modulation over parietal electrodes was seen when subjects were switching between either attentional or intentional sets. While an intentional set switch began with a medial frontal modulation, attentional set switching began with a lateral frontal modulation. Implementing a new attentional set was associated with modulation of relatively early visual potentials, while implementing a new intentional set was associated with modulation of later response-related potentials. The results confirm that task switching consists of a number of constituent processes which may be taxed to different degrees depending on whether a task-switch paradigm requires subjects to change the way in which they select stimuli or responses.

Attention↗

PubMed2004

Orienting attention to locations in perceptual versus mental representations.

Extensive clinical and imaging research has characterized the neural networks mediating the adaptive distribution of spatial attention. In everyday behavior, the distribution of attention is guided not only by extrapersonal targets but also by mental representations of their spatial layout. We used event-related functional magnetic resonance imaging to identify the neural system involved in directing attention to locations in arrays held as mental representations, and to compare it with the system for directing spatial attention to locations in the external world. We found that these two crucial aspects of spatial cognition are subserved by extensively overlapping networks. However, we also found that a region of right parietal cortex selectively participated in orienting attention to the extrapersonal space, whereas several frontal lobe regions selectively participated in orienting attention within on-line mental representations.

Adult↗

PubMed2003

Brain potentials associated with conscious aftereffects induced by unseen stimuli in a blindsight subject.

The study is of brain activity in a blindsight subject (D.B.), who reports conscious visual afterimages of stimuli of which he is unaware when they are presented. This contrast offered a unique opportunity to study event-related potential recordings of conscious versus unconscious visual phenomena generated by the very same stimulus in the identical locus of the visual field. The behavioral results confirmed the reliability of the difference in the subject's report for inducing stimuli versus their aftereffects. The rationale of the event-related potential analysis was to subtract "on" signals from "off" signals, the latter associated with the onset of conscious events and the former for events that remained unconscious. Because there are inherent differences in on and off potentials, the subtractive resultants for the blind hemifield were compared with the same subtractions for the good hemifield when the subject was aware both of the stimuli and their afterimages. A differential pattern in subtractive resultants emerged with a strong anterior left frontal focus for the blind field and a posterior focus for the intact field. The results are compared with other studies suggesting an anterior focus for conscious visual events.

Afterimage↗

PubMed2003

Brain activations during visual search: contributions of search efficiency versus feature binding.

We investigated the involvement of the parietal cortex in binding features during visual search using functional magnetic resonance imaging. We tested 10 subjects in four visual search tasks across which we independently manipulated (1) the requirement to integrate different types of features in a stimulus (feature or conjunction search) and (2) the degree of search efficiency (efficient or inefficient). We identified brain areas that were common to all conditions of visual search and areas that were sensitive to the factors of efficiency and feature binding. Visual search engaged an extensive network of parietal, frontal, and occipital areas. The factor of efficiency exerted a strong influence on parietal activations along the intraparietal sulcus and in the superior parietal lobule. These regions showed a main effect of efficiency and showed a simple effect when inefficient conditions were compared directly with efficient pop-out conditions in the absence of feature binding. Furthermore, a correlation analysis supported a tight correspondence between posterior parietal activation and the slope of reaction-time search functions. Conversely, feature binding during efficient pop-out search was not sufficient to modulate the parietal cortex. The results confirm the important role of the parietal cortex in visual search, but suggest that feature binding is not a requirement to engage its contribution.

Adolescent↗

PubMed2003

The posterior cingulate and medial prefrontal cortex mediate the anticipatory allocation of spatial attention.

The purpose of this study was to identify brain regions underlying internally generated anticipatory biases toward locations where significant events are expected to occur. Subjects fixated centrally and responded to peripheral targets preceded by a spatially valid (predictive), invalid (misleading), or neutral central cue while undergoing fMRI scanning. In some validly cued trials, reaction time was significantly shorter than in trials with neutral cues, indicating that the cue had successfully induced a spatial redistribution of motivational valence, manifested as expectancy. The largest cue benefits led to selectively greater activations within the posterior cingulate and medial prefrontal cortex. These two areas thus appear to establish a neural interface between attention and motivation. An inverse relationship to cue benefit was seen in the parietal cortex, suggesting that spatial expectancy may entail the inhibition of attention-related areas to reduce distractibility by events at irrelevant locations.

Adult↗

PubMed2002

Components of switching intentional set.

Despite the intuition that we can shift cognitive set on instruction, some behavioral studies have suggested that set shifting might only be accomplished once we engage in performance of the new task. It is possible that set switching consists of more than one component cognitive process and that the component processes might segregated in time. We recorded event-related potentials (ERPs) during two set-switching tasks to test whether different component processes were responsible for (i) set initiation and reconfiguration when presented with the instruction to switch, and (ii) the implementation of the new set once subjects engaged in performing the new task. The response switching (RS) task required shifts of intentional set; subjects selected between responses according to one of two conflicting intentional sets. The results demonstrated the existence of more than one constituent process. Some of the processes were linked to the initiation and reconfiguration of the set prior to actual performance of the new task. Other processes were time locked to performance of new task items. Set initiation started with modulation of medial frontal ERPs and was followed by modulation over parietal electrodes. Implementation of intentional set was associated with modulation of response-related ERPs.

Adult↗

PubMed2002

The response of left temporal cortex to sentences.

The meaning of a sentence differs from the sum of the meanings of its constituents. Left anterior temporal cortex responds to sentences more strongly than to unconnected words. We hypothesized that the anterior temporal response to sentences is due to this difference in meaning (compositional semantics). Using positron emission tomography (PET), we studied four experimental conditions (2 x 2 factorial design): In one condition, subjects read normal sentences. In a second condition, they read grammatically correct sentences containing numerous semantic violations (semantically random sentences). In a third condition, we scrambled the word order within the normal sentences, and, in a fourth condition, the word order was scrambled within the semantically random sentences. The left anterior temporal pole responded strongly to sentences compared to scrambled versions of sentences. A similar although weaker response occurred in the left anterior superior temporal sulcus and the left posterior middle temporal gyrus. A subset of voxels within the left anterior temporal pole responded more to semantically random sentences and their scrambled versions than to normal sentences and the corresponding scrambled versions (main effect of semantic randomness). Finally, the grammatical and the semantic factor interacted in a subset of voxels within the anterior temporal pole: Activity was higher when subjects read normal sentences compared to their scrambled versions but not for semantically random sentences compared to their corresponding scrambled versions. The effects of grammar and meaning and, most importantly, the interaction between grammatical and semantic factors are compatible with the hypothesis that the left anterior temporal pole contributes to the composition of sentence meaning.

Adult↗

PubMed2002

Filtering of distractors during visual search studied by positron emission tomography.

We investigated the sensitivity of brain areas to the presence of filtering operations during overt visual search in crowded displays. Task conditions involved either visual search or predetermined simple eye movements for the detection of target digits. Furthermore, visual displays either contained letter foils that required filtering or contained only target digits. Brain imaging using positron emission tomography showed extensive overlap between areas involved in overt visual search and eye movements. Selective filtering of foils affected visual processing in ventral areas associated with object recognition and in primary visual cortex.

Adult↗

PubMed2001

Task-switching and memory retrieval processing: electrophysiological evidence.

Participants attempted to retrieve phonological or imagery-based information under conditions where either the two kinds of retrieval were required in separate blocks, or where frequent switches between retrieval tasks were required within blocks. Electrophysiological indices of processes engaged in pursuit of accurate memory judgments, elicited by contrasting ERPs evoked by correctly identified new test items, differed according to retrieval task only when the tasks were completed in separate blocks. The principal conclusion suggested by these findings is that the requirement to alternate frequently between the two retrieval tasks attenuated the engagement of task-specific processes that form part of a retrieval attempt.

Analysis of Variance↗

PubMed2001

Orienting attention in time.

Temporal information is essential for effective perception and action in the dynamic environment in which we exist. However, our ability to use information about time intervals flexibly to direct attention to an expected point in time has until recently been unexplored. Here we report a series of behavioural, neuroimaging and electrophysiological experiments that investigate and define the ability to orient attention in the temporal domain. These studies reveal that we are able to orient attention selectively to different time intervals, enhancing behavioural performance. These effects are mediated by a left-hemisphere dominant frontal-parietal system, which partially overlaps with the networks involved in spatial orienting. The optimisation of behaviour by temporal orienting appears to be achieved via motor-related mechanisms, in contrast to the typical perceptual enhancements produced by spatial attention. From a more general perspective, these findings illustrate the flexibility of attentional functions in the human brain.

Adaptation, Psychological↗

PubMed2001

Heterogeneity of cingulate contributions to spatial attention.

Functional magnetic resonance imaging was used to investigate activation patterns within the cingulate region during tasks based on spatial attention. Subjects were asked to detect targets which appeared either at the site indicated by a cue or on the opposite side. A "cue effect" was identified by the presence of shorter reaction times to validly than invalidly cued targets, showing that an anticipatory bias had been generated in the direction of the cue. Target detection accuracy was consistently above 90% although cue effects and reaction times displayed substantial variations, from one task session to another. Activation within the anterior cingulate region was seen in 16 of the 26 sessions but showed no correlation with reaction time. Posterior cingulate activation was seen in only 6 of the 26 sessions. However, a random effects analysis showed that the task-related signal change in this region was strongly correlated with the speed of target detection. A post hoc analysis indicated that this correlation was significant only when cue effects were present. No other part of the cerebral cortex displayed significant correlations with reaction times or cue effects. These results suggest that the cingulate component of the attentional network has at least two functionally segregated sectors, an anterior one in BA 24/32 and a posterior cingulo-retrosplenial one in BA 23/29/30. The posterior sector appears to be associated with the speed of detecting spatial targets, especially when attention is under the influence of a cue-induced anticipatory bias. The anterior cingulate focus did not display such a relationship in our tasks and is likely to mediate other aspects of attentional deployment such as performance monitoring, response selection or target identification.

Adult↗

PubMed2001

Orienting attention to instants in time.

My colleagues and I have investigated whether the temporal framework can be used to guide selective attention, and have applied non-invasive methodology to reveal the brain systems and mechanisms involved. Our findings show that we are able to orient attention selectively to different points in time, enhancing behavioral performance. These effects are mediated by a left-hemisphere dominant parietal-frontal system, which partially overlaps with the networks involved in spatial orienting. The neural system for temporal orienting also includes brain areas associated with motor preparation and anticipation, suggesting that sensorimotor areas with different specializations can contribute to attentional orienting depending on the stimulus attributes guiding selection. The optimization of behavior by temporal orienting involves enhancement of the latency and amplitude of event-related potentials that are associated with motor responses and decisions. The effects are distinct from those during visual spatial attention, indicating that behavioral advantages can be conferred by multiple types of neural mechanisms. Taken together, the findings illustrate the flexibility of attentional functions in the human brain.

Attention↗

PubMed2001

The role of phospholipase A(2) and cyclooxygenase in renal toxicity induced by microcystin-LR.

We have shown previously that exposure to microcystin-LR (MCLR) causes renal toxic effects in isolated perfused rat kidney. That study was extended further to approach the perspective of pharmacological blockade of renal toxic effects by MCLR through the use of experimental therapeutic agents. An isolated kidney perfusion system was utilized and samples of urine and perfusate were collected at 10min intervals to determine the levels of inulin, sodium, potassium and osmolality. Dexamethasone (20microg ml(-1)) and indomethacin (10microg ml(-1)) were administered in the beginning of the perfusion and MCLR was employed in a dose of 1microg ml(-1) after an internal control of 30min to evaluate the perfusion pressure (PP), renal vascular resistance (RVR), glomerular filtration rate (GFR) and urinary flow (UF). Dexamethasone and indomethacin antagonized the toxic effects of MCLR on PP, RVR, GFR and UF. Histologic analysis of dexamethasone and indomethacin treated groups did not show any vascular or interstitial alterations. MCLR potentially impairs the renal function, probably causing vascular and glomerular lesions and, promoting renal alterations through direct or indirect actions. These data seem to indicate that the renal alterations promoted by MCLR involves also phospholipase A(2) and arachidonic acid-derived mediators.

Analysis of Variance↗

PubMed2001

The attentive homunculus: now you see it, now you don't.

The nature of the neural system that directs our attention toward selective items in the extrapersonal world is a longstanding and interesting puzzle. The ability to image the human brain at work non-invasively using positron-emission tomography or functional magnetic resonance has provided the means to investigate this issue. In this article, I review the contributions of brain imaging toward the characterization of attentional control in the human brain. The majority of experiments to date have investigated visual spatial orienting. A consistent pattern of brain areas has been revealed, comprising most notably the posterior parietal cortex around the intraparietal sulcus and frontal regions including the frontal eye fields. The brain areas implicated in the control of visual spatial attention were noted to resemble those involved in the control of eye movements, and direct experimental comparisons supported a tight link between the two systems. The findings suggested a sensible view of the attentional 'homunculus' as a distributed neural system related to the control of eye movements. Eye movements form perhaps the most basic orienting response, and can be shifted rapidly and efficiently based on multiple frames of reference. Some attention experiments using objects and features instead of spatial locations as the target of selection also obtained similar patterns of parietal-frontal activations, rendering further support to this view of the attentional control system. Some recent experiments, however, have cautioned against a premature conclusion regarding the ubiquity of the attentional control system revealed by studies of visual spatial attention. Different parietal and frontal regions become engaged when attention is shifted along non-spatial dimensions, such as when attention is directed toward a particular motor act or toward a specific point in time. In these cases, the neural system resembles those involved in the control of limb movements. The attentional homunculus thus begins to dissolve. The alternative view suggested is that attentional control may be a property of specialized parietal-frontal systems that transform perception into action. Future studies will be needed to validate this view of attention, or to provide a more mature understanding of its true nature.

Attention↗

PubMed2001

Hunger selectively modulates corticolimbic activation to food stimuli in humans.

Functional magnetic resonance imaging (fMRI) was used to determine whether visual responses to food in the human amygdala and related corticolimbic structures would be selectively altered by changes in states of hunger. Participants viewed images of motivationally relevant (food) and motivationally irrelevant (tool) objects while undergoing fMRI in alternately hungry and satiated conditions. Food-related visual stimuli elicited greater responses in the amygdala, parahippocampal gyrus. and anterior fusiform gyrus when participants were in a hungry state relative to a satiated state. The state-dependent activation of these brain structures did not generalize to the motivationally irrelevant objects. These results support the hypothesis that the amygdala and associated inferotemporal regions are involved in the integration of subjective interoceptive states with relevant sensory cues processed along the ventral visual stream.

Adult↗

PubMed2001

The noradrenergic alpha2 agonist clonidine modulates behavioural and neuroanatomical correlates of human attentional orienting and alerting.

We examined whether the known noradrenergic attenuation of the alerting effect (the beneficial effect of a warning cue) results from an underlying effect of noradrenaline on temporal orienting (orienting toward a particular moment in time). Following a within-subjects, counterbalanced design, 10 healthy human volunteers received placebo, 200 microg clonidine or 1 mg guanfacine (alpha2 agonists) in three separate testing sessions. Subjects were scanned by fMRI while performing attentional orienting tasks containing spatially informative, temporally informative, non-informative or no cues. The alerting effect primarily activated left-lateralized prefrontal, premotor and parietal regions. Clonidine, but not guanfacine, impaired behavioural measures of the alerting effect while attenuating activity in the left temporo-parietal junction. Replicating previous results, the temporal orienting task activated left parietal and frontal cortex, while parietal cortex was activated bilaterally during spatial orienting. Of these networks, clonidine, but not guanfacine, attenuated left prefrontal cortex and insula activity during temporal orienting and attenuated right superior parietal cortex activity during spatial orienting,. To complement these neuroanatomical changes, clonidine produced selective behavioural effects on both temporal and spatial orienting. The anatomical dissociation between the effects of clonidine during temporal orienting versus alerting suggests that noradrenergic modulation of the alerting effect does not result only from an underlying effect on temporal orienting. Furthermore, we have demonstrated lateralized neuroanatomical substrates for the noradrenergic modulation of human attentional orienting in the spatial and temporal domains.

Adolescent↗