Two different attentional networks have been associated with visuospatial attention and conflict resolution. In most situations either one of the two networks is active or both are increased in activity together. By using functional magnetic resonance imaging and a flanker task, we show conditions in which one network (anterior attention system) is increased in activity whereas the other (visuospatial attention system) is reduced, showing that attentional conflict and selection are separate aspects of attention. Further, we distinguish between neural systems involved in different forms of conflict. Specifically, we dissociate patterns of activity in the basal ganglia and insula cortex during simple violations in expectancies (i.e., sudden changes in the frequency of an event) from patterns of activity in the anterior attention system specifically correlated with response conflict as evidenced by longer response latencies and more errors. These data provide a systems-level approach in understanding integrated attentional networks.
The Harvard community has made this article openly available. Please share how this access benefits you. Your story matters AbstractI Previous neuroimaging studies of perceptual priming have reported priming-related decreases in the extrastriate cortex. However, because these experiments have used visual stimuli, it is unclear whether the observed decreases are associated speciªcally with some aspect of visual perceptual processing or with more general aspects of priming. We studied withinand cross-modality priming using an auditory word stem completion paradigm. Positron emission tomography (PET) images were obtained during stem completion and a ªxation task. Within-modality auditory priming was associated with blood ºow decreases in the extrastriate cortex (bilateral), medial/ right anterior prefrontal cortex, right angular gyrus, and precuneus. In cross-modality priming, the study list was presented visually, and subjects completed auditory word stems. Crossmodality priming was associated with trends for blood ºow decreases in the left angular gyrus and increases in the medial/right anterior prefrontal cortex. Results thus indicate that reduced activity in the extrastriate cortex accompanies withinmodality priming in both visual and auditory modalities. I
Sequential learning is an important aspect of cognitive processing. Neuropharmacological evidence acquired in laboratory animals suggests that striatal dopaminergic mechanisms may be important for processing of this form of learning. However, because experiments conducted on dopamine deficient patients have reported contradictory evidence, the role of dopamine and the striatum remains unclear in human sequential learning. We used a newly developed dynamic molecular imaging technique to determine whether striatal dopamine is released during performance of a sequential learning task. In this study we localized striatal regions where dopamine receptor ligand (11C-raclopride) was displaced from receptor sites, during performance of a motor sequence learning (serial reaction time) task. The results suggest that the task induces release of endogenous dopamine in the posterior two-third of dorsomedial aspect of left putamen and the anterior part of the body of caudate bilaterally. The activations of the left putamen and the right caudate coincided with the activations observed earlier during performance of a motor planning task. Since these activations are associated with the selection and execution of a response, the activation in the left caudate, which was not observed in motor planning, is probably associated with the detection of a change in the 'context', and in the formulation of a new 'rule'. Thus, the results suggest that sequential learning involves two striatal dopaminergic mechanisms, one for the detection of a change in context, and the other for selection and execution of the response.
Involvement of dopamine neurotransmission in human emotional processing is unclear but animal studies have indicated that it is critical for processing of fear response. In this experiment we examined dopaminergic involvement in the processing of human emotions. We used a novel dynamic molecular imaging technique to detect and map dopamine released during presentation of emotional stimuli. The technique exploited the competition between endogenously released dopamine and its ligand for receptor occupancy and involved dynamic voxel-wise measurement of the rate at which a dopamine receptor ligand (18F-Fallypride) was displaced from receptor sites during emotional processing. An increase in the rate indicated dopamine release. We found that the rate of ligand displacement increased significantly in the left amygdala, left medial temporal lobe (MTL) and left inferior frontal gyrus. The results provide the first direct evidence of dopaminergic modulation of human emotional processing and suggest that the modulation occurs at multiple levels of processing. This finding indicates that the neurocognitive models of human emotion should take into account dopaminergic effects, and that, there is a need to investigate whether manipulation of the dopaminergic system could be an alternate strategy for treatment of conditions in which emotional processing is impaired.
Striatal dopamine is associated with the processing of rewarded motor tasks. Its involvement in mediating unrewarded tasks is, however, unclear. We used a recently developed PET technique to dynamically measure the rate of displacement of a dopamine receptor ligand raclopride in healthy volunteers performing a finger opposition task. Rapid displacement of the ligand from the posterior putamen and the caudate immediately after the task initiation suggested striatal dopamine release during task performance. Since dopamine release was observed in the striatal areas that are implicated in unrewarded tasks by neuroimaging studies, the results demonstrate that the PET method can be used to extend the findings of conventional neuroimaging techniques, that do not provide information about signal transduction.
The distinction between implicit and explicit retrieval of learned material is central to recent thinking about the neural systems underlying memory. Word stem completion is one task in which subjects can be instructed either to make a deliberate recall (explicit instruction) or to be told to complete the stem with any appropriate word (implicit instruction). Positron emission tomography (PET) studies have indicated that during implicit retrieval, there is reduced blood flow in right posterior areas, whereas some tasks of explicit retrieval involve frontal and hippocampal activation. However, there is no information about the timing of these activations or how implicit and explicit retrieval might be related.We used word stem completion tasks similar to those used in the PET studies, but used high-density electrical recording designed to allow localization of the regions involved in the tasks and to provide temporal information. We found reduced activity for primed words in right posterior cortex corresponding to previous PET results. The reduction occurred within the first 200 msec after input, suggesting early interaction with the information stored in this area. Similar reductions observed during explicit recall of the previously presented words indicate that priming is similar under implicit and explicit conditions. In addition, when priming was not an adequate basis for response, then frontal areas were active. Retrieval of unprimed words under implicit instruction elicited right frontal activation, whereas explicit retrieval activated frontal areas bilaterally. Left frontal and hippocampal activations appear to occur only when the retrieval involved use of the words from the list studied previously. Key words: priming; word stem completion; implicit memory; explicit memory; hippocampus; frontal cortex; parietal cortex; temporal cortexRecent experiments using a variety of techniques have supported the view that memory is not a unitary faculty and that different types of memory have distinct neural networks (Tulving and Schacter, 1990;Schacter et al., 1993;Nyberg et al., 1996). One such distinction is between explicit (intentional) and implicit (unintentional) retrieval (Graf and Schacter, 1985;Schacter et al., 1993).Of particular importance are tasks in which the same material can be retrieved implicitly under one instruction and explicitly under another. In the word stem completion task, subjects learn a list of words and then later, three-letter strings are used as cues for retrieval. When the instruction is to pronounce the first word that comes to mind beginning with the three-letter stem, the subject need not be aware of any relationship to the previously learned words. Nonetheless, previously learned words are recalled with significantly higher than chance probability Buckner et al., 1995). In this task, explicit retrieval strategies can be evoked by instructing the subject to retrieve words from the studied list.Under implicit instructions, stems of the studied words (primed stems) show a reduction o...
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