Title: Functional specialization in parietal cortex: fMRI and lesion studies of selective attention
Other Titles: Functionele specialisatie in de pariëtale cortex: fMRI en letselstudies van selectieve aandacht
Authors: Gillebert, CĂ©line
Issue Date: 15-May-2012
Abstract: Visual attention refers to the set of cognitive processes that prioritize visual information according to current goals and expectations. Visual attention not only modulates the process of selecting between competing stimuli. It also has an impact on for instance the expectancy of ensuing information and on access of information to visual short-term memory. To explain different attentional processes within a unified framework, previous studies proposed the concept of the attentional priority map. The attentional priority map is a topographical representation of the environment in which each location is weighted by its sensory salience and its behavioral relevance. Despite conceptual advances in understanding the mechanisms of visual attention (Chapter 1), clinical neurology and functional neuroimaging studies in healthy individuals yielded contrasting findings regarding the cortical structures through which such mechanisms are executed. The main goal of my doctoral research project has been to characterize the functional neuroanatomy of visual attention in the human parietal cortex, with particular emphasis on the process of selecting between competing stimuli. To this end, we combined different study populations, experimental paradigms, neuroimaging methods and computational modeling (Chapter 2). In the first part of my work, we studied stroke patients with lesions in the superior and inferior parietal cortex. Lesions of the right inferior parietal lobule are commonly associated with spatial attentional disorders, such as neglect and extinction. Conversely, functional imaging studies in the intact brain have implicated a more superior region, the intraparietal sulcus (IPS), in spatial attention. In Chapter 3 we investigated whether IPS is critical for spatially selective attention. We used a spatial cueing task in which subjects received a prior spatial cue indicating the relevant location. We manipulated the need to reorient or to select between competing stimuli by changing the validity of the prior spatial cue or by adding a distracter to the display, respectively. Focal lesions of the posterior and middle segment of IPS gave rise to a contralesional deficit in attentional reorienting and selection between competing stimuli, indistinguishable from that seen after classical inferior parietal lesions. Furthermore, functional magnetic resonance imaging (fMRI) data showed that the deficit could not be accounted for by functional effects at a distance affecting inferior parietal cortex. This study established the critical role of the posterior and the middle IPS segments in spatially selective attention. Next, we used fMRI in healthy volunteers to investigate how different attentional processes, namely selection between competing stimuli and reorienting after an invalid spatial cue, precisely map onto the underlying cytoarchitecture of the parietal cortex. In Chapter 4, we directly contrasted these two attentional processes within the same event-related fMRI experiment by means of thesame spatial cueing task as used in patients. Our results confirmed a functional dissociation between cytoarchitectonic area PF in the temporoparietal junction (TPJ) of the right hemisphere, and areas hIP1/hIP3 in left and right IPS. Right PF was primarily involved in reorienting attention to behaviorally relevant singletons, whereas hIP1/hIP3 were mainly involved in selection between competing stimuli. The functional selectivity of areas PF and hIP1/hIP3 was corroborated by their distinct patterns of resting-state functional connectivity with other brain areas. This study demonstrated the differential contribution of area PF and areas hIP1/hIP3 to spatially selective attention. After investigating the functional neuroanatomy of distinct attentional processes, we studied the interactions between vision and selective attention by means of fMRI in healthy volunteers. Previous studies demonstrated that the selection deficit induced by IPS lesions has a strong spatial determinant: it is more pronounced when stimuli are configured along a horizontal configuration axis compared to a vertical or diagonal configuration axis. In Chapter 5 we investigated the origin of this spatial anisotropy. Our analysis of fMRI data indicated that the effect of configuration axis arises at the level of the attentional priority map in IPS rather than at the level of a sensory map in early visual cortex. It also revealed that adding a competing distracter to a target not only enhances activity in the middle IPS segment but also strengthens feedback connectivity from IPS to early visual areas. These findings supported the idea that the middle IPS segment plays a critical role in calibrating attentional priorities and exerts top-down control on early visual cortex to select between competing stimuli. In the last part of my doctoral research project, we examined the relationship between selective attention and access to visual short-term memory by means of fMRI in healthy volunteers (Chapter 6). We used a change detection paradigm: subjects were presented with a variable number of targets and distracters and had to remember the targets for a brief time period. We then evaluated how activity in the parietal cortex changed as a function of the number of targets versus distracters in the display and in visual short-term memory. Our fMRI analysis showed a functional dissociation between right TPJ and IPS. Right TPJ was selectively activated when a single target was presented in the display. In contrast, activity levels in the middle and posterior segments of IPS increased when either targets or distracters were added to a single target, but not when both targets and distracters were added to the display. Furthermore, activity levels in middle IPS were positively correlated with the number of targets, but not distracters, in visual short-term memory. Overall, the response profile in middle IPS suggested that items accessing VSTM receive differential weights depending on their behavioral relevance, and secondly, that a suppressiveeffect originates during the selection phase when multiple targets and multiple distracters are simultaneously present. Conversely, the response profile in right TPJ indicated a role in spotting target singletons. To conclude, the work for my doctoral research project clearly showed that different brain areas in the parietal cortex have dissociable functions and interact with one another to mediate spatially selective attention. It also contributed to a better understanding of how selective attention influences early visual processes and interacts with visual short-term memory. Importantly, the findings led to a functional neuroanatomical model of spatially selective attention (Chapter 7). According to our model, TPJ plays a role in detecting behaviorally important singletons, whereas the posterior and middle segments of IPS are critically involved in attentional enhancement of relevant stimuli and the compilation of a priority map, respectively. This model cannot only explain functional processes in the intact brain, but also provides a framework to disentangle distinct attentional deficits in patients with brain lesions.
Table of Contents: 1 INTRODUCTION
1.1 Cognitive processes of visual attention
1.2 Disorders of visualattention
1.3 Functional neuroimaging of visual attention
1.4 A network view on visual attention
1.5 Thesis aims

2.1 Psychophysical methods
2.2 Single-case approach
2.3 Magnetic Resonance Imaging
2.4 Functional neuroimaging

3.1 Abstract
3.2 Introduction
3.3 Materials and Methods
3.4 Results
3.5 Discussion
3.6 Supplementary Material

4.1 Abstract
4.2 Introduction
4.3 Materials and Methods
4.4 Results
4.5 Discussion

5.1 Abstract
5.2 Introduction
5.3 Materials and Methods
5.4 Results
5.5 Discussion
5.6 Conclusion

6.1 Abstract
6.2 Introduction
6.3 Materials and Methods
6.4 Results
6.5 Discussion
6.6 Conclusion
6.7 Supplementary Material

7.1 Selective attention and the parietal cortex: novel lesion evidence
7.2 Functional neuroanatomical model
Publication status: published
KU Leuven publication type: TH
Appears in Collections:Laboratory for Cognitive Neurology
Laboratory for Experimental Psychology
Research Group Experimental Neurology

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