Spatial Neglect and Attention Networks By Maurizio Corbetta & Gordon L. Shulman

1. Spatial Neglect and Attention Networks By Maurizio Corbetta & Gordon L. Shulman Group 3, Week 11 Alicia Iafonaro Kimberly Villalva Tawni Voyles

2. 1. How do the authors address the issue that spatial neglect can involve damage to different regions . . . What is meant by physiology? R hemisphere lesions in several different locations within the R hemisphere cause neglect of the L visual field. To explain this, the authors propose that the diverse variety (heterogeneousness) of lesions found in spatial neglect patients masks a greater uniformity (homogeneousness) at the level of physiology, with common physiological abnormalities in remote neural systems specialized for spatial processing. Physiology refers to how organs, cells, and biomolecules carry out the chemical or physical functions that exist within a living system. The authors are saying that one might expect a certain kind of behavioral response to result from damage to a particular structure, usually because said behaviors were previously thought to have involved the structure. However, this was not the case with spatial neglect. An assortment of lesions of the R hemisphere resulted in neglect of the L field.

3. 2. The authors describe the dorsal frontoparietal network. Explain its function and the neural components of it, and explain figure 3. Ips / SPL – Intraparietal Sulcus / Superior Parietal Lobule FEV – frontal eye field TPJ – temporal parietal junction (Inferior Parietal Lobule & Superior Temporal Gyrus) VFC – ventral frontal cortex (Inferior Frontal Gyrus / Medial Frontal Gyrus)

4. . . . Figure 3 a • Dorsal frontoparietal regions are activated following a central cue to shift attention. • The time course of the response to the cue shows bilateral activity from R intraparietal sulcus with a contralateral dominance indicating spatial selectivity.

5. . . . Figure 3 b • Occipital & dorsal frontoparietal regions show spatially selective attentional modulations following a stimulus-driven shift of attention. • Note the strong spatially selective response in R intraparietal sulcus & visual cortex for shifting and attending to contralateral rather than ipsilateral stimulus streams.

6. . . . Figure 3 c • The L image shows 5 contralateral polar angle maps along R IPS. • The R image shows the activations in these maps during a VSTM task. • The bottom graph shows that although L IPS & R IPS contains contralateral polar angle maps, R IPS was equally activated by VSTM load in the contralateral & ipsilateral hemifields. • The L IPS was only modulated by load in the contralateral hemifield.

7. . . . Figure 3 d • BOLD activity was measured following an auditory cue to attend a peripheral location. • Activity in the L FEF and R FEF is highly correlated across trials, but a contralateral signal is superimposed on the positively correlated noise. • This correlated noise is partly explained by the presence of strong correlations at rest between homologous regions. • The locus of attention is only weakly predicted by reading activity in the visual cortex or FEF which codes for the attended location.

8. Inter-hemispheric competition may play a key role in the efficient control of spatial attention by the dorsal network. • Sylvester found that attention-related BOLD activity in dorsal frontoparietal & occipital regions contralateral to an attended location was only modestly predictive of the locus of attention. • Prediction probability was greatly increased by subtracting the activity of homologous L & R hemisphere regions. 3. What can be said about HOW attention is controlled, and where in the brain is this control exercised? • The locus of attention may be efficiently coded by the two hemispheres through a difference signal implemented by interactions via either the corpus callosum or subcortical routes. • This predicts that abnormalities in the computation of the locus of attention should correlate with abnormal interhemispheric interactions or response imbalances between the L & R hemisphere dorsal attention network.

9. 4) Discuss physiological correlates of the egocentric spatial bias and explain figure 4. Statistical map of BOLD activations during a Posner spatial attention task (are cued to peripheral location and detect a subsequent target. Right hemisphere acute neglect patients show hypoactivation of both hemispheres (right greater that left) that partly recovers at the chronic stage. The dark shading in the anatomical image indicates the distribution of structural damage.

10. Figure 4 Left columns show statistical maps of activity in parietal cortex. (blue line) Right column shows average time course of activity time-locked to the presentation of the cue (red line). Result of right hemisphere hypoactivation acute patients show a large imbalance of BOLD activity in IPS/SPL, with relatively greater left than right. Activity in both hemisphere is lower than normal. Imbalance normalizes at the chronic stage.

11. Figure 4 Top graph neglect patients show low correlations in BOLD spontaneous activity between homologous regions of left and right parietal corext. Correlations recovers at the chronic stage, the bottom graph. Left parietal cortex- blue line Right parietal cortex- red line

12. Figure 4 Abnormal physiological signals in the dorsal attention network of neglect patients are functionally significant. Left graph- left parietal activity was stronger in subjects with more sever neglect, indexed by longer response times to contralesional versus ipsilesional visual targets. Right graph- reduced interhemispheric correlation within frontoparietal regions of the dorsal attention network correlates with the severity of neglect of the left visual field. Indexed by longer response times to contralesional versus ipsilesional visual targets

13. 5. Some effects are localized to damaged right hemisphere. • Cognitive-communication problems that can occur from right hemisphere damage include difficulty with the following: • attention • left-side neglect • memory • organization • orientation • problem solving • reasoning • social communication

14. 5. What are these neglect symptoms or features? • Damage to the right hemisphere impairs attention to the left hemifield, whereas damage to the left hemisphere can be compensated. • Essentially, the individual no longer acknowledges the left side of his/her body or space. • These individuals will not brush the left side of their hair, for example, or eat food on the left side of their plate, as they do not see them or look for them.

15. 6. What was Mesulam’s theory? How does Kinsbourne’s theory differ? Discuss support for the theories. • Mesulam -- standard theory of neglect postulates that the right hemisphere controls shifts of attention to both sides of space. While the left hemisphere only controls attention to the right side • Kinsbourne -- opponent-process theory proposes that each hemisphere promotes orienting in a contralateral direction, but the strength of this bias is stronger in the left than right hemisphere. • Several studies, however, have reported larger activations for contralateral than ipsilateral stimuli (contralateral bias) in some left than right dorsal regions. • These asymmetries in contralateral bias are consistent with either the standard or opponent process theory, depending on whether the bias in the right hemisphere was completely absent (standard theory) or was simply present to a lesser degree (opponent process).

16. 6. Could VSTM explain symptoms? Discuss the evidence. • A recent study suggested that one factor might involve high loading of VSTM. Under conditions that involved high VSTM load and spatial filtering of distracters, left and right hemisphere activations in regions of IPS showed the postulated visual field profile. • Despite this result, the presence of the standard visual field organization under high VSTM load does not satisfactorily explain the laterality of neglect. • Contralesionalneglect is correlated with conditions that do not involve high VSTM loads, such as simple detection of a single visual target.