Dynamic spatial coding within the dorsal frontoparietal network during a visual search task

Abstract

To what extent are the left and right visual hemifields spatially coded in the dorsal frontoparietal attention network? In many experiments with neglect patients, the left hemisphere shows a contralateral hemifield preference, whereas the right hemisphere represents both hemifields. This pattern of spatial coding is often used to explain the right-hemispheric dominance of lesions causing hemispatial neglect. However, pathophysiological mechanisms of hemispatial neglect are controversial because recent experiments on healthy subjects produced conflicting results regarding the spatial coding of visual hemifields. We used an fMRI paradigm that allowed us to distinguish two attentional subprocesses during a visual search task. Either within the left or right hemifield subjects first attended to stationary locations (spatial orienting) and then shifted their attentional focus to search for a target line. Dynamic changes in spatial coding of the left and right hemifields were observed within subregions of the dorsal front-parietal network: During stationary spatial orienting, we found the well-known spatial pattern described above, with a bilateral hemifield representation in the right hemisphere and a contralateral preference in the left hemisphere. However, during search, the right hemisphere had a contralateral preference and the left hemisphere equally represented both hemifields. This finding leads to novel perspectives regarding models of visuospatial attention and hemispatial neglect.

Figure 1. Paradigm separating spatial orienting (SO) and visual search (VS).

After a fixation period, a central cue indicated the relevant hemifield for the next trial. When stimuli appeared, subjects had to search covertly for a target line among the 6 positions of the cued hemifield. Task difficulty was modified by the number of distractors with nonvertical orientations in the relevant hemifield (easy: 2; difficult: 5).

Figure 2. Contralateral preference of activated voxels during spatial orienting (SO) and visual search (VS).

Contralateral preference visualized by t-tests for all voxels activated by SO and VS (p<0.005, RFX, cluster threshold: 50 mm²). Negative (red) and positive (blue) t-values indicate a preference for the left and right hemifield, respectively. Voxels with t-values<−2.26 or >2.26 show a significant contralateral preference (p<0.05) and are indicated by dark blue or red, respectively. White color indicates voxels involved in the process with no preference for either hemifield. (a) Histograms with t-values for all activated voxels within predefined ROIs of the dorsal FPN. The range of t-values represented by each bar is 1.13. The significance level (p<0.05) for contralateral preference to the left (−) or right (+) is indicated by black lines on the x-axis of each histogram. Additionally, the percentage of voxels with a significant contralateral preference for the left or right hemifield is shown on the appropriate side of the histograms. (b) Dorsal posterior view of the flattened left and right hemisphere with representation of t-values on the surface. (c) Histograms with t-values for all activated voxels within control ROIs MC (motor cortex) and VO (visual occipital). T: mean t-value of activated voxels; STD: standard deviation of t-values; Yellow dotted lines: predefined anatomical ROIs.

Figure 3. Contralateral preference of activated voxels during easy and difficult search conditions.

Contralateral preference visualized by t-tests for all voxels activated by easy search and difficult search conditions (p<0.005, RFX, cluster threshold: 50 mm²). The color convention and the significance level for hemifield preference are described in Fig. 2. (a) Histograms with t-values for all activated voxels within predefined ROIs of the dorsal FPN. The range of t-values represented by each bar is 1.13. The significance level (p<0.05) for contralateral preference to the left (−) or right (+) is indicated by black lines on the x-axis of each histogram. As in Figure 2, the percentage of voxels with a significant contralateral preference for the left or right hemifield is given on the appropriate side of the histograms. (b) Dorsal posterior view of the flattened left and right hemisphere with representation of t-values on the surface. (c) Histograms with t-values for all activated voxels within control ROIs MC (motor cortex) and VO (visual occipital). T: mean t-value of activated voxels; STD: standard deviation of t-values; Yellow dotted lines: predefined anatomical ROIs.