Areas activated by spatial attention shifts in the macaque monkey: an fMRI study.

• ¹ Katholieke Universiteit Leuven, Neurosciences, Belgium
• ² University Hospital Leuven, Neurology, Belgium
• ³ Massachusetts General Hospital, Harvard Medical School, Radiology, United States

fMRI experiments in human have revealed a critical role of the superior parietal lobule (SPL) in shifting spatial attention, a finding unpredicted by human lesion studies. At present it is unclear which area(s) in the monkey may compute spatial attention shifts. Such knowledge, however, would be critical to explore the causal involvement of a putative monkey homologue (of the human SPL) in shifting attention. We used fMRI to identify brain regions involved in the computation of spatial shifting of attention in monkeys.
The experimental paradigm was a modified version of a human fMRI experiment by Molenberghs et al. (Cereb. Cortex 2007) and was performed by 2 monkeys (Macaca Mulatta) in a Siemens 3T fMRI scanner. Monkeys were scanned at a voxel size 1.25 isotropic, with 8-channel receiver coils and contrast-agent (MION).
Stimuli used in this experiment consisted of 2 yoked pairs of shapes, a circle together with a diamond and a triangle with a square. Each pair contained one relevant and one irrelevant stimulus and this was fixed and trained beforehand. The display always contained one of the two pairs, which were presented on the horizontal meridian (9.25˚ eccentricity, left and right). The monkey fixated in the center and had to respond manually when the relevant stimulus dimmed. An event consisted of the replacement of the current stimulus pair by the other pair. In half of the trials this change between pairs elicited a spatial shift in attention as the relevant stimulus was replaced by the irrelevant stimulus of the next pair. In 50% of the trials the change in shape was not associated with an attentional shift since the relevant stimulus was replaced by the relevant stimulus of the next pair. Only event sequences without dimming, and where monkeys fixated for at least 3 or more successive trials were analyzed.
Several frontal, parietal and occipital areas showed a strong effect of the direction of sustained attention: The frontal eye fields (FEF), area 46v, 45A, lateral intraparietal area (LIP), dorsal V4, 3 patches within superior temporal sulcus and areas TE/TEO were activated for sustaining attention to the contralateral attended stimulus, while being deactivated for attention to the ipsilateral stimulus.
Irrespective of the direction of attention, spatial shifts elicited mildly higher activity within the IPS (both medial and lateral bank), in areas V6/V6A, as well as in two medial frontal patches, areas 6M and 9M.
These results suggest that the IPS and/or the POS in monkeys may contain a functional homologue to the superior parietal region activated during spatial attention shifts in humans. Sustained spatial attention enhances fMRI activity in contralateral and suppresses activity in ipsilateral frontal, parietal and visual areas relative to the attended hemifield.