Abstract
A balance of mutual tonic inhibition between bi-hemispheric posterior parietal cortices is believed to play an important role in bilateral visual attention. However, experimental support for this notion has been mainly drawn from clinical models of unilateral damage. We have previously shown that low-frequency repetitive TMS (rTMS) over the intraparietal sulcus (IPS) generates a contralateral attentional deficit in bilateral visual tracking. Here, we used functional magnetic resonance imaging (fMRI) to study whether rTMS temporarily disrupts the inter-hemispheric balance between bilateral IPS in visual attention. Following application of 1 Hz rTMS over the left IPS, subjects performed a bilateral visual tracking task while their brain activity was recorded using fMRI. Behaviorally, tracking accuracy was reduced immediately following rTMS. Areas ventro-lateral to left IPS, including inferior parietal lobule (IPL), lateral IPS (LIPS), and middle occipital gyrus (MoG), showed decreased activity following rTMS, while dorsomedial areas, such as Superior Parietal Lobule (SPL), Superior occipital gyrus (SoG), and lingual gyrus, as well as middle temporal areas (MT+), showed higher activity. The brain activity of the homologues of these regions in the un-stimulated, right hemisphere was reversed. Interestingly, the evolution of network-wide activation related to attentional behavior following rTMS showed that activation of most occipital synergists adaptively compensated for contralateral and ipsilateral decrement after rTMS, while activation of parietal synergists, and SoG remained competing. This pattern of ipsilateral and contralateral activations empirically supports the hypothesized loss of inter-hemispheric balance that underlies clinical manifestation of visual attentional extinction.
Highlights
Visual attention depends upon the balance of tonic inhibition exerted between bilateral posterior parietal cortices (Kinsbourne, 1977; Muri et al, 2002; Battelli et al, 2009)
In comparing voxels commonly active in both hemispheres, we found that IPL, Superior Parietal Lobule (SPL) (BA5), Superior Occipital Gyrus (SoG), Lingual, Middle Occipital Gyrus (MoG), and MT+ were affected at inter-hemispheric level as well similar to INTRA-PARIETAL SULCUS (IPS) (Figures 3A,B)
(3) The evolution of activation of IPS and its network-wide synergists relates to changes in attentional accuracy over serial runs; whereas, immediately, activation of right IPS is associated with right and left field accuracy, subsequently, synergists as IPL, Lingual gyrus, SoG, MoG, and MT+ likely relate to recovery
Summary
Visual attention depends upon the balance of tonic inhibition exerted between bilateral posterior parietal cortices (Kinsbourne, 1977; Muri et al, 2002; Battelli et al, 2009). One hypothesis is that the damaged posterior parietal cortex is unable to “compete” against the uninhibited activity of the intact homologue, which in turn hyper-orients attention to the ipsi-lesional visual field leading to extinction of targets in the contra-lesional space (Kinsbourne, 1977). Inferences have been drawn from clinical neuropsychological observations (Battelli et al, 2001; Corbetta et al, 2005). This approach is severely limited because lesions have widespread, unpredictable effects that make it challenging to disentangle their direct sequel from ensuing disruptions in inter-hemispheric balance (Pascual-Leone et al, 2005)
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