Near and far space: Understanding the neural mechanisms of spatial attention

Abstract

Visuospatial neglect is a multicomponent syndrome, and one dissociation reported is between neglect for near (peripersonal) and far (extrapersonal) space. Owing to patient heterogeneity and extensive lesions, it is difficult to determine the precise neural mechanisms underlying this dissociation using clinical methodology. In this study, transcranial magnetic stimulation was used to examine the involvement of three areas in the undamaged brain, while participants completed a conjunction search task in near and far space. The brain areas investigated were right posterior parietal cortex (rPPC), right frontal eye field (rFEF), and right ventral occipital cortex (rVO), each of which has been implicated in visuospatial processing. The results revealed a double dissociation, whereby rPPC was involved for search in near space only, whilst rVO only became necessary when the task was completed in far space. These data provide clear evidence for a dorsal and ventral dissociation between the processing of near and far space, which is compatible with the functional roles previously attributed to the two streams. For example, the involvement of the dorsal stream in near space reflects its role in vision for action, because it is within this spatial location that actions can be performed. The results also revealed that rFEF is involved in the processing of visual search in both near and far space and may contribute to visuospatial attention and/or the control of eye-movements irrespective of spatial frame. We discuss our results with respect to their clear ramifications for clinical diagnosis and neurorehabilitation.

Figure 1

Diagram depicting the trial progression. A central fixation cross was presented for 500 ms, followed immediately by the 10‐item search array. This array remained present until the participant responded with a button‐press (Present or Absent), which could be at any time after display onset. TMS was delivered at 10 Hz for 500 ms from the beginning of array onset.

Figure 2

Diagram showing the averaged location of each of the stimulated sites: rPPC (MNI co‐ordinates: x = 30, y = −70, z = 28 mm), rFEF (x = −493, y = 183, z = −34 mm), and rVO (x = 28, y = −95, z = 5 mm). The position was verified using each participants' MRI scan co‐registered to their skull co‐ordinates using BrainSight™ software. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]

Figure 3

Graphs showing the mean RT (in ms) for each condition of TMS and distance. There are different graphs for each of the three stimulation sites: rPPC (A), rFEF (B), and rVO (C). Error bars represent the SEM across participants and an asterisk indicates a significant difference (P < 0.025).