Spatial neglect in near and far space investigated by repetitive transcranial magnetic stimulation

Abstract

Localized repetitive transcranial magnetic stimulation was used to disrupt visuospatial perception in the near and far space of six healthy volunteer subjects. In addition to the baseline condition, they were stimulated over the right posterior parietal cortex, the right or left dorsal occipital cortex or the right ventral occipital cortex, during the brief presentation of a transected horizontal line. Subjects had to indicate whether the part of the line to the left or right of the transection appeared longer. The stimulus display was back-projected on a screen at a viewing distance of either 50 or 150 cm ("near" and "far" space, respectively). Reaction times and choices were measured. In a forced-choice paradigm, subjects showed "pseudoneglect", the natural tendency of neurologically intact subjects to perceive the left side of a centrally transected line as slightly longer than the right. These errors occurred more for lines in near space than for lines in far space. Magnetic stimulation of the right posterior parietal cortex or the right ventral occipital lobe selectively induced a significant shift to the right in the perceived midpoint for near- and far-space lines, respectively. The results reproduced in normal subjects the dissociation between neglect in near and far space that has been described in patients with different right-hemisphere lesions. This dissociation supports the contention that there is a dorsal/near space-ventral/far space segregation of processing in the visual system which reflects the behavioural goals of the two putative visual streams.

Dissociating the neural mechanisms of distance and spatial reference frames

“…The role of rPPC in processing near space is in accordance with previous research (Bjoertomt et al, 2002;Halligan & Marshall, 1991;Lane et al, 2013;Mennemeier et al, 1992;Weiss et al, 2000), as is its relationship with egocentric processing (Committeri et al, 2004;Galati et al, 2000;Hillis et al, 2005;Medina et al, 2009;Neggers et al, 2006;Vallar et al, 1999). Neuropsychological research has indicated that the right hemisphere is involved in egocentric processing in near space (Iachini, Ruggiero, Conson & Trojano, 2013), although this finding was based on extensive fronto-parietal lesions in only four patients making it difficult to determine the precise brain area underlying their impairment .…”

“…The chosen site was the intersection of the precentral and superior frontal sulci, a location that has repeatedly been used with TMS and confirmed as a functional locus (Grosbras and Paus, 2002;Paus, 1996;Ro et al, 1999). The rVO site was determined using the averaged scalp co-ordinates reported by Bjoertomt et al (2002), who also used this same site to examine near and far space processing.…”

“…Taking this neuropsychological data together with that gathered from techniques including neuroimaging and electroencephalography, there is converging evidence that the dorsal stream including right posterior parietal cortex (rPPC) is important in near space, while ventral stream areas (such as right ventral occipital, rVO) are involved in far space processing (Bjoertomt, Cowey & Walsh, 2002;Butler, Eskes & Vandorpe, 2004;Halligan & Marshall, 1991;Mennemeier et al, 1992;Shelton, Bowers & Heilman, 1990;Valdés-Conroy, Sebastián, Hinojosa, Romá & Santaniello, 2014;Vuilleumier et al, 1998;Weiss et al, 2000). The frontal eye fields (FEF), known to be involved in the orientation of visual attention, have also been associated with far space processing (Cowey et al, 1994;Rizzolatti, Matelli & Pavesi, 1983).…”

Time course of the involvement of the ventral and dorsal visual processing streams in a visuospatial task

“…In contrast the involvement of the dorsal stream is based on its established visuo-spatial specialisation (Avidan et al, 2003;Ellison et al, 2003;Bjoertomt et al, 2002). Corthout et al, (2007) have recently delineated the arrival time of visual information in extra-striate visual cortex, whether feed-forward or feed-back, as between 50 and 100ms.…”

TMS can reveal contrasting functions of the dorsal and ventral visual processing streams

“…As previously discussed, neuropsychology and neuroimaging have identified many areas of specialisation interacting in broad streams of processing in these domains. However, our aim is to investigate the differential involvement of two areas in these three tasks: right lateral-occipital area (LO) known to be an important region in object recognition in the ventral stream (Ferber et al, 2005, Avidan et al, 2003, Lerner et al, 2002Kourtzi & Kanwisher, 2001) and an area of right posterior parietal cortex (PPC) known from many studies to have visuospatial specialisation (Avidan at al., 2003, Ellison et al, 2003Bjoertomt et al, 2002). As these areas are part of the established processing streams, if either is involved in the processing of a task, a deficit in performance should be demonstrated following disruption by TMS.…”