Mapping of the human visual cortex using image-guided transcranial magnetic stimulation

Institute of Bioengineering, Faculty of Medicine, Universidad Miguel Hernández, San Juan 03550, Spain.
Brain Research Protocols (Impact Factor: 1.82). 11/2002; 10(2):115-24. DOI: 10.1016/S1385-299X(02)00189-7
Source: PubMed


We describe a protocol using transcranial magnetic stimulation (TMS) to systematically map the visual sensations induced by focal and non-invasive stimulation of the human occipital cortex. TMS is applied with a figure of eight coil to 28 positions arranged in a 2x2-cm grid over the occipital area. A digitizing tablet connected to a PC computer running customized software, and audio and video recording are used for detailed and accurate data collection and analysis of evoked phosphenes. A frameless image-guided neuronavigational device is used to describe the position of the actual sites of the stimulation coils relative to the cortical surface. Our results show that TMS is able to elicit phosphenes in almost all sighted subjects and in a proportion of blind subjects. Evoked phosphenes are topographically organized. Despite minor inter-individual variations, the mapping results are reproducible and show good congruence among different subjects. This procedure has potential to improve our understanding of physiologic organization and plastic changes in the human visual system and to establish the degree of remaining functional visual cortex in blind subjects. Such a non-invasive method is critical for selection of suitable subjects for a cortical visual prosthesis.

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    • "An initial training before the experimental session was carried out to determine the optimal site of occipital stimulation for inducing reliable phosphenes. To this aim, a functional mapping procedure for phosphene induction was used (Fernandez et al., 2002); this type of protocol has been previously used to probe excitability of the visual cortex (Romei, Gross, & Thut, 2012; Bolognini, Senna, et al., 2010; Romei et al., 2007, 2009; Silvanto, Muggleton, Lavie, & Walsh, 2009; Bolognini & Maravita, 2007) based on findings of phosphenes to originate from the striate cortex ( V1; Sparing et al., 2002; Cowey & Walsh, 2000; Corthout, Uttl, Walsh, Hallett, & Cowey, 1999; Amassian et al., 1994; Meyer, Diehl, Steinmetz, Britton, & Benecke, 1991) and extrastriate areas V2/ V3 (Kammer, Puls, Erb, et al., 2005; Cowey & Walsh, 2000; Potts et al., 1998). Participants sat in an armchair, wearing a specially designed blindfold to prevent any light perception and an elastic swimming cap to mark the stimulation sites. "
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    ABSTRACT: Merging information derived from different sensory channels allows the brain to amplify minimal signals to reduce their ambiguity, thereby improving the ability of orienting to, detecting, and identifying environmental events. Although multisensory interactions have been mostly ascribed to the activity of higher-order heteromodal areas, multisensory convergence may arise even in primary sensory-specific areas located very early along the cortical processing stream. In three experiments, we investigated early multisensory interactions in lower-level visual areas, by using a novel approach, based on the coupling of behavioral stimulation with two noninvasive brain stimulation techniques, namely TMS and transcranial direct current stimulation. First, we showed that redundant multisensory stimuli can increase visual cortical excitability, as measured by means of phosphene induction by occipital TMS; such physiological enhancement is followed by a behavioral facilitation through the amplification of signal intensity in sensory-specific visual areas. The more sensory inputs are combined (i.e., trimodal vs. bimodal stimuli), the greater are the benefits on phosphene perception. Second, neuroelectrical activity changes induced by transcranial direct current stimulation in the temporal and in the parietal cortices, but not in the occipital cortex, can further boost the multisensory enhancement of visual cortical excitability, by increasing the auditory and tactile inputs from temporal and parietal regions, respectively, to lower-level visual areas.
    Journal of Cognitive Neuroscience 12/2012; 25(5). DOI:10.1162/jocn_a_00347 · 4.09 Impact Factor
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    • "Some of the procedures for measuring the MT have been adopted for measuring the PT, including the method of constant stimuli (MOCS) [8], a truncated version of the method of limits (the Rossini-Rothwell procedure [21], [28], used in [22], [29]), and the modified binary search algorithm (MOBS [30], [31], used in [32]–[34]). These procedures, however, have limitations. "
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    ABSTRACT: To calibrate the intensity of transcranial magnetic stimulation (TMS) at the occipital pole, the phosphene threshold is used as a measure of cortical excitability. The phosphene threshold (PT) refers to the intensity of magnetic stimulation that induces illusory flashes of light (phosphenes) on a proportion of trials. The existing PT estimation procedures lack the accuracy and mathematical rigour of modern threshold estimation methods. We present an improved and automatic procedure for estimating the PT which is based on the well-established Ψ Bayesian adaptive staircase approach. To validate the new procedure, we compared it with another commonly used procedure for estimating the PT. We found that our procedure is more accurate, reliable, and rapid when compared with an existing PT measurement procedure. The new procedure is implemented in Matlab and works automatically with the Magstim Rapid(2) stimulator using a convenient graphical user interface. The Matlab program is freely available for download.
    PLoS ONE 07/2011; 6(7):e22342. DOI:10.1371/journal.pone.0022342 · 3.23 Impact Factor
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    • "This conclusion is strengthened by recent works investigating the crossmodal modulation of visual phosphenes induced by sTMS. The application of sTMS to the occipital visual areas in the human brain can elicit phosphenes, i.e., bright spots of light in specific regions of the visual field (Fernandez et al., 2002; McKeefry et al., 2009). Phosphenes are generated within coextensive regions of the cortex and could be induced by application of TMS to virtually all early visual areas, including the striate cortex (V1), extrastriate areas (V2/V3), and cortico-cortical tracts projecting from V2/V3 back to V1 (Kammer et al., 2005a,b). "
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    ABSTRACT: Most of current knowledge about the mechanisms of multisensory integration of environmental stimuli by the human brain derives from neuroimaging experiments. However, neuroimaging studies do not always provide conclusive evidence about the causal role of a given area for multisensory interactions, since these techniques can mainly derive correlations between brain activations and behavior. Conversely, techniques of non-invasive brain stimulation (NIBS) represent a unique and powerful approach to inform models of causal relations between specific brain regions and individual cognitive and perceptual functions. Although NIBS has been widely used in cognitive neuroscience, its use in the study of multisensory processing in the human brain appears a quite novel field of research. In this paper, we review and discuss recent studies that have used two techniques of NIBS, namely transcranial magnetic stimulation and transcranial direct current stimulation, for investigating the causal involvement of unisensory and heteromodal cortical areas in multisensory processing, the effects of multisensory cues on cortical excitability in unisensory areas, and the putative functional connections among different cortical areas subserving multisensory interactions. The emerging view is that NIBS is an essential tool available to neuroscientists seeking for causal relationships between a given area or network and multisensory processes. With its already large and fast increasing usage, future work using NIBS in isolation, as well as in conjunction with different neuroimaging techniques, could substantially improve our understanding of multisensory processing in the human brain.
    Frontiers in Psychology 03/2011; 2:46. DOI:10.3389/fpsyg.2011.00046 · 2.80 Impact Factor
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