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Cognitive Brain Research 14 (2002) 300–302www.elsevier.com/ locate /bres
Short communication
Functional anatomy of top-down visuospatial processing in the humanbrain: evidence from rTMS
a , a b a*´Andre Aleman , Dennis J.L.G. Schutter , Nick F. Ramsey , Jack van Honk ,a b a b´Roy P.C. Kessels , Johannes M. Hoogduin , Albert Postma , Rene S. Kahn ,
aEdward H.F. de HaanaHelmholtz Instituut, Psychological Laboratory, Utrecht University, Heidelberglaan 2, 3584 CS, Utrecht, The Netherlands
bDepartment of Psychiatry, University Medical Center, Utrecht, The Netherlands
Accepted 31 January 2002
Abstract
The hypothesis was tested that visuospatial mental imagery relies on processing in the posterior parietal lobe. Using repetitivetranscranial magnetic stimulation (rTMS) in a cross-over, sham-controlled design, we compared involvement of right posterior parietalcortex with primary visual cortex. Subjects received rTMS over the parietal and occipital cortices during 20 min, after which theyperformed a behaviorally controlled visuospatial mental imagery task. Performance deteriorated significantly after rTMS over the parietal,but not occipital, cortex. These data support a causal link between parietal activation and top-down spatial processing. 2002 ElsevierScience B.V. All rights reserved.
Theme: Neural basis of behavior
Topic: Cognition
Keywords: Parietal cortex, Visuospatial function; rTMS; Imagery
1. Introduction applied over the right posterior parietal lobe. The parietallocation was contrasted with a control location in the
Recent studies have suggested that spatial imagery, i.e. occipital cortex, which has been suggested to be involvedperforming spatial mental operations based on internally in depictive (as opposed to spatial) imagery processinggenerated images (as opposed to externally presented [11].percepts), mainly involves the right parietal cortex[15,20,21], in addition to prefrontal areas [6]. However,these studies applied functional imaging methods such as 2. MethodsfMRI, revealing the cortical structures activated during acognitive task, but they cannot establish whether this The spatial imagery task was first described by Pod-activation is essential to task performance. By inducing gorny and Shepard [17] (cf. Ref. [9]). We used a modifiedtransient inactivation in cortical areas, rTMS can provide version of this task. Subjects had to indicate whether aevidence for a causal relation. In the present study we cross-mark, presented in a 4 by 5 grid on a computertested the hypothesis whether spatial mental imagery screen, would fall on an imaginary letter or not (this coulddepends on parietal cortex activation, using rTMS. We be one of eight different letters, ‘c’, ‘f’, ‘h’, ‘j’, ‘l’, ‘p’, ‘s’,predicted that magnetic-field-induced inhibition of brain ‘u’). A lowercase letter was presented before the grid, andactivity worsens spatial mental imagery performance when subjects were asked to image the corresponding uppercase
letter into the grid. The letter is not actually presented inthe grid, but must be imaged by the subject. For example,*Corresponding author.
E-mail address: [email protected] (A. Aleman). after a fixation point a lowercase letter ‘f’ is presented (for
0926-6410/02/$ – see front matter 2002 Elsevier Science B.V. All rights reserved.PI I : S0926-6410( 02 )00109-X
A. Aleman et al. / Cognitive Brain Research 14 (2002) 300 –302 301
150 ms), followed by an empty grid with the X-mark at the The order of the conditions was counterbalanced overlower right corner (100 ms). The grid subtended |1.58 of participants, to control for possible practice effects. Realvisual angle. The subject had to decide whether the target rTMS conditions were compared with the sham conditionwould fall on an uppercase letter ‘F’ or not. of the other location to avoid type 2 errors due to the
The task consisted of 32 trials, four trials for each of the possibility of slight effects of sham rTMS on informationletters (two ‘on’ and two ‘off’ trials for each letter), in processing in the targeted brain area [12]. In the statisticalpseudorandom order. Our modification of the task was that analysis, ANOVAs were used to examine whether con-we allowed the X-mark to appear only in cells in which the ditions would significantly differ.chance that the X-mark would cover a letter was equal(thus, no X-marks appeared in the most left column, asmost uppercase letters would cover these cells). Dependent 3. Resultsmeasures were reaction time and number of correctresponses (accuracy). Results revealed a significant effect (increase in reaction
Eight subjects, aged between 20 and 25 years, partici- time, RT) of rTMS at P4, compared to sham rTMSpated in the study after having provided written informed (ART5181 ms, P50.006). There was no effect of realconsent. Only subjects without neurological and psychiat- rTMS at Oz compared to sham. The two sham conditionsric disorders and who were naive to rTMS were included. did not differ significantly from each other. As can be seenSubjects were unaware of the aim of the study. All in Fig. 1, reaction times were higher in the parietal than inprocedures were approved by the local ethics committee of the occipital condition for all participants. None of thethe University faculty. Subjects were stimulated continu- conditions affected the percentage of correct responses,ously with rTMS during 20 min, after which they per- consistent with other studies that show reaction times to beformed the imagery task. a more sensitive measure than accuracy in cognitive TMS
Real rTMS and sham rTMS (which controls for the studies. The overall mean reaction time on the imagerycharacteristic sound and sensation of real rTMS on the task was 870 ms (standard deviation 240).scalp) were applied to the Oz scalp position and to the P4 In conclusion, the results from the present brain map-position (according to the international 10-20 EEG sys- ping experiment with rTMS provide compelling evidencetem), corresponding to Brodmann area 17 and right for posterior parietal mediation of spatial mental imagery.Brodmann area 7, respectively [4,5]. TMS at these posi- Thus, our finding is consistent with the hypothesis thattions has been shown to result in inactivation of primary top-down visuospatial processing depends on parietalvisual cortex [3] and posterior parietal cortex [16], respec- structures, and does not necessarily involve primary sen-tively. rTMS was carried out with a 2 Tesla Neopulse sory areas [8,20]. In this regard, it has been suggested thatstimulator (Neotonus Inc., iron-core coil). rTMS is capableof changing the cortical excitability of regions of the brain.Frequencies of around 1 Hz produce inhibition, whereasfrequencies over 10 Hz have been shown to producefacilitation [19]. There is evidence that these effects alsodepend on duration (i.e. number of pulses applied) andintensity of stimulation (i.e. stimulating above or below themotor threshold, which is the intensity that enables visuali-sation of thumb movement). Research using motor evokedpotentials and brain imaging indicates that the interin-dividual variability of effects is reduced from 1600 pulsesonwards [13], while the focality of effects increases whenkeeping the intensity below the motor threshold [14]. Here,frequency of 2 Hz was used at 90% of motor threshold for20 min continuously. With these parameter settings we arenot only below motor threshold and well above 1600pulses, but also within safe limits [18]. A purpose-builtcoil holder was used to secure coil position duringstimulation. For sham stimulation in the air, the coil washeld close to but not in contact with the scalp, and rotated908. Subjects were tested in four sessions, each session ona different day: (1) real rTMS at the Oz position, (2) shamrTMS at the Oz position, (3) real rTMS at the P4 position,and (4) sham rTMS at the P4 position. Subjects performed Fig. 1. Difference of reaction times between the real rTMS and shamthe imagery task immediately after the 20 min of rTMS. rTMS conditions for the Oz and P4 positions.
302 A. Aleman et al. / Cognitive Brain Research 14 (2002) 300 –302
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