Current Biology 16, 1479–1488, August 8, 2006 ª2006 Elsevier Ltd All rights reserved DOI 10.1016/j.cub.2006.06.057 Article Concurrent TMS-fMRI and Psychophysics Reveal Frontal Influences on Human Retinotopic Visual Cortex Christian C. Ruff, 1,2,3, * Felix Blankenburg, 1,3 Otto Bjoertomt, 1,3 Sven Bestmann, 1,3,4 Elliot Freeman, 1,2 John-Dylan Haynes, 5 Geraint Rees, 1,3 Oliver Josephs, 3 Ralf Deichmann, 3 and Jon Driver 1,2,3 1 UCL Institute of Cognitive Neuroscience University College London 17 Queen Square London WC1N 3AR United Kingdom 2 UCL Department of Psychology University College London 26 Bedford Way London WC1H 0AP United Kingdom 3 Wellcome Department of Imaging Neuroscience University College London 12 Queen Square London WC1N 3BG United Kingdom 4 Sobell Department of Motor Neuroscience and Movement Disorders University College London Queen Square House Queen Square London WC1N 3BG United Kingdom 5 Max-Planck Institute for Human Cognitive and Brain Sciences Stephanstrasse 1a D-04103 Leipzig Germany Summary Background: Regions in human frontal cortex may have modulatory top-down influences on retinotopic visual cortex, but to date neuroimaging methods have only been able to provide indirect evidence for such func- tional interactions between remote but interconnected brain regions. Here we combined transcranial magnetic stimulation (TMS) with concurrent functional magnetic resonance imaging (fMRI), plus psychophysics, to show that stimulation of the right human frontal eye-field (FEF) produced a characteristic topographic pattern of acti- vity changes in retinotopic visual areas V1-V4, with func- tional consequences for visual perception. Results: FEF TMS led to activity increases for retino- topic representations of the peripheral visual field, but to activity decreases for the central field, in areas V1- V4. These frontal influences on visual cortex occurred in a top-down manner, independently of visual input. TMS of a control site (vertex) did not elicit such visual modulations, and saccades, blinks, or pupil dilation could not account for our results. Finally, the effects of FEF TMS on activity in retinotopic visual cortex led to a behavioral prediction that we confirmed psychophys- ically by showing that TMS of the frontal site (again com- pared with vertex) enhanced perceived contrast for peripheral relative to central visual stimuli. Conclusions: Our results provide causal evidence that circuits originating in the human FEF can modulate ac- tivity in retinotopic visual cortex, in a manner that differ- entiates the central and peripheral visual field, with func- tional consequences for perception. More generally, our study illustrates how the new approach of concurrent TMS-fMRI can now reveal causal interactions between remote but interconnected areas of the human brain. Introduction Activity in human visual cortex does not depend solely on current input from the retina. Neuroimaging has shown that early retinotopic areas (including V1) can ex- hibit activity changes even when no visual stimulus is present as a result of factors such as directed attention [1–5] or saccades in darkness [6]. The sources for such modulation of occipital sites are debated but are widely thought to include influences from frontal regions [7–11]. Although it has often been suggested that human frontal cortex may modulate activity in posterior sensory corti- ces in a ‘‘top-down’’ manner [8, 10, 12–16], such influ- ences have rarely been shown directly. New methodo- logical approaches may be required for direct study of any such causal influences between remote but inter- connected regions in the human brain. Here we combined functional magnetic resonance im- aging (fMRI) with concurrent transcranial magnetic stim- ulation (TMS), which is technically demanding to imple- ment in the scanner but is now achievable [17–19]. In this way, we studied directly whether stimulating over a par- ticular region of frontal cortex (human frontal eye-field, FEF) could modulate fMRI activity in remote occipital visual areas V1-V4 and thus tested for causal influences on retinotopic visual cortex. We applied frontal TMS over the right posterior middle frontal gyrus, just ventral to the junction of superior fron- tal sulcus and ascending limb of precentral sulcus, in each individual (see red star in Figure 1A for schematic, Figure S1 for TMS site in individual brains, and supple- mental text for TMS-localization procedures). This par- ticular frontal site is widely held to correspond to human FEF on the basis of prior neuroimaging [20], electrical stimulation [21], and purely behavioral TMS studies [22–26]. We chose this specific frontal region as the ini- tial target for TMS for three reasons. First, it is often ac- tivated in PET or fMRI studies of directed attention [7] or saccade plans [20, 27], and so it might in principle relate to the occipital modulations observed in such para- digms. Second, recent elegant work using invasive FEF microstimulation in monkeys indicates that influ- ences of this frontal site on visual cortex have some *Correspondence: [email protected]
10
Embed
Concurrent TMS-fMRI and psychophysics reveal frontal influences on human retinotopic visual cortex
This document is posted to help you gain knowledge. Please leave a comment to let me know what you think about it! Share it to your friends and learn new things together.
Transcript
Current Biology 16, 1479–1488, August 8, 2006 ª2006 Elsevier Ltd All rights reserved DOI 10.1016/j.cub.2006.06.057
ArticleConcurrent TMS-fMRI and PsychophysicsReveal Frontal Influences on HumanRetinotopic Visual Cortex
Christian C. Ruff,1,2,3,* Felix Blankenburg,1,3
Otto Bjoertomt,1,3 Sven Bestmann,1,3,4
Elliot Freeman,1,2 John-Dylan Haynes,5
Geraint Rees,1,3 Oliver Josephs,3
Ralf Deichmann,3 and Jon Driver1,2,3
1UCL Institute of Cognitive NeuroscienceUniversity College London17 Queen SquareLondon WC1N 3ARUnited Kingdom2UCL Department of PsychologyUniversity College London26 Bedford WayLondon WC1H 0APUnited Kingdom3Wellcome Department of Imaging NeuroscienceUniversity College London12 Queen SquareLondon WC1N 3BGUnited Kingdom4Sobell Department of Motor Neuroscience
and Movement DisordersUniversity College LondonQueen Square HouseQueen SquareLondon WC1N 3BGUnited Kingdom5Max-Planck Institute for Human Cognitive
and Brain SciencesStephanstrasse 1aD-04103 LeipzigGermany
Summary
Background: Regions in human frontal cortex may havemodulatory top-down influences on retinotopic visualcortex, but to date neuroimaging methods have onlybeen able to provide indirect evidence for such func-tional interactions between remote but interconnectedbrain regions. Here we combined transcranial magneticstimulation (TMS) with concurrent functional magneticresonance imaging (fMRI), plus psychophysics, to showthat stimulation of the right human frontal eye-field (FEF)produced a characteristic topographic pattern of acti-vity changes in retinotopic visual areas V1-V4, with func-tional consequences for visual perception.Results: FEF TMS led to activity increases for retino-topic representations of the peripheral visual field, butto activity decreases for the central field, in areas V1-V4. These frontal influences on visual cortex occurredin a top-down manner, independently of visual input.TMS of a control site (vertex) did not elicit such visualmodulations, and saccades, blinks, or pupil dilation
could not account for our results. Finally, the effects ofFEF TMS on activity in retinotopic visual cortex led toa behavioral prediction that we confirmed psychophys-ically by showing that TMS of the frontal site (again com-pared with vertex) enhanced perceived contrast forperipheral relative to central visual stimuli.Conclusions: Our results provide causal evidence thatcircuits originating in the human FEF can modulate ac-tivity in retinotopic visual cortex, in a manner that differ-entiates the central and peripheral visual field, with func-tional consequences for perception. More generally, ourstudy illustrates how the new approach of concurrentTMS-fMRI can now reveal causal interactions betweenremote but interconnected areas of the human brain.
Introduction
Activity in human visual cortex does not depend solelyon current input from the retina. Neuroimaging hasshown that early retinotopic areas (including V1) can ex-hibit activity changes even when no visual stimulus ispresent as a result of factors such as directed attention[1–5] or saccades in darkness [6]. The sources for suchmodulation of occipital sites are debated but are widelythought to include influences from frontal regions [7–11].Although it has often been suggested that human frontalcortex may modulate activity in posterior sensory corti-ces in a ‘‘top-down’’ manner [8, 10, 12–16], such influ-ences have rarely been shown directly. New methodo-logical approaches may be required for direct study ofany such causal influences between remote but inter-connected regions in the human brain.
Here we combined functional magnetic resonance im-aging (fMRI) with concurrent transcranial magnetic stim-ulation (TMS), which is technically demanding to imple-ment in the scanner but is now achievable [17–19]. In thisway, we studied directly whether stimulating over a par-ticular region of frontal cortex (human frontal eye-field,FEF) could modulate fMRI activity in remote occipitalvisual areas V1-V4 and thus tested for causal influenceson retinotopic visual cortex.
We applied frontal TMS over the right posterior middlefrontal gyrus, just ventral to the junction of superior fron-tal sulcus and ascending limb of precentral sulcus, ineach individual (see red star in Figure 1A for schematic,Figure S1 for TMS site in individual brains, and supple-mental text for TMS-localization procedures). This par-ticular frontal site is widely held to correspond to humanFEF on the basis of prior neuroimaging [20], electricalstimulation [21], and purely behavioral TMS studies[22–26]. We chose this specific frontal region as the ini-tial target for TMS for three reasons. First, it is often ac-tivated in PET or fMRI studies of directed attention [7] orsaccade plans [20, 27], and so it might in principle relateto the occipital modulations observed in such para-digms. Second, recent elegant work using invasiveFEF microstimulation in monkeys indicates that influ-ences of this frontal site on visual cortex have some
Figure 1. Stimulation Sites and Interleaved TMS and fMRI Protocol
(A) Frontal (red star, over right human FEF) and vertex-control (blue star) TMS sites on a normalized brain template (see Figure S1 for TMS sites on
each individual’s brain).
(B and C) Schematic timecourse of TMS relative to MR volume acquisition during combined TMS-fMRI. (B) Trials with visual stimuli on the screen
during TMS; (C) trials without visual stimuli. For each trial, three TMS trains were delivered in the 570 ms gaps between acquisition of subsequent
image volumes, and seven rest scans were included between successive trials. Visual stimuli (when present, as in [B]) remained visible during all
three TMS trains and during the acquisition of the three image volumes after the TMS trains.
physiological plausibility in the primate brain at thesingle-unit level [28]. Finally, TMS to right human FEFcan affect some visual judgements behaviorally in bothhemifields [22–25, 29]. Here we propose that this mightreflect remote influences on activity in retinotopic visualcortex. We tested this directly by measuring humanbrain activity through the use of blood-oxygenation-level-dependent (BOLD) contrast fMRI in humans. TheBOLD signal provides an index of neuronal populationactivity [30–34] and allowed us to measure any TMS-evoked remote effect on multiple visual areas of thehuman brain concurrently.
In separate scanning sessions, we applied TMS dur-ing fMRI either to right FEF or to a control site at the ver-tex. We selected the vertex site in order to control fornonspecific effects of TMS because vertex TMS wouldnot be expected to affect visual cortex except by non-specific means (see Experimental Procedures and Sup-plemental Data for further rationale). We applied TMS toeither site at four different intensities, allowing us toidentify any visual brain areas showing activity changesdue to the intensity of TMS rather than merely its pres-ence versus absence. Participants had to fixate cen-trally, with no other task during scanning, to ensurethat any remote physiological influences of TMS on ac-tivity in visual cortex could not be contaminated byTMS-induced changes in behavior. We administeredTMS either while subjects passively viewed a blank dis-play or while they were presented with bilateral movingand changing visual stimuli designed to activate manyvisual regions (see Figures 1B and 1C). We could thustest whether any TMS influences on activity in visual cor-tex might depend on the level of bottom-up activationvia visual inputs.
We found that increasing the intensity of FEF TMSproduced a characteristic pattern of activity modula-tions in early retinotopic visual areas V1-V4. These
activity changes arose in a top-down manner regardlessof current visual input, in accord with previous fMRI find-ings that visual cortex can show activity changes even inthe absence of visual stimuli, e.g., during directed atten-tion [3] or saccades in darkness [6]. By contrast, TMS tothe control site (vertex) produced no such influences onvisual cortex, thus demonstrating the specificity of theFEF TMS effects. Further analyses showed that thoseeffects were not due to eye movements, blinks, or pupildilation.
The specific retinotopic pattern of fMRI modulationscaused in visual cortex by FEF TMS led to a new predic-tion for perceptual effects that we confirmed in separatepsychophysical TMS work outside the scanner. Takentogether, our results provide causal evidence that thehuman frontal eye-field can modulate activity in earlyretinotopic visual cortex in a manner that differentiatesthe central from the peripheral visual field, with corre-sponding consequences for perception.
Results
Concurrent TMS-fMRI
In both fMRI sessions (right FEF or vertex control), wemaximized sensitivity for early visual cortex (areas V1-V4) by using an occipital surface coil for fMRI in combi-nation with retinotopic mapping of cortical visual areasfor each individual participant. Although TMS does notinduce eye movements [22–26], we were careful to as-sess and eliminate any possible influences on visual cor-tex from blinks, pupil dilations, or losses of fixation (allmeasured throughout scanning). We also took consider-able care to avoid MR artifacts from concurrent use ofTMS (see Experimental Procedures and SupplementalData).
We used two complementary analysis approaches forthe fMRI data. Group analyses of activity across the
TMS-fMRI Reveals Frontal Influences on Visual Cortex1481
Figure 2. Frontal TMS: Activity Changes in the Group Analysis for Stereotactic Space
The brain displays and associated graphs show (A) significant negative correlations or (B) significant positive correlations of BOLD with frontal-
TMS intensity. In the central images these effects are shown as 2D projections of the whole-volume SPM(T) onto a transparent schematic of the
MNI template brain (used here so that no region is hidden) and as renderings onto a transverse slice of the mean structural scan. All thresholds
are set to T > 3 and p < 0.05 (corrected for multiple comparisons at cluster level). The graphs on either side show single-subject plots of the mean
signal intensity (different colors for different subjects, group average in black) in the left-hemisphere regions circled by red in the glass brains (left
graphs) or for the right-hemisphere regions circled by blue (right graphs) for the two highest versus two lowest TMS intensities (see also Figure S2
for results at each of the four TMS intensities separately). The plots show that the described effects in the calcarine and occipital-pole regions
were consistently present across subjects, both when visual stimuli were present (dotted lines) and when they were absent (solid lines) during
TMS. Overall activity in these visual regions was higher with visual stimulation (dotted) than without (solid), but the impact of high versus low
intensity of frontal TMS was additive to this.
image volume (EPI images covering occipital cortex andextending into temporal cortex were acquired with thevisual surface coil) identified any regions in stereotacticspace that reliably displayed activity changes as a func-tion of TMS intensity (or mere TMS presence). We alsoused standard retinotopic mapping procedures [35]within each individual, in conjunction with cortical flat-tening [36, 37], to visualize the topography of any TMSeffects on early retinotopic areas.
Group Analyses in Stereotactic Space
Group whole-volume analysis revealed two bilateral setsof occipital regions with activity levels related to FEFTMS intensity. A significant negative relationship be-tween BOLD signal and TMS intensity was found in bilat-eral regions close to the occipital poles (these regionstherefore represented central visual locations), withstronger FEF TMS leading to lower activity there (Fig-ure 2A; see also Table S1). The opposite pattern, of sig-nificantly higher activity with stronger FEF TMS, wasfound for bilateral regions in anterior-calcarine sulci (rep-resenting the more peripheral visual field; Figure 2B; seealso Table S1). These opposite effects on anterior-cal-carine sulci versus occipital poles were present in eachparticipant, as shown in plots of mean activity for these
regions under high or low TMS intensities (see graphson either side of Figures 2A and 2B; see also Figure S2).These plots additionally demonstrate that the influence ofFEF TMS intensity on these occipital regions was equiva-lent during the presence or absence of visual stimuli (Fig-ures 2A and 2B), even though overall activity was higherduring visual stimulation. No region in the acquired vol-umes displayed any interaction of frontal-TMS intensitywith the presence versus absence of visual stimuli.
By contrast, increased intensity of vertex TMS did notelicit any significant activity changes in visual cortex (thecorresponding results in Figure 2 show no significant ef-fects). We formally confirmed this difference betweenthe TMS sites (i.e., frontal versus vertex TMS) duringscanning by extracting the mean signal from sphericalregions of interest (ROIs, 6 mm radius) centered in theregions that displayed activity changes during FEFTMS (see circles in Figure 2). For both the occipital-pole (central visual field) and anterior-calcarine (periph-eral visual field) regions, the modulatory effects of TMSintensity (two highest versus two lowest intensities)were significantly bigger for FEF than for vertex TMS(2 3 2 repeated-measures ANOVA on the signals fromthese ROIs; interaction of TMS intensity 3 TMS site,p < 0.05, for each ROI). Pairwise comparisons showed
Current Biology1482
Figure 3. Frontal TMS: Mean Effects for Different Eccentricity Sectors in Retinotopic Visual Areas
(A) An illustrative flatmap of retinotopic visual areas in one subject and hemisphere (see Figure S3 for all others). The flatmaps confirm BOLD-
signal increases with stronger frontal TMS in retinotopic representations of the peripheral visual field but BOLD-signal decreases instead for
central-visual-field representations around the foveal confluence. The voxel-wise correlation of BOLD with TMS intensity is plotted as a standard-
ized T value (in relation to voxel-wise residuals of the model) according to the color bar at bottom. Hot colors indicate positive and cold colors
indicate negative correlations with TMS intensity. The foveal representation is indicated approximately by the cross, and borders of all mapped
visual areas are indicated by black lines.
(B–E) The correlation of TMS intensity with BOLD (quantified as T value) was extracted from each individual flatmap, separately for four different
eccentricity sectors in each region (see Supplemental Data). (B) Mean effect of frontal-TMS intensity for each area and eccentricity sector, av-
eraged across flatmaps and voxels within each sector (this measure is conservative, given the larger effects at peak voxels). The most central
sector is outlined in dark gray, and the most peripheral sector is outlined in light gray. The effects are color coded according to the scale below.
(D) An analogous representation, but now for differences between effects of frontal versus vertex TMS. Both (B) and (D) indicate that increased
intensity of frontal TMS produced activity increases for peripheral-visual-field representations in V1–V4 (‘‘outer’’ segments in these graphs) but
activity decreases in the most central eccentricity sector. Panels (C) and (E) plot the corresponding mean TMS-induced effect with its standard
error ([C] for frontal TMS; [E] for frontal-minus-vertex difference) for the most central and the most peripheral eccentricity sectors when data are
averaged across visual areas (leftmost two bars) or separately for areas V1–V4 (data are pooled across dorsal and ventral subdivisions). In all
these retinotopic visual areas, increased frontal-TMS intensity produced activity increases for the peripheral sector but activity decreases for
the central sector (stars indicate p < .05 in paired t tests).
that TMS intensity had significant effects only for FEFTMS (paired t tests, all p < 0.05) but not for vertex TMS(all not significant [n.s.]). Finally, the differences in TMSeffects between the ROIs (occipital poles versus calcar-ine sulci, i.e., the differential effects for central versusperipheral visual field) were also significantly strongerfor FEF than for vertex TMS (2 3 2 repeated-measuresANOVA, interaction of ROI 3 TMS site, p < 0.05). Takentogether, these initial group analyses in stereotacticspace indicate that TMS intensity over the FEF, but notthe vertex, modulated activity in occipital cortex differ-entially for representations of the peripheral versus cen-tral visual field. As discussed below, we confirmed thispattern in further detail by examining individually flat-mapped retinotopic visual areas.
Individual Retinotopic Analyses
A topographic pattern of FEF TMS effects on fMRI activ-ity in occipital visual cortex was reliably present in earlyretinotopic visual areas for all participants and hemi-spheres (Figure 3A shows one example; Figure S3 showsall). Specifically, we found activity increases with stron-ger FEF TMS in peripheral visual field representationsfor each retinotopic visual area (notably, including evenV1), whereas activity decreases were located in repre-sentations of the central visual field around the fovealconfluence. Although individual flatmaps (Figure S3)
show minor variations, as typical for such data, the over-all pattern was clearly present in each.
We confirmed this consistency by quantifying the pat-tern across subjects. We divided each of the areas V1–V4 into four sectors representing different eccentricitiesin the visual field (see Supplemental Data and [38]) andthen extracted the inter-participant mean effect of FEFTMS intensity on BOLD signal for each such sector(see Figure 3B, where ‘‘inner’’ segments are less eccen-tric and ‘‘outer’’ segments are more eccentric). Figure 3Cshows the mean effect of FEF TMS intensity for the mostperipheral (light bars) and for the most central (darkbars) retinotopic sector in visual cortex. Averagedacross areas V1–V4 (leftmost pair of bars in Figure 3C),activity in the peripheral sector was significantly in-creased by higher-intensity FEF TMS, but activity inthe central sector was instead significantly decreasedby higher-intensity FEF TMS (t tests, both p < 0.001).This same pattern also applied significantly when eachretinotopic area was considered individually (Figure 3C;t tests, all p < 0.05, except for the trend in the peripheralV4 sector). In direct paired comparisons, the FEF TMSinfluence was significantly different for the peripheralthan for the central sector in all visual areas (Figure 3C;asterisks indicate p < 0.05 in paired t tests).
These retinotopic analyses show that TMS over righthuman FEF had distinct effects on fMRI activity in
TMS-fMRI Reveals Frontal Influences on Visual Cortex1483
representations of the peripheral versus central visualfield in early retinotopic visual areas. This accords withthe spatially normalized group analysis presented earlier(Figures 2A and 2B), but the retinotopic analyses (Fig-ure 3) additionally show that this topographic patternof influences holds for multiple areas of early retinotopichuman visual cortex, including even area V1.
Thus far, the retinotopic analyses only considered ac-tivity in visual areas during FEF TMS. We next comparedthis directly to the vertex-TMS scanning data by calcu-lating the differences between FEF- and vertex-TMS in-tensity effects for each eccentricity sector in each reti-notopic visual area (Figure 3D). This analysis showedessentially the same pattern as for the FEF TMS dataalone because only that TMS site produced the topo-graphic pattern of changed activity in retinotopic visualcortex (again consistent with the group analysis in ste-reotactic space, where vertex TMS was found to haveno effect on occipital cortex). We found significant dif-ferences between the influences of FEF versus vertexTMS on the central versus peripheral sectors, bothwhen data were pooled across visual areas and foreach region alone (2 3 2 repeated-measures ANOVAs;p < 0.05 for all interactions between the TMS site andthe central versus peripheral sector). Figure 3E showsthese differences between FEF and vertex TMS-inten-sity effects for the most peripheral and most centralretinotopic sectors. Note that a similar pattern is appar-ent in Figures 3E (difference of FEF and vertex TMS) and3C (FEF-TMS effects only). Thus, even when directly ac-counting for any potential nonspecific effects of TMS(via comparison with the vertex TMS data), we still foundthat stronger FEF TMS led to significantly increasedfMRI activity for sectors representing the peripheral vi-sual field, but to decreased activity instead for sectorsrepresenting the central visual field, in every retinotopicvisual area (compare light and dark bars for each pair inFigure 3E).
On-line eye-tracking throughout scanning (see Exper-imental Procedures) measured eye position, blinks, andpupil diameter. None of these factors can explain the ob-served pattern of FEF TMS effects on retinotopic visualcortex (see Figure S4 and supplemental text). The ef-fects on visual cortex also cannot be plausibly attributedto any possible cross-modal influence of the ‘‘clicking’’sound or somatosensory impact of TMS. Such nonspe-cific effects were equivalent for frontal and vertex TMS,with similar activation of auditory and somatosensorycortices by these TMS sites here (see Figure S5 and sup-plemental text).
In sum, these fMRI results show directly that TMS offrontal cortex, over right human FEF, causally modu-lated activity in retinotopic visual cortex (V1–V4) in atop-down manner. Stronger FEF TMS led to a specificretinotopic pattern of increased activity for the periph-eral visual field but led to decreased activity for centralvisual-field representations, whereas this pattern wasnot produced by control TMS to the vertex.
Psychophysical Study
The fMRI results described above suggest a behavioralprediction that we tested in a further psychophysical ex-periment. We could now predict that TMS to the frontalsite (over right FEF) may enhance peripheral vision
relative to central vision for both hemifields. Given thatearly visual areas were modulated by FEF TMS, includ-ing even V1, we tested this behavioral prediction by us-ing visual stimuli and a judged property that shouldinvolve early visual cortex; namely, the perceived con-trast of Gabor patches. We applied TMS to the samefrontal (right FEF) or vertex sites as before but now didso during a psychophysical task that required partici-pants to judge which of two concurrent stimuli (one cen-tral and one peripheral Gabor patch, the latter presentedrandomly on the left or right) appeared higher in per-ceived contrast (see [39] for a similar measure). Centralfixation was again ensured with on-line eye tracking.The central patch had a fixed (25%) contrast, whereasthe peripheral patch on the left or right varied in contrastvia an adaptive algorithm (see Experimental Proceduresand Supplemental Data). We derived the point of subjec-tive equality (PSE) between central and peripheral con-trasts by fitting psychometric functions to the behavioraldata (e.g., see Figure 4B). Separate PSEs were deter-mined for each visual hemifield for TMS at the frontalor vertex site.
We chose these particular stimuli and this task forseveral reasons. Although extrapolating from fMRI sig-nals to visual perception often requires many caveats,in the specific case of contrast there is already someevidence that BOLD increases in early visual cortexcan be associated with increases in contrast perception[40–42]. Moreover, perceived contrast can be enhancedby top-down influences (e.g., by attention [39]), whichmight extend to the present top-down influences fromFEF TMS also. Finally, it is often argued (e.g., [8, 10,43]) that top-down increases in baseline occipital activ-ity may lend a competitive advantage to correspondingvisual stimuli when presented. Based on these findingsand suggestions, we predicted that the topography oftop-down occipital activity changes found during FEFTMS in our fMRI experiment may lead to enhancementsof perceived contrast for peripheral relative to centralstimuli.
The psychophysical results accorded with these pre-dictions derived from our fMRI results, indicating thatthe effects of FEF TMS on activity in visual cortex canhave perceptual consequences for vision. Perceivedcontrast judgements were altered systematically byFEF as compared to vertex TMS, with peripheral stimulihaving stronger perceived contrast relative to centralstimuli during FEF TMS (see Figures 4B–4C). Moreover,this pattern applied equivalently for either peripheralhemifield, again just as expected from our fMRI resultsin retinotopic cortex during FEF TMS. This outcomewas confirmed in a 2 (frontal or vertex TMS) 3 2 (periph-eral patch on left or right) repeated-measures ANOVA ofthe PSE data, which showed a reliable effect of TMS site(F(1,6) = 7.69, p < 0.05) but no effect or interaction due tohemifield. Note that this effect corresponded to a lateralshift in the psychometric functions (see example inFigure 4B); although the PSEs differed significantly asa result of TMS site, the slopes of the underlying psycho-metric functions did not (all terms n.s. in a correspondingANOVA on slopes). Finally, for completeness we alsocompared the two TMS conditions (which were run incounterbalanced order) to a no-TMS condition run atthe end of each session (see Figure S6).
Current Biology1484
Figure 4. Frontal TMS Enhances Perceived Contrast for Peripheral Relative to Central Visual Stimuli for both Hemifields
(A) Frontal (red star) and vertex-control (blue star) TMS sites, selected according to the same criteria as in the neuroimaging experiments (cf.
Figure 1A).
(B) Psychometric curves fitted to the psychophysical data of an illustrative participant (who had also taken part in neuroimaging) for one hemifield
when the individual was judging which of two concurrent Gabor patches appeared higher in contrast (either the central patch of fixed contrast or
a peripheral patch of varied contrast, unpredictably on the left or right). Separate psychometric functions were obtained with frontal TMS (red
curve) or vertex TMS (blue curve) co-occurring with the visual displays in counterbalanced order. The intersection of the dashed horizontal line
with either curve indicates the point of subjective equality (PSE) value for the peripheral patch (contrast at which the patch was perceived as
equivalent to the fixed central patch) in the corresponding TMS condition; note the lateral shift of the psychometric curve as a result of frontal
versus vertex TMS.
(C) Inter-participant mean contrast-value differences between central and peripheral stimuli at the derived PSE (in percent of contrast of central
patch) for both TMS conditions and both hemifields. Because of the subtraction of contrast values at the PSE (central minus peripheral contrast
value), higher values represent more enhancement of peripheral relative to central perceived contrast. The graph shows that, as compared with
vertex TMS (blue bars), frontal TMS (red bars) significantly enhanced peripheral relative to central perceived contrast in both hemifields (stars
indicate p < 0.05 for main effect of TMS site in ANOVA, in the absence of significant effect or interaction due to hemifield; see also Figure S6
for no-TMS data).
In sum, TMS of right human FEF significantly en-hanced perceived contrast for peripheral visual stimulirelative to central stimuli in both hemifields. This ac-corded with the pattern of peripheral enhancement butcentral suppression that we observed for early retino-topic visual cortex in the fMRI experiments during TMSof the same frontal site.
Discussion
By combining fMRI with concurrent TMS in the scanner,we found that stimulating a region of frontal cortex (righthuman FEF) could produce systematic remote effectson fMRI signal in early human retinotopic cortex, includ-ing even area V1. These effects could not be attributedto blinks, changes in pupil size, or losses of fixation.The direct comparison with vertex TMS suggests thatthese effects also did not reflect any nonspecific TMSeffects, such as the associated ‘‘clicking’’ sound. Ourresults thus provide causal evidence that signals origi-nating in human frontal cortex are capable of modulatingactivity in early human visual cortex, as previouslyproposed only on much more indirect grounds [8, 10,12–14].
The present effects of frontal TMS on visual cortextook a specific retinotopic form, with stronger TMS ofright FEF increasing fMRI activity for representationsof the peripheral visual field but reducing activity forthe central field in all retinotopic visual areas. ThisfMRI pattern led to a novel behavioral prediction that weconfirmed with psychophysics by showing that TMS tothe same frontal site (versus vertex) enhanced perceived
contrast for peripheral relative to central visual stimuli.Although it can be difficult to extrapolate from fMRIeffects to visual perception, for the specific case ofcontrast a relation to fMRI signals in early visual cortexhas been established [40–42]. This permitted our newapproach of using a pattern of remote activity changesfound with concurrent TMS-fMRI to derive (and confirm)a prediction for behavioral effects of TMS.
Our results echo but also extend recent findings frommonkey studies. Elegant work by Moore and colleagueshas shown that electrical microstimulation of macaqueFEF neurons with implanted microelectrodes, at intensi-ties too low to elicit a saccade, can modulate activity inV4 neurons with spatially corresponding receptive fields[28, 44]. At an abstract level, our results accord well withthose monkey studies in establishing a causal effect ofFEF on occipital visual cortex, now for the human brain.However, the studies differ in more concrete details. Forinstance, we showed that human FEF can influence eventhe earliest retinotopic visual areas (V1, V2, and V3).Moreover, the present effect of FEF TMS on visual cor-tex was independent of the concurrent changing andmoving visual input, whereas the previous FEF-microsti-mulation effects on single-unit firing in V4 depended onthe visual preferences of the individual neuron and onthe preferred static stimulus being present for sometime prior to microstimulation [28, 44]. Such differencesin the details of our findings and the recent monkey workmay be explained by methodological aspects, and onemust be cautious in extrapolating from fMRI findingsto single-unit findings or vice versa (see also [4] for a dis-cussion of this issue). Here we indexed neural activity
TMS-fMRI Reveals Frontal Influences on Visual Cortex1485
from large populations by using BOLD-contrast fMRI,which may correlate better with local field potentials[31, 33] and synchronized population activity [34] thanwith spiking output. It has been suggested that BOLD-contrast fMRI may more closely index the input into anarea than its local firing rates [30, 32, 45]. For this veryreason, fMRI may be particularly sensitive to top-downinfluences [34, 46], as here. It should also be notedthat TMS is very different from microstimulation andwill target sizeable neural populations [47]. Most cru-cially, however, the present findings are fully consistentwith other demonstrations that fMRI signal changes invisual cortex can arise without the presence of a visualstimulus (e.g., during directed attention [3, 4]). The pres-ent study shows directly that human FEF is a plausiblesource for such modulations. Moreover, it corroboratesa new methodology for studying causal influencesbetween brain areas; unlike the invasive approachesemployed in monkeys, this methodology can now beused in humans.
The general point that TMS to frontal cortex can havesome remote physiological effects in the human brainwas first demonstrated in a pioneering PET study [48],which showed that frontal TMS (again over FEF) couldlead to some changes in PET activity for posterior brainregions, such as the parieto-occipital sulcus. Moreover,one recent EEG study reported that TMS over a similarfrontal site can change voltage fluctuations recordedfrom electrodes over posterior scalp positions [49]. Al-though PET and EEG studies cannot examine retino-topic visual cortex in any detail (because of methodolog-ical limitations; see Supplemental Data), here we wereable to maximise power for visual cortex (albeit inevita-bly with less power for more anterior structures such asfrontal or parietal cortex) by using fMRI with an occipitalsurface-coil in conjunction with individual retinotopicmapping. This allowed us to show that TMS of humanFEF can affect early retinotopic visual areas, includingeven V1, with a specific topographic pattern. The newmethodological combination of TMS during retinotopicfMRI of visual cortex now opens up many possibilitiesfor future work, including TMS to further frontal or pari-etal sites in the same or opposite hemispheres.
The specific pattern of FEF TMS influences we foundin human visual cortex may have implications for furtherresearch on the structure, function, and connectivity ofthe human FEF. The effects on visual cortex here arosebilaterally (despite right FEF stimulation) and affectedeven area V1, for which monosynaptic connectionswith the FEF have not been reported so far in the ma-caque brain [50, 51]. Although humans might differfrom monkeys, we suspect that the FEF-occipital cir-cuits underlying the present effects may be poly- ratherthan mono-synaptic and might involve intervening fron-tal [52], parietal [50, 51, 53], or subcortical [54] brain re-gions. Future extensions of the present method couldcombine TMS with whole-brain fMRI to examine anyroles for intervening regions and pathways and mighteven test the contribution of transcallosal connectionsvia split-brain patients [55]. However, our main aimhere was to characterize any frontal influences on reti-notopic visual cortex; we were able to achieve this aimby maximizing our power to detect such effects withan MR surface-coil centered over occipital cortex.
Moreover, the bilateral nature of the fMRI effects fromright-FEF TMS accorded well with the bilateral psycho-physical result we found for the same TMS site, whichaffected perceived contrast for both visual fields.
It is also noteworthy that the present results revealeddistinct effects of FEF TMS on peripheral versus centralvisual-field representations. This difference may accordwith some known anatomical details of macaque FEF,where the central and the peripheral visual field appearfunctionally differentiated by two neuronal subpopula-tions. These code for either large saccades and periph-eral visual locations or small saccades and more centrallocations, and they are mainly connected to occipital re-gions via separate pathways involved in more peripheralor more central vision, respectively [50, 51]. Subdivi-sions and anatomical connections for human FEF arenot as well established as in monkeys, but there arenow some initial demonstrations that the peripheral vi-sual field may be represented spatiotopically in humanFEF [56], in a patch of cortex readily targeted by TMS.Our results encourage further research into the questionof whether the peripheral and central visual field mightbe separately represented within human FEF, with dis-tinct connections to occipital cortex, in analogy to themacaque brain.
The nature of the fMRI effects on visual cortex as a re-sult of FEF TMS here accorded well with our psycho-physical TMS findings, which showed that perceivedcontrast was enhanced for peripheral relative to centralvisual stimuli in both hemifields during stimulation of thesame frontal site. Such an enhancement for peripheralvisual stimuli may conceivably play a functional role dur-ing saccade planning and execution or during covertattention to the visual periphery, consistent with theknown involvement of the FEF in those situations [7,10, 27, 57]. Our results may also reconcile seemingly dis-crepant results from prior, purely behavioral TMS stud-ies that had likewise reported bilateral effects on visualjudgments during stimulation of right human FEF.Some of those prior behavioral TMS studies found en-hancements of visual judgments [22, 23], whereas othersreported impairments instead [24, 25]. Although thoseprior behavioral studies differed from each other in sev-eral methodological details, our fMRI and psychophysi-cal results highlight a previously overlooked factor. Theprevious reports of visual judgments facilitated by TMSof right FEF had presented visual targets more eccen-trically [22, 23] than those reporting behavioral impair-ments instead [24, 25]; the latter studies used morecentral targets (w2� visual angle). Our fMRI resultsdirectly show that TMS of right FEF has opposing effectson representations of the peripheral versus central visualfield within retinotopic visual cortex, consistent with theperceptual effects that we found psychophysically.
At a more general level, our findings highlight the factthat TMS not only may affect the targeted cortical site inisolation but can also result in remote physiologicaleffects on interconnected brain regions (as foundhere for visual cortex after FEF TMS), which may havefunctional consequences (as found here for visualperception). This could challenge some conventionalinterpretations of purely behavioral TMS effects; thoseinterpretations have often considered only the targetedbrain-site alone. However, this does not limit the utility
Current Biology1486
of TMS, which can now be used in combination withfMRI, as here, to study influences between brain regions(see also [48, 58]), as well as the causal roles of the tar-geted site in inducing such effects.
Conclusions
The present results establish that TMS of human frontalcortex, over the right human FEF, can causally modulatefunctional activity in early retinotopic visual cortex, ina systematic fashion that distinguishes the central andperipheral visual fields, with corresponding perceptualconsequences. More generally, our study illustrates howcombining TMS and fMRI now allows the direct studyof causal functional interactions between remote butinterconnected areas of the human brain.
Experimental Procedures
Participants
The same four male participants (29–35 years) took part in both neu-
roimaging experiments, and there were seven male participants (29–
36 years, three of whom also took part in fMRI) in the psychophysical
studies. All were right-handed and reported normal vision and no
history of neurological or psychiatric illness. They participated with
informed consent in accord with local ethics.
Neuroimaging Experiments: Setup and Stimulation
Functional data were acquired on a 1.5 T whole-body scanner
(SONATA, Siemens, Erlangen, Germany) with a custom visual surface
coil (Nova Medical, Boston, MA) with maximum sensitivity over
occipital cortices and extending into temporal cortex. A multi-slice
gradient echo EPI sequence was used to acquire BOLD contrast