The Neural Signature of Phosphene Perception

Transcription

1 Human Bain Mapping 31: (2010) The Neual Signatue of Phosphene Peception Paul C.J. Taylo, 1,2 * Vincent Walsh, 2,3 and Matin Eime 1 1 School of Psychology, Bikbeck College, London WC1E 7HX, United Kingdom 2 Institute of Cognitive Neuoscience, Univesity College London, London WC1N 3AR, United Kingdom 3 Depatment of Psychology, Univesity College London, London WC1N 3AR, United Kingdom Abstact: Atificial pecepts (phosphenes) can be induced by applying tanscanial magnetic stimulation (TMS) ove human visual cotex. Although phosphenes have been used to study visual awaeness, the neual mechanisms geneating them have not yet been delineated. We diectly tested the two leading hypotheses of how phosphenes aise. These hypotheses coespond to the two competing views of the neual genesis of awaeness: the ealy, feedfowad view and the late, ecuent feedback model. We combined online TMS and EEG ecodings to investigate whethe the electophysiological coelates of conscious phosphene peception ae detectable ealy afte TMS onset as an immediate local effect of TMS, o only at longe latencies, afte inteactions of TMS-induced activity with othe visual aeas. Stimulation was applied at the intensity theshold at which paticipants saw a phosphene on half of the tials, and bain activity was ecoded simultaneously with electoencephalogaphy. Phosphene peception was associated with a diffeential patten of TMS-evoked bain potentials that stated ms afte stimulation and encompassed a wide aay of posteio aeas. This patten was diffeentiated fom the TMS-evoked potential afte stimulation of a contol site. These findings suggest that conscious phosphene peception is not a local phenomenon, but aises only afte extensive ecuent pocessing. Hum Bain Mapp 31: , VC 2010 Wiley-Liss, Inc. Key wods: phosphenes; tanscanial magnetic stimulation; electoencephalogaphy; evoked potentials; visual peception; visual cotex; awaeness; consciousness INTRODUCTION Tanscanial magnetic stimulation (TMS) is a technique that can be used to test whethe a cotical aea is necessay fo a given function, by tansiently inteacting with the Suppoted by the Medical Reseach Council (MRC), UK. *Coespondence to: Paul C.J. Taylo, School of Psychology, The Heny Wellcome Building, Bikbeck College, Toington Squae, London WC1E 7HX, United Kingdom. pc.taylo@bbk.ac.uk Received fo publication 26 August 2009; Accepted 12 Octobe 2009 DOI: /hbm Published online 20 Januay 2010 in Wiley Online Libay (wileyonlinelibay.com). nomal undelying patten of neual activity and studying the consequences. Fo example, if TMS is applied to pimay visual cotex (V1) then paticipants can epot having peceived an atificial flash-like visual pecept, o phosphene. TMS-induced phosphenes can be used to exploe the neual dynamics undelying visual peception [Antal et al., 2003; Cowey and Walsh, 2000; Gothe et al., 2002; Rauschecke et al., 2004; Walsh and Pascual-Leone, 2003]. Phosphenes have been found to show seveal popeties: they occu moe often as the intensity of stimulation is inceased, and a theshold intensity of stimulation can be detemined fo individual paticipants at which phosphenes ae elicited on about half of tials [Kamme et al., 2001; Stewat et al., 2001]. This phosphene theshold can be educed (i.e., it becomes easie to elicit a phosphene) if TMS is fist applied to connected aeas such as posteio paietal cotex [Silvanto et al., 2009], o the fontal eye VC 2010 Wiley-Liss, Inc.

2 The Neual Signatue of Phosphene Peception fields [Silvanto et al., 2006] whee TMS can also facilitate peception of visual stimuli [Gosbas and Paus, 2003]. Duing phosphene theshold stimulation, cotical excitability immediately befoe the pulse pedicts whethe o not a phosphene is elicited [Romei et al., 2008a], suggesting that the peceptual diffeences between consecutive stimulation tials ae diven by inheent vaiability in the cental nevous system. The pesence of phosphenes demonstates that atificial conscious pecepts can be geneated noninvasively in humans, but the time couse and patten of the neual activity that ae esponsible fo such pecepts ae not yet known. Obtaining an electophysiological measue of these atificial pecepts would offe new insights into how and when duing visual pocessing phosphenes ae geneated. This could enable a deepe intepetation of pevious studies that have used phosphenes and povide a novel measue fo futue wok to exploit. We ecoded the bain activity in esponse to occipital TMS by combining online TMS with electoencephalogaphy (EEG). TMS intensity was adjusted to phosphenetheshold so that a phosphene would be peceived on appoximately half of all tials. TMS-EEG can show causal inteactions between bain aeas with high tempoal esolution [Dive et al., 2009; Ilmoniemi et al., 1997; Miniussi and Thut, in pess; Taylo et al., 2008]. This method was applied hee to chaacteize the TMS-evoked potential (TEP) in esponse to occipital TMS by compaing electical bain esponses that wee time-locked to TMS onset on phosphene-pesent and phosphene-absent tials. The ationale of this design was that the subtaction of the TEP on phosphene-absent fom phosphene-pesent tials would isolate the diffeential activity elated to conscious peception of the phosphene and emove the nonspecific esponses to the acoustic o somatosensoy atifact that accompany TMS, but ae identical acoss expeimental conditions. To chaacteize the visual cotical esponse that is tiggeed by TMS, but is independent of conscious phosphene peception, we also compaed TEPs tiggeed by occipital TMS to TEPs elicited by stimulation of a contol site in paietal cotex. We tested two contasting hypotheses with espect to the time couse of neual activity that is elated to phosphene peception. The ealy hypothesis pedicts that the TEP on phosphene-pesent tials should stat to diffe fom the TEP on phosphene-absent tials within a vey shot time inteval (i.e. seveal tens of milliseconds) afte the TMS pulse. Such a esult would suggest that vaiations in the initial esponse of the visual aeas diectly stimulated by the TMS pulse detemine whethe o not a phosphene is peceived. In othe wods, phosphenes ae peceived wheneve the stimulated aea itself is in a paticulaly excitable state at the time of the TMS pulse, and phosphene peception does not equie any inteactions with othe aeas. Such ealy effects on cotico-spinal excitability have been demonstated afte TMS of the pimay moto cotex M1 [Hess et al., 1987] and might be pedicted if all aeas espond to TMS in a simila fashion such that the moto-evoked potential can be egaded as a diect (albeit peipheal) analog of the occipital TEP. By contast, an altenative late theoy pedicts that diffeences between the patten of neual activity on phosphene-pesent and phosphene-absent tials, as eflected by TEPs, will not emege immediately afte TMS onset, but ae instead obseved with a consideable delay (e.g., 150 o 200 ms afte the TMS pulse). Accoding to this account, conscious phosphene peception is not simply detemined by diffeences in local cotical excitability, but is instead linked to inteactions between a wide ange of visual aeas. Fo example, phosphene peception and phosphene-elated potentials might only aise afte the feedfowad activation of highe-ode visual aeas, followed by ecuent loops that involve the stimulated site. Such additional pocessing fom ecuent loops of activation has been suggested to be a citical peequisite fo phosphenes [Pascual-Leone and Walsh, 2001] and fo nomal conscious visual peception [Lamme and Roelfsema, 2000]. Additional electophysiological evidence consistent with this late theoy comes fom ecent wok whee ERPs wee measued in esponse to backwad-masked visual stimuli that wee pesented at peceptual theshold [Del Cul et al., 2007]. In that study, only boad and elatively late ERP modulations (positive deflections in the P3 component stating ms poststimulus) coelated with whethe o not the paticipant peceived the taget. While the ealy hypothesis suggests that phospheneelated potentials afte occipital TMS ae functionally analogous to moto-evoked potentials following M1 TMS, the late hypothesis claims that conscious phosphene peception and its associated phosphene-elated potentials ae simila to the conscious peception of extenal visual stimuli and its electophysiological coelates. Ou esults demonstate that diffeential ERP modulations associated with conscious phosphene peception emege elatively late ( ms afte TMS onset) and ove a wide egion of aeas, a patten unlike the immediate effects of M1 TMS. Howeve, phosphene-elated potentials still emeged substantially ealie than the ERP coelates of conscious visual peception peviously obseved. MATERIALS AND METHODS Paticipant Sceening All paticipants wee ight-handed and gave infomed consent fo a TMS potocol appoved by the Bikbeck College Psychology School Ethics Committee. Pevious studies have elicited phosphenes fom appoximately half of paticipants without extensive taining [Romei et al., 2008a] and hee potential paticipants wee sceened as to whethe o not they wee saw phosphenes duing ight occipital TMS. The sceening task was simila to that used 1409

3 Taylo et al. Figue 1. Task. TMS was applied eithe ove ealy visual cotex o ove a paietal contol site while paticipants fixated centally: phosphene pesent/absent judgments wee made 1,000 ms afte the TMS pulse, duing the intetial inteval. [Colo figue can be viewed in the online issue, which is available at wileyonlinelibay.com.] in the main expeiment (Fig. 1, descibed in the next section). A gay fixation point was pesented on a cathode ay tube monito 150 cm fom the paticipant. TMS was applied at a vaiable andom delay of 1,000 1,500 ms afte the onset of the fixation point, which then emained onsceen fo a futhe 1,000 ms. This was followed by a 2,000-ms intetial inteval, duing which the monito was blank and the paticipants wee to epot vebally whethe a phosphene was pesent o absent (note that manual esponses wee used in the main expeiment, see below). This task was designed to povide an epoch fo eventelated potential (ERP) analysis compising only a measue of the bain esponse to the TMS pulse that did not include any eye movements, blinks, o manual esponses. Paticipants wee theefoe instucted to fixate with eyes open while the fixation point was pesented and to espond as accuately as possible (and to blink o move thei eyes) only duing the 2,000-ms blank intetial inteval afte fixation offset. Fo TMS, we used a figue-8 flat coil with an intenal diamete of 70 mm (Magstim Rapid 2 Machine, Whitland, Wales, United Kingdom). Single pulses of TMS wee applied ove the ight occipital lobe, with the initial stimulation site 2 cm dosal and 1 cm lateal of the inion (electode position Iz) [Kamitani and Shimojo, 1999; Silvanto et al., 2005]. To ensue that the cuent expeiment would be compaable to pevious phosphene studies the coil was placed flush with the electode cap diectly on the scalp and not on top of any EEG electodes, necessitating emoving electode Oz fom the electode aay (see Event-Related Potentials below). The coil was held with the handle pointing towads the ight (and the cuent theefoe flowing lateal-to-medial, fom ight to left), an efficient coil oientation fo inducing phosphenes [Kamme et al., 2001]. Duing sceening, stimulation intensity was inceased fom 50% of maximal stimulato output in 5 10% steps up until 90% output was eached o a phosphene was epoted, whicheve occued fist. Paticipants wee excluded if no phosphenes had been epoted afte 10 consecutive stimulation tials using 90% stimulation ove each of nine points spanning a 2 cm 2 cm gid centeed on the stating point. Fifteen of twenty-seven subjects failed this citeion leaving 12 to paticipate in the study (mean age: 24 yeas; age ange: 19 33; eight wee female). Once the paticipants had been selected, the next stage of expeimental pepaation was to optimize the TMS sites used. The active TMS site was defined functionally as the point on the 2 cm 2 cm gid, which poduced the stongest phosphenes, as epoted vebally by the paticipant. At the end of each session, the location of the stimulated sites was plotted using Bainsight steeotactic infaed egistation to each subject s stuctual MRI scan. Figue 2. TMS sites. (A) Right occipital TMS. The dots epesent the MNI coodinates of each site in each paticipant supeimposed ove thei aveage stuctual bain image. The locations of TMS sites wee ecoded using infaed steeotactic egistation to evey paticipant s stuctual MRI scan. TMS sites ae tightly centeed in the ight occipital lobe (mean, x ¼ 9, y ¼ 81, z ¼ 19). (B) Contol site TMS. The sites ae clusteed in the ight supeio medial paietal cotex (mean, x ¼ 13, y ¼ 37, z ¼ 57). 1410

4 The Neual Signatue of Phosphene Peception The active TMS sites, which wee defined functionally, wee nevetheless clusteed tightly aound the classical occipital phosphene site in the calcaine sulcus (see Fig. 2). The TMS intensity was also chosen functionally and tailoed to each paticipant so that thee would be high enough numbes of both phosphene-pesent and also phosphene-absent tials to deive obust ERP aveages, i.e., appoximately half of each. To detemine the phosphene theshold, paticipants pefomed sequences of ten consecutive tials using each candidate stimulation intensity, epoting phosphene pesence o absence vebally duing the intetial inteval. If thee o moe tials elicited phosphenes in each sequence of ten, then the stimulation intensity was educed by 2% of the maximal stimulato output and the pocedue epeated; if not, then the pevious intensity that had been tied was used fom the stat of the expeiment. The mean phosphene theshold intensity was 70% of maximal stimulation output (ange 55 80%). A contol site was also stimulated to contol fo the acoustic, somatosensoy, and electical atifacts that accompany TMS of any aea on the scalp and which ae nonspecific to the site stimulated, and at the same lateal distance fom the midline as the occipital TMS site in each individual paticipant, ensuing no diffeences in the extent to which the active and contol TMS pulses could have acted as cues to oient spatial attention to one side of space. The contol site was also plotted to be level with electode position CPz in the anteio posteio diection so that the distance and elative position between the contol TMS site and midline electode Cz would be appoximately simila to that between the phosphene site and midline electode POz, allowing us to display whethe TMS effects on the ERP diffeed ove diffeent sites. The contol sites clusteed aound supeio medial paietal cotex (see Fig. 2). In addition, this site does not ovelie any cotical aea that has been associated in pevious studies with phosphenes, moto-evoked potentials, o the contol of visual spatial attention. Main Expeiment The main expeiment compised 16 blocks of 40 tials. The task, fixation point, and timing wee the same as duing sceening (see Fig. 1), but athe than using vebal epots, paticipants executed manual button pesses made with the ight hand, using the index o thid finge to indicate phosphene pesence o absence, espectively. Block ode was pseudoandomized and countebalanced: each goup of fou consecutive blocks included two blocks with occipital TMS on evey tial, one with contol site TMS, and one without any TMS. Paticipants wee instucted to espond manually to the pesence o absence of a phosphene in the intetial inteval even with contol site TMS o without TMS, when phosphenes wee not pedicted. The ationale of using contol TMS and no TMS conditions was as follows. Similaities between the TEPs on phosphene-absent tials duing active TMS o contol TMS would consist of the ERP esponse to the somatosensoy and auditoy stimulation accompanying TMS and any commonalities in how the two stimulated sites espond to TMS. The contol site TMS condition was included to find these similaities. Any diffeences between the TEPs on phosphene-absent contol TMS and phosphene-absent occipital TMS tials would theefoe be attibutable to diffeences in how the two aeas eact to the intefeence of neual activity caused by TMS. This would then enable detemination of which pats of the TEP fom occipital TMS phosphene-absent tials wee due to the somatosensoy-acoustic atifact and which wee due to effects of TMS on visual cotex. The no TMS block was included simply to demonstate conclusively that in the absence of TMS, and befoe paticipants epoted the absence of a phosphene, thee was little o no change in the ERP and that any activity changes pesent in the EEG afte TMS of the contol site wee diven by the TMS. Oveall, paticipants epoted that phosphenes wee pesent on appoximately half (45%) of occipital TMS tials. Thee wee phosphenes epoted on only 8% of contol site tials: six paticipants epoted the pesence of phosphenes on a small numbe of contol site tials. Two of those paticipants epoted a phosphene on only one tial (out of a total of 160 contol site tials), and the othe fou had scoes of 7, 9, 36, o 80 tials. In these paticipants, the contol site may have been nea a ecently epoted paietal phosphene aea [Mazi et al., 2009]: note that tial numbes on which the paticipants epoted a phosphene at this contol site wee too low to fom a eliable gand aveage and theefoe only phosphene-absent tials wee included in the cuent analysis. Event-Related Potentials EEG was DC-ecoded continuously at 1,000 Hz with a TMS-compatible ERP amplifie (BainAmp DC, Bainpoducts, Gemany) capable of ecoding a veidical EEG without TMS o echaging atifacts within 50 ms of a TMS pulse [Venieo et al., 2009]. EEG was ecoded with minimal filteing (DC-450Hz, no notch) fom a whole-head montage of custom-built Ag-AgCl electodes (each with a built-in 5-kX-esisto) at positions C3, C4, CP5, CP6, Cz, F3, F4, F7, F8, FC5, FC6, FPz, Fz, P3, P4, P7, P8, PO3, PO4, PO7, PO8, POz, Pz, T7, and T8. Hoizontal EOG was ecoded fom the left and ight temples. The gound was at AFz and the active efeence on the left ealobe. Electode impedance was kept below 10 kx. Data wee eefeenced to the aveage of the left and ight ealobes, and an HEOG signal was fomed fom linea deivation of the left and ight EOG electodes. Data wee then epoched to fom 5,600-ms segments containing the whole tial. Then the TMS atifact was emoved fom the data though linea intepolation of the data between 5 ms befoe and

5 Taylo et al. ms afte the TMS pulse [Fuggetta et al., 2006; Taylo et al., 2008]. Filteing used a notch 50-Hz filte and then a Buttewoth zeo phase filte with low cutoff of 0.01 Hz and high cutoff of 40 Hz (12 db/octave). Data wee then epoched into 600-ms peiods, stating 100 ms befoe the time of TMS onset. Baseline coection used the 100 ms pio to the TMS pulse. Automated ERP atifact ejection emoved tials with eye movements by eliminating tials whee the HEOG signal exceeded 30 lv. Blinks wee emoved by deleting tials if the signal at FPz exceeded 60 lv, and othe movement-elated atifacts wee emoved by eliminating any tials whee the signal fom any electode exceeded 80 lv. A minimum citeion of 30 tials pe condition was set to ensue a sufficiently high signal-to-noise atio of the ERP aveages, which wee time-locked to TMS onset. No blocks fell below this level. The mean numbe of tials pe condition pe paticipant was as follows: occipital TMS phosphene pesent, 132; occipital TMS phosphene absent, 109; contol site TMS phosphene absent, 102; no TMS phosphene absent, 111. ERP effects wee chaacteized statistically by compaing the mean TEP amplitude within thee diffeent time windows centeed on successive TMS-evoked components (70 140, , ms) acoss conditions. The phosphene effect was analyzed by compaing phosphene-pesent and phosphene-absent tials duing occipital TMS. A second analysis compaed the phosphene-absent tials duing occipital and contol-site TMS to detemine any effect of TMS site. Fo both analyses, the spatial distibution of any effect was investigated by pooling electodes into thee goups compising a posteio goup ove visual cotex (electodes PO3, P3, POz, Pz, PO4, P4, PO8), a cental goup (T7, CP5, C3, Cz, C4, CP6, T8), and a fontal goup (FC5, F7, F3, Fz, F4, FC6, F8). To detemine effects at the individual electode level and with fine tempoal esolution, each time window was divided into sequential 20-ms time bins, compaing the means between conditions with Student t-tests, and coecting fo multiple compaisons by adopting the citeion that significant diffeences should last fo at least two consecutive 20-ms time bins. RESULTS The TMS-Evoked potential TMS was applied at the theshold fo phosphene peception eithe to occipital cotex o to a contol paietal site (Figs. 1 and 2), and TMS of eithe site poduced TEPs of 10 lv amplitude (see Fig. 3). The TEP pofile consisted Figue 3. The mean amplitude of the ERP afte occipital TMS on phosphene-pesent (top), phosphene-absent tials (middle), and contol site phosphene-absent tials (bottom) calculated in 20-ms time bins. The nonspecific TMS atifact (i.e., the ERP esponse to somatosensoy o acoustic stimulation accompanying TMS) is evident as a cental positivity in all sites. 1412

6 The Neual Signatue of Phosphene Peception of an ealy fontocental negativity fom 70 to 120 ms, a centopaietal positivity fom 140 to 220 ms, and a posteio positivity ove visual aeas that was appaent fom 240 ms afte TMS onset. The fist two components that wee pesent both fo occipital and contol-site TMS pimaily eflect sensoy-specific auditoy and tactile ERP components tiggeed by TMS. The thid posteio component was much moe ponounced fo occipital elative to contol-site TMS. Two sets of analyses wee pefomed. The main analyses tested which aspects of the TEP wee sensitive to phosphene peception by compaing ERPs in esponse to occipital TMS on phosphene-pesent and phosphene-absent tials. A second set of analyses investigated the impact of stimulation site on TEPs by compaing ERPs tiggeed when TMS was applied to the occipital site o to the centopaietal contol site. The Phosphene-Evoked Potential Figue 4 shows the TEP diffeences between phosphene-pesent and phosphene-absent tials following occipital TMS. When paticipants epoted a phosphene, thee was a geate visual positivity ove a wide ange of posteio but not fontal aeas. To chaacteize this effect, the mean amplitudes of phosphene-pesent and phosphene-absent tials wee compaed acoss the thee goups of electodes (posteio, cental, and fontal). Phosphene peception (the compaison between phosphene-pesent and phosphene-absent tials) did not affect the ealy phase of the TEP fom 70 to 140 ms (no effect of phosphene peception and no inteaction of phosphene peception with electode goup, all Fs 0.7, all Ps > 0.4). In contast, bain activity between 160 and 200 ms afte TMS onset was modulated by phosphene peception [F(1,11) ¼ 11.0; P < 0.01]. The lack of any inteaction between phosphene peception and electode goup showed that the phosphene effect was wide-spead [F(2,22) ¼ 0.3; P ¼ 0.7]. The effect was also statistically significant at the individual electode level and ove a wide aea, with all diffeences between 160 and 200 ms being diven by a elative positivity on phosphene-pesent tials [C3, C4, CP5, Cz, F3, F4, FC5, Fz, P3, P4, PO4, POz, Pz: all ts (11) 2.2, Ps < 0.05]. The late phase of the TEP ( ms) was also modulated by phosphene peception [F(1,11) ¼ 10.9; P < 0.01]. Additionally, this late modulation was accompanied by effects of electode goup and an inteaction of electode goup and phosphene peception (all Fs 4.5, all Ps < 0.05), indicating spatial heteogeneity acoss the diffeent electode goups. Phosphene effects wee statistically significant within the posteio and cental goups [posteio goup: F(1,11) ¼ 13.9; P < 0.01, cental goup: F(1,11) ¼ 9.9; P < 0.01], but only maginally significant at the fontal goup of electodes [F(1,11) ¼ 4.2; P ¼ 0.066]. Follow-up analysis conducted fo individual electodes between 280 and 400 ms showed significant diffeences between phosphenepesent and phosphene-absent tials at most posteio and cental electodes [individual electodes C3, C4, CP5, CP6, Cz, FC5, FC6, P3, P4, P8, PO3, PO4, PO7, PO8, POz, Pz, T8: all ts (11) 2.2, Ps < 0.05]. In contast, no eliable diffeential effects of phosphene peception wee obseved at the fontal electodes F3, F4, F7, F8, FZ, o FPz [all ts (11) < 2.0, Ps > 0.05]. Effects of Stimulation Site on the TMS-Evoked Potential The second analysis compaed the TEPs afte TMS of the occipital site and the contol paietal site, and only included phosphene-absent tials, to avoid any confound with conscious phosphene peception. The contol site was lateal to cento-paietal electode position CPz (see Mateials and Methods and Fig. 2) and has not been epoted to elicit phosphenes o moto twitches. Figue 5 shows the diffeence wavefom obtained by subtacting the TEPs in esponse to contol-site TMS fom TEPs following occipital TMS, fo phosphene-absent tials. An initial dipola activity patten ove ight posteio visual aeas close to the occipital TMS location (ight-lateal positivity and cental negativity) was appaent between 70 and 140 ms afte TMS onset, although this diffeential ERP modulation was highly vaiable acoss paticipants and did not each statistical significance (no effect of TMS site o inteaction between site and electode goup: all Fs 0.7, all Ps > 0.4). At longe post-tms latencies, TEPs wee moe positive fo occipital as compaed to contol site TMS, and this diffeence was boadly distibuted. Duing the 160- to 200-ms time window, the TEP afte occipital TMS was moe positive than the TEP following contol site TMS [main effect of TMS site: F(1,11) ¼ 8.4; P < 0.05] ove a wide ange of electode locations [no inteaction of TMS site and electode goup: F(2,22) ¼ 0.5, P ¼ 0.6]. In the late time window ( ms), thee wee again effects of TMS site [F(1,11) ¼ 46.2, P < 0.01], but in this case this effect diffeed acoss electode goups [inteaction between TMS site and electode goup: F(2,22) ¼ 4.5, P < 0.05]. TMS site affected the ERP within evey electode goup when analyzed sepaately [posteio goup: F(1,11) ¼ 68.8, P < 0.01, cental goup: F(1,11) ¼ 21.3, P < 0.01, fontal goup: F(1,11) ¼ 15.5, P < 0.01] so that inteaction (TMS site by electode goup) was diven by a diffeence in degee of the TMS site effect, with weake effects fontally. To illustate the time couse and elative amplitude of these TEPs, Figue 6 shows TEPs fo all tial conditions obtained at the posteio paieto-occipital electode POz whee these late effects of phosphene peception and of TMS site wee maximal. DISCUSSION Phosphene peception was linked to a diffeential TMSinduced bain activation ove cental and posteio visual aeas, with two positive components between 160 and 200 ms and 280 and 400 ms afte TMS onset. This was shown 1413

7 Taylo et al. Figue 4. The phosphene effect. This is the diffeence in the mean amplitude of the ERP on phosphenepesent and phosphene-absent tials duing occipital TMS (the diffeence between the top two panels of Fig. 3). Effects each statistical significance stating fom 160 ms. Figue 5. The topogaphy of the diffeence between phosphene absent tials afte occipital and contol site TMS (active minus contol: the diffeence between the lowe two panels in Fig. 3). 1414

8 The Neual Signatue of Phosphene Peception The time couse of the phosphene effect in compaison to nonspecific TMS atifacts. This shows the ERPs fom the electodes neaest to the occipital TMS site (top left: PO3; top ight: PO4; lowe middle: POz; positive deflections plotted downwads). Duing occipital TMS, thee is a elative positive deflection on phosphene-pesent tials (ed) stating at 160 ms. This effect is Figue 6. supeimposed on the positive component, which is elicited by the somatosensoy and acoustic stimulation that accompanies the TMS pulse and is pesent on all TMS tials. The gay-shaded aea shows the 50-ms time window following the TMS pulse in which EEG ecoding is not possible. by compaing the TEPs elicited duing visual cotical stimulation at phosphene theshold intensity between tials whee phosphenes wee peceived and tials whee no phosphenes wee elicited. The elatively late onset and boad topogaphy of this phosphene-elated potential, and the fact that electical bain activity at ealie latencies was unelated to phosphene peception, povide no suppot fo the hypothesis that phosphenes ae an immediate and local consequence of TMS-induced activations of visual cotex, but ae consistent with the late hypothesis. Phosphene peception was linked to diffeential electical activity that was measued ove a boad aea of paieto-occipital, paietal, and cental cotex. The effects immediately adjacent to the TMS coil wee moe shotlived than those futhe away, and effects wee geneally ealie and moe ponounced ove the stimulated ight hemisphee. The second positive wave ( ms) was even moe boadly distibuted acoss posteio and cental electodes than the phosphene-elated potential measued between 160 and 200 ms afte TMS onset. Even though the limited numbe of EEG ecoding sites used in this study pecludes fim conclusions about the neual geneato pocesses esponsible fo the phosphene-elated potential obseved hee, the oveall topogaphy of these effects suggests that the neual activation esponsible stated locally at the stimulated site and then spead to othe aeas. This 1415

9 Taylo et al. geate spead of activation (even to the contalateal hemisphee) was then elated to phosphene peception. Compaing phosphene-pesent and phosphene-absent tials when stimulation paametes ae constant contols fo the somatosensoy and acoustic atifacts accompanying TMS. On phosphene-absent tials, TMS-induced visual cotical activity was insufficient to induce phosphene peception. On phosphene-pesent tials, TMS paametes wee identical, but esulted in conscious phosphene peception. The TEP pofiles on phosphene-pesent and phospheneabsent tials had a simila oveall shape. Both included two discete posteio components with a tendency towad ight latealization, with lage amplitudes on phosphenepesent tials. The geneal similaity of the mophology of phosphene-absent and phosphene-pesent TEPs and the link between TEP amplitudes and phosphene peception may point towad a theshold mechanism fo the geneation of conscious phosphene peception. It has been suggested that phosphene peception can eflect tansient changes in cotical excitability and that these changes can be detected even pio to TMS onset [Komssi and Kahkonen, 2006; Romei et al., 2008a,b]. While such changes in excitability could have been linked to diffeences in bain activity between phosphene-pesent and phosphene-absent tials that ae evident immediately afte TMS onset, the ealy phase of the TEP in esponse to occipital TMS was unelated to phosphene peception, with phosphene-evoked activity only emeging ms afte TMS. This suggests that spontaneous changes in cotical excitability ae not the sole facto involved in phosphene peception, o that thei biasing effects on peceptual pocessing become effective only 160 ms afte the initial TMS-induced activation of visual cotex. This time couse is consistent with a ole of e-entant pocesses in phosphene peception. Recuent loops of activation acoss subcotical o cotical egions would be one possible intepetation of the two discete phases of phosphene-elated TEP modulations [Lamme and Roelfsema, 2000; Pascual- Leone and Walsh, 2001]. It should howeve also be noted that the absence of ERP diffeences between phosphenepesent and phosphene-absent tials in the fist 160 ms afte TMS onset does not necessaily imply that thee was no diffeential neual activity at all duing this time inteval. Because the stength of EEG signals ecoded at the scalp suface depends on the geomety and oientation of the undelying neual geneato pocesses, it is possible in pinciple that some ealy phosphene-elated bain activity was pesent, but was not detected by EEG ecodings. Using a logic simila to the one employed hee, a ecent ERP study has investigated electophysiological coelates of conscious visual peception in esponse to backwadmasked taget stimuli pesented at peceptual theshold [Del Cul et al., 2007]. An enhanced P3 component stating ms afte TMS was obseved on tials whee paticipants epoted having seen the taget, elative to tials whee tagets wee not peceived. It is notable that in the pesent study, the fist diffeential ERP effect that was linked to phosphene peception emeged 100 ms ealie than the awaeness-elated P3 modulation obseved by Del Cul et al. [2007]. Although it is difficult to diectly compae a study whee ERPs wee tiggeed by extenal visual stimuli to a situation whee electical bain activity is elicited by TMS, this latency diffeence may eflect the diffeent causal outes fo conscious peception. Tiggeing phosphenes by diectly stimulating occipital cotex obviously eliminates any involvement of pestiate stages of visual pocessing, such as the etinogeniculate pathway. If the activation of highe-ode visual aeas is associated with the emegence of peceptual awaeness, and if this activation equies a simila amount of time egadless of whethe the initial occipital activation was poduced as a consequence of etinal stimulation o diectly via TMS, electophysiological coelates of conscious peception should indeed be obseved substantially ealie in esponse to occipital TMS than in esponse to extenal visual stimuli. In addition to compaing the effects of TMS to occipital cotex with and without phosphenes, we wee able to show that TMS to occipital cotex and to the contol site (medial paietal cotex) both poduced a lage TEP consisting of an ealy cental negativity (fom 70 to 140 ms afte the TMS pulse), a posteio positivity ( ms), and a late and moe boadly widespead positivity ( ms), independently of phosphene peception (see Fig. 3). These TEPs ae at least patly diven by that which is common to all conditions, namely, the somatosensoy and acoustic stimulation that accompanied each TMS pulse. This site-unspecific TEP pofile is stikingly simila to the esults of pevious TMS-EEG studies that have stimulated moto o fontal aeas [Komssi and Kahkonen, 2006; Nikouline et al., 1999], and which also epoted stong positive cental maxima at 175 ms afte TMS, suggesting good eplicability of nonspecific TMS-induced ERP esponses acoss paticipants, expeimental paadigms, and stimulation/ecoding systems. Howeve, thee wee also substantial diffeences between TEPs tiggeed in esponse to occipital vesus contol-site TMS: electodes neaest the contol site showed a elative ealy negativity, wheeas those neaest to the occipital site showed a late positivity, suggesting that TMS-induced ERP modulations ae not simply detemined by the distance between the electode and the TMS coil. TMS does not affect the electical signal at adjacent electodes in a unifom way, but athe depends on the popeties of the stimulated cotex [Silvanto et al., 2008], suggesting that diffeent aeas will have diffeent TEP pofiles. The cuent esults not only show that TEPs tiggeed by occipital TMS ae distinctive, but also that thei modulation is linked to phosphene peception. Both obsevations may offe useful indices of visual connectivity fo futue studies of visual function and awaeness. Fo example, the cuent demonstation of bain activity changes that ae linked to phosphene peception could be extended in futue wok by testing whethe, when and how 1416

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