transcranial magnetic stimulation ( tms ) of human visual cortex , at an intensity above a distinct threshold , induces illusory visual perceptions called phosphenes [ 13 , 14 ] .
it is thought that phosphenes originate from early visual cortex ( v1/v2 ) [ 1316 ] and depend on the integrity   and excitability [ 17 , 18 ] of the occipital region .
the perceived phosphene lies within the visual hemifield contralateral to the stimulated cortical hemisphere , at a location reflecting the retinotopic organization of visual cortex .
for example , tms of superior right occipital cortex elicits a phosphene in the lower - left quadrant of the visual field .
this spatial specificity enabled us to use phosphene perception as a probe measure of cortical excitability , in two related experiments , under conditions of transient or sustained spatial attention .
this allowed us to test directly whether spatial attention influences excitability of visual cortex , as revealed by phosphenes induced by tms of visual cortex , which bypasses the retinogeniculate pathway .
attention was covertly ( without displacement of gaze ) directed toward a particular location ( left or right ) during a task involving real visual stimuli .
but on some trials , instead of real visual stimuli , tms was applied , which ( when sufficiently intense ) produced a phosphene , either within the attended location or in the symmetric location in the opposite hemifield . this allowed us to measure whether conscious perception of tms - induced phosphenes can be influenced by spatial attention . the tms intensity needed to induce a conscious phosphene ( i.e. , the phosphene threshold [ pt ] )
is widely held to provide a measure of visual - cortex excitability [ 17 , 18 ] .
hence , if we were to find that pts are affected by spatial attention , this would imply a direct effect of attention on visual - cortex excitability as well as the corresponding awareness . at or above threshold intensity
, tms phosphenes were experienced as small , brief , illusory white flashes of light , clearly localized at a particular position in the contralateral hemifield [ 14 , 19 , 20 ] .
pts were determined by adjusting tms intensity as a function of the participant 's response , according to an adaptive converging - staircase algorithm [ 19 , 21 ] .
tms trials were randomly interleaved with trials in which real visual stimuli were presented for judgement at the same eccentricity in space as the possible tms phosphene .
variation of the side to which covert attention was directed for the real visual task allowed measurement of pts with and without spatially congruent attention .
importantly , we never gave tms simultaneously with real visual stimuli here , unlike in some other studies that used tms to suppress perception of visual stimuli [ 2224 ] .
instead , trial types were interleaved unpredictably such that tms inputs substituted for real visual stimulation on some trials . in experiment 1 , participants maintained gaze on  a central point throughout , while covertly directing spatial attention toward either the left or the right hemifield on each trial , according to a randomly determined prior cue signal that preceded the presentation of the target ( figures 1a1d ) . a briefly illuminated led ( 100 ms ) , on one side or other of the fixation point acted as an attentional cue , indicating the side on which a peripheral visual target was likely to appear for that trial . on two - thirds of trials , after a variable interval , a real , suprathreshold visual target then appeared briefly , and participants had to press a button as quickly as possible . for 80% of these  visual  trials ,
the external target was on the side to which attention had been directed ( validly cued ) ; for an unpredictable 20% , the target appeared in the opposite hemifield ( invalidly cued ) .
reaction times for detection of visual targets were faster to validly cued than to invalidly cued targets ( 358  21 ms versus 393  23 ms ( mean  sem ) , z = 2.52 , p = 0.012 ; see figure  1e ) .
this confirms that spatial attention was indeed directed to the cued location for the external task . on one - third of the trials , randomly interleaved with the valid and invalid visual trials , no external visual target appeared . instead
, a single tms pulse was applied to one side or the other of the occiput , ipsilateral or contralateral to the hemifield that had been cued for attention . on these tms trials ,
our critical finding was that pts were significantly lower for trials on which the phosphene appeared in the hemifield that had been cued for attention ( 51.6% of maximum stimulator output [ mso ] , compared with 56.2% mso for the other side ; z = 2.52 , p = 0.012 ; see figures 1f and 1 g ) .
this indicates that transiently cued spatial attention enhances visual awareness , even for direct tms of visual cortex , which bypasses the pathway from the eye , hence precluding feedforward thalamic gating of initial afferent sensory input .
it is conceivable that the heightened thresholds for the unattended noncued hemifield in experiment 1 were due , at least partly , to the fact that stimuli on the noncued side ( real visual or tms - induced phosphene ) were less frequent overall .
surprise  factor and thereby possible differences in criteria for reports associated with different probabilities of event occurrence on one side versus the other .
the particular side on which the external target had to be detected was now constant throughout each block .
for each block of 96 trials , participants were instructed to maintain their covert attention continuously on the lower quadrant of one particular hemifield while they retained central fixation ( confirmed with eye - tracking ) .
visual stimuli , which were presented simultaneously and bilaterally ( see figures 2a2d ) , consisted of a variable number ( one to four ) of target rectangles , independently selected on the two sides for each trial .
participants had to indicate , via button - press , the number of such rectangles on the attended side only . on a critical 50% of randomly interleaved trials in each block ( with attention sustained to one quadrant for the external visual task ) ,
a tms pulse was delivered to the right side of the occiput instead of visual stimulation .
observers had to report whether they experienced a phosphene ( which now fell within the target area used for real visual stimuli in the left hemifield ; see figures 2a2d ) .
as tms was now applied to only one hemisphere , more pt measurements could be obtained than in experiment 1 , allowing fitting of full psychometric functions to the response data ( see experimental procedures and figure  s1 in the supplemental data available online ) . in half of the blocks , attention was directed constantly to the left ( the side of possible phosphene appearance ) for the rectangle - counting task ; in the other half , it was directed to the right .
the critical finding was again that pts were significantly lower when spatial attention was congruent with the side of the phosphene ( 62.3% mso , compared with 69.4% mso for the other side ; z = 2.55 , p = 0.011 ; figures 2e and 2f ) .
moreover , whereas these thresholds differed reliably as a function of attention , the slopes of the underlying psychometric functions did not ( see experimental procedures and figure  s1 ) .
this is consistent with a genuine increase in cortical excitability as a result of spatial attention , rather than  a criterion shift .
recent demonstrations that attention can modulate activity in the human lateral geniculate nucleus ( lgn ) [ 25 , 26 ] reopened the long - standing question of whether thalamic gating of afferent inputs may contribute to , or even be necessary for , effects of spatial attention on visually evoked activity in striate [ 58 ] and extrastriate cortex [ 6 , 7 , 9 ] .
initial modulation of feedforward signals at the lgn might conceivably be a necessary prerequisite for effects of spatial attention to be propagated to  and enhanced at subsequent cortical areas , with possible amplification at successive stages .
on the other hand , many authors have previously suggested or implied that top - down influences on early visual cortex [ 1012 , 2831 ] , producing so - called baseline shifts [ 3234 ] , might arise without thalamic mediation .
typically such arguments against a critical role for the thalamus have been based on indirect evidence , from the relative timing of components in event - related potentials ( erps ) or from the absence of observable attentional effects in neuroimaging of the thalamus ( which , in some cases , might have been attributable to use of relatively low - resolution imaging in some cases ) . by contrast , the present experiments were able to test the issue directly by using cortical stimulation and demonstrate a positive effect that can not be attributed to thalamic gating of initial afferents .
this new evidence confirms prior proposals that top - down attentional influences ( e.g. , from frontal or parietal cortex ) change activity and excitability of visual cortex itself ( e.g. , [ 2 , 8 , 28 , 29 ] ) .
it is also compatible with studies in which the relative timing of erp components was taken to indicate that attentional modulation of visual cortex is more likely to reflect top - down feedback than feedforward thalamic gating [ 1012 , 2830 ] .
what , then , should we make of the clear evidence that visual activation of the lgn itself can be influenced by spatial attention [ 25 , 26 ] ?
we suggest that this may reflect recursive corticothalamic interactions , in line with the time at which spatial attention appears to influence early visual - cortex excitability [ 1012 , 28 , 30 ] .
although during  normal  visual perception such recursive processing may indeed contribute to spatial attention , our results show that thalamic gating of initial afferents is not a necessary precondition for attentional modulation of cortical excitability .
our results further demonstrate that attention can act directly on the cortex , enhancing excitability of neural populations that process the attended region in visual space .
this appears to be a very general neural mechanism that can affect the very different inputs and featural properties of our two types of stimuli ( i.e. , cortical tms as well as real optical stimulation ) and that might apply across different degrees of attentional load , as for experiments 1 and 2 here . in conclusion ,
our study shows directly that spatial attention enhances excitability in visual cortex to facilitate visual awareness , even when retinal stimulation , retinogeniculate conduction of sensory signals , and thalamic gating of feedforward afferents are ruled out .
in both experiments , participants wore earplugs and headphones , and stable viewing and head position were ensured with a chinrest and nose - bridge .
all participants had normal or corrected visual acuity and reported no history of neuropsychiatric illness or epilepsy .
there were eight participants ( aged 2429 yr , mean 26.5 yr , 4  females ) for experiment 1 .
each observer sat in a darkened room , 40 cm from a central fixation target ( red led ) , with the tms positioned over one side of the occiput .
trials were initiated by button - press . a small target light ( amber led )
was positioned at the center of the region of the visual hemifield contralateral to the tms coil that corresponded to the spatial location of the phosphene produced by suprathreshold tms ( individual range , 1023 lateral eccentricity ; mean 11.54  standard deviation [ sd ] 6.39 ) .
an identical target light was set symmetrically at the same eccentricity in the other hemifield .
the task for external visual targets was to report their onset via speeded button - press .
see figure  1 for the temporal sequence of events in different trial types , with timing information .
on tms trials , which were evident to the participants because of the audible click produced by the coil , they had to report verbally whether or not they had perceived a phosphene .
responses were entered into a pc , and tms output intensity was automatically adjusted according to the modified binary search ( mobs ) adaptive - converging algorithm .
the button for external visual targets was inappropriately pressed on 8%  1.9% ( mean  sd ) of tms trials .
the exact number of tms trials per subject depended on how quickly the mobs algorithm converged on to a pt ( mean 32  sd 9 tms trials ) .
pts were determined separately for trials in which attention had been cued to the locus of phosphene appearance ( see figure  1b ) and those in which the cue had directed attention to the opposite spatial location ( figure  1d ) . after pts had been determined for tms stimulation of one cortical hemisphere , the session was repeated with tms applied to the other hemisphere , to elicit phosphenes in the opposite hemifield .
, four phosphene thresholds were determined via mobs for each participant : for left and right hemifields , with each either validly or invalidly cued , respectively . in experiment 2 , eleven different participants ( aged 1931 yr , mean 23.5 yr , 6 females ) performed a sustained spatial - attention task . they had been selected from a sample of 27 participants for reliably perceiving phosphenes in a well - circumscribed location , with eyes open , at 90% of tms stimulator output or less ( see ) .
two participants were excluded from subsequent analysis because of excessive eye movement ( see supplemental data ) .
visual stimuli were presented in a darkened room on a 21 inch computer screen , refreshing at 60 hz , at a viewing distance of 45  cm .
a signal was presented at screen center for 2 s prior to the  start of each block , to instruct the observer to direct attention covertly to one side for the whole of the block .
the corners of each target area were demarcated by four small , gray marker dots , 2 pixels square and visible throughout the experiment , in order to help the observer sustain spatial attention on the relevant region for the visual task ( see supplemental data ) .
the target areas ( each 8 square ) were centered 8 below the horizontal meridian , at an eccentricity of 19 in the lower quadrants of both hemifields . in visual trials ( see figures 2a and 2c ) , participants had to report the number of target rectangles in the attended quadrant only , by pressing one of four keys with digits 2 to 5 of the right hand .
target stimuli were presented for 180 ms , between 0 and 540 ms after trial initiation .
visual stimulation was bilateral , with the number of rectangles randomly selected on each side , but participants only had to judge the attended side .
apart from this , there was no emphasis on speed for this difficult discrimination ( response latency was similar for all trial types : left correct , 1299 ms ; left incorrect 1309 ms ; right correct , 1285 ms ; and right incorrect , 1277 ms ; friedman test , (3 ) = 1.93 ; p = 0.57 ) .
on tms trials ( see figure  2b and 2d ) subjects reported the presence or absence of a phosphene via button - press of the left index or middle finger , respectively .
tms intensities were again varied from trial to trial by using mobs [ 19 , 21 ] , but whenever mobs converged prior to the end of a block the procedure was started again , in order to present as many phosphene stimuli as possible throughout a block .
the number of mobs convergences per subject did not differ between experimental conditions ( attend right , mean 7.00  sd 1.34 ; attend left , mean 7.18  sd 1.66 ) .
given the larger dataset in experiment 2 , pts could  now be determined by fitting full weibull psychometric functions   to the raw data as measured with mobs , by using the matlab toolbox psignifit ( see http://bootstrap-software.org/psignifit ) .
this allowed assessment not only of pts , but also of the slopes of the underlying psychometric functions ( see results and discussion and figure  s1 ) .
however , note that the pts determined by psychometric curve fits for experiment 2 were highly correlated ( r = 0.93 across all conditions , p <
0.001 ) with pts when determined by mobs convergence as in experiment 1 ( demonstrating that both methods are equally valid for our purposes ) .
importantly , the curve fits confirmed that the slopes of the psychometric functions in experiment 2 did not differ between attended and unattended sides ( z = 0.53 , p = 0.60 ) , further supporting our finding of significant differences in threshold .
the order of experimental blocks ( two blocks each of leftward or rightward attention ) and trial types was randomized .
subjects were repeatedly instructed to avoid eye movements , as confirmed by eye - tracking ( see supplemental data ) .
the z values given for comparisons of attended versus unattended conditions in the main text and figure legends were derived from wilcoxon signed - ranks comparisons .
tms was applied by using a 70 mm figure - of - eight coil ( super rapid , magstim , dyfed , wales , uk ) .
a two - joint holder was used to place the coil to one side or other of the occiput , approximately 4 cm above the inion , at a laterality for which phosphenes were reliably reported .
the initial rising phase of the induced biphasic current ( 250 s duration ) had a temporomedial orientation , optimal for inducing visual phosphenes .
in experiment 1 , the location of the real visual targets ( leds ) was matched in eccentricity to the center of each individual 's phosphene locus , reported before the collection of experimental data . in experiment 2 , the tms coil was held in place as in experiment 1 , but with the stimulation site , on the right side of the occiput , chosen to maximize spatial congruence of phosphenes with the fixed target area for external visual stimuli in the lower - left quadrant on the computer screen .
note that tms - evoked phosphenes are more readily elicited for lower quadrants of the visual field .
tms output intensity was varied according to the mobs algorithm [ 19 , 21 ] .
the termination criteria were a maximum of five reversals and a maximum last step size of 5% of the initial tms output range .
note that in this study , no visual stimuli were presented at the same time as tms ( unlike [ 2224 ] ) because pts are known to change with visual input ( e.g. , as a function of its luminance contrast ) .
indeed , this is one of the reasons why pts are widely considered to reflect excitability of visual cortex ( just as tms thresholds for induced movements reflect excitability of motor cortex ) .
sound , but this was equivalent across the different conditions of visual attention here , so it could not have confounded our results .
 summaryconscious perception depends not only on sensory input , but also on attention [ 1 , 2 ] . recent studies in monkeys [ 36 ] and humans [ 712 ] suggest that influences of spatial attention on visual awareness may reflect top - down influences on excitability of visual cortex . here 
 we tested this specifically , by providing direct input into human visual cortex via cortical transcranial magnetic stimulation ( tms ) to produce illusory visual percepts , called phosphenes . 
 we found that a lower tms intensity was needed to elicit a conscious phosphene when its apparent spatial location was attended , rather than unattended . 
 our results indicate that spatial attention can enhance visual - cortex excitability , and visual awareness , even when sensory signals from the eye via the thalamic pathway are bypassed . 
Results and Discussion
Experimental Procedures