sixteen university students ( 10 male and 6 female 
 with an age range of 1924 years ) participated in the experiment . all participants were 
 right - handed and had normal or corrected - to - normal vision .
they were all nave to the purpose 
 of the experiment and were paid after the experiment .
the spatial resolution of the crt monitor was 1024  768 pixels 
 and the refresh rate was set at 75 hz .
each trial started with a white fixation cross ( 1  1 ) centered on the dark 
 computer screen .
after 1,000 ms , a white outline box serving as a peripheral cue appeared for 100 ms 
 at one of the four peripheral locations ( indicated by dash - line boxes in figure 1 ) with equal probability . 
 two of the peripheral locations had an eccentricity of 7 ( left , right ) and the other two an 
 eccentricity of 21 ( left , right ) ( figure 
 1 ) . at 70
ms after the offset of the peripheral cue an outline box appeared at the 
 fixation location for 100 ms , reorienting the subject s attention back to the center . another 
 70 ms later , a white disk serving as the target appeared at either the same or the symmetric 
 opposite location of the peripheral cue , with equal probability .
the target remained present until 
 the participant responded , that is , pressed the space bar with the right index finger .
responding 
 was instructed to be as quickly and as accurately as possible . during each trial , subjects were 
 instructed to maintain their fixation at the central cross without shifting their gaze to the 
 peripheral locations .
this requirement was particularly checked by the experimenter during the 
 practice session , and all subjects could follow this instruction successfully before the main test 
 started .
an intertrial interval of 1,000 ms with a blank screen was inserted after the subject 
 responded .
the sizes of the cues and targets for different 
 stimulus eccentricities were manipulated as follows : in the same - size condition the cue subtended 
 1  1 and the target 0.6  0.6 for both stimulus 
 eccentricities ( 7 and 21 ) ; in the size - scaling condition , the peripheral cue at 
 7 was kept at 1  1 size while , at 21 , it was magnified to 
 2.8  2.8 ; the peripheral target at 7 was kept at 0.6 
  0.6 size while , at 21 , it was magnified to 1.68 , using a formula 
 for calculating cortical magnification as suggested by rovamo and virsu ( 1979 ) .
four blocks were trials of the same - size condition and 4 blocks 
 were trials of the size - scaling condition . there were 80 catch trials in which no targets were 
 presented and the participants had to withhold their responses .
ten catch trials were randomly mixed 
 with 60 target trials in each block . the number of trials for targets appearing in the left or right 
 visual field , at 7 or 21 eccentricity , and at a cued or uncued location , was 
 balanced in both the same - size and the size - scaling conditions .
one half of the participants 
 performed four blocks of the same - size condition first , and the other half performed four blocks of 
 the size - scaling condition first .
a practice block with 20 trials of the same - size condition and 20 
 trials of the size - scaling condition were tested before the main experiment .
the total error rate for each subject was lower 
 than 1% and therefore was not further analyzed .
response times for the correct trials of each 
 subject were first submitted to a descriptive analysis . those rts outside of 3 standard deviations 
 ( 3 sd ) from the mean were considered as outliers and were excluded from the 
 rt data .
a lower limit of 120 ms and a higher limit of 800 ms were further applied to double check 
 whether there remained rts that were physiologically impossible ( too short rts  less than 
 120 ms ) or were due to reduced concentration ( too long rts  longer than 800 ms ) .
sixteen university students ( 10 male and 6 female 
 with an age range of 1924 years ) participated in the experiment . all participants were 
 right - handed and had normal or corrected - to - normal vision .
they were all nave to the purpose 
 of the experiment and were paid after the experiment .
the spatial resolution of the crt monitor was 1024  768 pixels 
 and the refresh rate was set at 75 hz .
each trial started with a white fixation cross ( 1  1 ) centered on the dark 
 computer screen .
after 1,000 ms , a white outline box serving as a peripheral cue appeared for 100 ms 
 at one of the four peripheral locations ( indicated by dash - line boxes in figure 1 ) with equal probability . 

two of the peripheral locations had an eccentricity of 7 ( left , right ) and the other two an 
 eccentricity of 21 ( left , right ) ( figure 
 1 ) . at 70 ms after the offset of the peripheral cue an outline box appeared at the 
 fixation location for 100 ms , reorienting the subject s attention back to the center . another 
 70 ms later , a white disk serving as the target appeared at either the same or the symmetric 
 opposite location of the peripheral cue , with equal probability .
the target remained present until 
 the participant responded , that is , pressed the space bar with the right index finger .
responding 
 was instructed to be as quickly and as accurately as possible . during each trial
, subjects were 
 instructed to maintain their fixation at the central cross without shifting their gaze to the 
 peripheral locations .
this requirement was particularly checked by the experimenter during the 
 practice session , and all subjects could follow this instruction successfully before the main test 
 started .
an intertrial interval of 1,000 ms with a blank screen was inserted after the subject 
 responded .
the sizes of the cues and targets for different 
 stimulus eccentricities were manipulated as follows : in the same - size condition the cue subtended 
 1  1 and the target 0.6  0.6 for both stimulus 
 eccentricities ( 7 and 21 ) ; in the size - scaling condition , the peripheral cue at 
 7 was kept at 1  1 size while , at 21 , it was magnified to 
 2.8  2.8 ; the peripheral target at 7 was kept at 0.6 
  0.6 size while , at 21 , it was magnified to 1.68 , using a formula 
 for calculating cortical magnification as suggested by rovamo and virsu ( 1979 ) .
four blocks were trials of the same - size condition and 4 blocks 
 were trials of the size - scaling condition . there were 80 catch trials in which no targets were 
 presented and the participants had to withhold their responses .
the number of trials for targets appearing in the left or right 
 visual field , at 7 or 21 eccentricity , and at a cued or uncued location , was 
 balanced in both the same - size and the size - scaling conditions .
one half of the participants 
 performed four blocks of the same - size condition first , and the other half performed four blocks of 
 the size - scaling condition first .
a practice block with 20 trials of the same - size condition and 20 
 trials of the size - scaling condition were tested before the main experiment .
the total error rate for each subject was lower 
 than 1% and therefore was not further analyzed .
response times for the correct trials of each 
 subject were first submitted to a descriptive analysis .
those rts outside of 3 standard deviations 
 ( 3 sd ) from the mean were considered as outliers and were excluded from the 
 rt data .
a lower limit of 120 ms and a higher limit of 800 ms were further applied to double check 
 whether there remained rts that were physiologically impossible ( too short rts  less than 
 120 ms ) or were due to reduced concentration ( too long rts  longer than 800 ms ) . mean rts 
 for all experimental conditions are shown in table 
 1 
 .
mean rts were analyzed with a 2  2 
  2 repeated - measure anova , with stimulus size ( same - size and size - scaling ) , 
 stimulus eccentricity ( 7 and 21 ) , and cue - target 
 validity ( valid and invalid ) as within - subject variables .
the results of the three - factorial anova showed 
 significant main effects of stimulus eccentricity , f(1 , 15 ) = 
 11.308 , p < .01 , p = .430 , and 
 cue - target validity , f(1 , 15 ) = 48.153 , p < 
 .001 , p = .762 , but no main effect of 
 stimulus size , f(1 , 15 ) = 1.492 , p > .05 , 
 p = .090 . however , stimulus 
 size significantly interacted with stimulus eccentricity , 
 f(1 , 15 ) = 31.961 , p < .001 , 
 p = .681 . further analysis of this interaction using 
 paired t - tests with bonferroni correction showed a slower rt for 21 versus 
 7 stimuli , but only in the same - size condition ( 305 ms vs. 294 ms , p < 
 .05 ) , not in the size - scaling condition ( 305 ms vs. 306 ms , p = .875 ) 
 ( figure 2a ) , as expected from the effect of cortical magnification .
consistent with previous 
 observations ( bao & pppel , 
 2007 ; bao et al . , 
 2011 , 2012 , 
 2013 ) , the interaction between 
 stimulus eccentricity and cue - target validity was also 
 significant , f(1 , 15 ) = 33.445 , p < .001 , 
 p = .690 . a further paired t - test 
 on the magnitude of ior ( i.e. , the rt difference between valid and invalid trials ) 
 for the two stimulus eccentricities revealed a stronger ior magnitude for 21 versus 
 7 stimuli ( 34 ms vs. 22 ms , t(15 ) = 5.783 , p < 
 .001 ) , showing an eccentricity effect of ior ( figure 
 2b ) .
more importantly , no three - way interaction between stimulus size , 
 stimulus eccentricity , and cue - target validity was observed , 
 f(1 , 15 ) = 1.069 , p = .318 , 
 p = .067 .
this indicated that the eccentricity 
 effect of ior was not affected by the manipulation of stimulus size .
in other words , a result 
 pattern of stronger ior at 21 versus 7 eccentricity stayed the same in both the 
 same - size condition ( 37 ms vs. 23 ms ) and the size - scaling condition ( 30 ms vs. 21 ms ) .
the present study examined whether the difference 
 in cortical projection of the ior stimuli ( as inferred from the cortical magnification factor ) is 
 responsible for the eccentricity effect of ior . by measuring the ior magnitude at two different 
 stimulus eccentricities ( 7 and 21 ) in a same - size and a size - scaling condition
, we 
 observed a robust eccentricity effect of ior  stronger ior magnitude for 21 relative 
 to 7 stimuli  independent of the manipulation of stimulus size . since in the 
 size - scaling condition , both the cue and the target at the more peripheral location ( 21 ) 
 were m - scaled simultaneously , we were concerned with a potential confounding of the effects of cue 
 size and target size .
therefore , we further conducted a supplementary experiment with 10 subjects 
 using similar experimental design and procedures , but having only the peripheral target magnified 
 while keeping the peripheral cue size unchanged .
again , we observed a significant eccentricity 
 effect of ior ( i.e. , a larger ior effect for 21 ( 41 ms ) versus 7 ( 28 ms ) 
 eccentricity ) which was unaffected by the manipulation of target size .
the main result of this study indicates that 
 cortical magnification is not responsible for the eccentricity effect of ior , since the critical 
 manipulation in the study 
rt was influenced by stimulus size as expected , but ior , i.e. , rt 
 difference , was not .
stimulus size exerts its influence on rt likely by spatial summation of 
 luminous flux up to the size of receptive fields at the respective visual field location 
 ( ricc s law ; strasburger et al . , 
 2011 ) .
the latter can be estimated by a psychophysical procedure known as 
 westheimer s paradigm ( oehler , 
 1985 ) .
table 2 
 shows a comparison of the stimulus sizes used in the present 
 study and estimated mean receptive field sizes at the same location .
stimulus size was just above 
 mean receptive field size at 7 eccentricity , but was below it at 21 in the unscaled 
 condition and above it in the scaled condition .
so stimulus sizes compare to summation areas as 
 intended and are thus chosen appropriately .
the observation of a significant two - way 
 interaction between stimulus eccentricity and cue - target validity 
 in the present study , together with other previous evidence ( bao & pppel , 2007 ; bao et al .
, 2011 , 2012 , 2013 ) , support 
 the notion that the distribution of attention in the human visual field is not homogeneous but can 
 be characterized by ( at least ) two attentional systems ( see also pppel & bao , 2012 ) , one extending from the fovea to 
 approximately 10 to 15 eccentricity along the horizontal meridian and the other 
 beyond this perifoveal region up to the far periphery . although previous studies suggest that ior is 
 associated with a distributed neural network including the right parietal lobe , superior and middle 
 temporal gyrus , frontal eye field , orbitofrontal cortex , and some subcortical structures including 
 superior colliculus and cerebellum ( lepsien & pollmann , 
 2002 ; mayer , seidenberg , dorflinger , 
 & rao , 2004 ; ro , farne , & 
 chang , 2003 ; sapir , soroker , berger , 
 & henik , 1999 ; vivas , humphreys , 
 & fuentes , 2003 ) , it is still too early to specify the neural mechanisms 
 underlying the eccentricity of ior , since all these previous studies exploring the neural correlates 
 of ior have only examined the ior effects in a limited range of stimulus eccentricity , typically 
 within the perifoveal visual field .
a direct comparison between ior in the perifoveal and the 
 peripheral regions of the visual field with fmri technology comes however from a recent study by 
 lei and colleagues ( 2012 ) .
consistent 
 with previous studies , a stronger activation of the dorsal fronto - parietal network was evidenced for 
 the perifoveal ior , whereas the peripheral ior was significantly associated with a stronger 
 activation of the prefrontal cortex which indicates that different cortical mechanisms underlie the 
 eccentricity effect of ior . due to the limited sensitivity of fmri in their study , lei and 
 colleagues could not detect any potential subcortical contributions to the eccentricity effect of 
 ior . in a structural model of attentional control 
 ( bao & pppel , 2007 ) it has 
 been argued that different anatomical pathways might be responsible for this functional 
 dissociation .
it has been well established that the more peripheral regions of the visual field show 
 a stronger projection to the superior colliculus of the midbrain and from there to extrastriate 
 regions , compared to the foveal and perifoveal regions which possess a strong geniculo - striate 
 projection .
thus , the eccentricity effect of ior might reflect different attentional control 
 mechanisms which , on the one hand , are dominated by cortical and on the other hand by subcortical 
 processes . in more general terms
this functional dissociation of attentional control would highlight 
 the importance of subcortical processing in attention as has been pointed out already some decades 
 ago ( singer , zihl , & pppel , 
 1977 ; for review see , e.g. , laberge , 
 1995 ; sommer & wurtz , 
 2004 ) . although the eccentricity effect of ior in the 
 visual field can not be explained by cortical magnification , stimulus size and eccentricity certainly 
 have an influence on visual processing .
if reaction times at 7 and 21 eccentricities 
 were compared for the same - size and the size - scaling conditions , a clear difference was observed . in 
 the same - size condition
the more peripheral targets ( 21 ) resulted in significantly longer 
 reaction times compared to the perifoveal targets ( 7 ) as also observed in a previous study 
 ( zhou , bao , sander , trahms , & pppel , 
 2010 ) .
this difference was no longer present in the size - scaling condition 
 indicating that , indeed , the extent of cortical representation being equalized for the two 
 conditions is relevant for the speed of processing .
the fact that the reaction times , after 
 adjusting the size of the stimuli , were similar further strengthens the hypothesis for the 
 eccentricity effect of ior as not being due to speed differences in perifoveal and peripheral 
 processing . there remains , however , an interesting paradox with respect to the interaction between 
 stimulus size and eccentricity : one might have expected that reaction times for targets at 7 
 and 21 eccentricities under the size - scaling condition would correspond to the reaction time 
 observed for 7 eccentricity for the same - size condition , but this was not the case ; they 
 were the same , but on a higher level corresponding to the slower reaction time under the same - size 
 condition for 21 eccentricity .
this result might indicate an instationarity within the 
 experimental conditions as often observed in experiments using reaction time as critical indicator 
 ( pppel , schill , steinbchel , 
 1990 ; steinbach , dreden , & 
 pppel , 1991 ) .
possibly , when a peripheral target of larger size is used , 
 the overall saliency for all stimuli is increased , resulting in a lower level of general activation 
 which then leads to slower response times . however
, whatever may be the case this effect did not 
 compromise the key observation of the eccentricity effect of ior being independent of the 
 magnification factor .
this supports the basic notion of a robust phenomenon of different attentional 
 control in the visual field .
 inhibition of return ( ior ) as an indicator of attentional control is characterized by an 
 eccentricity effect , that is , the more peripheral visual field shows a stronger ior magnitude 
 relative to the perifoveal visual field . 
 however , it could be argued that this eccentricity effect 
 may not be an attention effect , but due to cortical magnification . to test this possibility , we 
 examined this eccentricity effect in two conditions : the same - size condition in which identical 
 stimuli were used at different eccentricities , and the size - scaling condition in which stimuli were 
 scaled according to the cortical magnification factor ( m - scaling ) , thus stimuli being larger at the 
 more peripheral locations . 
 the results showed that the magnitude of ior was significantly stronger 
 in the peripheral relative to the perifoveal visual field , and this eccentricity effect was 
 independent of the manipulation of stimulus size ( same - size or size - scaling ) . 
 these results suggest 
 a robust eccentricity effect of ior which can not be eliminated by m - scaling . underlying neural 
 mechanisms of the eccentricity effect of ior are discussed with respect to both cortical and 
 subcortical structures mediating attentional control in the perifoveal and peripheral visual 
 field . 
Methods
Participants
Materials and Procedure
Data Preparation
Results
Discussion