although blood oxygenation level - dependent ( bold ) functional mri 
 has assumed a role of great importance in systems neuroscience , our 
 understanding of factors determining the amplitude and spatial extent of the 
 bold effect under different conditions remains incomplete .
relevant parameters 
 include baseline values and reactive capacity for cerebral perfusion , oxidative 
 metabolism , and blood volume .
an understanding of how these contribute to the 
 bold response is important , since in general they may vary due to age or 
 disease , and also depending on the nature of the neural system targeted by an 
 applied stimulus . in particular , the exact nature and extent of the coupling 
 between changes in oxidative metabolism and perfusion increases during neuronal 
 activation is still the subject of debate . 
 while recent studies have focused on quantification of responses during 
 nonspecific activation of diffuse regions of sensory and motor cortex [ 14 ] , this topic has also arisen in the context of highly localized 
 responses in cortical columnar systems [ 5 , 6 ] . in the present study , we sought 
 to bridge the gap between these two regimes by looking at the effect of 
 selective activation of only part of a small - scale cortical columnar system on 
 the apparent bold response observed at a spatial resolution typical of studies 
 used in human subjects .
early optical imaging studies [ 79 ] suggested that although 
 evoked changes in oxidative metabolism exhibit a high degree of spatial 
 specificity , brain perfusion is regulated on a much coarser spatial scale .
if 
 this is true , then there might be profound implications for the bold mri 
 signal , especially when measured during manipulations such as monocular 
 stimulation , which preferentially activates the set of ocular dominance columns 
 projecting to the stimulated eye .
in particular , one might expect the spatial pattern 
 of perfusion response evoked by stimulation of a single eye to be similar to 
 that seen during binocular stimulation , despite a substantial reduction in the 
 metabolic response ( compared to binocular stimulation ) in columns projecting to 
 the occluded eye . since the bold signal reflects changes in the level of venous 
 deoxygenated hemoglobin , this gratuitous hyperperfusion in unstimulated ocular 
 dominance columns
could be expected to result in a higher bold signal at a 
 given level of perfusion increase ( considering spatial averages over multiple 
 columns , which would be applicable at commonly used spatial resolutions in 
 fmri ) .
the present study examines joint changes in perfusion and bold 
 signals during monocular and binocular stimulation , to test the hypothesis that 
 spatial decoupling of flow and metabolic responses during stimulation of only a 
 partial subset of the columnar regions distributed within primary visual cortex 
 leads to a significant shift in the ratio between spatially averaged bold and 
 perfusion signals ( measured using arterial spin - labeling ) . by combining 
 quantitative mri - based measures of these two physiological quantities ,
we hope 
 to provide new insight into the spatial precision with which cerebral blood 
 flow is regulated , as well as factors which determine bold contrast amplitude 
 in cortical tissues exhibiting columnar organization .
eight healthy subjects ( five males and three females ) 24  2.6 years old , one left eye and hand dominant 
 ( male ) and seven right eye and hand dominant , participated in the study .
the 
 subjects did not suffer from any known visual deficits except myopia 
 ( mri - compatible corrective glasses were fitted in these cases ) .
all gave 
 informed consent and the project was approved by the comit mixte dthique de 
 la recherche du regroupement neuroimagerie / qubec .
data from two of the 
 subjects was not analyzed due to the poor quality of the arterial spin - labeling 
 ( perfusion ) data .
subjects were fitted with a neoprene rubber mask which allowed 
 occlusion of one eye by a removable patch . the patch was applied and removed as 
 needed between the appropriate scans , by an experimenter , from the back of the 
 scanner bore .
each scanning session included eight six - minute acquisitions , 
 during which alternating one - minute blocks of baseline ( uniform grey screen 
 with central fixation point ) and one - minute blocks of visual stimulation ( black 
 and white checkerboard reversing contrast at a rate producing four white 
 periods per second within a square ) were presented , starting with baseline . 

during each scanning run , the subject received either binocular ( b ) or 
 monocular ( m ) stimulation to their nondominant eye with separate scans 
 conducted in the following order : b - b - m - m - b - b - m - m .
subjects were instructed to 
 direct their gaze at the central fixation point throughout all scans .
the nondominant 
 eye was selected for monocular stimulation to maximize the difference in 
 activation between the monocular and binocular trials given that there may presumably 
 be more extensive activation of v1 for the dominant eye .
mri data acquisition was carried out using a 
 siemens trio 3 tesla mri 
 system , at software revision va25a .
images reflecting relative 
 perfusion were acquired using a picore / q2tips arterial spin - labeling ( asl ) 
 acquisition [ 11 , 12 ] .
the spatial resolution was 3.4 mm  3.4 mm on a 64  64 matrix , 
 with 10 slices of 5 mm thickness .
other 
 sequence parameters included tr / te / alpha = 2 s/19 ms/90 and ti1/ti2 = 700 ms/1400 ms . a slab 
 thickness of 200 mm was used , with a 10 mm gap between the top of the label 
 slab and the most inferior image slice . 

mr signals were received using an 
 eight - channel receive - only head coil , with excitation and labeling performed 
 using the system body coil .
these scans were 
 at 1 mm isotropic resolution , acquired using an mprage sequence with 
 ti / tr / te / alpha = 900/2300/2.94/9 .
voxel size was 1.0  1.0  
 1.2 mm . flow and bold images were generated using the  surround 
 subtraction 
the sequence of flow images 
 was generated by computing the difference between each image and the average of 
 the previous and subsequent images .
the sequence of bold images was computed by 
 adding each image to the average of its two neighbors . for each run ,
the effect 
 and standard error maps were then generated by fitting a linear signal model to 
 each voxel in the flow and bold series .
the model consisted of a term 
 representing the three task epochs in the run convolved with a dual gamma 
 function including positive response plus undershoot , plus a third - order 
 polynomial .
multiple runs for each subject were then combined using a mixed - effects 
 model , followed by spatial normalization to the mni 152 brain and combination 
 of normalized maps for different subjects again using a mixed - effects model ( as 
 described in worsley et al . ) .
individual and multisubject maps were then 
 thresholded at p = .001 significance with correction for multiple 
 comparisons using the stat_threshold routine of the fmristat software package 
 .
regions of interest ( rois ) were generated using the neurolens 
 software package ( www.neurolens.org ) .
average bold statistical maps for each 
 subject were used to make a v1 roi by thresholding as described above .
voxels 
 exceeding threshold in the bold map but located outside the banks of the 
 calcarine sulcus as visualized in the t1-weighted structural scan were manually 
 edited from the roi , to ensure that the signals extracted were associated with 
 primary visual cortex and therefore contained tissue organized as ocular 
 dominance columns .
the effect values were then averaged within the roi for each 
 functional scan and tabulated as effect size  standard error .
values were converted to percent change as 
 needed by dividing the effect size by the constant ( dc ) term fit during linear 
 modeling and multiplying by 100 .
figure 1 shows mixed - effects bold and flow maps over all subjects 
 for monocular and binocular stimulation .
occlusion of input to one eye reduced 
 the amount of activation detected in extrastriate areas .
however , the 
 significance levels observed within primary visual cortex appeared similar 
 during both monocular and binocular stimulation .   averaged time course signals for flow and 
 bold are shown in figure 2 , revealing the initial bold signal overshoot and 
 poststimulus undershoot commonly observed in visual cortex during checkerboard 
 stimulation ( seen here during both monocular and binocular stimulation ) .
the 
 flow signal illustrates the lower signal - to - noise ratio generally obtained in 
 arterial spin - labeling measurements .
the bar graphs in figure 3 show average percent changes in bold 
 and flow signals within the v1 rois of all subjects .
the average percent change 
 in bold signal for monocular stimulation was 0.930.04 , which was significantly ( p < .05 ) less than the percent 
 change of 1.11  0.05 observed during binocular stimulation .
the percent flow 
 increase measured during monocular stimulation was 29  2 , also significantly 
 less than the percent change of 37  2 observed during binocular stimulation .
expressed 
 as a percent reduction in the response amplitude , withholding input from one of 
 the two eyes resulted in a 16  6% decrease in the bold response and 19  9% 
 decrease in the perfusion response . the percent changes in bold signal per percent signal increase in 
 flow ( i.e. ,
the quotient %bold  %flow ) during 
 monocular and binocular stimulation were found , respectively , to be 
 0.031  0.004 and 0.030  0.004 ( figure 4 ) .
the difference between these ratios was not 
 statistically significant , failing to support any difference in flow - metabolism 
 coupling during the two forms of stimulation .
in this study , we examined the coupling between bold and cbf 
 responses in primary visual cortex during monocular and binocular stimulation .
we 
 found that the bold and cbf responses to monocular visual stimulation were both 
 significantly reduced in v1 relative to the responses observed during binocular 
 stimulation ( figure 3 ) .
the ratio of bold to cbf effect sizes did not differ 
 significantly between the two stimulation conditions ( figure 4 ) , indicating 
 comparable coupling between flow and oxidative metabolism regardless of the 
 columnar fraction that was activated .
the results obtained in the present study do not support the 
  strong  form of the theory that tissue perfusion is regulated only on a coarse 
 spatial scale , irrespective of the spatial precision with which metabolism 
 might change .
this notion has been described previously as  watering the entire 
 garden for the sake of one thirsty flower  . to borrow the garden analogy , 

the experiments described here could be described as an attempt to measure the 
 total water intake of the garden , as well as the runoff of unused water , to 
 test this hypothesis .
our results are consistent with recent mri studies 
 showing that there is in fact sufficient spatial contrast in the perfusion 
 response , as imaged using arterial spin - labeling , to resolve columnar 
 structures in the visual cortex .
the study by duong et al .   found that 
 the early negative bold response ( initial dip ) also exhibited a high degree of 
 spatial localization , whereas the late positive bold response was more diffuse . 

it is important to remember that the apparent resolution of each signal is 
 dictated both by the underlying physiological regulatory precision and by the degree to which confounding vascular structures are superimposed on the 
 pattern of parenchymal activation .
based on our results and those from duong et 
 al . , it appears likely that the lack of precision in the late bold signal is 
 due primarily to obscuring effects from the macrovascular anatomy , rather than 
 a diffuse parenchymal bold effect .
it has been suggested by other authors 
 that the increase in the apparent precision of the initial dip arises because 
 bold signal increases in large draining veins do not arise until after the 
 initial transit of blood through the local capillary bed postulated to take 
 approximately one second .
given that functional signals of interest may exhibit bias due to 
 vascular anatomy , designating regions of interest using objective criteria is 
 an important part of quantitative neuroimaging studies . in the present study , the use of 
 phase - encoded retinotopic mapping to identify v1 in a separate mapping 
 experiment would have been the optimal approach , since this procedure yields a 
 set of voxels exhibiting a specific spatial trend in the polar angle or 
 eccentricity of their projection in visual field that is unlikely to appear in 
 a large vein .
this would have led to excessively long scan sessions , however . 
 instead , we used the fact that the optic radiations terminate in the calcarine 
 sulcus , making it very probable that activated regions lying in this anatomical 
 zone are in fact part of primary visual cortex .
it is still possible that the 
 bold activation maps used to create rois based on a simple  activation 
 minus baseline  contrast could contain a disproportionate contribution 
 from large draining veins .
these veins mix venous outflow from multiple visual 
 areas , including regions which do not exhibit eye - specific columnar 
 segregation , diluting any shift in flow - bold coupling present specifically in 
 v1 . in a pilot study of six subjects 
 performed at 1.5 t using retinotopic mapping to identify v1 ( but performed using 
 single - slice asl at 1.5 t )
we therefore do not feel that the results of 
 the present study are substantially impacted by our roi selection procedure .
moreover , 
 the relatively large voxel size and intense stimulus used in the present study 
 yielded diffuse regions of robust activation that did not appear to be limited 
 to macrovascular responses ( as can occur at higher spatial resolutions or with 
 weaker stimuli ) . by measuring total flow and bold responses in v1 during 
 activation of different columnar fractions , we were able to achieve high 
 signal - to - noise ratios ( snrs ) compared to studies that have used extremely high 
 spatial resolution to actually resolve the columns .
the purpose of the present 
 study was to provide insight into two questions : the first is whether there is in fact a 
 fundamental difference in the spatial precision with which perfusion and 
 oxidative metabolism is measured ; the second was to determine the impact of 
 partial activation of a cortical columnar system on the bold signal 
 characteristics observed at a customary fmri spatial resolution .
if there is 
 indeed a profound mismatch in the spatial extent of increases in oxidative 
 metabolism and flow , this should be apparent in the total average signal over 
 v1 . that none was found suggests that similar extents are likely to be found at 
 higher spatial resolutions .
however , it is notable that while removal of input from one of 
 the two eyes did result in a reduction of both bold and flow signals , the 
 response decreased by much less than one half .
this is consistent with detailed 
 autoradiographic studies showing that pronounced ocular dominance segregation 
 is mainly limited to cortical layer iv , with layers ii and iii actually exhibiting higher activation during binocular 
 than monocular stimulation [ 10 , 18 ] .
this is consistent with later human 
 neuroimaging studies , in which some regions showed higher apparent activity 
 levels during the appropriate monocular stimulation than during binocular input 
 [ 16 , 19 ] .
the columnar structure associated with ocular dominance is therefore 
 most appropriately viewed as reflecting a moderate bias in overall selectivity , 
 associated primarily with a single cortical sublayer , superimposed on numerous 
 other modulating influences . in light of the issues
discussed above , it is clear that the 
 columnar segregation of brain activation is not  all or nothing  
 during selective stimulation such as monocular or single - orientation 
 conditions .
much of the early research in this area , performed using optical 
 imaging methods capable of producing compelling maps of the columnar 
 architecture , examined the perfusion of orientation domains ( e.g. , malonek and 
 grinvald ) and not ocular dominance 
 columns although a number of authors have imaged ocular dominance using a 
 variety of other methods [ 5 , 6 , 10 , 19 , 20 ] . 

it would therefore be informative to replicate the present study using 
 different combinations of oriented stimuli .
it is also possible that certain 
 stimuli might achieve a higher degree of selectivity than the ones used in this 
 and prior studies .
perhaps under such conditions a small difference in 
 flow - bold coupling might become detectable .
future investigation of this topic 
 might include the use of different stimuli designed to selectively activate 
 pathways involved in stereopsis .
our results do not support the theory of spatially decoupled 
 responses in blood flow and oxidative metabolism during activation of a subset 
 of cortical ocular dominance columns .
the limited impact of monocular blockade 
 on flow and bold response amplitudes is also demonstrated , and should serve as 
 a caution that ocular dominance contrast is likely to be faint in hemodynamic 
 imaging methods .
 previous studies have suggested that during selective activation of a subset of the zones comprising a columnar system in visual cortex , perfusion increases uniformly in all columns of the system , while increases in oxidative metabolism occur predominantly in the activated columns . 
 this could lead to disproportionately large blood oxygenation level - dependent ( bold ) signal increases for a given flow increase during monocular ( relative to binocular ) stimulation , due to contributions from columns which undergo large increases in perfusion with little or no change in oxidative metabolism .   in the present study 
 , we sought to test this hypothesis by measuring bold - perfusion coupling ratios in spatially averaged signals over v1 during monocular and binocular visual stimulation . 
 it was found that , although withholding input to one eye resulted in statistically significant decreases in bold and perfusion signals in primary visual cortex , the ratio between bold and perfusion increases did not change significantly . 
 these results do not support a gross mismatch between spatial patterns of flow and metabolism response during monocular stimulation . 