Orientation-specific contextual modulation of the fMRI BOLD response to luminance and chromatic...

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FOrientation-specific contextual modulation of the fMRI BOLD responseto luminance and chromatic gratings in human visual cortex

J. Scott McDonald a, Kiley J. Seymour a, Mark M. Schira b, Branka Spehar b, Colin W.G. Clifford a,*

a School of Psychology, The University of Sydney, Griffith Taylor Building (A19), Sydney, NSW 2006, Australiab School of Psychology, UNSW, Sydney, NSW 2021, Australia

2223242526272829303132

a r t i c l e i n f o

Article history:Received 21 March 2008Received in revised form 11 August 2008Available online xxxx

Keywords:fMRIInhibitionSuppressionTilt illusionCentre–surround interactions

33343536373839

0042-6989/$ - see front matter � 2009 Published bydoi:10.1016/j.visres.2008.12.014

* Corresponding author.E-mail address: colinc@psych.usyd.edu.au (C.W.G.

Please cite this article in press as: Scott McResearch (2009), doi:10.1016/j.visres.2008

PRa b s t r a c t

The responses of orientation-selective neurons in primate visual cortex can be profoundly affected by thepresence and orientation of stimuli falling outside the classical receptive field. Our perception of the ori-entation of a line or grating also depends upon the context in which it is presented. For example, the per-ceived orientation of a grating embedded in a surround tends to be repelled from the predominantorientation of the surround. Here, we used fMRI to investigate the basis of orientation-specific surroundeffects in five functionally-defined regions of visual cortex: V1, V2, V3, V3A/LO1 and hV4. Test stimuliwere luminance-modulated and isoluminant gratings that produced responses similar in magnitude. LessBOLD activation was evident in response to gratings with parallel versus orthogonal surrounds across allthe regions of visual cortex investigated. When an isoluminant test grating was surrounded by a lumi-nance-modulated inducer, the degree of orientation-specific contextual modulation was no larger forextrastriate areas than for V1, suggesting that the observed effects might originate entirely in V1. How-ever, more orientation-specific modulation was evident in extrastriate cortex when both test and inducerwere luminance-modulated gratings than when the test was isoluminant; this difference was significantin area V3. We suggest that the pattern of results in extrastriate cortex may reflect a refinement of theorientation-selectivity of surround suppression specific to the colour of the surround or, alternatively,processes underlying the segmentation of test and inducer by spatial phase or orientation when no colourcue is available.

� 2009 Published by Elsevier Ltd.

1. Introduction

We use functional MRI to investigate the substrates of orienta-tion-specific contextual modulation in human vision. In each oftwo experiments, we compare the magnitude of the BOLD signalin response to stimulus blocks where the test and inducer are par-allel (but 90� out of phase spatially) with blocks where test and in-ducer are perpendicular. The motivation for these experiments isdescribed below.

Contextual modulation is a fundamental property of visual pro-cessing with moment-to-moment relevance to our perception ofattributes such as colour, lightness, contrast and orientation. Ef-fects of image context have been investigated extensively usingpsychophysical and electrophysiological methods, and their func-tional basis has been much debated by theoreticians interestedin the optimal coding of sensory information (for a review, see Sch-wartz, Hsu, & Dayan, 2007). Perhaps the ‘‘best studied test case” ofcontextual modulation is that of visual orientation processing (Sch-

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Clifford).

Donald, J., et al. Orientation-.12.014

wartz et al., 2007), and it is for that reason that we focus on orien-tation-selective contextual modulation here.

Visual neurons in cat and monkey cortex are excited by stimuliplaced within their classical receptive fields (CRF). The area adjacentto the CRF, when stimulated alone, does not elicit a response fromthe neuron. However, this extra-classical receptive field (ECRF) re-gion can profoundly modulate the neuron’s response to a stimuluswithin the CRF (Blakemore & Tobin, 1972; Chen, Kasamatsu, Polat,& Norcia, 2001; DeAngelis, Freeman, & Ohzawa, 1994; Jones, Grieve,Wang, & Sillito, 2001; Jones, Wang, & Sillito, 2002; Maffei & Fioren-tini, 1976; Nelson & Frost, 1978, 1985;Walker, Ohzawa, & Freeman,1999; Webb, Barraclough, Parker, & Derrington, 2003). For both catand monkey, contextual modulation tends to be suppressive. Theincidence of facilitation depends on the relative contrast of the stim-uli in the CRF and ECRF, typically occurring only when the stimulus inthe CRF has a low contrast (Polat, Mizobe, Pettet, Kasamatsu, & Nor-cia, 1998). Suppression predominates when the CRF is stimulatedwith high contrast stimuli. Of particular relevance to our study, themagnitude of suppression is strongly modulated by the relative ori-entations of the stimuli in the CRF and ECRF. Suppression tends to begreatest when the ECRF and CRF orientations are the same and isattenuated or abolished when they are very different (Gulyas, Orban,

specific contextual modulation of the fMRI BOLD response ... Vision

2 J. Scott McDonald et al. / Vision Research xxx (2009) xxx–xxx

Duysens, & Maes, 1987; Levitt & Lund, 1997; Li & Li, 1994; Li, Thier, &Wehrhahn, 2000; Nelson & Frost, 1978; Sillito et al., 1995; Cava-naugh, Bair, & Movshon, 2002). Indeed, at least two previous studiesspecifically identifying the surround orientations giving maximumand minimum suppression for each of a population of cells havefound maximum suppression for surrounds near the cell’s preferredorientation and minima around the orthogonal orientation (Gilbert& Wiesel, 1990; Sengpiel, Sen, & Blakemore, 1997). Consequently,we have chosen to use inducing gratings oriented parallel andorthogonal to the test stimulus in order to investigate orientation-specific contextual modulation. Our hypothesis is that the effect ofcontextual modulation should be maximal for parallel and minimalfor orthogonal test and inducing gratings. A direct comparison of thefMRIBOLDresponsetothesestimuli is thusameasureoftheorientation-specific component of contextual modulation in human visual cortex.

Perceptually, similarly oriented surround gratings tend to pro-duce the greatest effects on perceived orientation and contrast(Cannon & Fullenkamp, 1991; Gibson & Radner, 1937; Snowden& Hammett, 1998). The effect of an oriented surround on the per-ceived orientation of a central test stimulus, the tilt illusion(Fig. 1A), was first reported by Gibson and Radner (1937). The tiltillusion depends on the relative orientation of centre and surround(O’Toole and Wenderoth, 1977), indicating that it can be mediatedno earlier in the visual processing hierarchy than the first site ofsignificant orientation-selectivity, V1 (Hubel & Wiesel, 1966;Smith, Chino, Ridder, Kitagawa, & Langston, 1990; Xu, Ichida, Sho-stak, Bonds, & Casagrande, 2002). When centre and surround stim-uli are presented in separate eyes (dichoptic viewing) themagnitude of the tilt illusion is reduced by around 20% (Forte &Clifford, 2005; Wade, 1980) compared with presentation to thesame eye (monocular viewing). This incomplete interocular trans-fer indicates that the tilt illusion is mediated in part by neuralmechanisms receiving input from only one eye. The only area of vi-sual cortex known to contain a significant proportion of monocularneurons is V1 (Hubel & Wiesel, 1966), indicating that the tilt illu-sion is mediated at least in part within V1 itself. Thus, we predictthat orientation-specific contextual modulation of the fMRI BOLDresponse will be evident as early as V1.

The tilt illusion displays chromatic tuning, as do surround effectson perceived contrast (Singer & D’Zmura, 1994), such that the mag-nitude of the illusion is maximal for centre and surround modulatedalong the same axis of colour space. For centre and surround modu-lated along orthogonal axes of colour space, for example an L–M iso-lating (‘‘red–green”) central grating and a luminance-definedsurround grating, the magnitude of the illusion drops by around50% (Clifford, Spehar, Solomon, Martin, & Zaidi, 2003; Clifford, Pear-son, Forte, & Spehar, 2003; Forte & Clifford, 2005). Thus, the illusioncan be considered to involve colour-specific and colour-invariantcomponents. When centre and surround stimuli are modulated

Fig. 1. Contextual modulation of perceived orientation – the tilt illusion. (A) Theorientation of a vertical central grating appears repelled away from that of thesurround. (B) The illusion persists when a vertical test is presented in an annulussurrounded inside and out by an oriented inducer.

Please cite this article in press as: Scott McDonald, J., et al. Orientation-Research (2009), doi:10.1016/j.visres.2008.12.014

along orthogonal axes of colour space, full interocular transfer ofthe tilt illusion is observed (Forte & Clifford, 2005). This indicatesthat the colour-invariant portion of the tilt illusion is purely binocu-lar, and thus that the monocular component of the tilt illusion is col-our-specific. The binocular component of the tilt illusion, however, islargely colour-invariant (Forte & Clifford, 2005).

The observation that the monocular component of the tilt illu-sion is colour-specific implicates neurons in V1 coding both colourand orientation. Evidence for just such conjoint tuning of colourand orientation in human V1 comes from fMR adaptation (Engel,2005) and a recent application of multivariate pattern analysis tofMRI data (Sumner, Anderson, Sylvester, Haynes, & Rees, 2008).Single neurons selective for both colour and orientation have alsobeen reported in V1 of non-human primates (De Valois, Cottaris,Elfar, Mahon, & Wilson, 2000; Johnson, Hawken, & Shapley,2001; Lennie, Krauskopf, & Sclar, 1990; Leventhal, Thompson, Liu,Zhou, & Ault, 1995; Thorell, De Valois, & Albrecht, 1984), as wellas in areas V2 and V3 (Gegenfurtner, Kiper, & Fenstemaker, 1996;Gegenfurtner, Kiper, & Levitt, 1997). Thus, we predict that the ori-entation-selective component of contextual modulation in thefMRI BOLD response will be greater for test and inducer modulatedalong the same versus orthogonal axes of DKL colour space, andthat this difference will be evident as early as V1.

The tilt illusion is thought to be largely due to orientation-selec-tive inhibition of neurons responding to the test grating (Fig. 2). Asimilar mechanism could potentially account for the reduction inperceived contrast (Olzak & Laurinen, 1999). Specifically, orienta-tion-selective cortical neurons responding to the inducing gratinginhibit similarly tuned neurons responding to the test (Blakemore& Tobin, 1972; Clifford, Wenderoth, & Spehar, 2000; Wenderoth &Johnstone, 1987). When test and inducer differ slightly in orienta-tion (e.g. by 15�), this lateral inhibition biases the neural represen-tation of the orientation of the test such that the populationresponse (and hence the perceived orientation) is repelled awayfrom the inducing orientation (Clifford et al., 2000; Gilbert & Wie-sel, 1990). When test and inducer are parallel, the neuronal popu-lation response to the test is maximally inhibited. However, noorientation illusion is elicited because there is no asymmetry inthe inhibition and hence no bias in the population response. Thus,while a difference in inducer-test orientation is necessary to elicitthe perceptual tilt illusion, the underlying neuronal interactionsare best studied physiologically using parallel gratings.

Our use of a 90� shift in spatial phase between test and sur-round gratings in the parallel condition stems from our interestin the orientation-specific contextual modulation likely underlyingthe tilt illusion as distinct from phase-specific effects of contextualmodulation with parallel gratings as are evident psychophysicallyin perceived contrast (Olzak & Laurinen, 1999) and electrophysio-logically (Akasaki, Sato, Yoshimura, Ozeki, & Shimegi, 2002; DeAn-gelis et al., 1994; Xu, Shen, & Li, 2005). Relative spatial phase is nota meaningful parameter when test and surround are not paralleland hence is of no relevance in the tilt illusion, so we did not wantto confound orientation- and phase-specific effects by using in-phase test and surround in the parallel condition. While there issome evidence from single-cell studies to suggest that we mighthave observed a bigger effect if we had used in-phase test and sur-round in the parallel condition (Xu et al., 2005), our aim here wasto isolate the effect of orientation rather than to observe the max-imum effect.

In psychophysical studies of contextual modulation it is con-ventional to position the test region at fixation, for example a cir-cular test patch embedded in an annular inducer (Fig. 1A). Such aconfiguration has also been used to investigate orientation-specificcontextual modulation in an fMRI study (Williams, Singh, & Smith,2003). Williams et al. (2003) reported suppression of the responseto the test in retinotopic visual cortex in the presence of a parallel

Fig. 2. Relationship between neuronal population response and perceived orientation. (A) Hypothetical response of a population of orientation-selective neurons as afunction of the orientation neuronal peak tuning in response to a vertical (0�) test stimulus. (B) Hypothetical effect of an oriented surround on the gain of the neuronsresponding to the test illustrated for surround oriented at 0� (solid line) or 15� (dotted line). Orientation-selective cortical neurons responding to the inducing grating inhibitsimilarly tuned neurons responding to the test. (C) Predicted neuronal population response to a test stimulus oriented at 0� in the presence of a 0� surround. The prediction isobtained by multiplying the unmodulated population response in (A) by the gain – solid line in (B). The overall population response is strongly reduced (compare area underpopulation response curve with and without surround) but the effect on the response profile is symmetrical about 0� and hence the population continues to code the testorientation veridically – i.e. no tilt illusion. d. Predicted neuronal population response to a test stimulus oriented at 0� in the presence of a 15� surround. The prediction isobtained by multiplying the unmodulated population response in (A) by the gain – dotted line in (B). The overall population response is not as strongly reduced as in (C) butthe response profile is now biased away from the orientation of the surround and hence the population codes an orientation consistent with a repulsive tilt illusion. Thus,while a difference in inducer-test orientation is necessary to elicit the perceptual tilt illusion, the underlying neuronal interactions are best studied physiologically usingparallel gratings.

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ECbut not a perpendicular surround. However, foveal presentation of

the test stimulus did not allow areas V1–V3 to be consistently dis-tinguished, so quantitative data were only reported for a region-of-interest encompassing all three areas. Delineation of the bordersbetween retinotopic visual areas is easier in the periphery thanin the fovea. Here, we are interested in tracking orientation-spe-cific contextual modulation through the visual processing hierar-chy, so we follow the stimulus configuration of Zenger-Landoltand Heeger (2003) in using an annular test region surrounded in-side and out by the inducing stimulus (Fig. 3A). Psychophysicalpiloting with this stimulus revealed that the magnitude of therepulsive tilt illusion was of the same order (several degrees) asfor foveal test presentation (compare the perceived orientation ofthe central grating in Fig. 1A and that of the annular grating inFig. 1B: both are objectively vertical).

Simultaneous presentation of stimuli has been shown to sup-press the fMRI BOLD response across human visual cortex relativeto sequential presentation (Kastner et al., 2001). Surround suppres-sion of the response to collinear gratings has been specificallyinvestigated in areas V1–V3 (Zenger-Landolt & Heeger, 2003). Bothstudies found that suppression was greater in extrastriate areasthan in V1. When compared with behavioural data, Zenger-Landoltand Heeger (2003) found that the surround suppression observedin V1 was in good quantitative agreement with the psychophysicaleffect on perceived contrast. Our hypothesis about the link be-tween contextual modulation at the neuronal level and its expres-sion in the BOLD signal is that the contextual modulation weobserve will be primarily inhibitory and thus that it will reducethe magnitude of the BOLD signal in the regions of the corticalmaps being inhibited (Beck & Kastner, 2005; 2007; Kastner et al.,2001; Zenger-Landolt & Heeger, 2003). However, since the precise

relationship between neuronal inhibition and the BOLD signal re-mains uncertain (Attwell & Iadecola, 2002; Waldvogel et al.,2000), we conduct all statistical tests as 2-tailed rather thanassuming a priori that neuronal inhibition reduces the BOLD signal.

One possible confound we sought to avoid was different BOLDresponses elicited by different spatial orientations, the fMRI ana-logue of the psychophysical oblique effect (Furmanski & Engel,2000). To ensure that any differences in the responses to paralleland orthogonal blocks were due to the relative orientation of testand surround rather than the absolute orientation, each block con-sisted of a sequence of grating stimuli presented at each of 16 spa-tial orientations (Fig. 3B). In this way, it was ensured that thedistribution of orientations in both test and surround was the samefor parallel and orthogonal conditions.

Another important issue in using fMRI to study contextual mod-ulation is isolation of the response to the test. Specifically, in com-paring the response to a test stimulus presented in isolation with atest stimulus presented with a surround, there is a danger that acomponent of the response to the surround may be interpretedas a response to the test or that ‘‘haemodynamic stealing” by thesurround might inflate the measured effect of contextual modula-tion (Zenger-Landolt & Heeger, 2003). Here, we avoided any suchconfound by comparing two conditions each of which consistedof test and surround stimuli. Thus, we are measuring the orienta-tion-selective component of contextual modulation rather thanthe total magnitude of contextual modulation in any one condition.

We concentrated our analysis on those voxels within five func-tionally-defined regions of visual cortex: V1, V2, V3, V3A/LO1 andhV4 that gave significant responses to the stimulus in the testannulus presented in isolation (see Section 2.2.1). We do not claimto have isolated only the response to the test region of the stimulus

Fig. 3. Experimental stimuli. (A) The five different stimulus types used in the experiments, from left to right: test-only, inducer-only, orthogonal test-inducer, parallel test-inducer, blank screen. All conditions also have a fixation marker. (B) Schematic example of the stimulus orientations used in a typical block, in this case with orthogonal test-inducer.

when presented with an abutting surround as voxel receptive fieldsize in V1 at the eccentricity of the test annulus (2–3�) is of the or-der of 0.5� (standard deviation of fitted 2D Gaussian: Dumoulin &Wandell, 2008; Kay, Naselaris, Prenger, & Gallant, 2008; Larsson &Heeger, 2006). However, we would expect lateral interactions be-tween annulus and inducer to be reciprocal (i.e. if the test isorthogonal to the surround then the surround is orthogonal tothe test), so voxels close to the boundary should show contextualmodulation in the component of their response to the inducer aswell as in the component of their response to the annulus. Failureto exclude all response to the surround would thus not be expectedto bias the data as long as it was consistent across conditions.

A final issue involves the border between test and surround. It isconceivable that the activity of neurons whose receptive fields in-clude the border between test and surround might induce a differentBOLD response for parallel and orthogonal surrounds regardless ofany contextual modulation proper. For example, it might be thatthe salience of the border between test and surround differs betweenparallel and orthogonal stimulus blocks, or that segmentation pro-cesses are engaged differentially in the two conditions. Althoughwe know of no direct evidence in support of this potential criticism,it is something for which we have attempted to control in our secondexperiment by using an isoluminant test grating and a luminance-modulated surround. In such a stimulus, the colour-luminanceboundary provides a salient border and thus a strong cue to segmen-tation in both parallel and orthogonal stimulus blocks.

2. Methods and materials

2.1. Experimental procedures

2.1.1. Subjects and experimental sessionsOne female and three male subjects participated in the experi-

ments. All were experienced psychophysical subjects and had nor-mal or corrected to normal vision. Each subject participated in atleast 16 sessions. Six sessions were required for each of Experi-ments 1 and 2 and at least four sessions were used to establishthe retinotopic areas.

2.1.2. Scanner and experimental setupA Philips 3T scanner with a whole-head ‘‘birdcage” coil was

used to perform the MRI. The stimuli were presented on a Fara-

Pday-shielded flat panel LCD Philips monitor size 35 cm � 28 cmand resolution 1024 � 768. The monitor was calibrated so that pix-el value was linearly related to luminance output. Subjects lay inthe scanner and viewed the monitor through a mirror attachedto the head coil. The viewing distance to the monitor, when seenthrough the mirror, was 158 cm. Behavioural responses were indi-cated through an MRI-compatible LU400-PAIR Lumina responsepad (Cedrus Corporation, San Pedro, CA, USA).

While the subjects viewed the stimuli, a time series of 168 MRIvolumes were collected using a T2�-sensitive, boustrophedon, fieldecho (i.e. gradient echo) echo planar imaging (FEEPI) pulse sequence.The echo time (TE) was 30 ms, repetition time (TR) 2000 ms, flip an-gle of 90�, field of view (FOV) 220 mm � 87 mm � 220 mm, in-planeresolution 1.7 mm � 1.7 mm, slice thickness 3.0 mm. Twenty-nineslices were collected in an interleaved, ascending order, in the trans-verse plane and the scan covered most of the head including all of theoccipital and parietal lobes.

2.1.3. Stimulus and taskThe stimuli were sinusoidal gratings (1 cycle/�) presented in

blocks of 16 s. Each block consisted of 16 static images of one sec-ond duration. The stimuli were presented in a circular aperturewith a radius of 6.0� surrounded by a uniform field at the meanluminance of the stimulus (57 Cd/m2). They consisted of an annu-lar test region, which extended from 2.0� to 3.0� radius, and a sur-round inducer region which covered the remainder of the stimulusaperture inside and outside the test annulus.

There were four different stimulus configurations (Fig. 3A): testonly, inducer-only, parallel test and inducer, and orthogonal testand inducer. Each 16-s block contained 16 different orientationsof the stimuli, presented in pseudo-random order, such that everydiscrete stimulus orientation occurred once (Fig 3B). The paralleland orthogonal blocks differed only in the relative orientation oftest and inducer and not in the distribution of absolute orienta-tions. Hence any ‘‘oblique effects”, whereby horizontal and verticalorientations generate great BOLD response than diagonal orienta-tions (Furmanski & Engel, 2000) were balanced between blocks.There were 21 blocks in each session, four blocks of each of the fourstimulus configurations and five blank fixation periods. The blockswere ordered in a balanced design (each block type occurred anequal number of times before every other block type) and the first,last and every fifth block were blank fixation periods.

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Throughout all blocks, subjects’ attention was carefully con-trolled by requiring them to perform a demanding dimming task:detection of a brief luminance decrement of the fixation markerpresented at the centre of the display.

2.1.4. Experiment 1Both regions of the stimulus, test and inducer, were achromatic

luminance-modulated gratings presented at full Michelson contrast.

2.1.5. Experiment 2As for Experiment 1, the inducing regions of the stimulus con-

tained full contrast achromatic luminance gratings. However, thetest annulus was now a grating that was modulated along theL–M (red–green) isolating axis of DKL colour space (Derrington, Kra-uskopf, & Lennie, 1984). Isoluminance for the L–M axis was estab-lished separately for each subject prior to scanning usingminimum motion (Anstis & Cavanagh, 1983) and minimum flickertechniques under viewing conditions matched to those of the scan-ner. Isoluminance was then confirmed in the scanner for each sub-ject using only the minimum flicker technique.

2.2. Analysis procedures

2.2.1. Preprocessing and definition of regions of interestThe fMRI data were processed with the BrainVoyagerQX 1.9

software package. The data were preprocessed to correct for slicetime, head motion and linear trends. The functional scans werealigned to the anatomical data and transformed into Talairachcoordinates at an effective in-plane resolution of 3.0 mm. For eachsubject the cortical surface was then segmented, smoothed and in-flated so that we could identify the early retinotopic areas (V1, V2,V3, V3A/LO1 and hV4) as regions of interest (ROIs).

Standard retinotopic mapping procedures were used to identifythe visual areas. Subjects were presented with rotating wedge andexpanding ring stimuli (Engel, Glover, & Wandell, 1997; Wandellet al., 2007). Voxel-by-voxel analysis of the temporal profile ofthe response to these stimuli allowed us to visualize the mappingof the visual field onto an inflated representation of the corticalsurface (illustrated for the left occipital lobe of one subject, KJS,in Fig. 4A). The early retinotopic areas were then delineated withreference to canonical data from fMRI of human visual cortex(Larsson & Heeger, 2006; Wandell et al., 2007). In defining our reti-notopic regions of interest, we follow the nomenclature recom-mended by Wandell et al. (2007). Three specific points are worthhighlighting. First, our area V3 contains both dorsal V3 and its ven-tral counterpart, sometimes termed VP (e.g. Pitzalis et al., 2006).Second, since we were unable to separate areas V3A and LO1 withconfidence in all subjects, we collapsed them into one large regionlateral to V3d that we term V3A/LO1. Third, we have defined areahV4 as a hemifield map directly abutting the ventral portion of V3.

We then isolated voxels where the test-only stimulus blockevoked a strong BOLD signal to create a mask (Fig. 4B). To this endwe created four reference functions, one for each stimulus type (fix-ation alone was not modeled), via convolution with a model haemo-dynamic response function. These reference functions were used asthe design matrix in a general linear model. The response to test-onlystimulus blocks was contrasted against baseline. Serial correlationswere corrected for by pre-whitening, assuming the correlations fol-lowed a first-order autoregressive AR(1) process. Only voxels with asignificance p < 0.001 (Bonferroni corrected) were included in themask. This mask was then combined with the retinotopically-de-fined ROIs to produce masked regions of interest (mROIs).

2.2.2. Comparison of conditions using percentage signal changeFor each subject we extracted fMRI responses by averaging data

from all the voxels within each mROI. For each scan, we then aver-

aged the signal across all blocks of the same type. The fMRI re-sponse in each condition was calculated as the percentage signalchange (PSC) from fixation:

PSC ¼ 100 � ðt � bÞ=b ð1Þ

where t is the mean signal value across the block (offset by two TRsto allow for haemodynamic delay) and b is the baseline response tothe blank, fixation-only blocks. The PSC was then averaged, for eachsubject in each mROI, across sessions.

For each subject in each mROI, we defined an index of the ori-entation-selectivity of contextual modulation (m) as:

m ¼ ðPSCorth � PSCparaÞ=ðPSCorth þ PSCparaÞ ð2Þ

where PSCorth is the percent signal change for the condition wherethe test and inducing gratings are orthogonal and PSCpara is the per-cent signal change for the condition where the test and inducinggratings are parallel.

O3. Results

Fig. 5A shows a scatter plot of the percentage signal change inthe parallel versus orthogonal conditions for each of the regions(mROIs) V1, V2, V3, V3A/LO1, hV4 for each subject in each experi-ment. Two points are worth highlighting. First, the vast majority ofdata points (37 of 40) lie below the leading diagonal, indicating aconsistently lower BOLD response in the parallel than the orthog-onal condition. Second, the range of responses is similar for thetwo experiments (Experiment 1: luminance-modulated test –filled symbols; Experiment 2: isoluminant test – open symbols),indicating that any differences between the results are not due todifferential sensitivity to the test gratings.

The data for each region-of-interest, averaged across subjectsfor Experiments 1 and 2 separately, are shown in Fig. 5B. In eachexperiment, all regions followed the trend of showing greater per-cent signal change in the orthogonal than parallel condition. Acrossthe four subjects and two experiments, 2-tailed paired samples t-tests showed that differences in percent signal change betweenthe orthogonal and parallel conditions were significantly greaterthan zero within all five regions of interest: V1–t(7) = 3.27(p < 0.05); V2–t(7) = 4.05 (p < 0.01); V3–t(7) = 4.34 (p < 0.01);V3A/LO1–t(7) = 3.43 (p < 0.05); hV4–t(7) = 3.88 (p < 0.01). Compar-ing differences in percent signal change between the orthogonaland parallel conditions for the luminance-modulated and chro-matic test stimuli revealed a consistent trend across all regionsfor greater orientation-selectivity with the luminance-modulatedtest. This difference was significant at p < 0.05, 2-tailed, only forarea V3: V1–t(3) = 0.30; V2–t(3) = 0.16; V3–t(3) = 3.26; V3A/LO1–t(3) = 2.23; hV4–t(3) = 1.18. Although effect sizes were similar orlarger for regions V3A/LO1 and hV4 than for V3, the use of pairedsamples t-tests revealed much smaller between subjects variabilityfor V3 and hence only in this region was the orientation-selectivityfor Experiments 1 and 2 significantly different.

To gain an idea of the relative orientation-selectivity across vi-sual areas we wanted a measure that took into account the overallmagnitude of the BOLD signal change; this varied by almost a fac-tor of two between V1, where percent signal change was around3%, and areas V3A/LO1 and hV4 (Fig. 5B). To this end we con-structed an orientation-selectivity index as the difference betweenresponses in the orthogonal and parallel conditions as a proportionof their sum (see Eq. (2)). Inspection of the resulting pattern of data(Fig. 5C) suggests that with the luminance-modulated test there isa trend for the orientation-selectivity of contextual modulation toincrease as the cortical processing hierarchy is ascended. This qual-itative impression is supported by post-hoc comparison of the ori-entation-selectivity index for the luminance-modulated test

Fig. 4. Stages leading to definition of masked regions of interest (mROIs). (A) Example map on an inflated brain of peak response phase to a rotating wedge. Colours indicateposition of the wedge in the visual field eliciting peak response. The key beneath the figure indicates which areas of the visual field correspond to which colours. Black lines onthe main graphic indicate borders between visual regions determined manually. Data come from the left hemisphere of subject KJS. (B) Statistical map showing voxels wherethe response to the test-only HRF model was greater than baseline. From these voxels we extracted the response time course for masked regions of interest (mROIs)corresponding to each of the visual areas. (C) The same hemisphere with the ROIs labelled; gyri and sulci removed for clarity.

Ebetween areas V1 and hV4: t(3) = 4.54 (p < 0.05, 2-tailed). This isclearly not the case with the chromatic test, for which orienta-tion-selectivity showed essentially no variation across visual areas.

R4. Discussion

Analyses of percentage signal change in predefined regions ofinterest revealed less BOLD activation in response to gratings withparallel versus orthogonal surrounds across all the retinotopicareas of visual cortex we investigated (Fig. 5B). The direction ofthe effect is consistent with greater inhibition of neuronal activityfrom parallel surrounds, as predicted on the basis of human psy-chophysical data and primate single-cell electrophysiology andconsistent with the fMRI data reported by Williams et al. (2003).However, we cannot rule out the possibility that the observed pat-tern of results actually reflects less facilitation rather than greatersuppression from parallel than perpendicular surrounds. Futureexperiments employing asynchronous presentation of test and sur-round are planned to disambiguate these possibilities.

With the luminance-modulated test there was a trend for theorientation-selectivity of contextual modulation to increase asthe cortical processing hierarchy is ascended (Fig. 5C). However,when an isoluminant test grating was surrounded by a lumi-nance-modulated inducer, the degree of orientation-specific con-textual modulation was no larger for extrastriate areas than forV1, suggesting that the observed effects might originate entirelyin V1. Furthermore, the degree of orientation-selective contextual

modulation was significantly greater in area V3 when test and sur-round both consisted of luminance-modulated gratings than whenthe test stimulus was an isoluminant (L–M isolating) grating. Asimilar although non-significant trend was evident in areas V3A/LO1 and hV4.

One possible interpretation of the results would be to relate thedifference in the orientation-selectivity of contextual modulationof luminance and isoluminant gratings evident in area V3 to differ-ences in the magnitude of psychophysical effects on perceived ori-entation and contrast. Specifically, the perceptual effect of aluminance-modulated surround on a luminance-modulated testis greater than its effect on an isoluminant test; here we foundan analogous effect on the fMRI BOLD response in area V3. The pat-tern of results for a luminance-modulated test is clearly not consis-tent with the observed effect in V3 simply being inherited fromlateral interactions within earlier visual areas. Instead, they sug-gest the operation of orientation-selective mechanisms of contex-tual modulation within V3 that enhance the observedorientation-selectivity of contextual modulation in V3 when testand surround are both luminance-defined but not when a lumi-nance-modulated grating surrounds a chromatic test.

Such a refinement of the orientation-selectivity of contextualmodulation specific to the colour/luminance congruence of testand surround as the visual hierarchy is ascended would seem tobe at odds with the observation that the binocular, and thus pre-sumably higher-level, component of the tilt illusion is essentiallycolour-invariant (Forte & Clifford, 2005). However, Gegenfurtneret al. (1997) have demonstrated that in macaque essentially all

Fig. 5. Experimental data. (A) Scatter plot of percentage signal change (PSC) invoked by the parallel test-inducer as a function of PSC invoked by the orthogonal test-inducer. Symbolcolours indicate the subject: red – CC, green – EA, blue – KJS, black – JSM. Filled symbols indicate achromatic test stimulus, empty symbols indicate chromatic test. Symbol shapeindicates the visual area: circle – V1, square – V2, diamond – V3, 5-pointed star – V3a/LO1, 6-pointed star – V4v. Nearly all the points (37 of 40) lie below the line of equal PSC,indicating that parallel test-inducer stimulus elicited a smaller PSC than the orthogonal under nearly all circumstances. (B) (top) Percentage signal change for Experiment 1(achromatic test) averaged across subjects and as a function of visual area. Dark bars indicate PSC invoked by orthogonal test-inducer stimuli. Light bars indicate PSC invoked byparallel test-inducer stimuli. (B) (bottom) Percentage signal change for Experiment 2 (chromatic test). Red bars indicate PSC invoked by orthogonal test-inducer stimuli. Green barsindicate PSC invoked by parallel test-inducer stimuli. The data indicate that the orthogonal stimuli consistently invoke a higher PSC than the parallel stimuli. (C) Comparison oforientation-selectivity index (Eq. (2)) across experiments and visual areas. The bars indicate the difference between orthogonal test-inducer PSC and parallel test-inducer PSC as aproportion of their sum, as a function of visual area. The light–dark bars indicate the value for achromatic stimuli and the red–green bars indicate the value for the chromatic stimuli.All error bars represent ± one standard error of the mean. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Please cite this article in press as: Scott McDonald, J., et al. Orientation-specific contextual modulation of the fMRI BOLD response ... VisionResearch (2009), doi:10.1016/j.visres.2008.12.014

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V3 neurons are orientation-selective and that their colour proper-ties are much more like those observed in V1 than V2, cautioningagainst a simple hierarchical view of processing across the early vi-sual areas. Furthermore, Solomon, Peirce, and Lennie (2004) havereported that surround suppression in V1 is primarily driven byluminance contrast and is largely insensitive to chromatic contrast,even in cells whose classical receptive field is strongly colour-selective. In V2, on the other hand, surround suppression is morestrongly driven by chromatic modulation and there is a tendencyfor the surround to have the same chromatic signature as the clas-sical receptive field. If V2 contains colour-selective mechanisms ac-tively involved surround suppression, as suggested by Solomonet al. (2004), and does not simply inherit its surround propertiesin a feed-forward fashion from V1, then it is reasonable to specu-late that other extrastriate areas such as V3 might also containmechanisms capable of generating surround suppression with sim-ilar tuning to that of the classical receptive field.

An alternative interpretation of the results is that the differentpatterns of orientation-selective contextual modulation for lumi-nance-modulated and isoluminant test gratings reflect processesinvolved in segmenting the test region from the luminance-modu-lated surround. When both test and surround are luminance-mod-ulated gratings, the only cue to segmentation between them istheir relative phase (‘‘parallel” condition) or orientation (‘‘orthogo-nal” condition). However, when the test is an isoluminant grating,the colour-luminance boundary provides a strong cue to segmen-tation and a salient border in both parallel and orthogonal stimulusblocks. Thus, the greater levels of orientation-selective contextualmodulation with the luminance-modulated test might reflect dif-ferences in segmentation by phase- versus orientation-difference.In this case, the orientation-specific contextual modulation under-lying the perception of orientation in the tilt illusion might origi-nate entirely in V1, as has been argued for orientation-specificadaptation to luminance-modulated patterns (Larsson, Landy, &Heeger, 2006). Support for this idea comes from Experiment 2,where it can be seen that orientation-selectivity for extrastriateareas is no larger than for V1 when an isoluminant test grating isused (Fig. 5C). However, psychophysical evidence has indicatedthat the tilt illusion is largest precisely when test and inducer arehardest to segment (Durant & Clifford, 2006), suggesting that aclear separation between processes affecting the perception of ori-entation and more general processes of image parsing may notexist.

To date, a number of studies have employed the technique offMR adaptation to investigate the processing of orientation in hu-man visual cortex (Boynton & Finney, 2003; Engel, 2005; Fang,Murray, Kersten, & He, 2005; Larsson et al., 2006; Montaser-Kouh-sari, Landy, Heeger, & Larsson, 2007). These adaptation studieshave used not only stimuli defined by luminance or chromaticmodulations but also second-order textures (Larsson et al., 2006)and subjective contours (Montaser-Kouhsari et al., 2007). The ef-fects of adaptation can be considered as modulation by temporalcontext, as opposed to the modulation by spatial context studiedhere. Thus, natural extensions to the current work would includeinvestigating the neural basis of contextual effects with second-or-der patterns (e.g. Wenderoth, Clifford, & Ma Wyatt, 2001).

Acknowledgments

Scanning was carried out at the Symbion Clinical ResearchImaging Centre at the Prince of Wales Medical Research Institutein Sydney. We are grateful to Kirsten Moffat for expert technicalassistance. This research was funded by a grant to CC from The Uni-versity of Sydney.

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