INSERM U438, RMN Bioclinique, LRC CEA, Centre Hospitalier Universitaire, Pavillon B, BP 217, F-38043 Grenoble Cedex 9 and , 1 INSERM U371, Cerveau et Vision, 18 avenue du Doyen Lépine, F-69675 Bron Cedex, France
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Abstract |
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Introduction |
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One of the issues currently being debated is the level at which illusory contours are encoded (Gregory, 1972; Smith and Over, 1975
; Rock and Anson, 1979
; von der Heydt et al., 1984
; Grosof et al., 1993
; Shapley, 1994
; Hirsch et al., 1995
; Ffytche and Zeki, 1996; Mendola et al., 1999
). Imaging studies in humans have suggested that static illusory contours activate neuronal populations at the earliest levels of cortical processing. In a positron emission tomography (PET) study, activation was shown mostly in area V2 (Ffytche and Zeki, 1996
). In one functional magnetic resonance imaging (fMRI) investigation, activation of extrastriate areas has been reported with a possible weak participation at the border of areas V1 and V2 (Hirsch et al., 1995
). In another fMRI study, similar static illusory contours have been shown to induce mainly activation of higher-order organized areas including V3A, V4v, V7 and V8. The activation of low-level areas (V1 and V2) was not, however, ruled out (Mendola et al., 1999
).
It is well known that processing within the visual system occurs along two main pathways, the occipito-temporal ventral stream and the occipito-parietal dorsal stream. Previous studies of cortical activation by illusory contours have used static stimuli which are likely to preferentially trigger neurons of the ventral stream (Hirsch et al., 1995; Ffytche and Zeki, 1996
; Mendola et al., 1999
).
Much less is known concerning the processing of moving illusory contours and the involvement of the dorsal stream. Two imaging studies have focused on the perception of illusory movement during viewing of static stimuli (Zeki et al., 1993; Tootell et al., 1995b
). Strong activation was found within and around area V5, a cortical region known to be important for the perception of motion. In another imaging study, the apparent motion of illusory contours (Ramachandran, 1985
) has been reported to evoke activation in V5 (Goebel et al. 1998
).
It was therefore of interest to assess whether moving illusory contours mainly activate area V5, as most moving stimuli do, and/or whether they activate areas V1 and V2 as at least some of the static illusory contours do (Hirsch et al., 1995; Ffytche and Zeki, 1996
). Moving illusory contours were generated by means of rotating Pacman inducers (Bravo et al., 1988
) configured in such a way that the subject perceives an illusory rectangle moving vertically across the visual field in a saccadic fashion (Fig. 1a
). Activations were assessed by means of fMRI. Part of this work has previously appeared in abstract form (Seghier et al. 1999
).
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Materials and Methods |
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Thirteen healthy volunteers (age 32 ± 10 years) were examined. All subjects (12 males, 1 female) gave their informed consent to participate to the study. They had normal (n = 11) or corrected-to-normal (n = 2) vision. Nine of the volunteers were trained fMRI subjects.
Experiments
All subjects performed an initial fMRI experiment (experiment 1) aimed at identifying the functional responses to a moving illusory rectangle. Three additional control experiments were performed on two subjects: one to obtain the responses to stationary illusory contours (experiment 2), another to localize the border between functional areas V1 and V2 (experiment 3) and a third to identify with certainty the functional area V5 (experiment 4). On these two subjects, whom we will refer to as the control subjects C1 and C2, the four experiments were performed during the same session, without repositioning.
Paradigms
For all experiments, a block paradigm was applied, which alternated between reference and stimulation epochs.
Visual Stimuli
Visual stimuli were generated by means of a PC equipped with VisionWorks software (Vision Research Graphics, Durham, NC, USA). They were transmitted to the subjects by means of an LCD videoprojector (Eiki LC 6000, Eiki Industrial Company, Japan), a backprojection screen and a system of mirrors. The visual field spanned by this setup was approximately 15° horizontal x 11° vertical.
Experiment 1
The visual stimuli presented during the stimulation epochs aimed at inducing the perception of an illusory rectangle moving vertically (Fig. 1a). Ten white Pacman inducers (henceforth referred to as pacmen) were positioned on a dark background on the edges of a rectangular grid. Four adjacent pacmen among them were configured so as to induce the perception of the illusory Kanizsa rectangle. Vertical movement of this rectangle was achieved by the sequential display of four different configurations of the ten pacmen. Sequential configurations differed in a 90° rotation of the individual pacmen, clockwise for those in the left column and counterclockwise for those in the right column. Thus, while individual pacmen rotated, the illusory rectangle appeared to move downwards, in a saccadic fashion (Fig. 1a
). Refreshment rate of the sequential configurations was 1.7 Hz, corresponding to an angular velocity of the illusory rectangle as viewed by the subjects of 3.2°/s.
During the reference epochs, sequential configurations of pacmen were presented which were similar to those displayed during the stimulation epochs (Fig. 1b). The sequential configurations differed from those presented during the stimulation epochs mainly in that most of the pacmen were tilted. In the experiments on the control subjects (C1 and C2) they were tilted by 10° and in all other experiments by 45°. Thus, the perception of illusory staircase shapes during the reference epochs was avoided. As during the stimulation epochs, sequential config- urations differed in a 90° clockwise rotation of the individual pacmen in the left column and a 90° counterclockwise rotation of those in the right column. In order to reduce possible visual after-effects, a black transition screen was presented when switching from stimulation to reference epochs. The MR measurements performed while viewing the transition screen were discarded during data processing. A white fixation dot was presented throughout the experiment.
The illusory rectangle as viewed by the subjects subtended 2° horizontal x 1.7° vertical. The rectangles were displayed symetrically with respect to the vertical meridian. The radius of the pacmen extended over 0.45°. Thus the support ratio of these contours (Shipley and Kellman, 1992), i.e. the ratio of the total length of the borders actually drawn to the perimeter of the rectangle, is 0.48. Luminances of the pacmen and of the dark background were respectively 201 and 56 cd/m2, as measured on the projection screen.
Experiment 2
The visual stimulus presented during the stimulation epochs was a single static Kanizsa rectangle, centrally placed. During the reference epochs, the four pacmen were tilted by 10° to avoid the perception of the illusory rectangle. During stimulation and reference epochs, the pacmen flickered synchronously at 4 Hz (Hirsch et al., 1995).
In this experiment, the illusory contour as viewed by the subjects (C1 and C2), subtended 3.3° horizontal x 2.6° vertical, the radius of the pacmen subtended 0.7°, and the support ratio was 0.48. Luminances of the pacmen and of the dark background were respectively 201 and 56 cd/m2, as measured on the projection screen.
Attentional tasks
In order to maintain the subject's attention, attentional tasks were built into the paradigm in experiments 1 and 2. The fixation point (0.09°) was intermittently dimmed and subjects were instructed to report the occurrence of these (rare) events by means of a button press. Furthermore, during the stimulation epochs in experiment 1, the vertical movement of the illusory rectangle, while occurring usually from top to bottom, occurred four times from bottom to top. Subjects were also instructed to report these reversals.
Experiment 3
The visual stimulus presented during the stimulation epochs aimed at inducing functional responses at the borders of areas V1 and V2. This was achieved by using a hourglass-shaped stimulus centered on the vertical meridian. The latter is known to be represented at the border between areas V1 and V2. The stimulus was composed of two segments filled with a contrast-reversing checkerboard pattern (8 Hz flickering). The two segments had their apices at the fixation point and were centered on the upper and lower vertical meridian respectively. The angle of the segments was 40°. Maximum eccentricity was 7° (Engel et al., 1997; Hasnain et al., 1998
). During the reference epochs, only the fixation point was displayed.
Experiment 4
The visual stimulus presented during the stimulation epochs aimed at inducing robust functional responses in V5. A stimulus consisting of a set of low-contrast (gray on white, 16% contrast), continuously expanding, concentric rings was presented (Tootell et al., 1995b). Maximum eccentricity of the rings was 10°. The expansion rate of the rings was 7°/s. During the reference epochs, a snapshot of the rings was shown continuously. In order to prevent the perception of a motion after-effect (Tootell et al., 1995b
), a transition screen was presented during the first 4 s of each reference epoch. The screen presented the same concentric rings, this time continuously contracting.
MR Acquisition
Subjects were examined at 1.5 T on a clinical MR scanner (Philips ACS II). The body coil was used for excitation. A circular surface coil (diameter = 13 cm) was used for detection. This coil was centered just beneath the occipital pole of the subject's head. Positioning of the volume was performed on scout images acquired in the sagittal plane.
Subsequent to the functional MR scans, a high-resolution three- dimensional phase-contrast (PC) (Dumoulin et al., 1989) MR angiography scan was eventually ran from the volume functionally examined. This scan provided anatomical (T1-weighted) as well as vascular information about the volume of interest.
For the 11 subjects taking part only in experiment 1, the volume of interest imaged in the functional scans was composed of six adjacent, axial, slices (5 mm thickness each). This volume was oriented parallel to the bicommissural plane and centered so as to encompass most of the calcarine sulcus. The volume was measured five and four times during the reference and stimulation epochs respectively. This sequencing was repeated four times (total measurement time ~9 min). Presentation time of the transition screen was ~15 s. The functional scans applied a conventional gradient-recalled echo (GRE) MR sequence. The major parameters of this sequence were: TR = 80 ms, TE = 40 ms, acquisition matrix = 45 x 64, FOV = 154 x 220 mm2, flip angle = 10°, bandwidth per pixel = 14 Hz, acquisition time per slice = 2.45 s, reconstruction matrix = 128 x 128.
For the experiments performed with the two control subjects (C1 and C2), a single-shot EPI GRE MR sequence was used. The major parameters of this sequence were: TR = 2s, TE = 50 ms, acquisition matrix = 64 x 64, FOV = 192 x 192 mm2, flip angle = 80°, reconstruction matrix = 64 x 64. The volume of interest imaged in the functional scans was composed of eight adjacent, coronal slices (5 mm thickness each). This volume was oriented orthogonal to the bicommissural plane and also centered so as to encompass most of the calcarine sulcus. In experiments 1 and 2, the volume was measured 17 times during each of the reference, stimulation and transition screen' epochs. This sequencing was repeated five times (total measurement time 8 min 36 s). In experiments 3 and 4, the volume was measured 16 times during the control and stimulation epochs. This sequencing was repeated four times (total measurement time 4 min 22sec).
Data Processing
MR images were analyzed off line by means of a software package developed under the IDL 5.2 programming language (Research Systems Inc., Boulder, CO, USA). Following image co-registration (Roth et al., 1996) and pixelwise baseline correction, functional responses were detected by means of cross-correlation analysis (Bandettini et al., 1993
). The cross-correlation coefficient was calculated pixelwise between a box-car function (representing the paradigm) and the set of measurements. In practice, we first identified pixels which presented a cross-correlation coefficient above a predefined threshold (typically 0.4). We retained only the pixels which were members of clusters counting a minimum number of activated pixels (typically six) (Forman et al., 1995
). In a second step, the cross-correlation coefficient of the average signal intensity within each cluster was determined. We kept only the clusters with an average cross-correlation coefficient above a second threshold (typically 0.5). The functional maps were obtained by superimposing the activated pixel clusters onto the anatomical images derived from the three-dimensional PC MR scan. The functional areas were identified in each subject on the basis of the Talairach atlas (Talairach and Tournoux, 1988
).
For the sake of presentation, the functional maps (corresponding to individual slices) were also combined into a single functional projection map. This projection map was obtained by superimposing the maximum intensity projections of the activation clusters onto averaged intensity projections of the corresponding anatomical images. The projections were performed perpendicularly to the individual image planes (projection maps thus were also oriented transversally). Following radiological practice, the left and right hemispheres are displayed on the right and left respectively.
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Results |
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The results obtained for the two control subjects in response to moving (experiment 1) and to static illusory contours (experiment 2) are summarized in Table 2. Again, this table gives the number of pixels found activated within the areas of interest, namely V1/V2, LOS/KO and V5, together with the mean cross-correlation coefficients in those areas. Activation was also found in the cuneus, the lingual gyrus, the superior parietal gyrus and the cerebellum. Note that the number of pixels and the cross-correlation coefficients are larger for moving than for static illusory contours in areas V1 /V2 as is also apparent in Figure 4
. The volume of the activated areas was on average 1.4 ± 0.4 cm3 in V1/V2 and 0.4 ± 0.2 cm3 in V5 for the moving illusory contours, and 0.5 ± 0.05 cm3 in V1/V2 for the static illusory contours.
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Discussion |
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In Experiment 1, the stimuli presented during the reference and the stimulation epochs differed in the 45° tilt of the pacmen. A priori, part of the activations detected could be ascribed to this mere difference. The continuous rotation of the pacmen in reference and stimulation epochs lowers the perception of this difference, however. Moreover, in the two experiments on the control subjects, pacmen were tilted by only 10° and similar results were obtained. Since most cortical neurons respond similarly to orientations differing by 10°, we therefore assume that the activations observed in experiment 1 are not to be ascribed to a greater sensitivity to vertical and horizontal elements than to oblique ones.
Because the fixation task was the same during the reference and the stimulation epochs, it is also unlikely that the activations reported here result from differences in spatial attention. Artefactual activations due to eye movements appear unlikely since the subjects were instructed to report the dimming of the fixation spot, a task which is difficult to perform without precise fixation of the spot. From monkey studies, this task is known to provide an efficient means to stabilize the eye position (Wurtz, 1969). There is motion correspondence between pacmen in the the stimulation epochs which is not perfectly matched in the reference epochs. Therefore, a contribution from this effect to the activations detected may not be ruled out completely.
For all these reasons, we consider that the activations observed are due without ambiguity to the perception of moving illusory contours. The inter-subject variability observed (Table 1) may furthermore reflect functional and anatomical differences in visual areas between subjects (Hasnain et al., 1998
) and/or differences in the ability of the brain to perform perceptual grouping (McCourt and Paulson, 1994
). The volumes activated with the control subjects (Table 2
) were found larger than with the 11 subjects who performed only experiment 1 (Table 1
). This may be ascribed partly to the larger cerebral volume explored with the control subjects and partly to the difference in MR acquisition techniques used in these two series of experi- ments.
The additional control experiments demonstrate that moving illusory contours, in contrast to kinetic boundaries, activate area V5 and its satellites. Involvement of area V5 has been confirmed by the co-registration of the activity evoked by the moving illusory contours (Fig. 4e) and that generated by an expanding set of concentric rings (Fig. 4f
), a stimulus known to activate area V5 and its surrounding areas (Tootell et al., 1995b
). Activation of that region is not surprising, given the strong perception of movement generated by the stimulus and the fact that the V5 area has repeatedly been shown to be activated by real (Zeki et al., 1991
; Watson et al., 1993
; Tootell et al., 1995a
), by illusory (Zeki et al. 1993
; Tootell et al., 1995b
) and by apparent (Goebel et al. 1998
) moving stimuli. What is surprising in our results is that the activity in area V5 is often weaker (in terms of cross- correlation coefficients) than in areas V1/V2 (Tables 1 and 2
) and that it was not observed for all subjects (Table 1
).
Activation in low-level areas has been reported with static illusory contours (Kanizsa type) in two earlier studies, one using PET (Ffytche and Zeki, 1996) and one using fMRI (Hirsch et al., 1995
). In their recent fMRI studies, Mendola et al. do not rule out these findings but insist on the predominant activations obtained in high-level cortical regions, including V3A, V4v, V7 and V8 (Mendola et al., 1999
). The activation patterns obtained in our study show that moving as well as static illusory contours activate low-order, retinotopically organized visual areas, especially V1. Involvement of area V1 during the perception of moving illusory contours is clearly shown in Figures 2 and 3
, where activations are located along the calcarine sulcus, the location of area V1 (Holmes 1918
; Tootell et al., 1998
). The activation of area V1 has furthermore been confirmed by the co-registration, for the two control subjects, of the activities evoked by a static illusory contour (Fig. 4a
), by the moving illusory contour (Fig. 4c
) and by the hourglass-shaped stimulus centered on the vertical meridian (Fig. 4b
), a stimulus known to activate neurons located at the frontier between areas V1 and V2 (Tootell et al. 1995a
; Engel et al. 1997
).
The patterns of activity obtained here differ from those reported previously (Hirsch et al., 1995; Ffytche and Zeki, 1996
) for static illusory contours. In these studies, the activations which were assigned to V2 (Hirsch et al., 1995
; Ffytche and Zeki, 1996
) and possibly to V3 (Ffytche and Zeki, 1996
) were located more laterally than in our study. It could be argued that the area labeled LOS/KO in our data corresponds to the cortex assigned to V2 and/or V3 in these earlier reports. In contrast, the active area labeled V1/V2 in our results is located more medially and overlaps the calcarine sulcus in most instances (Figs 24
), thus including area V1.
In their report on the regions activated by apparent motion of illusory contours, Goebel and his collaborators (Goebel et al., 1998) observed a weak activation in V1, contrasting with a robust activity in V5. The discrepancy with our results is likely due to differences in the stimuli used. In their study the two illusory contours were positioned 9° apart, symetrically with respect to the vertical meridian, thus stimulating each hemi- sphere alternately. We used a distance of 1.7° to separate the four positions of the illusory contours which straddled the vertical meridian. The motion with our stimulus was less saccadic and involved activation of regions representing the vertical meridian. In the study of Goebel and collaborators, apparent motion must involve neurons located in both hemispheres along the repres- entation of the horizontal meridian. A difference in stimulus velocity, 3°/s in our study and 17°/s in Goebel's study, could also have contributed to the stronger activation of V1 in our study. This interpretation is consistent with the findings of a recent fMRI study showing that the V1 activation decreases linearly as speed increases during the perception of real motion (Chawla et al., 1998
). In preliminary measurements, we have also observed that the activation within V1 is stronger with 3°/s than with 10°/s.
With static illusory contours, V1 activation was not observed with PET (Ffytche and Zeki, 1996), but was not entirely dismissed with fMRI (Hirsch et al., 1995
; Mendola et al., 1999
). Our results (see Table 2
) on the two subjects tested with the static illusory contours confirm that area V1 is activated by these stimuli, but suggest that the activation by moving illusory contours is stronger than with static illusory stimuli. Thus, animation such as 4 Hz flickering (Hirsch et al., 1995
), reversal of contrast polarity (Mendola et al., 1999
) or movement seems to enhance activation in V1.
The detection of activations in low-order visual areas for static as well as for moving illusory contours suggests that perceptual grouping involves neural processing in these areas. This is consistent with the results of animal studies showing activations in areas V1 and V2 by illusory contours (von der Heydt et al., 1984; Redies et al., 1986
; Grosof et al., 1993
; Shapley 1994
; Sheth et al., 1996
; Baumann et al., 1997
; Leventhal et al., 1998). Moreover, activation of the primary visual cortex with Kanisza-type illusory contours is in keeping with results from psychophysical studies that argue for local mechanisms in the perception of such figures (Dresp and Bonnet, 1991
, 1993
; Davis and Driver, 1994
) and with a recent report of perception of illusory contours in patients with spatial neglect (Vuilleumier and Landis, 1998
).
The observation of activity in area V1 due to illusory stimuli is of particular interest in the context of the debate concerning the cortical regions activated by mental images, with several reports claiming that only areas outside area V1 are activated (Roland and Gulyas, 1994; D'Esposito et al., 1997
; Braun et al., 1998
; Mellet et al., 1998
), while others report evidence that area V1 is involved in visual imagery (Le Bihan, 1993
; Kosslyn et al., 1993
, 1995
, Chen et al., 1998
). Although perception related to illusory contours differs from mental imagery, the strong activation of area V1 by moving illusory contours in our results is in keeping with the idea that low-order visual areas are activated by high- order processes such as visual imagery or illusory perception. These results suggest that V1 is a distributing center, rather than simply a spatially transformed replication of the retina (Zeki and Shipp, 1988
).
In conclusion, these fMRI results provide additional ground to the hypothesis that low-level processes are involved in perceptual grouping and in illusory contour perception. This supports the idea that visual information processing is a cooperative task distributed over several interconnected brain modules (Bullier and Novak, 1995). Whether such activity is generated through local computations involving horizontal connections or whether they require feedback input (Hupé et al., 1998
) from higher-order areas remains a subject for future investigation.
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Notes |
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Correspondence should be addressed to Michel Dojat, Ph.D., INSERM U438, CHU de Grenoble, Pavillon B, BP 217, F-38043 Grenoble Cedex 09, France. Email: mdojat{at}ujf-grenoble.fr.
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Footnotes |
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