Department of Neurobiology, Weizmann Institute of Science, Rehovot 76100, , 1 Sourasky Medical Center, Tel Aviv 64239 and , 2 Tel Aviv University, Tel Aviv 69978, Israel
R. Malach, Department of Neurobiology, Weizmann Institute of Science, Rehovot 76100, Israel. Email: rafi.malach{at}weizmann.ac.il.
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Abstract |
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Introduction |
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To what extent are completion effects evident in the activity of specific areas within human visual cortex? Recently, an electrophysiological study using high-density electrical mapping (Doniger et al., 2000) has reported a focus of event-related potential (ERP) activity correlated with visual completion processes. The focus of this activity appears to correspond to a high-order object-related region situated in occipito-temporal cortex, termed the lateral occipital complex (LOC) (Malach et al., 1995
). Because ERP measurements are drastically different from blood oxygenation level dependent (BOLD) functional magnetic resonance imaging (fMRI) in their spatio-temporal profile, we have conducted a functional imaging study in order to map in detail the involvement of the LOC and neighboring areas in the completion of visual images.
Another phenomenon, which is reminiscent of the completion process, is perception of a whole shape in displays containing illusory contours although not an image completion phenomenon in the strict sense, illusory contours and particularly the Kanizsa triangle, can be viewed as producing an activation wave that spreads beyond the local object features. Again, the object-related LOC was found to be strongly activated by illusory contours (Mendola et al., 1999). Finally, in a previous study, (Lerner et al., 2001
) we have found that the LOC was hardly affected when object and face images were cut into halves and exchanged positions. While this effect could indicate that optimal feature size is smaller than half a face, it is also compatible with a completion effect. It may be that in such images the neuronal network completes each half of the face or object into the full-object representation. Thus, converging evidence suggests the LOC as a likely candidate to mediate the operation of completion effects.
In the present work, using fMRI, we mapped the manifestation of completion effects in human visual cortex by comparing the cortical activation produced by intact, partially occluded and scrambled images of familiar and unfamiliar objects. Note that in the latter two conditions, the set of the local image features was matched. Our results reveal the presence of significant completion effects in high-order visual cortex that go beyond the local feature structure of object images.
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Materials and Methods |
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Subjects were scanned in a 1.5 Signa Horizon LX 8.25 GE scanner equipped with a quadrature surface coil (Nova Medical Inc., Wakefield, MA), which covered the occipital, posterior parietal and posterior temporal lobes. BOLD contrast was obtained with gradient-echo echo-planar imaging (EPI) sequence (TR = 3000, TE = 55, flip angle = 90°, FOV = 24 x 24, matrix size 80 x 80). The scanned volume consisted of 17 nearly axial slices of 4 mm thickness and 1 mm gap. This covered most of the cerebral hemispheres, with the exception of the top part of the parietal lobe (~15 mm). Spin-echo T1-weighted high resolution (1 x 1 x 1 mm) anatomical images (124 slices, 1.2 mm thickness) and 3-D spoiled gradient echo sequence (TR/TE = 40/9, flip angle = 30°, image matrix 256 x 256, 22 min) were acquired on each subject in order to allow exact cortical segmentation, reconstruction and volume statistical analysis of signal changes during the experiment.
Experimental Procedure
Subjects
Eleven healthy subjects (six males, aged 1950 years, two left-handed) with normal or corrected-to-normal vision took part in one or more experiments (six subjects in experiment 1, five subjects in experiment 2 and five subjects in experiment 3; two subjects took part in all studies). All subjects were given detailed instructions for the experiment and provided written informed consent. The Tel Aviv Sourasky Medical Center approved the experimental protocol.
Stimuli
The stimuli, generated on a PC, were presented with an LCD projector (Epson MP 7200) onto a translucent screen. Subjects viewed them in a mirror positioned ~45° above the forehead.
Experiment 1.
The stimuli were presented in a pseudo-random short epoch design epochs containing visual stimuli were alternated with epochs of blanks. The visual stimuli were line-drawings of animals (126 pictures, size of 14 x 12°) obtained from various commercial CD databases and converted to line drawings through high-pass and thresholding using Adobe Photoshop 5.0 software package (Adobe Systems Inc.). There were three types of visual epochs in the experiment (see Fig. 1): whole epochs that contained the full animal images; grid epochs that consisted of the same images but with superimposed grid of green vertical stripes (each stripe 1.2 x 12°, the borders of the stripes were smoothed); and scrambled epochs, in which positions of object-stripes were changed relative to their positions in the grid epochs two examples of these are shown by dashed gray arrows (Fig. 1a
). Note that the grid and scrambled epochs consisted of the same group of local features, because the feature arrangement within each individual object stripe remained unchanged. The whole condition consisted of a richer set of local object features since no obstruction was involved in these images. This is illustrated by the four small circles with local object features bellow the images (Fig. 1a
); note, though, that the whole condition did not contain the opaque grid. A red fixation point was presented in the center of all pictures. Each epoch consisted of 18 different images presented in 9 s (500 ms per image) with inter-epoch intervals of 6 s blank screen containing a fixation point only. At the beginning and end of the scan there were long blank periods (21 and 9 s, respectively). Epochs of different conditions were interleaved in pseudo-random order. A schematic representation of the temporal sequence of events during an experiment is illustrated in Figure 1c
. Each scan consisted of 21 epochs seven epochs for each category. The subjects were requested to covertly name the objects in all epoch types. In the scrambled cases they were asked to guess if they could not recognize the images.
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Experiment 3.
The third experiment had exactly the same experimental conditions and short-epoch design as experiment 1, but in this case unfamiliar novel objects (fractal patterns, Fourier descriptors and self-made images, 126 pictures, size 14 x 12°) were used (Fig. 10). The subjects were asked to fixate on a central red point and passively view the images.
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The representation of vertical and horizontal visual field meridians was mapped in the same subjects to establish the location of retinotopic areas based on anatomical and functional characteristics (DeYoe et al., 1996; Tootell et al., 1996
). Details of the procedure are as previously published (Grill-Spector et al., 1998
). Briefly, visual stimuli consisted of triangular wedges that were presented either vertically (upper or lower) or horizontally (left or right) meridians. The wedges included either gray-level natural images or black and white objects-from-texture pictures. The visual stimuli were presented at a rate of 4 Hz in 18 s blocks alternated with 6 s blanks. Subjects were required to fixate on a small cross in the center of images.
Measuring Naming Performance
For each subject, naming performance was measured outside the magnet ~4 weeks after the fMRI scan. Subjects were asked to overtly name the animal images as specifically as possible. Performance was measured as percentage of correct naming. The order of stimuli was the same as that in the fMRI scan, except that each stimulus was presented for 500 ms, followed by a 1500 ms blank to allow sufficient time for overt naming of each image.
Data Analysis
All fMRI data were processed using the BrainVoyager 4.2 software package (Goebel et al., 1998a,1998b
) and in-house software. The cortical surface was reconstructed from the subject's structural MRI. Surface reconstruction included the segmentation of the white matter by grow-region function, the smooth covering of a sphere around the segmented section and the expansion of the reconstructed white matter into the gray matter. Following surface reconstruction, the brain was inflated so as to allow sulcal as well as gyral cortex to be viewed. The surface was cut along the calcarine sulcus and unfolded into the flattened format.
Data analysis was done separately for each subject in each scan. The preprocessing procedure consisted of head motion correction and high-frequency temporal filtering. The first three images of each functional scan were rejected and a lag of 3 s was used to take account of the hemodynamic response delay. The fMRI activation (2-D images) were overlaid on corresponding 2-D anatomical slices, incorporated into the 3-D data sets through trilinear interpolation and transformed into Talairach space (Talairach and Tournoux, 1988). Finally, this fMRI activation was placed on the subjects' unfolded cortical surfaces.
Statistical analysis was based on the general linear model (Friston et al., 1995). The signal time-course was correlated for each voxel with a reference function. The reference function for each experimental condition (predictor) was a delayed (3 s) square wave corresponding to the stimulus condition (i.e. whole, grid and scramble). An average time-course was achieved from all voxels within a-priori defined regions of interest (ROI). Three-dimensional statistical maps were obtained according to the degree of correlation to the reference function. Only voxels whose correlation value was > 0.2 (P < 105, uncorrected) were included in the statistical calculations.
Multi-subject Analysis
In addition to subject-by-subject analysis, we also analysed the data in a multi-subject approach. To obtain the multi-subject maps, the time-courses of subjects were converted into Talairach space, z-normalized and concatenated. The statistical tests were performed on the concatenated time-course.
Internal Localizer Test
To obtain highly accurate measures of the activation level within the regions of interest, we took advantage of the short-block presentation and adopted a procedure which we termed the internal localizer approach. In this procedure a subset of the epochs served to localize regions of interest, while another subset, not used in the statistical localization tests, was used to evaluate the activation level. Note that this approach has the advantage that the localizer test is done on epochs which were in the same scan in which the activation level was measured (rather than the more common separate localizer scan), thus minimizing inaccuracies due to head motion between scans. However, the measured activity in the other epochs is unbiased, since these epochs were not included in the statistical localizer test. Specifically, for each localizer test (e.g. whole versus scrambled) two statistical tests were conducted. In the first test, five of the epochs were used as anatomical localizers, while second and fourth epochs of the same conditions (i.e. whole and scrambled) were not included in the test, and the level of activation during these epochs was measured separately. In the second test, the same localizer test was conducted, but this time the second and fourth epochs were included in the localizer test, while the third and fifth epochs of the same conditions were excluded from the test, and their activity level was measured separately.
The average activation from epochs which were used for anatomical localization are presented in the figures as Localizer. The data obtained from all epochs, which were not included in the localizer test (but consisted of the same type of stimuli), were averaged within each subject. These data were averaged across subjects and were presented separately in the histograms.
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Results |
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In order to uncover the neural mechanisms mediating visual completion, we conducted the object-completion experiment 1 shown in Figure 1 (see Materials and Methods for more details). The experiment consisted of three types of images: whole, grid and scrambled. Note that the set of local features is identical (up to a change in absolute position) in the grid and scrambled conditions, while the whole condition contains a richer set of local object features. This is illustrated in Figure 1a
by the four local object features sampled by the small aperture. Note also that the global relationship between adjacent stripes is maintained in the grid condition, but is completely disrupted in the scrambled condition. Figure 1b
illustrates the fact that, because the global configuration of local elements is maintained in the grid condition, image completion effects are plausible, while these effects are essentially abolished in the scrambled condition.
In considering the various possible outcomes of this experiment, one can envision two extreme situations (with numerous additional variations and gradations of these possibilities). One possibility is that object areas are completely dominated by very local, independent, object-feature representations. In such a representation we would expect a higher activation to the whole image compared to the occluded cases, since presumably the occlusion removes a substantial fraction of local object features. However, no difference in activation is expected in this case between the grid and scrambled conditions, since an identical set of local features is present in the two conditions. The other extreme case is a network that manifests full completion effects here we would expect that activation to the whole and the grid images should be identical, since the neuronal representation is capable of completely filling in the occluded object-parts through figure completion effects. On the other hand, the scrambled images should produce much less activation, since they presumably do not lead to figure completion effects.
Before reporting the fMRI data, we present the behavioral findings to document the fact that the paradigm did indeed elicit perceptual completion. Figure 2 shows the naming performance of the nine subjects (six subjects that were scanned in the experiments 1 and 2 and three new subjects) measured outside the magnet for the same images presented during the fMRI scan, but with an average delay of 4 weeks (and no less than 3 weeks) after the scan.
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Mapping Object-related Areas in the Human Cortex
In order to reveal the cortical activation associated with completion, we first had to functionally define object-related areas. To that end we compared, in visually responsive regions, the relative activation to whole line drawings versus scrambled images. Figures 3 and 4 show the results of such tests on single- and multi-subject maps, respectively. As can be seen, there was a clear transition in the relative activation by the whole compared to the scrambled images, so that as one moves from retinotopic to more anterior visual areas, there was increasingly dominant activation by the whole object images. The regions most dominated by whole images (colored in green/blue) were located in the most anterior-lateral aspects of the visually activated regions and corresponded to the LOC (Malach et al., 1995
; Lerner et al., 2001
). Table 1
presents the foci of this activation in Talairach coordinates (Talairach and Tournoux, 1988
). In contrast, early retinotopic areas showed a slightly higher activation to the scrambled images (orange colors), most likely due to the high spatial frequencies introduced by the occluding bars.
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The results obtained from each of these regions are shown in Figure 5. As can be seen, and in agreement with our previous studies, in the LOC, both in the LO and pFs subdivisions, the activation was significantly higher to the whole images compared to the scrambled images. Importantly, the activation to the grid images was significantly higher compared to the scrambled images (P < 0.05 in LO and P < 0.005 in pFs). Considering that in terms of the local feature structure the grid and scrambled images were identical, this increased activation can be attributed to the operation of non-local completion effects that transcend the grid lines. On the other hand, the activation to whole images was significantly higher compared to the grid images (P < 0.05). This indicates that the completion processes were not sufficient to fully compensate for the occlusion of image parts. In earlier retinotopic areas V4/V8 and Vp, the trend observed in LOC was reversed. Here the activation to both the grid and scrambled images was significantly higher compared to the whole images (P < 0.05 for the grid condition in both areas and scrambled condition in area Vp; P < 0.005 for the scrambled condition in V4/V8). This trend may reflect a higher activation due to discontinuities and corners introduced into the image by the grid occluder.
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It could be argued that object-related regions, defined by their preferential activation to whole compared to scrambled images, are not the optimal sites to search for the operation of completion effects it could be that object completion requires additional processing operations which are allocated in other cortical areas. To examine this possibility we mapped cortical regions that showed the highest preferential activation to the grid stimuli compared to the scrambled stimuli (statistical test grid > scrambled, whole images ignored).
Figure 6 shows the activation pattern revealed by preferential activation to grid images compared to scrambled images. The voxels that were preferentially activated by the grid stimuli were located in the LOC, anterior to retinotopic areas designated by white dotted lines. To examine the relationship of these voxels to object-related regions we superimposed the borders of clusters preferentially activated by whole images (whole versus scramble test), indicated by the purple contour, on these maps. It is clear that the two activity maps overlap substantially. Note also that early retinotopic areas were not activated by either test. Thus, it can be concluded that the same cortical regions, which show the highest object-related activation, also show the highest completion effects.
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To what extent are the results of the completion experiment sensitive to the specific paradigm design? To explore this issue we repeated the completion experiment, but modified two important aspects: image exposure duration and the subject's task. Thus, we changed the task to a one-back memory task in which the subject had to identify whether two consecutive images were identical or not. It should be noted that performing this task on the scrambled images was much more difficult and undoubtedly involved a higher attentional demand. Second, images were presented for 250 ms, followed by 250 ms fixation such duration is sufficient for completion effects (Rauschenberger and Yantis, 2001), but precludes lengthy contemplation and scanning of the image.
The results of this modified experiment performed on five subjects are shown in Figure 8. Voxels were chosen using the internal localizer approach (see Materials and Methods). The results are quite similar to the results obtained in experiment 1 (see Fig. 5
). In particular, there was a small but not statistically significant difference in the activation during the grid condition between the two experiments (LO, 0.95 ± 0.1% in the first experiment and 0.91 ± 0.2% in the second; pFs, 1.04 ± 0.1% in the first experiment and 0.95 ± 0.2% in the second). This effect may be due to the shorter presentation duration (see Discussion).
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It has been reported that previous exposure may change both the behavior and fMRI signal in high-order object areas (Biederman and Cooper, 1991; James et al., 2000
). In the present experiments, the design was not intended to examine such effects since we used a large number of different images. However, to examine the potential impact of such exposure in the experiment, we analysed separately the activation in grid epochs in which subjects saw the image for the first time (three epochs), with those epochs which followed prior exposure to the whole image (three epochs). The results of this analysis are shown for LO and pFs in Figure 9a
and for the recognition performance in Figure 9b
. In both regions there appeared to be a significant trend for a slightly higher activation after exposure to the whole stimulus compared to prior to exposure (in LO P = 0.007; in pFs P = 0.048). A similar but not statistically significant (P = 0.2) trend could be observed in the recognition performance (Fig. 9b
).
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To explore whether the completion effects we have observed so far were unique to familiar objects, we ran a third experiment in which unfamiliar objects were used instead. The design of the experiment is shown in Figure 10. It was identical to experiment 1 (Fig. 1
), with the exception that unfamiliar fractal and Fourier descriptors served as the objects.
As in experiments 1 and 2, we defined the regions of interest in the LOC by the contrast whole > scrambled. Based on anatomical criteria (see Materials and Methods) we divided the region of activation into LO and pFs foci. The level of completion effects was estimated, as in experiments 1 and 2. Again, the activation during the whole and scrambled epochs was obtained independently from visual epochs not included in the localizer test (see Materials and Methods). The results of this analysis are shown in Figure 11. In both LO and pFs, the activation was significantly higher to the whole images compared to the scrambled images. Moreover, the activation to the grid images was significantly higher compared to the scrambled images (P < 0.05 in LO and P < 0.005 in pFs). Thus, we can conclude that the completion effects observed in experiments 1 and 2, using familiar animal figures, can be extended to unfamiliar shapes.
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Discussion |
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The use of rectangular stripes as occluders allowed us to compare the activation to occluded and scrambled images which contained an identical set of local features since the local object elements in each inter-stripe region were identical in both images (see Fig. 1a,b). Note that our definition of local pertains to sizes not larger than the width of an inter-stripe region. Our finding of a significantly higher activation in the occluded case compared to the scrambled case indicates that completion processes that go beyond the local feature structure contribute to the object-selective activation in high order object areas, i.e. LOC. Our study could not provide an exact measure of the spatial range of the completion effects involved. It is possible that the completion involved the entire object template or, alternatively, large object chunks that span more than a single stripe and were consequently disrupted in the scrambling operation.
Regardless of their range, the completion effects found here provide further demonstration of the operation of long-range holistic processes in high-order human object areas. Such effects were reported in the case of illusory contours (Hirsch et al., 1995; Mendola et al., 1999
; Hasson et al., 2001
) and in perception of object volume (Moore and Engel, 2001
). The results also illustrate the similarity between naming performance, which also shows a strong completion effect (Fig. 2
) and fMRI activation.
On the other hand, the activation to the occluded figure did not reach the level of activation produced by the whole, unoccluded images. We can envision a number of possible reasons for this lower activation level. First, it could be that for some images the occlusion was too extensive and prevented the operation of full completion effects. This possibility is actually reflected in the behavioral results, which show a slightly lower recognition level for the occluded as compared to the whole images (Fig. 2).
Alternatively, it could be that the representation of objects in the LOC contains a mixture of neurons, some sensitive to local features while others are activated by more holistic aspects of the images. Such a mixture can not be differentiated at the resolution of fMRI. One way to by-pass such resolution limit is through object adaptation effects (Grill-Spector et al., 1999). Interestingly, a recent report has demonstrated, using adaptation effects, that LOC neurons can be sensitive to the global configurational aspects of objects rather than their local feature structure (Kourtzi and Kanwisher, 2000a
). This result supports the notion that at least some of the neuronal populations in the LOC show full completion effects.
Finally, an interesting possibility is that the partial completion actually reflects a rapidly evolving transformation of the object representation from one dominated by local features to a more holistic representation manifesting completion effects (Sekuler et al., 1994; Rauschenberger and Yantis, 2001
). Intriguing single unit evidence for such behavior was provided recently by R. Born and his colleagues in macaque MT, where the neuronal population underwent, in the span of ~100 ms, a transformation from component to holistic response to moving plaids (Pack et al., 2000). One way to explore this possibility is through rapid masking paradigms (Grill-Spector et al., 2000
), which may allow the temporal separation of early and late events in object representation. The slight reduction in grid activation when presentation time was shortened (Fig. 8
) in the second experiment is compatible with this notion. However, this question will need a detailed parametric study to be clarified.
A Separate Area for Object Completion?
It could be argued that, since the process of picture completion is so fundamental to human vision, it may have engaged specialized mechanisms located separately from the more automatic aspects of whole-object representation. We searched specifically for such regions, but found that they largely coincided with the regions that were preferentially activated to the whole images (Fig. 6). Thus, we can conclude that the cortical mechanisms involved in completion effects co-localize with regions activated by the unoccluded object image.
This finding relates to the more general question of the manifestation of recognition difficulty in fMRI activation. It may appear intuitively that more difficult recognition processes necessitate more computational resources, engage more intense attentional effects and consequently should involve enhanced cortical activation. As our completion results show, the inverse is true; pictures that are more difficult to recognize produce a weaker activation level. This result is in agreement with previous studies that showed that image degradation which makes recognition more difficult reduces rather than enhances fMRI activation (Malach et al., 1995; Haxby et al., 2000
).
More recently, in a paradigm quite similar to the completion experiment reported here, it was found that fMRI activation in high-order object areas is enhanced as the level of object occlusion is reduced (Goodale, 1998). In fact, the only instance in which enhanced recognition performance was associated with reduced fMRI activation was in priming effects and even here it is not clear whether this reduction is not confounded by concurrent adaptation effects (Goodale, 1998
; Grill-Spector and Malach, 2001
). Thus, it appears that the level of fMRI activation is correlated to the recognition performance rather than to recognition effort this should put important constraints on the type of models relevant to human recognition.
Hierarchical Organization within the Lateral Occipital Complex
In a previous report (Lerner et al., 2001) using a gradual object scrambling paradigm, we have found evidence for a putative posterior-to-anterior hierarchical axis extending from retinotopic areas into the LOC itself, so that sensitivity to image scrambling was gradually increased at higher levels of the putative hierarchy. Here, using the whole versus scrambled test, we found a further suggestion for such organization. This was particularly evident when combining the activation maps across subjects (Fig. 4). The putative hierarchy was clearer in the right hemisphere and was manifested in the gradual transition from blue to green colors, indicating increased selectivity to whole object representation as one moves anteriorly. In the present results this hierarchical trend appeared to split into two anatomical branches (Fig. 4
, arrows) a dorsal branch corresponding to the LO region and a ventral branch corresponding to the pFs. However, the possibility can not be ruled out that the anterior foci may form a continuous band, had additional object categories been used.
Sensitivity to Experimental Conditions
Comparing the results of completion experiments 1 and 2 showed that the main completion effect was not dependent on a lengthy contemplation of the images, since using short (250 ms) image presentation times did not affect the results substantially. Similarly, task-related effects such as attentional load and recognition strategy were changed substantially in the one-back memory task compared to the naming task used in experiment 1 and yet these did not affect the completion effects either. As pointed out above, the attentional demands in the one-back task are higher for the occluded objects compared to the whole objects, yet the activation level was actually lower again ruling out the possibility that the effect was due to enhanced attention.
Priming Effects
Our experimental design, which employed a large number of object images (126 images), was not optimized for the study of priming effects. In particular, as we and others have shown previously, adaptation effects are greatly minimized by the introduction of a large number of intervening object images between repetitions (Grill-Spector et al., 1999; Henson et al., 2000
). Nevertheless, we did find a small increase in activation to the grid condition when it followed a prior exposure to the whole objects. This enhancement is compatible with priming effects recently reported for occluded objects (James et al., 2000
).
Feedback Effects
The images used in the completion experiments allow us to address another question which is of high interest; namely, to what extent activity of high-order visual areas modulates the activation in lower-order areas. In comparing the activation of low-order retinotopic areas to the grid images versus the scrambled images, we would expect that the bottom-up, feed-forward activation in these areas should be the same, since the local feature structure is identical in the two sets of stimuli. However, if activity in high-order visual areas contributes to the activity in early areas, we would expect the grid stimuli to produce a higher activation compared to the scrambled images in early retinotopic areas, since the LOC was significantly more activated by the grid images. Our results did not find evidence for such feedback activation. This can be seen in Figure 5, which shows that in retinotopic areas V4 and Vp the activation to the grid and scrambled images was similar, or even slightly smaller to the grid compared to the scrambled condition. Furthermore, the absence of preferential activation to the grid compared to the scrambled images, in retinotopic areas, again supports the argument that this preferential activation is confined to high-order visual areas and does not trickle down to lower, retinotopic, areas. Obviously, this conclusion is true for the specific conditions of this experiment. Other feedback effects, such as spatial and anticipatory attention effects, have been amply demonstrated in early retinotopic areas (Tootell et al., 1998
; Brefczynski and DeYoe, 1999
; Kastner, 2000
). However, our results are compatible with the notion that early areas emphasize local feature representation, while holistic completion effects are more exclusively associated with high-order object areas.
Completion Effects for Unfamiliar Objects
It could be argued that the data presented so far are un-related to completion effects per se, but are driven by higher-order processes such as naming or semantic access produced by the familiar animal images. The results of our experiment 3, using unfamiliar, novel objects, rule out this interpretation. In such images, completion effects must rely on basic Gestalt geometric effects, rather than semantic familiarity effects. Interestingly, the level of completion observed for familiar and unfamiliar objects was quite similar. The results further confirm the involvement of the LOC in representing both familiar and unfamiliar objects (Malach et al., 1995; Kourtzi and Kanwisher, 2000b
).
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Notes |
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References |
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