1 Graduate School of Neurosciences, Department of Visual System Analysis, AMC, University of Amsterdam, PO Box 12011, 1100 AA Amsterdam,, 2 The Netherlands Ophthalmic Research Institute, PO Box 12141, 1100 AC Amsterdam, The Netherlands and , 3 Laboratory of Visual Perception, Cuban Neuroscience Center, Apartado 6880, Cubanacan, Habana, Cuba
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Abstract |
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Introduction |
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Gestalt psychologists, on the other hand, have emphasized the role of grouping laws in perceptual organization (Rock and Palmer, 1990). This view is supported by findings that global similarity influences the strength of local feature discontinuities in texture segregation (Enns, 1986
; Nothdurft, 1994
), and also by the finding that similarities may interfere with segregation (Callaghan, 1989
; Rivest and Cavanagh, 1996
; Moller and Hurlbert, 1997
). Segregation furthermore depends on information about surface layout defined by binocular disparity (He and Nakayama, 1994
). Also, thresholds for motion and colour segregation are lower, and segregation is faster, for broad vertical target strips than for thin ones (i.e. with identical boundary lengths), suggesting a role for fast region-based segmentation processes (Moller and Hurlbert, 1996
).
The relative roles of boundary formation and surface filling-in or grouping have been particularly addressed in the models of Grossberg on preattentive vision, segmentation and figure ground segregation. In these models, a boundary contour system (BCS) detects boundaries between regions that are filled-in by the feature contour system (FCS). In older versions of the model, the BCS leads the filling-in by the FCS (Grossberg and Mingolla, 1985; Grossberg et al., 1989). In later versions (Grossberg, 1994
), however, the two systems interact to form boundary and surface representations that are mutually consistent, and that may explain filling-in phenomenology (Arrington, 1994
). In other models, surface signals are used to sharpen boundary signals (Poggio et al., 1985
; Lee, 1995
).
Neuronal correlates of segregation and grouping have been studied in recent times: neuronal synchrony (Singer and Grey, 1995), as well as response rate modulation in early visual areas (Kapadia et al., 1995
; Lamme et al., 1993
, 1998a
), seem to play a role as correlates of perceptual grouping. Activity mimicking perceptual filling-in has been found in areas V3 and V2 (De Weerd et al., 1995
). Neuronal correlates of boundary detection on the basis of feature differences (Sillito et al., 1995
; Chaudhuri and Albright, 1997
) or discontinuities (Grosof et al., 1993
) have been found in V1. Also, a correlate of figureground segregation and global scene perception is found in response modulations in V1 (Lamme, 1995
; Zipser et al., 1996
). Primary visual cortex obviously also plays an important role in the encoding of basic stimulus features (Hubel and Wiesel, 1968
; Schiller et al., 1976
). This area therefore seems an ideal substrate to investigate the interrelations between feature detection and grouping, and boundary detection and filling-in.
We recorded from awake macaque monkey primary visual cortex while the animals were viewing displays like the one in Figure 1. Modulation of responses were observed depending on whether the receptive fields of the neurons responded to line elements of the figure or the background, as we have shown before (Lamme, 1995
; Zipser et al., 1996
). In this study, we focus on the temporal aspects of these modulations, in particular with respect to the modulations evoked by the figureground boundary and surfaces. The use of implanted electrodes enabled us to obtain new and very detailed spatio-temporal profiles of the neural image that is cast on V1 when the figureground display is presented. It appeared that the modulations exhibit a temporal sequence of processing, going from local feature detection, followed by the detection of texture boundaries, to a neural representation of the relative figureground relationships of the surfaces in the scene.
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Materials and Methods |
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Stimuli were presented on a 21 in. computer monitor, driven by a #9 GxiTC TIGA graphics board. The display resolution was 1024 x 768 pixels, the refresh rate was 72.4 Hz. The screen subtended 28° x 21° of visual angle. Trial initiation consisted of the appearance of a 0.2° red fixation spot on a texture of randomly oriented line segments. Monkeys were trained to maintain fixation on this spot. Fixation was considered maintained when the eyes did not at any time leave an imaginary 1.0° x 1.0° window centred around the spot. Eye movements were monitored with scleral search coils, according to the modified double magnetic induction method (Bour et al., 1984). When stored on disk, eye movements were digitized at 400 Hz. Three hundred milliseconds after fixation, the stimulus appeared on the screen, consisting of oriented line segments, so organized that a 4° square figure could be segregated from a full screen background. One stimulation sequence occurred per trial. The animals were rewarded with a drop of apple juice for maintaining fixation until the fixation spot turned off (500 ms after stimulus onset), and subsequently making a saccadic eye movement to the position of the square figure. In part of the recording sessions, the animals were also rewarded simply after having maintained fixation.
Recording of Neuronal Activity
Neural activity was recorded with surgically implanted, Trimel-coated platinumiridium wires of 25 µm diameter, with exposed tips of 50150 µm. Impedances ranged from 100 to 350 k, at 1000 Hz. These wires were implanted in the operculum of area 17 (V1) in two macaque monkeys. The obtained signals were amplified (40 000x), band-pass filtered (7505000 Hz), full-wave rectified, and then low-pass filtered (<200 Hz). This resulted in a low-frequency signal, representing the amount (or envelope) of high-frequency (i.e. spiking) activity (Legatt et al., 1980
), without any bias for high-amplitude spiking neurons, as might be the case when (arbitrary) amplitude thresholds are used to record multi-unit activity (MUA). This low-frequency signal was digitized (400 Hz), stored on disk and analysed off-line. For further analysis, 16 channels, selected from the implanted electrodes on the basis of signal-to-noise quality of the responses, were recorded simultaneously in each of the two monkeys. Aggregate receptive fields (RFs) of the neurons contributing to each channel were assessed with moving dark bars over a bright background while the animal was fixating. Exact RF positions and sizes were determined off-line from these responses. First, the peak of the activity that was evoked by the moving bar was timed. To compensate for response latency, 50 ms was subtracted, and the position of the bar at that moment in time was calculated. Eight or 16 orientations were used, and each orientation that evoked a response thus resulted in an estimate of the RF position. These position estimates were averaged to obtain the final RF positions that were used in this study. The size of the RF was determined from the response to a bar at the optimal orientation. It was calculated at what range of positions of the bar a response was obtained that exceeded background activity. The responses were considered to exceed background activity when 10% of the peak activity was reached. The width of the bar (0.2°) was subtracted from the thus obtained size. RF eccentricity ranged from 1.3° to 5.45°, and diameter from 0.18° to 1.4° (mean 0.52°). Orientation selectivity was moderately expressed in the MUA. The median orientation selectivity ratio (average response level while a bar of optimal orientation moved over the RF divided by average response for least effective orientation) was 1.94 (mean 2.19, range 1.169.03). All recording sites could be driven from either of the two eyes. Strong ocular dominance, as has been reported for layer 4C cells (Hubel and Wiesel, 1977
) was absent. Given that electrodes were implanted at a range of depths, and given the binocularity of the signals, it is highly unlikely that the moderate orientation tuning should be regarded as a sign that the recordings expressed mainly layer 4C activity. Instead, taking the RF sizes, tuning ratio and ocular dominance together, we roughly estimate that the electrodes sampled neuronal activity over a distance of some 200300 µm.
Receptive field positions and sizes stayed stable (within ~0.2°) for many months of recording in these animals. Tuning characteristics could slowly change over periods of weeks or months. The results presented here were collected during weeks of recording in each monkey. The sets of stimuli were presented in randomized or interleaved ways, so as to avoid possible electrode drifts biasing the results.
Stimulus Positioning and Complementary Stimulus Pairs
We presented the figureground stimuli with the figure at various positions relative to the aggregate RFs. As a result, many responses were obtained with a RF either inside or outside the square figure (Fig. 2a). Figure and background are composed of orthogonal orientations. This would result in different RF stimulation, depending on the position of the figure relative to RF. Therefore, we always used complementary stimulus pairs, i.e. a particular orientation that was in one trial used for the figure was in another trial used for the background, and vice versa (Lamme, 1995
; Lamme et al., 1998b
). To achieve this, two full-screen video pages were generated for each orientation, of which a small part was used for the figure and a large part of the other one for the background. Then, only the boundary between the figure texture and the background texture was changed for the different positions of the figure. Thus, by measuring and averaging V1 responses to the complimentary stimuli, we could ensure that, regardless of whether the RF falls in the figure or on the background, the RF was exposed on average to the same set of local features. Notice, however, that when the RF falls on the figureground edge, the complementary stimulation is only strictly balanced if the RFs behave like linear spatial filters; we will come back to this point in the Results section.
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To calculate post-stimulus time responses, 500 ms epochs following stimulus onset were averaged from those trials where the animal had fixated and responded with a correct saccade. The mean of the signal obtained at 030 ms after stimulus presentation was subtracted from the signal. For all practical purposes, this can be considered the amount of activity that was present while the animal fixated the pre-stimulus texture (Lamme, 1995; Lamme et al., 1998b
). Some sites exhibited activity that was locked to the monitor frame rate (72.4 Hz). Therefore a digital 72.4 Hz notch filter was applied. The displayed responses were additionally smoothed with 121 windows.
For all electrodes, we wanted to sample 15 positions of the RF relative to figure and ground (Fig. 2). However, for any specific position of the figure some RFs were inside the figure, while others were on the boundary or the background. We therefore used a more extensive set of figure positions that enabled simultaneous recording from electrodes, such that each electrode was used an equal number of times (~200 averages per position, per monkey) for all 15 positions. To get a population average response for all positions of `RF relative to figure' (Figs 2 and 3
), responses from the different electrodes were averaged. This was done such that those responses were averaged together that are identical with respect to the relative positions of figure and RF. In other words, RF1 (3.5° relative to figure centre) is averaged with RF2 (3.5° relative to figure centre) and with RF3 (3.5° relative to figure centre), etc., which may all be different absolute positions of the figure. In this alignment procedure, vertical positions of the RF relative to the figure were rounded to the figure position step interval (0.5°).
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Results |
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Neural activity was recorded with a square texture figure (Fig. 1) at various positions (Fig. 2a
) relative to the V1 RFs. Responses from all electrodes of both monkeys were averaged to obtain population average responses for each position of the figure relative to the RF (Materials and Methods: data analysis). Complementary stimulus pairs (Materials and Methods) were used in order to have both orientations contribute equally to figure and ground responses (note that at the boundary between figure and ground this would only strictly hold if the RFs behave like linear spatial filters; we will come back to this point below). Figure 2b
shows these average population responses. Fifteen positions can be discerned, three such that the background is overlying the RFs, nine such that the figure (or its boundary) is overlying the RFs and three again with the background overlying the RFs. Thin lines in Figure 2b
show the average response to background far away from the figure (>1.5° from the edge). Control experiments showed, in line with previous results (Lamme, 1995
) that the background response 1.5° away from the edge is not different from background responses obtained much further away (up to 8°) or responses to background without any figure present in the display. The difference between the background response and the responses to the 15 positions is indicated by grey shading. These differences are plotted for the 15 positions in Figure 2c
.
Several features of these responses confirm our earlier results on figureground modulation (Lamme, 1995). When the RF is on the background, responses are uniformly lower than when the receptive field is on the figure. In Figure 3a
, we plotted the average of the amplitude between 150 to 500 ms of the difference responses obtained at the different positions (black line and squares). Figure versus ground response modulation is as large (after ~150 ms) for figure responses close to the boundary between figure and ground as for responses within the centre of the figure. Also, there is no modulation at the immediate outside of the boundary between figure and ground (open squares).
What we are interested in here is how this modulation evolves temporally from stimulus onset; in particular, the time interval before 150 ms shows specific features that have not been observed before. In Figure 3a we show (in various colors) 10 ms time slices of the difference responses of Figure 2c
. The figure could be read as having a vertical scrolling bar going through the responses of Figure 2c
and plotting the response amplitude at each position. Points that deviate significantly (P < 0.01) from zero have been marked as solid circles, non-significant deviations as open circles. Before the neurons start to respond (2030 ms) the slice is flat. But also at the peak of the neuronal response (5060 ms) the slice is flat, indicating that the figureground modulation is strongly delayed with respect to the response itself. Between 70 and 80 ms the first hints of modulation are observed, which are strongest at the boundaries between figure and ground. Between 90 and 100 ms, we observe an intriguing phenomenon; the modulation obtained at the boundaries is now strongly present, while the modulation for the remainder of the figure surface is still almost absent. The edge modulation peaks at 115125 ms, while at that point figure surface modulation is still evolving. Only at 150160 ms is the modulation for the whole figure surface at full strength. Modulation subsequently decays very slowly, but remains uniform for all positions inside and including the figure boundaries until the end (420430 ms).
Some other points in Figure 3a than those discussed above are marked as significant deviations from zero, for example some of the `surface' responses of the 7080 or the 90100 ms intervals. Does that imply that some form of surface modulation already occurs very early, almost at the same time as boundary modulation? Note that some of these points are significant in the 7080 ms stretch, but then again fall out of significance at 90100 ms, or the other way around. Some of the points in Figure 3a
will be false positively assigned as significantly different from zero, simply because we are dealing with multiple comparisons (200 samples/position). However, it cannot be fully excluded that some very weak and transient early surface enhancement occurs. But to conclude that surface enhancement occurs before 100 ms would also not be warranted (see also below).
Responses are thus identical for all 15 positions up to ~70 ms after stimulus onset. The early activity is thus only determined by the local features presented in the receptive field, which are identical for all positions by means of the complementary stimulus pairs. Remarkably, responses up to 70 ms are also identical for the two positions where the figureground boundary is overlying the RF. The complementary stimulus design in this situation would only strictly hold in case of linear summation within RFs. Apparently, the aggregate RFs behave like linear spatial filters at the population response level, at least up to 70 ms.
An important question is whether the effects observed between 70 and 100 ms at the boundary are mediated by local RF tuning mechanisms, or by other local or global mechanisms. An obvious local mechanism would be selectivity for the orientation (or any other feature) of the locally present texture. We averaged background responses to the texture that yielded the largest response and responses from the least effective texture. The difference between these two is shown in Figure 3b, in composite with the average response to background, and the difference responses obtained for the figureground edge. Tuning to the orientation of the texture is somewhat later than the response itself, but clearly earlier than the figureground boundary enhancement. From this we conclude that the figureground boundary enhancement is caused by a mechanism that is different from local RF tuning.
To get objective measures of the latency differences between the various responses we used two methods: one based on a statistical significance criterion, the other based on a time to peak criterion, which is independent of the number of averages used. For the first method, we determined for each sample of the responses whether it was significantly different from zero at the P = 0.01 level. Because there are 200 samples/response, many `significant' samples can be expected to occur just by chance. Simply looking for the first significant sample thus would lead to erroneous latency estimates. An often used and more robust measure is to look for consecutive stretches of significant samples (Maunsell and Gibson, 1992; Munk et al., 1995
; Roelfsema et al., 1998
). The start of such a stretch would be a much more reliable estimate of latency. Here, we determined the start of the first continuous 50 ms epoch of response (or enhancement) that was significant at the 1% level. The results of this analysis are shown in shades of green in Figure 3c
. The second method of latency estimation was to calculate the time at which the amplitude of the response (or the enhancement) reached 50% of its peak value. The results of this calculation are shown in shades of red in Figure 3c
. Both methods yielded similar results in three respects: latencies of the response itself (3040 ms, method 1; 50 ms, method 2) are always shorter than the latency of orientation tuning (50 ms; 57.5 ms), which is in turn always shorter than the latencies of figureground edge enhancement (57.570 ms; 90 ms). Figureground edge enhancement is always faster than enhancement obtained for central figure surface positions (90107.5 ms; 112.5122.5 ms).
A remarkable feature is observed for the latency measures that are obtained with the second method (Fig. 3c, red bars): latencies of the figureground enhancement are shortest at the boundary (90 ms), and increase in ~7.5 ms steps for every 0.5° towards the centre of the figure (120 ms). In other words, there is a gradual increase in latency, going from the figureground edge towards the centre.
Feature, Boundary and Figure Surface Relationships at Individual Sites
Like we did in Figure 3 for the population response, we calculated latencies of response, orientation selectivity and figureground enhancement at the boundary and the centre of the figure for individual recording sites. In individual cases, method 1 could not be used due to signal-to-noise limitations; although figureground enhancement was significant at the P < 0.01 level in 25 of the 32 cases, too few responses showed reliable consecutive stretches of significant samples. Latencies of all four response types could be calculated for 29 of the 32 sites according to method 2 (when 50% of peak was reached before 30 ms or after 250 ms, results were discarded). Latency of response was calculated from the average of the two most extreme background positions, latency of orientation tuning from the difference in response for two texture orientations at background, latency of boundary enhancement from the two edge positions, and latency of surface centre enhancement from the three most central surface positions. The distributions of these four types of latencies are shown in Figure 4a
, their means in Figure 4b
. Significant differences are found between the latencies of all four types of responses (one-way ANOVA P = 6 x 1012; for individual comparisons, see Fig. 4b
). At the population mean level we thus observe a sequence of processing, starting with RF-based orientation-selective responses, followed by response enhancement caused by the figureground boundary, which is then again followed by response enhancement related to the figureground surface relationships.
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This finding is further strengthened by the following consideration. The boundary between figure and ground was placed from one position to the next in discrete 0.5° steps. Because of this, for some small and awkwardly positioned RFs the boundary was projected only in the periphery of the RF, instead of its centre, or in some cases might even have `missed' the RF. For these electrodes, the `edge' responses could therefore be diluted by `surface' responses. Electrodes for which this might have occurred are shown in light grey in Figure 5c. When these are excluded, all sites show that edge enhancement occurs before figure surface enhancement.
These results show that for the population of V1 neurons, the filling-in process is induced by the boundary detection. However, whether they also indicate that boundary detection at each individual site induces the filling-in at that site remains unresolved, since we were unable to establish whether the latency difference between boundary and surface signals was significant at each site.
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Discussion |
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At the population response level, a clear temporal sequence is observed in the processing of a figureground display like Figure 1. First, cells start to respond, and this is very shortly followed by orientation selectivity for texture elements. At considerable longer latency the boundary between figure and ground elicits a response enhancement. This is followed by response enhancement for the figure surface, with longer latencies for positions further away from the figureground edge (Fig. 3c
). This suggests a process of filling-in starting from the edge. This is corroborated by the finding that at individual recording sites edge enhancement may occur at a range of times, but is always followed by surface enhancement.
Eye Movement Controls
Our results were obtained in monkeys that had to fixate within a window of 1° x 1°. At first sight, it might therefore be surprising that dramatic changes in response were observed for shifts in stimulus position as small as 0.5°. However, monkeys fixated much more precisely on the 0.2° fixation spot than demanded. On average, 95% of fixations were within 0.2°. This can also be observed in our finding that some of the RFs were as small as 0.2°.
Another concern is whether the stimuli induce small eye movements within the window of fixation that might contribute to the effects reported. We have published many controls for this, showing that this is not the case (Lamme, 1995; Zipser et al., 1996
; Lamme et al., 1998b
). The eye movement controls that we performed on the data presented here have been published elsewhere (Lamme et al., 1998b
), together with additional experiments showing that the effects are not attributable to eye movements or spatially focused attention.
Feature, Boundary and Surface Detection
Three qualitatively different phases can thus be observed in the spatio-temporal response profiles that are unrolling in V1 after the presentation of a segregating texture. We argue that three different processes, separated by different temporal dynamics and brought about by different properties of the neurons and their connections within V1 underlie these phases. It appears that local feature detection governs the initial response phase in V1: almost as soon as cells start to fire, orientation selectivity is expressed in their responses. This is in line with other evidence showing that orientation selectivity is present in the early spikes of V1 neurons (Celebrini et al., 1993), and in early synaptic potentials, that arise from lateral geniculate nucleus (LGN) input alone (Ferster et al., 1996
). This response phase is therefore most likely generated from feedforward processing from LGN to V1 and through the cortical pathways within V1 (Mitzdorf and Singer, 1979
; Lund, 1998).
The initial feedforward phase is followed by the detection of surface boundaries. The modulation of the response that is caused by the figureground boundary occurs much later than the expression of orientation tuning (Fig. 3b). From this we conclude that a different mechanism underlies to it. The recent findings of some V1 cells being sensitive to orientation contrast (Sillito et al., 1995
) or abutting gratings (Grosof et al., 1993
) are most likely underlying this second response phase. Specific sensitivity for orientation contrast was also reported by Caputo (Caputo, 1998
). Another candidate would be endstopping (Hubel and Wiesel, 1968
, 1977
), as the line segments are typically truncated at the boundary. End-stopping has also been coined as a mechanism to create selectivity for cornerlike features, and the abutting orthogonal line segments at the boundary form such corner features. This phase may be mediated by feedforward mechanisms in combination with horizontal connections.
Finally the cells seem to represent the figureground relationships of surfaces in the scene with a higher response level for the figure than for the background. This higher level persists for as long as the stimulus is on. The third phase is clearly an expression of influences from beyond the classical receptive field (Allman et al., 1985; Gilbert and Wiesel, 1990
; Knierim and Van Essen, 1992
; Lamme, 1995
; Kapadia et al., 1995
; Zipser et al., 1996
). These have often been interpreted in terms of RF centres and (inhibitory or excitatory) surrounds. We have, however, shown that this representation is rather cue-invariant (Zipser et al., 1996
) and bears no direct relation to the RF properties of the V1 cells (Lamme, 1995
). Moreover, it seems to be most closely related to perceptual interpretation of the scene (Kapadia et al., 1995
; Lamme, 1995
). It is also absent in the anaesthetized animal (Lamme et al., 1998b
). It is very likely that the third phase is an expression of horizontal connections within V1 (Gilbert, 1992
) in combination with feedback from extrastriate areas (Salin and Bullier, 1995
). In fact, we have evidence that the surface signals, and not the boundary signals, are abolished by extra-striate lesions (Lamme et al., 1998a
).
Our results strongly argue in favour of a model of preattentive vision where boundary detection precedes and initiates surface filling-in (also called colouring). Boundary signals are very sharp right from the start, i.e. there seems to be no boundary contraction process (Lee, 1995). The interaction between boundary signals and surface signals seems to be mostly one way, from boundary to surface. That is not to say that surface signals never influence boundary formation. We used rather clear-cut, segmentable images, and it might be that when there is more noise or ambiguity, an influence of surface signals on the neural representation of the boundary is seen.
The Timing of Boundary versus Surface
In these experiments, the response modulation at the centre of the figure lagged the boundary enhancement by ~30 ms. Given the size of the figure, this would correspond to a speed of filling-in of 67°/s. Propagation speed of brightness filling-in was estimated in several studies. Speeds of 110150°/s (Paradiso and Nakayama, 1991), 510°/s (Paradiso and Hahn, 1996
), 140 180°/s (Rossi and Paradiso, 1996
) and 19°/s (Davey et al., 1998
) have been reported. Our results are within the same range. Using so called `phantom contour' stimuli, Rogers-Ramachandran and Ramachandran (Rogers-Ramachandran and Ramachandran, 1998
) found a fast (15 Hz) texture contour system, and a slower (7 Hz) surface discrimination system. In his masking experiments, Caputo (Caputo, 1998
) found a latency of 4080 ms for segregation contours and a latency of 120 ms for the spreading of surface filling-in. Although these results are difficult to compare, they both suggest that boundary detection is twice as fast as surface-filling-in. We find something similar when we compare the latencies of the boundary and surface signals with the latency of the feature detection system; surface latency minus feature latency (122.5 57.5 = 65 ms) is about twice as long as boundary latency minus feature latency (95.0 57.5 = 37.5).
The kind of filling-in discussed here also referred to as colouring is a rather different process than the much slower type of filling-in that is observed when surrounding stimulus properties `invade' a region of the visual field that receives no visual input, as is the case for the blind spot, or for artificial scotomata (non-stimulated regions). In those cases, it takes ~510 s (Ramachandran and Gregory, 1991; De Weerd et al., 1995
) before this type of filling-in starts to operate.
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Notes |
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Address correspondence to Victor A.F. Lammle, Graduate School of Neurosciences, Department of Visual System Analysis, AMC, University of Amsterdam, PO Box 12011, 1100 AA Amsterdam, The Netherlands. Email: v.lamme{at}amc.uva.nl.
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