Abstract
Perceptual grouping is a multi-stage process, irreducible to a single mechanism localized anatomically or chronometrically. To understand how various grouping mechanisms interact, we combined a phenomenological report paradigm with high-density event-related potential (ERP) measurements, using a 256-channel electrode array. We varied the relative salience of competing perceptual organizations in multi-stable dot lattices and asked observers to report perceived groupings. The ability to discriminate groupings (the grouping sensitivity) was positively correlated with the amplitude of the earliest ERP peak C1 (about 60 ms after stimulus onset) over the middle occipital area. This early activity is believed to reflect spontaneous feed-forward processes preceding perceptual awareness. Grouping sensitivity was negatively correlated with the amplitude of the next peak P1 (about 110 ms), which is believed to reflect lateral and feedback interactions associated with perceptual awareness and attention. This dissociation between C1 and P1 activity implies that the recruitment of fast, spontaneous mechanisms for grouping leads to high grouping sensitivity. Observers who fail to recruit these mechanisms are trying to compensate by using later mechanisms, which depend less on stimulus properties such as proximity.
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Notes
It was shown that increase in stimulus luminance increases the amplitudes of each of the early ERP peaks: C1, P1, and N1 (Johannes et al. 1995). The ERP modulation in Johannes et al. study was found when luminance was changed by a factor of nearly 40. As we show below, we found that the changes in lattice aspect ratio caused changes in the amplitudes of peaks C1 and P1 in different directions on different peaks. This means that the changes in peak amplitudes in response to the manipulation of aspect ratio were not driven by the small changes in lattice luminance.
In both stimulus- and response-based analyses we segmented data relative to stimulus onset.
The relationship between number of single trials and amplitude of the resulted averaged ERP is not linear, so it is impossible to calculate a normalization factor.
In a pilot experiment we determined the number of trials to be used in this study, to achieve a tradeoff between the statistical power needed for measuring changes in peak amplitudes, on the one hand, and the number of stimulus conditions, on the other.
An alternative method of calculating attraction coefficients, using only responses a and b, produced similar values of attraction coefficients (Supplementary Table 1) and led to the same division of observers to two groups as the method presented in the text. In the alternative method L = log{[N(b) + 1/6]/[N(a) + 1/6]}. We chose the method that took into account responses c and d, because it allowed us to use a larger number of trials in the analyses of ERP.
The values of mean-error plots do not exactly correspond to the values of the ERP curves, because the plotted values are averaged over groups of channels (used in the statistical analysis), whereas the ERP curves correspond each to a representative channel from the group.
See Fig. S1 and accompanying text in Electronic Supplementary Material for the motivation behind this selection.
The contribution of the superficial sources is especially significant for small EEG signals, such as early evoked potentials, which might be generated by a small dipole layer in the primary sensory cortex (Srinivasan 2004). Therefore, we believe that the distribution of scalp potentials in our data roughly reflects the underlying generators (within the known topographical limitations of EEG). Based on this argument, we believe we are in a position to draw conclusions about the topography of ERP components at the scale of large cortical areas.
Note that our selection of the peak latency and the areas of activity were independent of the hypotheses tested in the ANOVAs. Therefore the effects within the pre-selected latencies and areas are not false positives that could arise had we selected the latencies and areas using an exhaustive search for differences between conditions across the spatiotemporal matrix of 256 channels and 125 time samples.
Although low spatial frequencies deem critical for perceptual grouping, it is incorrect to assume that only low frequencies are needed for the task. Janez (1984) showed that grouping is possible with low frequencies filtered out from the stimulus. Consistent with this idea, our results indicate that a broad spectrum of spatial frequencies is used for grouping, manifested by ERP activity in left and right hemispheres, with higher activity in the right hemisphere indicating a greater role of low spatial frequencies.
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We thank Tatiana Tyukina for technical support, and Misa Kawashima and Maiko Maeda for assistance in running the experiments.
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221_2007_1214_MOESM2_ESM.eps
Figure S1. A. Mean-error plots of the normalized amplitude of C1 activity for four aspect ratios (averaged across orientations) in the high-sensitivity group. B. Mean-error plots of the normalized amplitude of P1 activity for four aspect ratios (averaged across orientations) for all participants. In both panels, the amplitudes are shown for 28 channels organized into 4 chains of 7 electrodes (colored green in Fig. 3D). Numbers on the abscissa correspond to the ordinal numbers of electrodes within each chain, from left to right in the frontal plane (EPS 4772 kb)
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Nikolaev, A.R., Gepshtein, S., Kubovy, M. et al. Dissociation of early evoked cortical activity in perceptual grouping. Exp Brain Res 186, 107–122 (2008). https://doi.org/10.1007/s00221-007-1214-7
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DOI: https://doi.org/10.1007/s00221-007-1214-7