Summary of the tasks.

<p>A: EEG Mooney rapid visual presentation task, with delayed response. Meaningful (perceived as faces or guitars) objects appear among noise images. Mooney faces and Mooney guitars are shown randomly with a likelihood of 1/30 (between the 10^th and 20^th presented images) at each trial masked backward and forward by a randomization of itself (each picture 150 ms). Subjects had to report the presence of a target (Mooney face or Mooney Guitar or none of these objects) at the end of the trial (inter-stimulus interval is 2000 ms). B: Mooney dynamic stimuli - Time-line of one run; for clarity, representative snapshots are represented in separated boxes (in the experiment movies run continuously and smoothly). Accordingly, only 4 snapshots are shown for each movie – faces rotate from inverted to upright in 12 s movies separated by a 3 s black screen. Subjects were instructed to provide a motor report, when they perceived the face, as quickly as possible.</p

To Perceive or Not Perceive: The Role of Gamma-band Activity in Signaling Object Percepts

<div><p>The relation of gamma-band synchrony to holistic perception in which concerns the effects of sensory processing, high level perceptual gestalt formation, motor planning and response is still controversial. To provide a more direct link to emergent perceptual states we have used holistic EEG/ERP paradigms where the moment of perceptual “discovery” of a global pattern was variable. Using a rapid visual presentation of short-lived Mooney objects we found an increase of gamma-band activity locked to perceptual events. Additional experiments using dynamic Mooney stimuli showed that gamma activity increases well before the report of an emergent holistic percept. To confirm these findings in a data driven manner we have further used a support vector machine classification approach to distinguish between perceptual <i>vs.</i> non perceptual states, based on time-frequency features. Sensitivity, specificity and accuracy were all above 95%. Modulations in the 30–75 Hz range were larger for perception states. Interestingly, phase synchrony was larger for perception states for high frequency bands. By focusing on global gestalt mechanisms instead of local processing we conclude that gamma-band activity and synchrony provide a signature of holistic perceptual states of variable onset, which are separable from sensory and motor processing.</p></div

Normalized time-frequency plots in 2D scalp maps (experiment 2).

<p>These maps are plotted for the channels marked as a black point in the 2D topographies in five consecutive time windows of 200 ms. The red-dashed row is associated with higher gamma-frequencies: activity is increased not only in occipital electrodes but seems to change its ‘centre of gravity’ during the time to more parieto-frontal areas. The blue-dashed row shows topographic maps for low-gamma. Gray row show the deactivation at the lowest frequencies (beta) that seems to have their source in central regions usually reported as motor areas.</p

SVM classification results.

<p>Frequency data from occipito-parietal electrodes were used as classification features to separate between perceptual states. Only subjects with >10 non perceived trials were used. A group average of accuracy, sensitivity, specificity and balanced accuracy is shown for each frequency band. These bands are matching the time-frequency results (<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0066363#pone-0066363-t001" target="_blank">Table 1</a>). We performed a permutation test for each subject and all p values were below threshold (p<0.001). Classification was successful for all tested subjects.</p

Time-frequency representations and topographies in the perception (A) and no perception (B) conditions– Experiment 2.

<p>A: group average responses to Mooney dynamic stimuli locked to response onset (blue line), baseline corrected and normalized for the baseline interval (fixation and stimulus); B: group average activity for the non- perceived trials. Topography plots for high and low gamma response are shown for perception and no perception conditions respectively. The topographies are averaged across the time window (−800 – 0 ms) for the higher gamma-band (60–75 Hz) and lower gamma-band (30–45 Hz; top right and bottom right, respectively). The gamma-band signal is expressed as relative power change during perception compared to baseline, averaged across all channels. Note that the frequency band where the effect size is highest is the higher gamma-band. Boxes highlight low (30–45 Hz) and high (60–75 Hz) gamma bands, here and in subsequent figures.</p

Head-in-head plots for the imaginary part of coherency at each frequency band.

<p>ImCoh is represented between all channel pairs time-averaged for the second before the button press. The difference between perception and no perception conditions is shown. Each small black dot corresponds to the position of the reference electrode in terms of connectivity. Note the link between occipital and frontal sites in the gamma range. Colorbar codes connectivity.</p

No perception<i>vs.</i> Perception statistics for experiment 2.

<p><a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0066363#s3" target="_blank">Results</a> of statistical t-tests when comparing perception and no perception during the second before button press. We tested four frequency bands. p and t values are shown and differences were considered significant for p<0.0025 (corrected for multiple comparisons). Perception show increased activity for higher gamma frequencies.</p

Group average ERP for experiment 1 - Mooney Rapid Visual Presentation.

<p>Baseline is set to −250 ms to 0 ms. ERP peaks negatively around 220 ms and is consistent across subjects (line points means standard deviation). Source distribution maps show sLORETA standardized current density for the three different peaks (hot colors signals the highest current density reconstruction values).</p

Time-frequency plot for the posterior channels– Experiment 1.

<p>Data are locked to the onset of the oddball salient frame (A: Face; B: Guitar), baseline corrected for the pre-stimulus interval and normalized for the baseline interval. Significant differences revealed prominent gamma activity for face trials at low frequency (30–45 Hz, 373–685 ms; z = 4.14; p corrected<0.001) as well as for guitar trials both for low (30–45 Hz, 370–600 ms; z = 5.06; pcorrected<0.0001) and high frequencies (60–75 Hz; 166–641 ms; z = 10.33; pcorrected<0.0001) when comparing with the baseline. Blue line marks the beginning of the target stimuli; Color scale: normalized units.</p

The topography of high frequency oscillations.

<p>We found distinct high frequency activity patterns during the ambiguous object recognition/perceptual decision task. The position of the electrodes per subject belonging to each of the clusters were marked as dots (in three distinct colors) in the coregistered brain. These patterns have distinct sources in the brain as represented by the correspondent dots. B) The example TF plots are a group average of all the represented electrodes of that cluster (N = 3 subjects for “blue” labels; N = 4 subjects for “orange” and “green” labels). Data (dB) are presented for the Mooney faces condition but these results could be generalized for the other conditions (see Figure A in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0186428#pone.0186428.s001" target="_blank">S1 File</a>). The black line segmented “blobs” in the plots depict the TF spectral–temporal patterns which are significant (blue z = -3.12, p<0.0017; orange, z = 3.11, p<0.0018; green, z = 3.09, p<0.0019). The dashed lines mark the start and end of stimulus.</p