Experimental observation.

<p>(a) The scheme of the electrode position the typical set of registered EEG traces. Different segments of the EEG recording are named I, II, III, which correspond, respectively, to the 1-sec time interval preceding the cube presentation (<i>before perception</i>), ∼ 1-sec interval of the cube observation (<i>perception</i>), and 1-sec interval after the cube observation (<i>after perception</i>) and (b) The values of (triangles) and (circles) illustrating the relation between the power of alpha and beta waves in intervals I and II obtained by the statistical analysis of the 40-min experimental session of each of the 10 subjects. The horizontal dashed lines indicate threshold values defining a > 40% decrease of alpha-activity (line 1) and a > 20% increase of beta-activity (line 2) used to identify different perception scenarios. The solid red boxes highlight the subjects (2,3,9) following the first scenario. Other subjects are associated with the second scenario. (c,d) 3-D histograms illustrating the distribution of the statistical measure <i>L</i>(<i>f</i>, <i>t</i>) calculated by <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0188700#pone.0188700.e006" target="_blank">Eq (3)</a> which indicates the location of the maximal spectral component during the 40-min session for two subjects demonstrating the (c) first (subject #9) and (d) second (subject #7) perception scenarios.</p

Brain-computer interface for estimation and control of alertness.

<p>(a) Schematical illustration of the experimental setup. (b) Control characteristics <i>G</i>_1,2,3(<i>t</i>) describing the degree of alertness during the processing of visual stimuli, obtained from three subjects of the group. The vertical dashed lines indicate the moments of time when the external disturbance (<i>t</i>_<i>EP</i>) was applied and the feedback message about the attention decrease (<i>t</i>_<i>FB</i>) was sent. The horizontal dash-dotted lines indicate the values of , , calculated by averaging <i>G</i>_1,2,3(<i>t</i>) over time intervals <i>t</i> < <i>t</i>_<i>EP</i>, <i>t</i>_<i>EP</i> > <i>t</i> > <i>t</i>_<i>FB</i> and <i>t</i> > <i>t</i>_<i>FB</i>. (c) Values of , and averaged over the group of eight subjects. The error bars indicate the standard deviation of these values among all participants.</p

Examples of Necker cube images.

<p>The ambiguity of the Necker cube is controlled by contrast parameter <i>I</i>. The left-hand image with <i>I</i> = 0 corresponds to the fully left-oriented cube, whereas the right-hand image with <i>I</i> = 1 to the fully right-oriented cube. The middle image with <i>I</i> = 0.5 has the highest ambiguity.</p

Effect of motivation.

<p>(a) Colored areas containing dependencies of the percentage of type-1 events on the number of the cube presentations for participants belonging to GROUP I (motivated subjects) and GROUP II (unpaid volunteers). (b) Percentage of type-1 events averaged over participants belonging to GROUP I (left circle) and GROUP II (right circle). The error bars show the standard deviation for each group.</p

Spectral properties of two different perception scenarios.

<p>Upper row: Typical EEG trials associated with perception of ambiguous images illustrating (a) first and (b) second perception scenarios. The colored solid lines show the temporal dependences of the dominant frequencies of the first and second maximal spectral components during the perception. The line color indicates the frequency band within which these spectral components occur at the current moment of time: green refers to delta band (1-4 Hz), red to alpha band (8-12 Hz), and purple to beta band (20-30 Hz). The double vertical lines limit the time interval of the button pressing. Lower row: Coefficients 〈<i>F</i>_{<i>α</i>,<i>β</i>,<i>δ</i>}〉 characterizing the location of the maximal spectral components averaged over all channels and time intervals Δ<i>τ</i>_I,II,III corresponding to different segments during perception for subjects belonging to (c) group 1 and (d) group 2. The error bars indicate standard deviations for each group. The horizontal bars with stars show significant differences in contributions of the alpha and beta components according to the statistical analysis using paired t-test.</p

Effect of cube ambiguity.

<p>(a,b) 3-D histograms illustrating dominant frequencies of the maximal spectral component during sessions with (a) high and (b) low ambiguity for the same subject. (c) Percentage of type-1 events observed in sessions with low (left red circle) and high (right blue circle) ambiguity, averaged over all participants. The error bars indicate the deviation of these values among all participants. (d) Percentage of type-1 events as a function of the number of presentations of cubes with low (lower curve) and high (upper curve) ambiguity.</p