Stimuli. (A) A sample frame of the stimuli. Two circular arrays of discs are presented within the two annuli. The triangle in the green annulus has the same area as the disc (0.52° in diameter); it is a target only when the green annulus is attended. (B) Locations of seven concentric annuli (1~7). Each annulus is 1.45° in width; the innermost circle has diameter 1.3° and the outermost one has diameter 21.6°. The circular lines were not visible to the observer in the experiment. (C) The IR (interior random broadband flicker) stimulus configuration: annuli 1 and 4. Regularly updating search arrays (flicker) were superimposed on annulus 4, always on a green background. Irregularly updating search arrays (random broadband flicker, rbbf) were superimposed on annulus 1, always on a red background. (D) The ER (exterior random broadband flicker) stimulus configuration: annuli 4 and 7. Flicker was superimposed in annulus 4 on a green background and the rbbf was superimposed on annulus 7 on a red background. (E) A temporal sequence of stimulus frames. Each pair of vertically aligned green and red bars represents a video frame (8.3 ms), red for red annulus and green for green annulus. The green bars (upper trace) represent regular flicker; the red bars (lower trace) represent random broadband flicker (rbbf). The longer vertical bars represent annuli with search arrays superimposed (e. g., A, F, and G) and the shorter vertical bars represent annuli without search arrays superimposed (C, D). A triangle above a longer vertical bar indicates that the search array contains a triangle. The stimuli were composed of four types of frames: Frames in which a search array was (1) absent in both annuli (C), represented by a vertically aligned pair of short green and red bars, (2) present in both annuli (A), represented by a pair of long bars, (3) present in the green annulus but not in the red annulus (F), represented by a long green and a short red bar, and (4) present in red annulus but not in the green annulus (G), represented by a short green and long red bar. Each trial begins with two colored concentric annuli (without superimposed search arrays) presented for 2.5 seconds (C or D). Then, for 50 seconds, search arrays are superimposed on the two annuli and updated independently. Finally, two colored concentric annuli without superimposed search arrays are presented for 1.5 seconds. The concentric red and green annuli are visible continuously, only the superimposed search arrays are updated.

Averaged of 10 observers’ hit probability as a function of flicker frequency. The error bars indicate standard errors. Inserted icons indicate experiment conditions: ‘A’ indicates the attended annulus and ‘f ’ indicates the annulus superimposed with flicker. Hit probability is calculated in the attended annulus. A and B show the hit probability for the stimulus configuration with interior random broadband flicker (IR), C and D show the hit probability for the stimulus configuration with exterior random broadband flicker (ER). When attending to an annulus with flicker (A and C), hit probability decreases as the flicker frequency increases. When attending to an annulus with random broadband flicker (B and D), the hit probability is uncorrelated with the frequency of the unattended flicker stimulus.

Power spectra at one channel (#85 of the Geodesic Sensor Net, Electrical Geodesics, Inc.) of one observer (S13) for the stimulus configuration with interior random broadband flicker (rbbf). The head in the top-middle indicates the channel located over the right occipital cortex. Inserted icons indicate experiment conditions: ‘A’ indicates the attended annulus and ‘f ’ indicates the annulus superimposed with flicker. The left and the right columns show power spectra for attention to the annulus with flicker (attF) and attention to the annulus with rbbf (attR) conditions respectively. Flicker frequency f increases from top (2.5Hz) to bottom (10.9Hz). The arrows indicate the flicker frequency in each spectrum in which the flicker-frequency peak could be distinguished from surrounding noise.

Power spectra at one channel (#85) of one observer (S13) for the stimulus configuration with exterior random broadband flicker (rbbf). Conventions are same as in Fig.3.

Attention-dependent SSVEP power and SSVEP power difference averaged over 10 observers for flicker frequencies f ranging from 2.5Hz to 20Hz. In the inserted icons, ‘f ’ indicates the annulus superimposed with flicker, and ‘R’ indicates the annulus superimposed with random broadband flicker (rbbf). The left column shows the results for the interior rbbf (IR) stimulus configuration; the right column shows the results for the exterior rbbf (ER) stimulus configuration. The head indicates that channel 85 located over right occipital cortex. (A) SSVEP power versus flicker frequency f for attF (red solid curves) and attR (blue solid curves). The noise is estimated in a narrow frequency band (+/−1 Hz) centered at the flicker frequency and is plotted as a dashed line for attF (red) and attR (blue). (B) SSVEP power difference (attF minus attR) versus flicker frequency. Flicker frequencies with significant SSVEP power difference are indicated by large filled circles. (C) SSVEP power difference averaged over 10 observers is plotted for all channels and all flicker frequencies. X axis represents channels’ positions, and Y axis represents flicker frequency. Channels’ positions are organized according to channel groups: LPF – Left Prefrontal; RPF – Right Prefrontal; LFR – Left Frontal; RFR – Right Frontal; LCE – Left Central; RCE – Right Central; MID – Midline, order from the frontal to occipital pole; LPR – Left Parietal; RPR – Right Parietal; LTM – Left Temporal; RTM – Right Temporal; LOC – Left Occipital; ROC – Right Occipital. Only statistically significant differences are plotted, others are masked (black).

Attention-dependent phase histograms and phase-locking index (PLI) differences. (A) and (B): Phase histograms of single-cycle Fourier coefficients and corresponding power spectra at one channel (#85) for one observer (S13). Inserted icons indicate experiment conditions: ‘A’ indicates the attended annulus and ‘f ’ indicates the annulus superimposed with flicker. When the flicker frequency was at 2.5 Hz (left column), the phase histograms are more uniformly distributed during attR (B) than during attF (A). At 9.2Hz (right column), there are no significant difference in phase histograms between attF and attR. Below each phase histogram, the corresponding power spectrum is also shown. When the phase histograms are uniformly distributed (left column in B), no peak at the flicker frequency is observed. (C) Phase-locking index (PLI) difference (attF minus attR). PLIs were averaged across 10 observers, only statistically significant PLI differences are shown. Conventions are same as in Fig.5.

Attention-dependent topographic distributions of SSVEP power and SSVEP power differences. Inserted icons indicate experiment conditions: ‘A’ indicates the attended annulus and ‘f ’ indicates the annulus superimposed with flicker. Topographic distributions of SSVEP power are shown for interior (A) and exterior (B) rbbf (random broadband flicker) configurations at flicker frequency f = 2.5 Hz (left column in A and B) and f = 9.2 Hz (right column in A and B). The filled green circles indicate channels at which the signal-noise ratio (the ratio of SSVEP power to noise power) was greater than 3. (C, D) Topographic distributions of SSVEP power difference of attF minus attR (first minus second row). The channels with significant attention modulation are marked: filled gray circles indicate significant attention differences in both SSVEP power and PLI, magenta circles indicate only significant SSVEP power differences.

Attention-dependent SSVEP power differences in alpha band for one observer (S13) for 6 positions of the random broadband flicker (rbbf) annulus. Inserted icons indicate different stimulus configurations; ‘f ’ indicates the annulus superimposed with flicker, and ‘R’ indicates the annulus superimposed with rbbf, random broadband flicker. For each stimulus configuration, significant SSVEP power differences (attF minus attR) are plotted for flicker frequencies in the alpha band (8.6, 9.2, 10 and 10.9 Hz). X axis represents channels’ positions, and Y axis represents flicker frequency. Conventions are same as in Fig.5.

Uncorrelated power spectrum and power difference. The uncorrelated power spectrum is calculated by setting the power at flicker frequency and its harmonic frequencies to zero and summing in each one Hz band. Inserted icons indicate experiment conditions; ‘A’ indicates the attended annulus. (A) Uncorrelated power spectrum averaged over all flicker frequencies for each experiment condition, and averaged across all 10 observers for one channel (#68) is plotted for four experimental conditions: attF(IR)(green line), attR(IR)(blue line), attF(ER)(black line) and attR(ER)(red line). The head indicates that Channel 68 is located in the middle parietal. (B, C) Uncorrelated power difference (attF minus attR) for interior rbbf (B) and exterior rbbf (C) configurations averaged across 10 observers. X axis represents channels’ positions, and Y axis represents frequency. Conventions are same as in Fig.5.