For each event-related scan, the time course of the MR signal intensity was first extracted by averaging the data from all the voxels within the predefined ROI. The average event-related time course was

then calculated for each type of trial, by selectively averaging to stimulus onset and using the average signal intensity during the fixation trials as a baseline to calculate percent signal change. Specifically, in each scan we averaged the signal intensity across the trials for each type of trial at each of 12 corresponding time points (s) starting from the stimulus onset. These event-related time courses of the signal intensities were then converted to time courses of percent signal change for each type of trials by subtracting the corresponding value for the fixation Alectinib chemical structure trials and then dividing by that value. Because M-sequences have the advantage that each type of trials was preceded and followed equally often by all types of trials, the overlapping BOLD responses due to the short interstimulus interval

are removed by this averaging procedure (Buracas and Boynton, 2002). The resulting time course for each type of trials was then averaged across scans for each subject and then across subjects. The V1 model with its original model parameters as in Li, 1999 and Li, 2002 was used to simulate V1 responses to the texture image of low luminance bars with a foreground region (i.e., 2 × 2 bars) like in our experiments. 3 Methyladenine The model mechanisms include (1), direct inputs to V1 neurons from each bar according to the classical receptive fields, and (2), interactions between V1 neurons by the intracortical connections implementing contextual influences (such as surround suppression) of the surround to the neural responses. At each grid location, the maximum response from all pyramidal model neurons was obtained. This maximum was averaged over all simulation

time steps within 50 ms (simulated by five membrane time constant of the model neurons). The saliency of each grid location is the Z-score of this maximum obtained as follows: take the difference between this maximum and the average of the maximums over all grid locations and then divide it by the standard deviation of all the maximums however ( Li, 1999). Saliency in the foreground region is the maximum of the Z-scores over the 4 × 4 bar region centered on the foreground region. The result for each orientation contrast (7.5°, 15°, 30°, and 90°) as plotted in Figure 2 was obtained by averaging the foreground region saliency from 24 simulations for 24 different background bar orientations evenly distributed between 0° and 180°. The saliency of the foreground region should be directly related to the strength of its attentional attraction (i.e., its cueing effect). We are grateful to Peter Dayan for reading the manuscript with helpful comments and Yan Song for help with dipole source localization.