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Figure 1 (A) Activation pattern during S1 pacing in isolated canine right atrial endocardial tissue (tissue no. 10). The S1 pacing was applied at the left edge of the representative tissue (B). (A) Color-coded isochronal activation map during S1 pacing, demonstrating the nonuniformity of the conduction velocity, which, along the central line during S1 pacing, is 122 cm/s. The site for S2 stimulation is marked by a black square. (B) Gross endocardial structures, including part of the crista terminalis (CT) and pectinate muscle bundles in the atrial tissue. These pectinate muscle bundles were either tightly attached to the atrial free wall ("ridge-like" structure, white arrows) or discontinuous with the atrial free wall ("bridge-like" structure, red probe). An asterisk marked the site for S1 pacing. To better correlate the endocardial activation patterns with the underlying anatomic structures, the endocardial tissue was displayed by a mirror image. The white outline in part B indicates the location of the recording plaque.