The middle occipital gyrus is the largest gyrus in the occipital lobe whose anterior boundary is the anterior boundary of the occipital lobe. The gyrus can be described as having two segments, an anterior and a posterior segment. The anterior segment is bound by the intraparietal sulcus superiorly and the inferior occipital sulcus inferiorly. The posterior segment’s superior boundary is defined by the intra-parietal sulcus, and its inferior boundary is defined by the location of the inferior occipital sulcus as found in the anterior segment of the gyrus. (Fig. 1)
Delineation of the middle occipital gyrus is done in the coronal view. Begin masking posteriorly, where the intra-occipital sulcus and the inferior occipital sulcus first come into view. Follow the intra-occipital sulcus to its internal end and draw a straight line out to the interhemispheric fissure. Secondly, follow the inferior occipital sulcus to its internal end and draw a straight line out to the interhemispheric fissure. Mask everything within these boundaries. Continue this step to the posterior end of the brain. (Fig. 2, Fig. 3)
Locate the first slice masked, and continue step 2 moving anteriorly. If the inferior occipital sulcus becomes unclear, reference the last slice where the sulcus was clear and approximate its location on the following slices.
Moving anteriorly, the intra-occipital sulcus becomes the intraparietal sulcus. Mask an angled line from the intraparietal sulcus down to the inferior occipital sulcus, and fill in everything within these boundaries.
Once the paracalcarine sulcus comes into view, mask from the internal endpoint of the intraparietal sulcus to the inferior occipital sulcus (or its approximated point) and fill in everything within these boundaries (Fig. 4). The paracalcarine sulcus can be found best on a medial view of the 3D object, or on a medial sagittal slice, and cuts the occipital lobe in half (Fig. 5).
Continue the previous step until the occipital notch appears at the inferior end of the brain. To locate the occipital notch, double click on the notch on the 3D object and locate the corresponding slice in the coronal view. (Fig. 6)