Why is sulcal labeling difficult?

This is a follow-up to an earlier post Why we label sulci. There will be two or three more posts; taken altogether, they will describe the sulcal labeling problem.

The labeling of sulci is a challenging problem. This is because, cortical sulci are highly variable. Sulci vary not just across individuals but even between the hemispheres of a single brain [1]. It might be useful when looking for ways to address this variability to classify this variation as follows:

Variation in physical features
Sulci vary in shape, in scale and in their placement (i.e. position and orientation) The figure below illustrates how the variability can make feature selection difficult.

The boxplot shows the length distribution for 18 subjects. The 10 types or classes of sulci shown cannot be identified solely on a length measurement. This poses a problem for feature selection and classification.
Figure credit: Meena Mani




Variation in branching
19th century illustrations such as those from Horsley [2], trace the wide variations along a sulcal fold. A whole nomenclature has developed since then to account for the branch variations possible along a single sulcus. (An example from the Ono atlas is illustrative--see figure below). For this reason, there is no gold standard in sulcal labeling; one neuroanatomist may disagree with another.

The figure to the left shows the pattern variations for a single sulcus (the posterior end of the superior frontal sulcus). Types A, B, C, D, are possible variations for this sulcus (for the 25 postmortem brains examined, 4 variations were found). The pattern in the two hemispheres of a single subject may differ; the left may be Type B and the right may be Type C. The lengths of the small segments and the connections they make to other sulci may also vary. Reproduced from Ono et al. [1].

Variation in number
Sulci may be continuous (present as one uninterrupted segment) in some individuals, fragmented (exist as multiple segments) in others and altogether absent in yet others. The larger primary sulci which start forming early in fetal development are the most consistent; the secondary and tertiary sulci are not always expressed.

References
1) Ono, M., Kubic, S. & Abernathy, C. (1892) "Atlas of the Cerebral Sulci", (Thieme, New York).
2) Horsley, V. (1892) "On the topographical relations of the cranium and surface of the cerebrum", In "Contribution to the surface anatomy of the cerebral hemispheres", pp.306-355, (Royal Irish Academy).


Posts on Sulcal Labeling
1) Why we label sulci
2) Why is sulcal labeling difficult ?
3) The use of a spatial distribution model in labeling sulci