Background:
The angular gyrus (AG) is an association area of the human cerebral cortex that plays a role in several processes, including auditory function. However, the precise anatomical location of the AG is not entirely clear. There are two common approaches for locating the AG based on gyral and sulcal landmarks: the ‘parallel’ and ‘count-back’ methods (as termed by the present authors). These two techniques do not always point to the same location on the cortex, thus making the macroanatomical locus of the AG rather ambiguous.

Material:
Twenty high-resolution brain MRIs of normal, right-handed human subjects chosen from an online database (OASIS).

Methods:
MRIs were sequentially chosen from OASIS and analyzed in MRIcron using two different visualization techniques: 1) skull-stripped surface renderings, and 2) serial sagittal slices. The AG was demarcated in the left and right hemisphere of each brain, as defined by the parallel and count-back methods. The reliability of each method for locating the AG was systematically assessed using both descriptive and inferential statistics, based on measures of hemispheric laterality

Results and discussion:
Examination of both methods for locating the AG showed poorer reliability in the left hemisphere compared to the right for both surface and more medial sites. Several anatomical factors were identified that compromised the reliability of the two methods.

Conclusions:
Our finding of poor reliability between the parallel and count-back methods suggests that the AG is sometimes difficult to identify, particularly in the left hemisphere. This places the traditional gross anatomical methods for locating the AG in question. Development of new techniques to define this area of human neuroanatomy is needed.