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Structure

The lunate sulcus is reliably present in the brains of great apes, where it forms the anterolateral boundary of the he primary visual cortex (V1). In humans, its presence is much more variable, and even when present, it does not correspond to a functional region- the lunate sulcus has been found to be non-homologous between apes and humans.

The location of the lunate sulcus in Homo sapiens can be understood in relation to the striate cortex. In human brains, the lunate sulcus can be found at the extent of the visual cortex (V1) at the calcarine sulcus- located on the medial surface of the hemispheres.

Some debate exists on the presence of the lunate sulcus in all modern day humans. Early research like the Ono et al.'s “Atlas of the Human Sulci” study reports that the lunate sulcus is present in 60% of right and 64% of left hemispheres in their sample of 25 individuals.

Alternatively, Allen et al. argue that “true” lunate sulci are much rarer in humans than in our hominoid counterparts. The study done by Allen et al. used 110 adult subjects, and a high resonance MRI machine to locate and identify the supposed lunate sulcus in modern day humans. Allen’s team found discontinuous, sulci segments that non contiguously form a possible “composite” lunate sulcus in a majority of their subjects. Among the 220 hemisphere samples, 3 (or 1.4% of the sample size) were found to have a more posteriorly placed, but continuous lunate sulcus that could be relatively comparable to that of our hominoid counterparts.

    An alternative to brain mapping via a high resonance MRI is the use of a stereoplotter, a way in which the lunate sulcus’ location can be pinpointed more accurately. A stereoplotter helps to translate the stable locations or landmarks of gyri and sulci to functional coordinates. The Oyen and Walker (1977) Stereoplotter was used to study the dorsal surface of 92 hominoid endocasts in a study done by Holloway. Utilization of the stereoplotter has been useful for quantifying the radial distances from the cerebral surface to a homologous point located within a once-living brain or found on an endocast (see Fig. 1). His findings would indicate that prior reconstructions of lunate sulcus’ morphology and location are inconsistent. Furthermore, the lunate sulcus’ position is not typically pongid. Holloway’s findings allow for a new line of thought on the location of the lunate sulcus; he hypothesizes that the lunate sulcus is more caudal or rostral than we previously thought. It is difficult to definitively prove where the lunate sulcus is located, but is easier to demonstrate where it is not.

If we use the Australopithecine brain as an example of an evolutionary intermediary between pongids and Homo, then we could recognize a significantly advanced intermediate brain. This statistic can be measured through the encephalization quotient where Australopithecines have approximately 60% of the human value, rather than the 40% for Pan troglodytes. For this reason, the position of the lunate sulcus can be used as a marker for the cognitive development of extinct species within the hominid lineage.

The hominoid visual brain structure volumes (in a broad sense) varies more than previously thought. Humans have relatively reduced primary visual cortex and lateral geniculate nucleus volumes compared to the allometric predictions we have created from other hominoids. These findings relevant to the human primary visual cortex may influence our understanding of the evolution and structure of the lunate sulcus, and its debated presence in humans today.