Lunate sulcus

File:Lunate_sulcus.png|alt=|thumb|324x324px|Comparison of the Affenspalte homologue, renamed the lunate sulcus by Sir Grafton Elliot Smith, in the primary visual cortex of chimpanzee versus human brains. Based on Smith’s observations from studying over 400 Egyptian human and ape brains, he believed that the sulcal patterns between humans and apes were similar. However, some neuroanatomists today disagree with Smith’s assertion that a lunate sulcus exists in humans, arguing that there is only an Affenspalte which is unique to apes. Specifically, in a high-resolution MRI study conducted by Allen et al. (2006), the researchers scanned and analyzed 220 human brains and found no sign of the lunate sulcus homologue. Based on this finding, they suggested that the claim asserting humans have a lunate sulcus homologue fails to account for and show appreciation of the extensive evolutionary reorganization of the visual cortex in humans. == Evolution ==

Analyzing variability in the location of gross anatomical landmarks such as sulci is an accepted method for studying evolutionary hominin brain reorganization. The position of the lunate sulcus in the occipital lobe has been studied in humans, early hominin endocasts, apes, and monkeys by researchers seeking to make inferences about the morphological evolution of brain regions associated with human visual versus cognitive behaviors. However, some scientists remain skeptical about whether the lunate sulcus is a valid and reliable indicator for studying volumetric changes in the V1 due to the inconsistencies of the sulcus’s presence and lack of histological correspondence with cytoarchitectonic boundaries in hominoids. Despite this, previous allometry studies have suggested that the lunate sulcus shifts from a lateral-anterior to a medial-posterior position as brain size increases. Such shifts have been credited with predicting whether the lunate sulcus will occur or not based on an increase or reduction in V1 volume, thus providing an explanation for inconsistencies in its presence and position in the occipital lobes. Moreover, a study conducted by de Sousa et al. (2010) compared the volumes of the V1 relative to the position of the lunate sulcus in three-dimensional reconstructed non-human hominoid brains to determine if an allometric relationship existed between V1 volume and lunate sulcus position. The researchers found that the position of the lunate sulcus does accurately predict V1 volume in apes, and that V1 volume in humans is smaller than would be expected based on our large brain size. Furthermore, other research suggests a more posteriorly positioned lunate sulcus from the early hominin fossil record. Based on all these findings, de Sousa et al. (2010) concluded V1 reduction began during early hominin evolution, given the more lateral-anterior position of the lunate sulcus in human and other primate brains today. ==References==