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Electronic Letters to:
- BehavioralSystemsCognitive:
Jonas Larsson and David J. Heeger- Two Retinotopic Visual Areas in Human Lateral Occipital Cortex
J. Neurosci. 2006; 26: 13128-13142[Abstract] [Full text] [PDF]
eLetters: Submit a response to this article
Electronic letters published:
The concept of homology applied to extrastriate cortical areas
- Marcello G Rosa (24 January 2007)
The concept of homology applied to extrastriate cortical areas 24 January 2007 
Marcello G Rosa,
Professor
Monash University, Melbourne, VIC 3800, AustraliaSend letter to journal:
Re: The concept of homology applied to extrastriate cortical areas
Marcello.Rosa{at}med.monash.edu.au Marcello G Rosa
One of the key findings of the paper by Larsson and Heeger is that the human lateral occipital cortex contains representation(s) of the central portion of the upper visual field in the dorsolateral cortex caudal to MT, and rostral to "dorsal V3". These representations would seem to occupy part of the region that corresponds to dorsal (lower quadrant) V4 in the macaque. Based on the fact that the visuotopic organization of this part of the brain appears to be different in humans and monkeys, the authors proceed to suggest the existence of two new visual areas of the human cortex (LO1 and LO2), which would have no simian homologues. This conclusion is unwarranted, as it reflects an erroneous expectation that homologue structures will look identical in different species.
Homologous structures in nature can, and often are, quite different. The concept of homology is based not on morphology or function, but rather on a common evolutionary origin (i.e., whether or not the structure in question was present in a common ancestor). This common origin is, in turn, often (if not always) reflected in a common embryological origin.
Larsson and Heeger report that the lateral occipital areas LO1 and LO2 show marked interindividual variability, with fewer than half of the examined hemispheres (14/ 30) showing a complete representation of polar angles in the upper quadrant, and a substantial proportion of the sample (7/ 30) showing no clear topographic order assessable via fMRI. This high degree of variability (which may also explain conflicting reports from other laboratories) is compatible with a developmental model based on activity-dependent formation of topographic maps, which would translate into multistable solutions for adult visual topography. We have proposed one such model, which specifically predicts a high degree of variability in topographic organization of the region of "fourth visual complex" as a result of its markedly delayed maturation relative to areas such as V1 and MT (Rosa and Tweedale, 2005). This variability is expected not only across primate species, but also between individuals of the same species. Critically, the final configuration of the maps is linked not only to phylogeny, but is the result of an epigenetic process that is strongly influenced by overall brain size (hence, length of corticogenesis). For example, New World and Old World monkeys of similar size have similar visual topographies in this region. Given that a pool of cells with similar embryological identity may form different maps in different individuals and/ or species, visuotopy alone constitutes a weak criterion for establishing homologies in high-order (or late-maturing) visual areas.
Whether the observed variability ultimately proves to be the result of variable visual topographies within areas, or variable topological relationships between areas, the rich data set and excellent analysis provided by Larsson and Heeger will provide a strong basis for further exploration of this question.
Reference: Rosa MGP, Tweedale R (2005) Brain maps, great and small: lessons from comparative studies of primate visual cortical organization. Philos Trans R Soc Lond B Biol Sci. 360: 665-691.
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