Dorsomedial area

The dorsomedial area, also known as DM or V6, is a subdivision of the visual cortex of primates first described by John Allman and Jon Kaas in 1975. DM is located in the dorsal part of the extrastriate cortex, near the deep groove through the centre of the brain (interhemispheric fissure), and typically also includes portions of the medial cortex, such as the parietoöccipital fissure. DM contains a topographically organized representation of the entire field of vision. Like area V5/MT (middle temporal area), DM receives direct connections from the primary visual cortex. Also similar to MT, DM is also characterized by high myelin content, a characteristic that is usually present in brain structures involved in fast transmission of information (Rosa et al. 2005).

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For many years, it was considered that DM only existed in New World monkeys. However, more recent research has suggested that DM also exists in Old World monkeys and perhaps humans. Another name for DM is "visual area 6" (V6). DM/ V6 is also sometimes referred to as the parietooccipital area (PO), although the correspondence is not exact (Galletti et al. 2005).

Properties

Neurons in area DM/V6 have unique response properties, including an extremely sharp selectivity for the orientation of visual contours, and preference for long, uninterrupted lines covering large parts of the visual field (Baker and collaborators, 1981; Lui and collaborators, 2006). However, in comparison with area MT, a much smaller proportion of DM cells shows selectivity for the direction of motion of visual patterns. Another notable difference is that cells in DM are attuned to low spatial frequency components of an image, and respond poorly to the motion of textured patterns such as a field of random dots. In contrast, cells in MT are often strongly responsive to such stimuli. These response properties suggest that DM and MT may work in parallel, with the former analyzing self-motion relative to the environment, and the latter analyzing the motion of individual objects relative to the background.

Pathways

The connections and response properties of cells in DM/ V6 suggest that this area is a key node in a sub-set of the "dorsal stream", referred to by some as the "dorsomedial pathway". This pathway is likely to be important for the control of skeletomotor activity, including postural reactions and reaching movements towards objects (Galletti and collaborators, 2003). The main "feedforward" connection of DM is to the cortex immediately rostral to it, in the interface between the occipital and parietal lobes (V6A). This region has, in turn, relatively direct connections with the regions of the frontal lobe that control arm movements, including the premotor cortex.

References

• Allman JM, Kaas JH (1975) The dorsomedial cortical visual area: a third tier area in the occipital lobe of the owl monkey (Aotus trivirgatus). Brain Res 100(3):473-487.
• Baker JF et al. (1981) Visual response properties of neurons in four extrastriate visual areas of the owl monkey (Aotus trivirgatus): a quantitative comparison of medial, dorsomedial, dorsolateral, and middle temporal areas. J Neurophysiol 45:397-416.
• Galletti C et al. (2003) Role of the medial parieto-occipital cortex in the control of reaching and grasping movements. Exp Brain Res 153:158-170.
• Galletti C et al. (2005) The relationship between V6 and PO in macaque extrastriate cortex. Eur J Neurosci 21:959-970.
• Lui LL et al. (2006) Functional response properties of neurons in the dorsomedial visual area of New World monkeys (Callithrix jacchus). Cereb Cortex 16(2):162-177.
• Rosa MG et al. (2005) Resolving the organization of the New World monkey third visual complex: the dorsal extrastriate cortex of the marmoset (Callithrix jacchus). J Comp Neurol 483:164-191.