Fig. 4.

A representation of the upper visual quadrant directly adjacent to V2. Topographic and architectural transitions at the boundaries of DM in one marmoset. (A, B). Data obtained from two different parasagittal levels across the dorsal extrastriate cortex (level A is more medial than B). These levels are indicated by the colored arrows in the inserts, which illustrate bidimensional reconstructions of the “third tier” densely myelinated field (DM) and immediately adjacent areas. The following conventions apply to both (A) and (B). Top left panel: Myelin-stained parasagittal section illustrating myeloarchitectural transitions near the site of the penetrations. These sections are within 320 µm of the nearest Nissl-stained section containing electrode tracks; they were chosen for the purpose of illustration as they demonstrate the myeloarchitectural patterns without much interference from artifacts due to the electrode tracks, while still allowing an accurate plotting of the nearby recording sites. The border between V2 and DM is clearly defined by an increase in myelination (indicated by the left black bar above the cortex). The rostral border of DM (right black bar) is subtler, as the adjoining fields are also rich in myelin. As detailed elsewhere (Rosa & Schmid, 1995), the primary criterion for defining the rostral border of DM in myelin stain is an increase in the separation between the inner and outer bands of Baillarger. The V1/V2 border is indicated by a black arrow and the zones of uncertainty between other areas are indicated by the black bars above the cortex. Top right: The same section as in (A), with overlaid locations of recording sites from nearby sections. Recording sites deemed to belong to V1 or V2 are indicated by white circles, those deemed to belong to area DM in black circles, and those deemed to be rostral to DM in white squares. Bottom: The receptive fields corresponding to the recording sites; note that these data (from Rosa et al., 2005) have been re-plotted in the appropriate orientation to follow the same convention used in other figures of this manuscript. The main trends in receptive field topography are indicated by colored arrows (green, V2; red, DM; purple, rostral to DM). In both (A) and (B), recording sites crossing V2 from caudal to rostral result in receptive fields that move away from the vertical meridian and toward the horizontal meridian (left receptive field map). At the border of the densely myelinated zone the receptive fields become larger and move into the upper visual field, approaching higher elevations near the vertical meridian as more rostral sites are sampled (middle receptive field map). At the rostral border of DM, the receptive fields move back away from the vertical meridian and toward the central visual field (right receptive field map). While different observers may place the borders at slightly different points, these trends remain robust indicators of the limits of area DM. Scale bars = 1 mm.