Cortical Connections of Area V2

The organization of corticocortical connections between areas V1 and V2 in human visual cortex has been studied using the neuronal tracer DiI (1,10-dioctadecyl-3,3,30,30-tetramethylindocarbocyanine perchlorate) in aldehyde-fixed post mortem brain tissue. DiI injections

Figure 6 Surface rendering of the two hemispheres from 10 brains illustrating the variability in the locations of areas 17 and 18. From Amunts et al. (2000).

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Figure 6 Surface rendering of the two hemispheres from 10 brains illustrating the variability in the locations of areas 17 and 18. From Amunts et al. (2000).

were placed either in area V1, identified by the presence of the stria of Gennari and cytochrome oxidase blobs, or in area V2, identified by the presence of cytochrome oxidase stripes. Following V1 injections, labeled fibers enter the white matter and then ascend into the gray matter of area V2. These fibers then either terminate in layers III and IV or travel horizontally through the gray matter to terminate a short distance away in V2 (Fig. 7A). Injections of DiI into V2 led to a different pattern of fiber projections. Rather than coursing through the white matter underlying V1, fibers coursed through the gray matter to terminate in layers I, II, III,

IVB, V, and VI and avoided terminating in layers IVA and IVC (Fig. 7B). The projections to layers IVB and V were highly clustered, forming 0.3-mm-wide clusters with a spacing of 0.6-1 mm. Some of the V2 injections failed to produce labeling of layer IVB. Presumably these V2 injections failed to hit the V2 thick stripes so the projections to layer IVB would not have been expected (Fig. 7C).

DiI injections into area V2 also produce clustered intrinsic projections. Figure 7D illustrates a transverse section throughV2 with an injection site in the superficial layers and clustered projections extending through layers II-V. The spacing of these intrinsic projections is approximately 1 mm, which is smaller than the distance between functional modules identified through cytochrome oxidase histochemistry (see later discussion).

The development of these feedforward and feedback connections between areas V1 and V2 has been studied using the DiI technique in post mortem brains ranging in age from 37 weeks of gestation to 4 postnatal months. At 37 weeks of gestation, injections to area V2 led to the labeling of fibers that originated only from deep cortical layers of V2 and ran through and perhaps terminated in layers V and VI of area V1. At 9 postnatal days, injections of V1 label fibers that are restricted to the deep layers of V2. No fibers were seen to terminate in superficial layers, but a few fibers ended in growth cones in layer IV. Injections of V2 produced both retrogradely labeled cells and anterogradely labeled fibers. Retrogradely labeled cells were observed in layers IVB and VI but not in supragranular layers II and III. Thus, feedforward projections first mature from layers IVB and VI, and the supragranular

Figure 7 Extrinsic and intrinsic connections of area V2. (A). Transverse section through V1 and V2 showing clustered terminations in layers III and IV of V2 following an injection of Dil into V1. (B) Projections from V2 to V1. Dil injection was made into V2 (right of arrowhead), and labeled fibers and terminals are observed in layers I, II, III, IVB, and V ofV1. (C) Projections from V2 to V1 that avoid layer IVB. DiI injection in V2 is located at the right of the arrowhead. Fibers are seen coursing from the injection site to terminate in layers I, II, III, and V of V1. (D) Intrinsic connections of V2. Transverse section showing projections following DiI injection into superficial layers of V2. Clustered projections extend from the injection site to terminate in layers II-V. Bar=1 mm. From Burkhalter and Bernardo (1989).

Figure 7 Extrinsic and intrinsic connections of area V2. (A). Transverse section through V1 and V2 showing clustered terminations in layers III and IV of V2 following an injection of Dil into V1. (B) Projections from V2 to V1. Dil injection was made into V2 (right of arrowhead), and labeled fibers and terminals are observed in layers I, II, III, IVB, and V ofV1. (C) Projections from V2 to V1 that avoid layer IVB. DiI injection in V2 is located at the right of the arrowhead. Fibers are seen coursing from the injection site to terminate in layers I, II, III, and V of V1. (D) Intrinsic connections of V2. Transverse section showing projections following DiI injection into superficial layers of V2. Clustered projections extend from the injection site to terminate in layers II-V. Bar=1 mm. From Burkhalter and Bernardo (1989).

projections, characteristic of adults, are not yet formed by 9 postnatal days. At 9 postnatal days, feedback projections from V2 to V1 are characterized by horizontally oriented fibers in deep layers V and VI, which contain vertical branches that reach into layer IVB. Some feedback fibers were observed in supra-granular layers, but these were usually obliquely oriented fibers that ramify in layer I.

At 7 postnatal weeks, feedforward projections from V1 to V2 begin to show the adult pattern of labeling. That is, labeled fibers enter V2 not only from the deep cortical layers but also from the superficial layers and begin to terminate in layers III and IV. In contrast, the feedback projections from V2 to V1 remain largely immature at 7 postnatal weeks. Feedback fibers terminate in layers I, IVB, V, and VI, just as was observed at 9 postnatal days. Fibers were still not observed in layers II and III, as in the adult. Nevertheless, there was some development of these feedback pathways in that more fibers were observed coursing through layers II and III to terminate in layer I. In addition, fibers in layer IVB were often oriented horizontally and some innervation of layer IVA was observed.

At 4 postnatal months, the projection from V1 to layer IV of area V2 became denser. Feedforward projections from V1 to V2 begin to take on the adult appearance at 4 postnatal months. At this time, injections of V2 produced retrograde labeling of neurons in layers II, III, IVB, V, and VI. In contrast, feedback projections from V2 to V1 are still immature at this time. Fibers were seen to terminate densely in layers I, IVB, V, and VI, but the projections to layers II and III remain weak at this time. Thus, at 4 postnatal months, the feedforward projections from V1 to V2 appear more mature than the feedback projections from V2 to V1. At 2 years of age, feedforward projections from V1 terminate in layers III and IV of V2. This projection is similar to that observed at 4 postnatal months but is more dense. Similarly, the feedback projection from V2 terminated densely in layers II, III, IVB, and V of area V1 and more weakly in layers I and VI. Similar to adults, this feedback projection was highly clustered with ~0.3-mm-wide clusters separated by ~0.3-mm-wide gaps.

The development of long-range local connections within areas V1 and V2 has been studied using the DiI technique in fixed prenatal and postnatal post mortem brains. At 37 weeks of gestation, injections of V2 give rise to labeled fibers in layers II, III, and V that extend horizontally for several millimeters. By 9 postnatal days, these fibers become denser and tend to form irregular clusters close to the injection site. In contrast, at 4 postnatal months, local connections within V1 are largely immature. Injections of V1 give rise to horizontally oriented fibers within layers IVB, V, and VI, whereas the labeling of layers I, II, and III is sparse. Thus, although V1 layer II-III neurons begin to project to area V2 by 4 postnatal months, they have yet to form the adult pattern of local horizontal connections within V1.

The intracortical connections of V2 with a more rostral region of human extrastriate cortex have been studied in post mortem brain tissue following a naturally occurring infarct in area 19. A ~ 1-cm-wide infarct was found in the superior lateral extrastriate cortex of a 92-year-old female who died of natural causes. The brain was recovered shortly after death and was later processed for degenerating axons. On the basis of previous studies of the interhemispheric connections of extrastriate cortex in humans, the infarcted region is located anterior to area V3A in a region that perhaps corresponds to area V6 of macaque monkeys. Dense regions of degenerating axons were found within 2-5 mm of the infarct and were distributed to more distant cortical regions, including areas V1, V2, V3, VP, V3A, and V4. Dense projections were seen in superior portions of V1, V2 and V4 and in areas V3 and V3A, whereas weaker projections were seen in inferior portions of V1, V2, V4, and in area VP. These results suggest that the infarct affected cortex that represented the inferior visual field, yet appeared to have extended into cortex that contained a representation of the superior visual field. In V2, the degenerating fibers were localized primarily in the infragranular layers, but, in its densest region, extended into the middle cortical layers. This pattern of labeling is suggestive of a feedback projection from the infarct to area V2. A similar laminar pattern of labeling was also observed in V1 and in extrastriate areas V3, VP, V4, V3A, and V5.

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