Discussion projecting bands roughly interdigitate with 21a projecting

T .H. Stewart et al. Brain Research 881 2000 47 –56 53 Fig. 6. An example of cell charts used to measure mediolateral density and width of the area 19 efferent bands. A A cell chart of the retrograde labeling in area 19 after injections in area 21a. Notice the branching of some of the bands small arrows. B The same cell chart overlaid with a 2D Gaussian kernel. Three transects along the mediolateral axis of a band are illustrated 1, 2 and 3. Three transects along the width of the bands are illustrated A, B and C. C The profile plots generated by transects 1, 2 and 3 in Fig. 6B. Note the uneven density along the transect. This was a common feature found in all cases. D The profile plots generated by transects A, B and C in Fig. 6B. The density of labeled cells along the transect peaks in a smooth Gaussian fashion. The consistency of density along the anterior posterior axis was found in all cases. A, anterior; L, lateral. Scale bar52 mm.

4. Discussion projecting bands roughly interdigitate with 21a projecting

bands [47]. 4.1. Tangential organization The 21a projecting band pattern in area 19 is more complex than the patchy organization of efferent and Efferent neurons in area 19 that project to area 21a are callosal neurons found in areas 17 and 18 organized in a pattern of irregularly shaped, mediolaterally [6,8,18,20,43,45,49]. One possible reason for the irregular elongated bands Figs. 3, 4, 5A, 6A. This pattern of structure of the bands may be nonuniform uptake of mediolaterally elongated bands may be a general organiza- retrograde tracer by cells in the injection site. This seems tional feature of area 19, since we previously reported that unlikely as the patchy pattern of staining in area 17 is LS projecting neurons in area 19 are also arranged in consistent between different animals and in cases where mediolaterally elongated bands [6]. Preliminary results different tracers are used. Also, the patches of staining in from double label studies indicate that within area 19, LS area 17 are consistent with other studies using retrograde 54 T tracer [8], indicating that the large aggregate injections [13] reports similar patches in layers 2 3, while our study used in this study are uniform and thus differences in found them through all layers except for layer 1. It is labeling density in area 19 are the result organizational possible that Dreher et al. missed labeling in layers 4 to 6 pattern of 21a inputs and not uneven distribution of tracer since labeled cells in these layers are more difficult to at the injection site. detect as they are sparser than labeled cells in layers 2 3. Another potential reason for the irregular structure of the Symonds and Rosenquist [50] also found patchy 21a bands may be retinotopy. This also seems unlikely as both efferent neurons in area 19 but they report heavier labeling 18 and 19 have similar retinotopy and magnification in layers 5 and 6. In the present study, the majority of factors [54], yet they demonstrate very different ana- labeled cells are found in layers 2 3 and 5 with less tomical structures i.e. patches versus bands. The com- labeling in layers 4 and 6. Difference in findings could be plexity of the bands may be linked to the thalamocortical due to the use of different tracers; Symonds and Rosen- connections in area 19. Area 19 receives an input primarily quist use HRP and 3H-leucine while WGA–HRP is used from the W-cells of the LGN while areas 17 and 18 receive for the coronal case in this study. These tracers rely upon thalamocortical input from X-, Y- and W-cells [12,48]. different uptake and transport mechanisms [57] and thus Variations in the type i.e. X, Y and or W of input may have resulted in variations in labeling. This difference between these areas may contribute to the differences in in laminar labeling may also be related to the smaller efferent organization. Furthermore, W- and Y-cell thalamic volume of tracer used by Symonds and Rosenquist com- inputs terminate in the CO blob columns in area 17 [5]. pared to this study. Since CO blob columns also contain 21a efferent neurons The relative hierarchy of cortical areas can be de- it would be worthwhile to determine if area 19 also termined by the laminar location of efferent cells and or receives direct W-cell thalamic input which is restricted to their axon terminals. For example, efferent cells found the efferent bands. Projections to area 19 from the C-layers primarily in the supragranular layers are indicative of an of the LGN terminate in patches in layers III and IV along ascending feed-forward pathway. In other words, the the coronal plane [25] and intraocular injections of WGA– receiving area is considered to be at a higher processing HRP result in patchy tangential label in area 19 with an level than the area containing the cells of origin. On the approximate spacing of 2.5 mm [1]. Therefore, it may be other hand, efferent cells located in the infragranular layers that termination sites of the LGN afferents are part of the are suggestive of a descending feedback pathway, with substructure of the area 19 efferent bands projecting to the area receiving the axon terminals being classed as a area 21a. lower processing area than the area containing the efferent Extrastriate regions in the cat can contain more complex cell bodies [16,40,51]. The hierarchy of an area, with a retinotopic maps, larger spacing of connections and more bilaminar distribution of efferent cells found in both variable patterns of label than the primary visual cortex supragranular and infragranular layers, is equivocal unless [43,45,54]. This increased variation in organization is the laminar distribution of the efferent cell terminals is also probably established during maturation of the extrastriate known in the areas they are projecting to [16]. In this areas. Corticocortical connections are frequently estab- study, retrogradely labeled efferent cells in area 19 are lished after eye opening, making the organization of these found in all layers, except layer 1, yet they are most projections more susceptible to individual visual ex- apparent in layers 2 3 and 5. This bilaminar-like dis- perience [7,10,29]. Also during development, activity may tribution makes it difficult to assign a hierarchical status to play a role in determining modular size and organization area 19 or area 21a, as the afferent distribution in area 21a [23,42] and this activity in extrastriate areas may differ is not examined. However, the area 19 to 21a projection is from primary visual cortex for two reasons. First, extras- classed as ascending by Felleman and Van Essen [16] who triate areas are more vulnerable to changes in organization based their findings on laminar data from Symonds and since input in these higher areas is altered by previous Rosenquist’s work [49,50]. Scanell et al. [41] collates visual processing in other visual areas. Secondly, the connectional data from numerous studies and also categor- correlation between incoming inputs decreases in extras- izes this projection as ascending based upon multiple triate regions [24]. Therefore, the increased variation and quantitative criteria. complexity witnessed in the area 19 efferent bands may be Based upon the tangential findings it was expected that the result of an organization based upon individual ex- there would be coronal columns of densely labeled cells, perience and the type of input and processing occurring in which would represent the dense patches of label within this area. the tangential bands, and in between the coronal columns there would be sparser labeling, which would represent the 4.2. Coronal organization lighter labeled regions of the tangential bands. However, labeling between the coronal columns is virtually absent, The patchy columns of label in the coronal plane Fig. less than expected from the tangential findings. One 2 support the idea that area 19 efferent bands have a possible reason for this is the TMB reaction product is substructure consisting of multiple patches. Dreher et al. unstable [37,57]. Although the tissue underwent a stabili- T .H. Stewart et al. Brain Research 881 2000 47 –56 55 zation procedure, some of the lighter stained cells may 1. Further studies examining the functional significance of have missed detection by fading as a consequence of these bands are necessary to establish what relationship, if dehydration or time the tissue was processed several years any, the efferent bands in area 19 have with parallel prior to the present analysis. Another potential reason that processing streams in the cat. labeled cells are not detected between the coronal columns is that the coronal plane is not the optimal plane for detecting subtle patterns in labeling density in the tangen- Acknowledgements tial direction [5,45] and therefore, the area 19 band pattern, seen in the tangential plane, may be underestimated in the We thank Virginia Booth and Eleanor To for surgical coronal plane. Nevertheless, there are a few coronal and technical assistance. We would also like to thank sections with faintly labeled cells between the cell columns Sarven Sabunciyan and Dawn Lam for their helpful as well as sections which do not have labeling and comments. This study was supported by MRC Canada. probably correspond to the unlabeled regions between the tangential bands. References 4.3. Primate comparison [1] P.A. Anderson, J. Olavarria, R.C. Van Sluyters, The overall pattern Like the cat, extrastriate organization in the primate is of ocular dominance bands in cat visual cortex, J. Neurosci. 8 more complex than the organization in the primary visual 1988 2183–2200. cortex, with extrastriate areas demonstrating greater size [2] M.E. Bickford, W. Guido, D.W. Godwin, Neurofilament proteins in Y-cells of the cat lateral geniculate nucleus: normal expression and and spacing in their periodic structures. For example, in V1 alteration with visual deprivation, J. Neurosci. 18 1998 6549– there are punctate CO blobs which project to thin CO 6557. stripes in V2. The interblob regions of V1 project to the [3] J.D. Boyd, J.A. Matsubara, Tangential organization of callosal interstripes in V2 and the thick stripes in V2 receive input connectivity in the cat’s visual cortex, J. Comp. Neurol. 347 1994 from layer 4B of V1 [27]. Even within V2 stripes, a 197–210. [4] J.D. Boyd, J.A. Matsubara, Modular organization of corticocortical substructure has been found. For example, V2 projections inputs and outputs of area 19, Soc. Neurosci. Abstr. 20 1994 1742. to V4 originate in submodular compartments within the [5] J.D. Boyd, J.A. Matsubara, Laminar and columnar patterns of thin CO stripes and interstripes [17]. Clusters of both MT geniculocortical projections in the cat: relationship to cytochrome projecting cells [46] and Cat-301 labeled cells [11,35] are oxidase, J. Comp. Neurol. 365 1996 659–682. found in the thick CO stripes, also representing a sub- [6] J.D. Boyd, J.A. Matsubara, Projections from V1 to lateral suprasyl- modular organization in these stripes. Furthermore, rather vian cortex: an efferent pathway in the cat’s visual cortex that originate preferentially from CO blob columns, Vis. Neurosci. 16 than being uniform, CO stripes in V2 consist of irregular 1999 1–12. aggregates of dark CO patches [52]. The MT-projecting [7] S. Clarke, G.M. Innocenti, Organization of immature intra-hemis- cells within the CO stripes colocalize with patches of pheric connections, J. Comp. Neurol. 251 1986 1–22. dense CO staining within the band, demarcating a sub- [8] B. Conway, J.D. Boyd, T.H. Stewart, J.A. Matsubara, The projection modular organization [46]. Like area 19 of the cat, V2 has from V1 to extrastriate area 21a: a second patchy efferent pathway that colocalizes with the CO blob columns in cat visual cortex, a band organization and within these bands there is a Cerebr. Cortex 10 2000 149–159. substructure. Despite some irregularity in the structure of [9] D.P. Crockett, S. Maslany, S.L. Harris, M.D. Egger, Enhanced these primate modules, the connections between modules cytochrome-oxidase staining of the cuneate nucleus in the rat reveals in different areas is an important anatomical component of a modifiable somatotopic map, Brain Res. 612 1993 41–55. visual parallel processing in this species. This may also be [10] C. Dehay, H. Kennedy, J. Bullier, Characterization of transient cortical projections from auditory, somatosensory, and motor cor- true in the cat with different periodic structures represent- tices to visual areas 17, 18, and 19 in the kitten, J. Comp. Neurol. ing different processing streams. 272 1988 68–89. [11] E.A. DeYoe, S. Hockfield, H. Garren, D.C. VanEssen, Antibody labeling of functional subdivisions in visual cortex: CAT-301

5. Conclusions