The main finding from this study is that neurons projecting from V1 to thick stripes and pale stripes in V2 are similarly organized by layer and CO compartment. For both stripe types, the major projection arises from layer 2/3. In this layer, ~85% of cells projecting to thick stripes and pale stripes are located in interpatches. There is also a prominent projection from layer 4B to thick stripes and pale stripes. In layer 4B the projecting neurons are also located preferentially in interpatches (75% for thick stripes, 72% for pale stripes), but the bias is less pronounced than for layer 2/3 neurons. There are also sparse projections from layer 4A and layer 5/6 to thick stripes and pale stripes. The projection from layer 4A favors interpatches, but curiously, the projection from 5/6 is random with respect to CO compartment.
Thick stripes and pale stripes derive their input not only from the same source in V1 but sometimes even from the same neurons. In one case, a dual tracer injection into adjacent pale and thick stripes, 20–27% of the retrogradely filled cells in layer 2/3 were double labeled (Sincich and Horton, 2002
). For the case shown in , dual tracer injections were made in a pale stripe and a thick stripe that were separated by several intervening stripes. Not surprisingly, the percentage of double-labeled cells was lower, amounting to only 10 –12% of the cells. Many factors can influence the percentage of double-labeled cells, including the exact location of the tracer injection sites, the efficiency of each tracer, and uptake by fibers of passage. Nonetheless, it is clear that a considerable number of layer 2/3 interpatch cells have manifold projections, sending their axons to both pale stripes and thick stripes, although the majority project exclusively to a single stripe type.
CO divides V1 into two complementary output streams: interpatches → pale and thick stripes and patches → to thin stripes. However, the segregation by CO compartment is not perfect. After pale stripe or thick stripe injection, 16% of cells in layer 2/3 are located in patches. In other layers, the segregation is even less strict (layers 4A, 4B) or absent altogether (layer 5/6). After thin stripe injection, 19% of cells are located in interpatches (Sincich and Horton, 2005
). Despite this overlap, the V2 projections from patches and interpatches appear to constitute separate systems, at least for the upper layers. The best evidence for this conclusion comes from two cases involving CTB-Au injection in a pale stripe and WGA-HRP injection in an adjacent thin stripe. Less than 0.5% of cells were double labeled [Sincich and Horton (2005)
, their ]. This result suggests that projections from patches and interpatches to V2 are truly separate, although the actual projecting cells may stray across CO compartments (whose exact borders, after all, are defined arbitrarily).
There is a fundamental difference in the patch versus interpatch systems regarding the distribution of labeled cells in V1 after small tracer injections in V2. After thin stripe injections, no matter how small, labeled cells are found in every patch, and their concentration falls off steadily as a function of CO density (Sincich and Horton, 2005
). In contrast, after small tracer injections into pale or thick stripes, labeled cells are often found in clusters within interpatches (; supplemental Fig. S11
, available at www.jneurosci.org
as supplemental material
). The most likely explanation is that interpatch labeling is confined to cells that share the same orientation tuning as cells at the V2 injection site. To test this idea, one could combine optical imaging with retrograde tracer injection. Optical imaging has shown that orientation columns are present within pale stripes and thick stripes but apparently absent in thin stripes (Malach et al., 1994
; Xiao et al., 2003
; Shmuel et al., 2005
; Lu and Roe, 2008
). This observation could explain why interpatches are filled only partially after small thick or pale stripe injections, whereas patches are filled diffusely (and no patches are skipped) after small thin stripe injections.
Livingstone and Hubel (1988)
reported that pale stripes and thick stripes receive entirely different sources of V1 input, with the former supplied by interpatches in layer 2/3 and the latter by a diffuse projection arising from layer 4B. However, in their first account of the V1-to-V2 projection, Livingstone and Hubel (1984)
reported that thick stripes receive confluent labeling from layer 2/3. In a later study, Livingstone and Hubel (1987)
reexamined the projection to thick stripes in the squirrel monkey. In these experiments, labeling was noted in layer 4B, “without any clear or consistent correspondence of the filled cells with the overlying blob pattern” (p. 3375). The authors stated, “we could easily have overlooked similar results in previous experiments, so we went back and re-examined 4 thick-stripe injections from our earlier experiments, and we did find faint labeling, again in layer 4B exclusively” (p. 3375). Thus it appears, combining both their 1984 and 1987 studies, that Livingstone and Hubel did identify a projection from both layers 2/3 and 4B to the thick stripes.
The idea that pale stripes and thick stripes receive completely different V1 input provided a key piece of support for splitting the visual system into ventral and dorsal streams (Zeki and Shipp, 1988
; Merigan and Maunsell, 1993
; Van Essen and Gallant, 1994
; Nassi and Callaway, 2009
). It also fit nicely with the discovery that pale stripes project to area V4 and thick stripes project to area MT (DeYoe and Van Essen, 1985
; Shipp and Zeki, 1985
). It was hypothesized that a V1 parvo-dominated upper layer projection flows via pale stripes to V4, while a magno-dominated projection goes from 4B via thick stripes to MT (Livingstone and Hubel, 1988
). Area MT also receives a direct projection from layer 4B (Shipp and Zeki, 1985
; Boyd and Casagrande, 1999
; Sincich and Horton, 2003
; Nassi and Callaway, 2007
). Given our finding that pale stripes and thick stripes are supplied by both layer 2/3 and layer 4B, it was important to learn whether their relative contributions differ. In fact, 23% of projecting cells to thick stripes originate from 4B, whereas for pale stripes, only 10% of projecting cells come from 4B. Layer 4B is thought to be dominated by the magnocellular LGN channel, although 4B pyramidal cells receive mixed input from 4Cα
(Yabuta et al., 2001
). Thus, our data support the contention that thick stripes receive a more robust magnocellular signal than do pale stripes.
At the same time, our results show that thick stripes and pale stripes have much of their V1 input in common. For both stripe classes, the most numerous projection comes from the inter-patches in layer 2/3. In some cases, the axons of individual V1 neurons ramify widely in V2 (Rockland and Virga, 1990
), supplying adjacent (or even nonadjacent) thick stripes and pale stripes, as shown by our dual tracer injections. Given this extensive shared V1 input, it is not clear why cells located within thick stripes and pale stripes have different physiological properties. Recently, for example, columns of disparity-tuned cells have been identified in thick stripes (Chen et al., 2008
; Kaskan et al., 2009
). Why are they absent in pale stripes, if pale stripes also receive input from disparity-tuned, oriented V1 cells? It is possible that many V1 cells projecting either to thick stripes or to pale stripes differ substantially in their functional properties, despite being located in the same cortical column and layer. Another possibility is that intrinsic local circuits in V2, fed by similar V1 inputs, generate de novo
the distinct receptive field properties of cells in thick stripes and pale stripes. Alternatively, cells’ receptive field properties may differ less as a function of stripe type than generally believed. Finally, the major thalamic input to layer 4 of V2 from the pulvinar has been inadequately characterized, and this input is anatomically distinct for pale and thick stripes (Livingstone and Hubel, 1982
; Levitt et al., 1995
). Undoubtedly, there are many anatomical and physiological factors that contribute to receptive field properties of V2 neurons, and it is necessary to work them out before the unique functions of this major visual cortical area can be fully understood.