Substrates for Interlaminar Connections in Area V1 of Macaque Monkey Cerebral Cortex

On the basis of our earlier studies of macaque visual cortical area V1 (Lund, 1973; Blasdel et al., 1985; Fitzpatrick et al., 1985) using Golgi impregnations and small intralaminar injections of horseradish peroxidase (HRP), a schema (reproduced here in Fig. 1) was outlined for spiny stellate neuron relays out of the thalamic recipient divisions of layer 4C (Lund, 1990). This diagram illustrates the finding that although the thalamic axons from magnocellular and parvocellular divisions of the lateral geniculate nucleus (LGN) terminate in the α and β divisions respectively of layer 4C (Hubel and Wiesel, 1972; Blasdel and Lund, 1983), the relays out of layer 4C seemed to fall into three sets. The lowermost set seemed to project in a very narrowly focused fashion to layer 4A and lower layer 3B, a set in the middle depth of layer 4C seemed to project to Open image in new window Figure 1. Diagram summarizing current information concerning the laminar distribution of thalamic inputs (to the left of diagram) from the lateral geniculate nucleus (LGN) to cortical area V1 in the macaque, and their further relays by thalamic recipient spiny stellate neurons of layer 4C. Thalamic axons P1 (to layer 4Cβ) and P2 (to layer 4A) appear to arise from different thalamic neuron populations, both situated in the LGN parvocellular laminae (Fitzpatrick et al., 1983; Blasdel and Lund, 1983). Thalamic axons M1 (to upper 4Cα) and M2 (terminals throughout 4Cα) arise from the magnocellular laminae of the LGN; axon population I, relaying to the upper layer blobs, comes from the intercalated layers of the LGN (Hendrickson et al., 1978; Fitzpatrick et al., 1983). The output from layer 4C is suggested to be a gradient, created by dendritic sampling of both parvocellular and magnocellular inputs by the postsynaptic spiny stellate neurons; 4C neurons are suggested to shift their projection target from layers 4A–3B to layer 4B as their primary LGN input shifts from the parvocellular layers to the magnocellular layers. (See Lund, 1990, for further discussion.) Diagram modified from Lund (1990) with permission. both layers 3B and 4B with more spreading axon arbors, and an upper set seemed to project principally to layer 4B. It was suggested that this distribution of projections could derive from a gradient of sampling of the magnocellular and parvocellular afferents by continuously overlapped dendritic fields of spiny stellate neurons through the depth of layer 4C; the three efferent sets of cells in layer 4C were seen as partially overlapping in depth.