Gilbert and Wiesel (1982) investigated the intrinsic connections in the cat visual cortex, revealing that individual neurons can communicate over long distances (up to 4 mm) horizontally, in directions parallel to the cortical surface. These connections are clustered, with an average periodicity of 1 mm, and are most apparent when viewed parallel to the cortical surface. The pattern was observed in more than half of the pyramidal and spiny stellate cells in the cortex and was seen in all cortical layers. The axons of some cells had clusters covering the same area in two layers, suggesting a relationship with columnar cortical architecture. Some pyramidal cells formed intrinsic connections exclusively, not projecting into the white matter. The axonal fields of all injected cells were asymmetric, extending for greater distances along one cortical axis than the orthogonal axis. These connections may be used to generate larger receptive fields or to produce inhibitory flanks in other cells' receptive fields. The classical view of cortical connectivity, derived from Golgi studies, is that axons run predominantly in a direction perpendicular to the cortical surface, from layer to layer. However, evidence suggests there may be more horizontal interaction within a given cortical area than Golgi studies indicate. Studies using anterograde and retrograde transport techniques have supported the existence of long-range intrinsic connections. Intracellular injection of horseradish peroxidase (HRP) allows for a more complete view of a cell's axonal arbor than Golgi impregnations. These studies revealed that afferent fibers from the lateral geniculate nucleus give off collaterals covering wide areas of cortex, innervating several ocular dominance columns. Cortical cells also project to points quite distant from their own dendritic fields but still within the same cortical area. The clustering of these connections is consistent with findings from axonal transport studies. The clustering of thalamic afferents is responsible for ocular dominance columns, while the functional correlate of the clusters of intrinsic cortical connections is not known. The authors used three-dimensional computer graphics to reconstruct cortical cells injected with HRP, revealing that axon collaterals are distributed in discrete, repeating clusters. This clustering pattern was observed in more than half of the pyramidal and spiny stellate cortical cells. The clustering pattern was also evident in layer 4c spiny stellate cells and in layer 5 pyramidal cells. The results suggest that intrinsic cortical connections are widespread and may be related to the columnar architecture of the cortex. The findings challenge the classical view of cortical connectivity and highlight the importance of horizontal connections in the visual cortex.Gilbert and Wiesel (1982) investigated the intrinsic connections in the cat visual cortex, revealing that individual neurons can communicate over long distances (up to 4 mm) horizontally, in directions parallel to the cortical surface. These connections are clustered, with an average periodicity of 1 mm, and are most apparent when viewed parallel to the cortical surface. The pattern was observed in more than half of the pyramidal and spiny stellate cells in the cortex and was seen in all cortical layers. The axons of some cells had clusters covering the same area in two layers, suggesting a relationship with columnar cortical architecture. Some pyramidal cells formed intrinsic connections exclusively, not projecting into the white matter. The axonal fields of all injected cells were asymmetric, extending for greater distances along one cortical axis than the orthogonal axis. These connections may be used to generate larger receptive fields or to produce inhibitory flanks in other cells' receptive fields. The classical view of cortical connectivity, derived from Golgi studies, is that axons run predominantly in a direction perpendicular to the cortical surface, from layer to layer. However, evidence suggests there may be more horizontal interaction within a given cortical area than Golgi studies indicate. Studies using anterograde and retrograde transport techniques have supported the existence of long-range intrinsic connections. Intracellular injection of horseradish peroxidase (HRP) allows for a more complete view of a cell's axonal arbor than Golgi impregnations. These studies revealed that afferent fibers from the lateral geniculate nucleus give off collaterals covering wide areas of cortex, innervating several ocular dominance columns. Cortical cells also project to points quite distant from their own dendritic fields but still within the same cortical area. The clustering of these connections is consistent with findings from axonal transport studies. The clustering of thalamic afferents is responsible for ocular dominance columns, while the functional correlate of the clusters of intrinsic cortical connections is not known. The authors used three-dimensional computer graphics to reconstruct cortical cells injected with HRP, revealing that axon collaterals are distributed in discrete, repeating clusters. This clustering pattern was observed in more than half of the pyramidal and spiny stellate cortical cells. The clustering pattern was also evident in layer 4c spiny stellate cells and in layer 5 pyramidal cells. The results suggest that intrinsic cortical connections are widespread and may be related to the columnar architecture of the cortex. The findings challenge the classical view of cortical connectivity and highlight the importance of horizontal connections in the visual cortex.