Organization of layers II–III connections in human visual cortex revealed by in vitro injections of biocytin

Abstract

In the search for cortical mechanisms subserving psychological phenomena, a better understanding of human cortical circuitry is crucial. In this report we describe aspects of intrinsic connectivity of supragranular layers in human visual cortex, revealed by extracellular injections of the anterograde tracer biocytin in vitro. Human cortical slices were obtained from visual association cortex in the posterior-medial portion of the dorsal bank of the occipital lobe, removed during neurosurgical tumor ablattions. Small iontophoretic injections of biocytin into layers II–III revealed intense Golgi-like staining of axonal projections emanating from the injection sites. Vertically descending axons are grouped in bundles 20 μm in diameter which are spaced 15 μm apart. Some of these axons enter the white matter and send long oblique and horizontal collaterals. The main horizontal spread of the axons could be observed in layers II–III and V. The bulk of projections extends to a distance of 1.5 mm in layers II–III and 1.1 mm in layer V. Few individual axons could be observed at greater distances. In contrast, layer IV is almost devoid of horizontal connections, forming a clear gap between supra- and infragranular layers. Axon collaterals in the infragranular layers project mostly in a descending oblique direction with long horizontal collaterals in lower layer VI.