Morphological Bases Of Spatial Summation Properties Of Cat V1 Neurons

Recent studies have reported that the size of the classical receptive field (CRF) and the extent of spatial summation of V1 neurons depend on stimulus contrast. We reexamined these properties for 48 V1 neurons in the cat and found that all the cells had a constant CRF size, whereas their spatial summation properties can be contrast-dependent (CD) or contrast independent (CID). Of the 29 CD cells, 17 showed facilitatory summation at low contrast (10%), but suppressive summation at high contrast (80%); the other 12 showed weak suppressive summation at low contrast, whereas the strength of suppression increased significantly at high contrast. The 19 CID cells showed similar facilitative (CIDf) or suppressive (CIDs) summation at low and high contrast, without changes in shape and/or peak location. We successfully labeled 11 CD cells and 10 CID cells with biocytin. The morphological results demonstrated that all the labeled CD cells were pyramidal cells, whereas all labeled CID cells were non-pyramidal cells, in which the CIDf cells were spiny stellates and the CIDs cells, smooth interneurons. There is thus a global distinction between summation properties of pyramidal and non-pyramidal cells, and between the smooth and spiny non-pyramidal cells as well. In the primary visual cortex, activity of neurons evoked by stimuli within the classical receptive field (CRF) can be modulated by stimuli in the extra-receptive field (ERF). This modulating effect can be facilitatory (F-ERF) or inhibitory (I-ERF) that play different roles in visual information processing. Little is known on the cellular and neurotransmitter receptor bases of the two types of long-range interactions. Using in vivo intracellular injection technique, combined with the technique of immunocytochemistry, we have studied the morphological and immunocytochemical features of the F-ERF and I-ERF neurons. Significant differences were found in both the dendrite and axon structures, as well as the density distribution of excitatory and inhibitory neurotransmitter receptors, between the two types of neurons.Neurons with F-ERF were mostly pyramidal cells, having widely distributed axon collaterals covering several cortical columns in the neighborhood; their dendrites have more complex branching covered by densely distributed spines with a larger head. In contrast, the axon collaterals of I-ERF neurons (mostly also pyramidal cells) were restricted within the same cortical column, and the dendrite branching was relatively sparse, with smaller and less spines on it. Moreover, F-ERF neurons have more Glu2/3 receptors and less receptors on the surface of dendrites and cell body, and the reverse is true for I-ERF neurons. As dendrite spines are the main targets of excitatory synaptic inputs, and the extent of axon collaterals represents the range of output connections with post-synaptic neurons, the differences in the input and output connections between the I-ERF and F-ERF neurons may underlie the distinct long-range modulation effects of the two types of neurons. GABAA...