Fractal Dimension as a Tool for Classification of Rat Retinal Ganglion Cells

Classification of neurons according to their morphological characteristics such as cell body size provides insight into their functional attributes. However this simple relationship does not hold for all mammals including the rat retinal ganglion cells (RGCs). The rat RGCs have been proposed to consist of a varying number of subtypes based on cell body size and dendritic spread. As dendritic morphology is an essential step towards understanding structure-function relationships of RGCs, this study aimed at using fractal analysis to classify these cells based on the dendritic branching complexity. Using D, the cell type that includes those with the largest cell bodies could be divided into two groups termed here simple and complex group. Both were further divided into an inner and outer subgroup depending on the level of dendritic branch stratification within the inner plexiform layer (IPL) of the retina. For the complex group D was significantly different between inner and outer cells (mean±sd; inner: 1.42±0.02; outer: 1.445±0.02; p<0.001). Simple and complex cell types differed significantly in both the inner and outer layer. Inner cells had a fractal dimension of 1.36±0.03 and 1.42±0.01 (p<0.001), whereas the outer cells had a fractal dimension of 1.37±0.01 compared to 1.45±0.02 (p<0.001) for the simple and complex types respectively. Our results extend those of previous reports in that the large cells can be classified into complex and simple cell types that are located in both the inner and outer IPL. Thus the visual processing of these cells is more complex then has been previously suggested.