Categorization of physiologically and morphologically characterized non-α/non-β cat retinal ganglion cells using fractal geometry

Non-α/non-β cat retinal ganglion cell images were obtained from the published literature, and a homogeneous group of cells was chosen as a standard for each currently accepted cell type (γ, δ and ε). The NIH box-counting method was chosen to determine the fractal dimension (Df) of all cells. The 'standard' values allowed comparisons with other morphologically and physiologically non-α/non-β classified cell types in the literature. We suggest, based on fractal analysis of the dendritic trees, that the morphologically defined γ, δ, and ε cells are distinct types. The W-tonic and W-phasic cell types were further divided into 2 subcategories (W-tonic₁, W-tonic₂, W-phasic₁, W-phasic₂). The fractal dimension, of the ε cells being equivalent to the W-tonic₁ group and γ cell type equivalent to the W-phasic₁ group. Delta cells may be equivalent to either the W-tonic₂ or the W-phasica₂ group. We discuss the value of the fractal dimension as an added morphological parameter for future morphophysiological classification schemes of vertebrate retinal ganglion cells.