Two types of luminosity horizontal cells in the retina of the turtle.

1. Two classes of luminosity horizontal cells have been identified in the retina of the turtle by intracellular recording. These cells differ in the area of retinal surface over which they are sensitive to light. For moderate light intensities the maximum response amplitude of ;type I' cells increases progressively as the radius of a circle of light is enlarged to over 750 mum. Responses of ;type II' cells instead reach maximum amplitude with circular illumination of about 300 mum radius.2. Both types are maximally sensitive to red light and have similar action spectra.3. The cells from which these responses originated were identified by intracellular marking with Procion Yellow dye. Both types were located in the inner nuclear layer. Type I cells had asymmetric, thick, tuberous processes and no apparent discrete somata. Type II cells had thin processes radiating symmetrically from rounded cell bodies.4. The different properties of the two types of cells can be simply explained assuming that the cells of type I are coupled by low-resistance electrical junctions while in cells of type II the junctional resistance is higher.5. Since type I and type II cells differ both morphologically and physiologically, they may have different functions.