1. Intracellular recordings have been made of the responses to light of single cones in the retina of the turtle. The shape of the hyperpolarizing response to a flash depends on the pattern of retinal illumination as well as the stimulus intensity. 2. Although changes in the stimulus pattern can produce changes in the effective stimulus intensity, the responses to certain patterns cannot be matched by any adjustment of stimulus intensity. 3. The initial portion of responses to large or small stimulating spots is proportional to light intensity; this allows comparison of responses when the amount of light on a cone is kept constant but the light on surrounding cones is changed. For equal light intensity on the cone, the response to a spot 2 or 4 μ in radius is smaller than that to a spot 70 μ in radius. 4. Responses to spots 70 and 600 μ in radius coincide over their rising phases and peaks without any adjustment of stimulus intensity. The responses to the larger spot, however, contain a delayed depolarization not present with the smaller spot. 5. During steady illumination of a cone with a small central spot, the response to transient illumination superimposed on the same area is greatly reduced. Illumination of cones in the near surround, however, produces a hyperpolarizing response, and illumination of cones in the more distant surround generates a delayed depolarization. 6. The results described above suggested that synaptic signals might impinge on cones. This possibility was tested by electrically polarizing one retinal cell while recording from another. 7. Currents passed through a cone within 40 μ of another cone can change the membrane potential of the latter. Not all cones within this distance show the interaction, however, and it has never been detected at distances greater than 50 μ. 8. Hyperpolarization of a horizontal cell with applied current can produce a depolarization of a cone in the vicinity. During this depolarization, the response of the cone to a flash is reduced in size and altered in shape. 9. It is concluded that the response of a cone to light may be modified by synaptic mechanisms which are activated by peripheral illumination.

SUMMARY1. Intracellular recordings have been made from single photoreceptors in the retina of the turtle. Histological sections of the retina made after injection of dye through the recording electrode reveal dye in the inner segments of single cones.2. Following a brief flash of light the cone undergoes a hyperpolarization which is graded with the intensity of the flash.3. The excitatory receptive field of a receptor is probably as small as the cross-section of a single cone, but accurate measurements are rendered difficult by scattering of light within the retina.4. The voltage drop produced by a current injected into the cell is increased during the response to light. Steady hyperpolarizing currents increase the size of the response to light; depolarizing currents of increasing strength reduce and then reverse the response.5. The results are consistent with the hypothesis that light activates the visual cell by decreasing the permeability of membrane channels which in darkness act as a shunt of the membrane.

It is well known that the receptor potentials evoked by flashes of light arise with a marked delay (HARTLINE et al., 1952; DE VOE, 1962).In the ommatidia of Limulus the dePolarization, V, produced by a brief subthreshold flash can be fitted fairly well bywhere Q is the quantity of light, S is a sensitivity constant, t is time after the flash and -r is a time constant. N is a number which characterises the apparent order of the delay and is usually about 10 in Limulus. After exposure to a bright light the sensitivity constant may be reduced by 103 to 104 and the time constant by 2-3. However, S and -: recover together and are related bySD \VD]where S D and -r D are for the dark adapted state and N' is a number which is not far from 10 (slightly less in the best experiments). A possible interpretation of these two empirical formulae is that there are about 10 stages of exponential delay, each with time constant 1:, between the absorption of light and the event which depolarizes the membrane. Changes of sensitivity are brought about by equal alterations of time constant and low frequency gain in all, or nearly all, of the stages. Since there are 10 sections, a relatively small change in time scale corresponds to a very large change in overall gain. The situation is analogous to a low-pass filter in which the d.c. gain and time constant of each stage is proportional to a leakage resistance in parallel with a condenser. Equations 1 and 2 apply to such a system if there are N stages and successive sections are connected by very large resistances or by some kind of isolating element, such as a pentode. 284

3. L-cells of both type I and type II are hyperpolarized by all visible wave-lengths, and their spectral sensitivity in the linear range resembles that of red cones. Their responses are not invariant with respect to colour, and their sensitivity to green relative to red stimuli increases during red backgrounds. These properties suggest that L-cells are activated mainly by red cones but also receive impingement from the red members of double cones.4. Spectral properties of red/green C-cells resemble those of green cones as modified by the recurrent action of L-cells. They can be explained assuming that red/green C-cells receive their principal impingement from green cones and subsidiary interactions from green/blue C-cells and the green members of double cones.5. The spectral sensitivity of the hyperpolarizing responses of green/ blue C-cells is ascribed to impingement from blue cones. Their depolarizing responses have complex properties which suggest that they are brought M. C. F. FUORTES AND E. J. SIMON about by the activity of both L-cells (probably through the blue cones) and red/green C-cells.6. It is concluded that the main properties of the responses of the horizontal cells can be explained by a simple circuit in which each horizontal cell is connected to a corresponding type of cone and the L-cells have a recurrent impingement on all cones. The scheme is modified by additional interactions which operate on the responses of each horizontal cell type.

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