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 of cone and horizontal cell responses to circles or annuli of light were made with the purpose of determining the properties of and the mechanisms underlying the horizontal-cell-mediated depolarization of cones which is evoked by surround illumination.2. A comparison of the responses of a cone and a near-by horizontal cell to a peripheral stimulus revealed a striking similarity in their time courses and amplitudes, indicating that a correlation exists between the depolarizing synaptic potential in the cone and the response of the horizontal cell.3. The depolarizing synaptic potential in cones was separated from the direct response of the cell to light by illuminating the periphery with an annulus during steady, bright illumination of the central cone. The synaptic potentials were graded with the intensity or area of peripheral illumination. In some cones a spike-like depolarization, which overshot the dark resting potential, occurred with bright illumination of the periphery.4. The effects of extrinsic current on the synaptic potential demonstrated that this response was generated by a change in membrane conductance consisting of two separate components with different timedependences and reversal levels. The slower of the two components, which often outlasts the stimulus, represents an increase in membrane conductance.5. The progressive decline in the amplitude of the responses of horizontal cells under a large spot from centre to periphery was found to result in a diminished feed-back effect in cones near the edge of the spot. This leads to a Mach-band effect during the plateau phase of cone responses, * Present address: Physiological Laboratory, Cambridge CB2 3EG, England. P. M. O'BRYAN suggesting that one function of the feed-back might be to enhance contrast discrimination.

SUMMARY1. Early receptor potentials (E.R.P.s) were recorded with internal electrodes in turtle cones by applying brief flashes from a xenon tube with a maximum photon density equivalent to 2-3 x 108 photons /sm-2 at the optimum wave-length.2. The E.R.P. was separated from the late receptor potential (L.R.P.) by superposing the flash on a step of light which was strong enough to saturate the L.R.P.3. In red-sensitive cones the E.R.P. consisted of a brief depolarizing phase (R1) followed by a hyperpolarizing phase (R2) of maximum amplitude 10 mV and duration 30-40 msec. R1 was small or absent in green-sensitive cones.4. With flashes of increasing intensity the E.R.P. approached its maximum exponentially with an exponential constant Q of about 108 photons um-2 which is of the same order as the reciprocal of the photosensitivity of porphyropsin; the implication of this result, which is considered in the theoretical section, is that the E.R.P. is proportional to the number of photoisomerizations.5. When tested with a constant xenon flash at varying times after the beginning of a bleaching light the E.R.P. declined exponentially with a similar value of Q. 6. After prolonged bleaches the E.R.P. recovered with a time constant of about 100 sec but much quicker recoveries were observed after relatively brief bleaches.7. The form and size of the E.R.P. are consistent with the accepted view that it arises from a redistribution of charge in the cone pigment molecule.

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