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1. Responses to monochromatic lights were recorded intracellularly from red cones, green cones, and luminosity horizontal cells (L-cells) in the retinae of turtles.

2. Both types of cones responded to small fields of illumination with graded hyperpolarizations. Red cones were only moderately more sensitive to deep red (680 nm) than to green (550 nm) light while green cones were much more sensitive to the green light than to the red. L-cells produced small responses for flashes of either colour covering small fields.

3. Stimulation of large fields with monochromatic lights of moderate or high intensity evoked large L-cell responses and composite responses in cones. These latter include the hyperpolarizing action of the light absorbed by the cone itself (direct response), its enhancement by illumination of the near surround, and the depolarizing effects of L-cell feed-back.

4. L-cells respond primarily to the activity of red cones; with sufficient intensity of the light, however, their responses are influenced also by green cones. As a result, if a red and a green light stimulate red cones equally, the L-cell response is larger for the green stimulus.

5. Green cones were depolarized by deep red lights of moderate intensity applied over large fields. These depolarizing responses include oscillations which follow closely oscillations in L-cells. Green light applied to the same large fields produced hyperpolarization of green cones.

6. Red cones were hyperpolarized by red or green light covering large fields, but the time course of their responses differed for the two colours, reflecting a corresponding difference in L-cell activity.

7. Red light in the form of an annulus produced large responses in central L-cells without eliciting direct responses in central green cones. In these conditions green cones developed depolarizing waves which included a large, sharp transient.

8. It is concluded from these and other results that the direct response of each cone is modified by two interactions: enhancement only from nearby cones of the same colour and depression controlled (through L-cell feed-back) by cones of all colours. In this way the response of any cone will change as the proportion of responses in cones of different colours changes, this proportion being a function of the wave-length of the light.

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