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1. The gain of the central response mechanism and the latency of the pure central response of on-centre ganglion cells were studied by recording from single optic tract fibres the responses evoked by slow square-wave stimuli applied against some steady background.

2. The concept of effective flux was introduced and defined: if any portion of a stimulus extends beyond Ricco's area of complete summation, then that stimulus has an actual flux, equal to the product of its area and luminance, but it also has an effective flux which is that fraction of its actual flux which equals the actual flux of another stimulus which, when it falls entirely within Ricco's area, evokes an isobolic pure central response or has the same adaptive effect upon the central response mechanism as the first stimulus.

3. The most significant finding was that when the cell responded with a pure central response to the incremental flux (the square wave) applied against a steady effective background flux, then the gain and the latency were functions exclusively of the sum of the two fluxes (the total flux), not of the incremental or background flux as such. This shows that the level of field adaptation of the central mechanism is reset within the latent period of the response to an incremental flux.

4. Increment sensitivity curves based on isobolic suprathreshold responses all had the same slope of 0·9, when the log of the incremental flux was plotted against the log of the total flux. A plot of log latency against log total effective flux had a slope of -0·1.

5. The stimulus—response relation derived from (3) and (4) was [Formula: see text] and [Formula: see text], where R is the response amplitude, F_et the total flux, F_e the incremental flux and K₁ and K₂ are constants.

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