Influence of adaptation level on response pattern and sensitivity of ganglion cells in the cat's retina.

1. The effect of background illumination on response pattern is correlated with its effect on visual sensitivity by analysing post-stimulus time-histograms obtained from single ganglion cells in the cat's retina at various levels of background illumination between zero and 2 x 10(6) photons (wave-length 523 nm).sec(-1).deg(-2) (via 5.7 mm(2) pupil).2. If background illumination did not exceed a critical value, about 10(3) photons (523 nm).sec(-1).deg(-2) (via 5.7 mm(2) pupil), stimulation of the centre of a receptive field resulted in either sustained excitation (i.e. increase in discharge rate) during ;on' and cessation of the excitation at ;off' (on-centre unit), or sustained inhibition (i.e. decrease in discharge rate) during ;on' and cessation of the inhibition at ;off' (off-centre unit). Within this low adaptational level, a ganglion cell maintained its maximum sensitivity regardless of whether the weak background light was presented or not.3. When background level exceeded the critical value up to 2 x 10(6) photons (523 nm).sec(-1).deg(-2), however, the simple, sustained responses changed into compound responses with two transient components of opposite polarities, either excitation at ;on' and inhibition at ;off' (on-centre unit), or inhibition at ;on' and excitation at ;off' (off-centre unit), and also the sensitivity began to decrease as the background increased, approximately obeying Weber's law.4. It is suggested that a ganglion cell gives simple-sustained response when its gain control mechanism remains inactive at a low background illumination below a critical level, whereas it gives compound-transient response when its gain control mechanism becomes active as background illumination exceeds the critical value.