Notes: Summary The colour of an object is changed by surround colours so that the perceived colour is shifted in a direction complementary to the surround colour. To investigate the physiological mechanism underlying this phenomenon, we recorded from 260 neurons in the parvo-cellular lateral geniculate nucleus (P-LGN) of anaesthetized monkeys (Macaca fascicularis), and measured their responses to 1.0–2.0° diameter spots of equiluminant light of various spectral composition, centered over their receptive field (spectral response function, SRF). Five classes of colour opponent neurons and two groups of light inhibited cells were distinguished following the classification proposed by Creutzfeldt et al. (1979). In each cell we repeated the SRF measurement while an outer surround (inner diameter 5°, outer diameter 20°) was continuously illuminated with blue (452 nm) or red (664 nm) light of the same luminance as the center spots. The 1.0–1.5° gap between the center and the surround was illuminated with a dim white background light (0.5–1cd/m2). During blue surround illumination, neurons with an excitatory input from S-or M-cones (narrowand wide-band/short-wavelength sensitive cells, NSand WS-cells, respectively) showed a strong attenuation of responses to blue and green center spots, while their maintained discharge rate (MDR) increased. During red surround illumination the on-minus-off-responses of NS- and WS-cells showed a clear increment. L-cone excited WL-cells (wide-band/long-wavelength sensitive) showed a decrement of on-responses to red, yellow and green center spots during red surround illumination and, in the majority, also an increment of MDR. The response attenuation of narrow-band/long-wavelength sensitive (NL)-cellls was more variable, but their on-minus-off-responses were also clearly reduced in the average during red surrounds. Blue surround illumination affected WL-cell responses little and less consistently than those of NL-cells, but often broadened the SRF also in the WL-cells towards shorter wavelengths. The M-cone excited and S-cone suppressed WM-cells were strongly suppressed by blue but only little affected by red surround illumination. The changes of spectral responsiveness came out clearly in the group averages of the different cell classes, but snowed some variation between individual cells in each group. The zero-crossing wavelengths derived from on-minus-off-responses were also characteristically shifted towards wavelengths complementary to those of the surround. The direction of changes of spectral responsiveness of P-LGN-cells are thus consistent with psychophysical colour contrast and colour induction effects which imply that light of one spectral region in the surround reduces the contribution of light from that same spectral region in the (broad band or composite) object colour. Surrounds of any colour also decrease the brightness of a central coloured or achromatic light (darkness induction). We calculated the population response of P-LGN-units by summing the activity of all WS-, WM- and WL-cells and subtracting that of all NS- and NL-cells. The SRF of this population response closely resembled the spectral brightness function for equiluminous lights rather than the photopic luminosity function. With red or blue surrounds, this population SRF was lowered nearly parallel across the whole spectrum to about 0.7 of the amplitude of the control. In a psychophysical test on 4 observers we estimated the darkness induction of an equiluminous surround in a stimulus arrangement identical to the neurophysiological experiment, and found a brightness reduction for white, blue, green and red center stimuli to 0.5–0.7 of the brightness values without surround. This indicates that the neurophysiological results may be directly related to perception, and that P-LGN-cells not only signal for chroma but also for brightness, but in different combinations. The results indicate that both an additive (direct excitation or suppression of activity) and a multiplicative mechanism (change of gain control) must be involved in brightness and colour contrast perception. As mechanisms for the surround effects horizontal cell interactions appear not to be sufficient, and a direct adaptive effect on receptors feeding positive or negative (opponent) signals into the ganglion cells receptive fields by straylight from the surround must be seriously considered. This will be examined in the following companion paper. The results indicate that changes of spectral and brightness responses in a colour contrast situation sufficient to explain corresponding changes in perception are found already in geniculate neurons and their retinal afferents. This applies to mechanisms for colour constancy as well in as much as they are related to colour contrast.