Owing to the limited dynamic range of a neuron's output, neural circui
ts are faced with a trade-off between encoding the full range of their
inputs and resolving gradations among those inputs. For example, the
ambient light level varies daily over more than nine orders of magnitu
de(1), whereas the firing rate of optic nerve fibres spans less than t
wo(2). This discrepancy is alleviated by light adaptation(3): as the m
ean intensity increases, the retina becomes proportionately less sensi
tive. However, image statistics other than the mean intensity also var
y drastically during routine visual processing. Theory predicts that a
n efficient visual encoder should adapt its strategy not only to the m
ean, but to the full shape of the intensity distribution(4-6). Here we
report that retinal ganglion cells, the output neurons of the retina,
adapt to both image contrast-the range of light intensities-and to sp
atial correlations within the scene, even at constant mean intensity.
The adaptation occurs on a scale of seconds, one hundred times more sl
owly than the immediate light response, and involves 2-5-fold changes
in the firing rate. It is mediated within the retinal network: two ind
ependent sites of modulation after the photoreceptor cells appear to b
e involved. Our results demonstrate a remarkable plasticity in retinal
processing that may contribute to the contrast adaptation of human vi
sion(7).