Primate horizontal cells, situated at the first synapse in the retina, offer a suitable locus for study of outer retinal adaptation. Sensitivity regulation falls short of Weber's law at low- to mid-photopic levels (Smith et al., 2001), and outer retinal adaptation is cone-specific and spatially local (Lee et al., 1999), Here I review recent results as to the time course of outer retinal adaptation, which could be instantaneous through some form of response compression, or might display a finite time course due to operation of feedback or feedforward mechanisms.

We recorded from primate horizontal cells in an in vitro preparation. In one set of experiments, probe stimuli (sinusoids or pulses) were added to slowly modulated adapting backgrounds. Results indicated that adaptation mechanisms showed a finite time course, on the order of about 10 milliseconds. In addition, characteristic distortions of responses to sinusoids of different frequencies could be accounted with such a time course, as could distortions to positive and negative going pulses. Nevertheless, it proved difficult to provide a comprehensive model of the adaptation process, a process that may not reflect operation of a unitary mechanism.