The flash electroretinogram (ERG) is a useful tool for assessing retinal function in both the laboratory and the clinic. It is a mass electrical potential that changes in a characteristic way in response to an increase in retinal illumination, and it can be recorded non-invasively using a corneal electrode. Recording non-invasively provides the opportunity for studying retinal function while maintaining an essentially normal physiological environment for the tissue. However, because it is a mass potential, the ERG represents the summed activity of all retinal cells. For specific information about retinal function, it is important to be able to separately analyse the contributions from the cells and circuits that combine to form the mass response.Much research on the flash ERG has focused on separating the various components of the response that correspond to different retinal cell types. As a result, it is well known that the initial waves of the dark-adapted (scotopic) ERG, the a-and b-waves, originate mainly from cells at early stages of retinal processing. Following an intense brief flash of light from darkness, the negative-going a-wave is generated by rod photocurrents (Penn & Hagins, 1969) and the positive-going b-wave by depolarizing bipolar-cell currents in combination with bipolar cell-dependent K + currents affecting Müller cells (Miller & Dowling, 1970;Stockton & Slaughter, 1989;Xu & Karwoski, 1994;Shiells & Falk, 1999;Robson & Frishman, 1995; for a review see Pugh et al. 1998). For weaker flashes, the a-wave is too small to be seen in the record, and the dominant component of the ERG is the b-wave.For very weak flashes from darkness, in several mammals, e.g. cats, monkeys, humans, and rats, a small post-

A cone photoreceptor releases glutamate at ribbons located atop narrow membrane invaginations that empty onto a terminal base. The unique shape of the cone terminal suggests that there are two transmitter microenvironments: within invaginations, where concentrations are high and exposures are brief; and at the base, where concentrations are low and exposure is smoothed by diffusion. Using multicell voltage-clamp recording, we show that different subtypes of Off bipolar cells sample transmitter in two microenvironments. The dendrites of an AMPA receptor-containing cell insert into invaginations and sense rapid fluctuations in glutamate concentration that can lead to transient responses. The dendrites of kainate receptor-containing cells make basal contacts and respond to a smoothed flow of glutamate that produces sustained responses. Signaling at the cone to Off bipolar cell synapse illustrates how transmitter spillover and synapse architecture can combine to produce distinct signals in postsynaptic neurons.

The electroretinogram (ERG) of anaesthetised dark‐adapted macaque monkeys was recorded in response to ganzfeld stimulation and rod‐ and cone‐driven receptoral and postreceptoral components were separated and modelled. The test stimuli were brief (< 4.1 ms) flashes. The cone‐driven component was isolated by delivering the stimulus shortly after a rod‐saturating background had been extinguished. The rod‐driven component was derived by subtracting the cone‐driven component from the mixed rod–cone ERG. The initial part of the leading edge of the rod‐driven a‐wave scaled linearly with stimulus energy when energy was sufficiently low and, for times less than about 12 ms after the stimulus, it was well described by a linear model incorporating a distributed delay and three cascaded low‐pass filter elements. Addition of a simple static saturating non‐linearity with a characteristic intermediate between a hyperbolic and an exponential function was sufficient to extend application of the model to most of the leading edge of the saturated responses to high energy stimuli. It was not necessary to assume involvement of any other non‐linearity or that any significant low‐pass filter followed the non‐linear stage of the model. A negative inner‐retinal component contributed to the later part of the rod‐driven a‐wave. After suppressing this component by blocking ionotropic glutamate receptors, the entire a‐wave up to the time of the first zero‐crossing scaled with stimulus energy and was well described by summing the response of the rod model with that of a model describing the leading edge of the rod‐bipolar cell response. The negative inner‐retinal component essentially cancelled the early part of the rod‐bipolar cell component and, for stimuli of moderate energy, made it appear that the photoreceptor current was the only significant component of the leading edge of the a‐wave. The leading edge of the cone‐driven a‐wave included a slow phase that continued up to the peak, and was reduced in amplitude either by a rod‐suppressing background or by the glutamate analogue, cis‐piperidine‐2,3‐dicarboxylic acid (PDA). Thus the slow phase represents a postreceptoral component present in addition to a fast component of the a‐wave generated by the cones themselves. At high stimulus energies, it appeared less than 5 ms after the stimulus. The leading edge of the cone‐driven a‐wave was adequately modelled as the sum of the output of a cone photoreceptor model similar to that for rods and a postreceptoral signal obtained by a single integration of the cone output. In addition, the output of the static non‐linear stage in the cone model was subject to a low‐pass filter with a time constant of no more than 1 ms. In conclusion, postreceptoral components must be taken into account when interpreting the leading edge of the rod‐ and cone‐driven a‐waves of the dark‐adapted ERG.

Microspectrophotometry studies show that zebrafish (Danio rerio) possess four cone photopigments. The purpose of this study was to determine the cone contributions to the zebrafish photopic increment threshold spectral-sensitivity function. Electroretinogram (ERG) b-wave responses to monochromatic lights presented on a broadband or chromatic background were obtained. It was found that under the broadband background condition, the zebrafish spectral-sensitivity function showed several peaks that were narrower in sensitivity compared to the cone spectra. The spectral-sensitivity function was modeled with L − M and M − S opponent interactions and nonopponent S- and U-cone mechanisms. Using chromatic adaptation designed to suppress the contribution of the S-cones, a strong U-cone contribution to the spectral-sensitivity function was revealed, and the contributions of the S-cones to the M − S mechanism were reduced. These results show that the b-wave component of the ERG receives input from all four cone types and appears to reflect color opponent mechanisms. Thus, zebrafish may possess the fundamental properties necessary for color vision.

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