Spectral Sensitivity Measured with Electroretinogram Using a Constant Response Method

<div><p>A new method is presented to determine the retinal spectral sensitivity function <i>S</i>(<i>λ</i>) using the electroretinogram (ERG). <i>S</i>(<i>λ</i>)s were assessed in three different species of myomorph rodents, Gerbils (<i>Meriones unguiculatus</i>), Wistar rats (<i>Ratus norvegicus)</i>, and mice (<i>Mus musculus</i>). The method, called AC Constant Method, is based on a computerized automatic feedback system that adjusts light intensity to maintain a constant-response amplitude to a flickering stimulus throughout the spectrum, as it is scanned from 300 to 700 nm, and back. The results are presented as the reciprocal of the intensity at each wavelength required to maintain a constant peak to peak response amplitude. The resulting <i>S</i>(<i>λ</i>) had two peaks in all three rodent species, corresponding to ultraviolet and M cones, respectively: 359 nm and 511 nm for mice, 362 nm and 493 nm for gerbils, and 362 nm and 502 nm for rats. Results for mouse and gerbil were similar to literature reports of <i>S</i>(<i>λ</i>) functions obtained with other methods, confirming that the ERG associated to the AC Constant-Response Method was effective to obtain reliable <i>S</i>(<i>λ</i>) functions. In addition, due to its fast data collection time, the AC Constant Response Method has the advantage of keeping the eye in a constant light adapted state.</p></div

Mean spectral sensitivity curves for different species measured with the AC Constant-Response Method.

<p><b>(A)</b> Mean <i>S</i>(<i>λ</i>) curves for mice with <i>λ</i>_max at 359 and 511 nm (n = 3). <b>(B)</b> Mean spectral sensitivity curves for rats with <i>λ</i>_max at 362 and 502 nm (n = 3). <b>(C)</b> Mean spectral sensitivity curves for gerbil with <i>λ</i>_max at 362 and 493 nm (n = 3). <b>(D)</b> Best fittings obtained for each species. For each species, spectral sensitivity curves directly obtained from FFT fits and underlying Gaussian curves representing individual UV and M cone spectral sensitivities are shown.</p

Residual analysis for the spectral sensitivity curves of different species.

<p><b>(A)</b> and <b>(B)</b> Results for mice and rats, respectively, where large differences relative to the adjustment curve were observed. <b>(C)</b> Results for gerbils where the differences were small.</p

Sensitivity curves determined by the residual method.

<p><b>(A)</b> Data obtained by using the AC Constant-Response Method for the gerbil. <b>(B)</b> Spectral sensitivity curve obtained using a Fast Fourier Transform (FFT) filter fitted to data points showed in (A). <b>(C)</b> Two peaks were found by fitting Gaussian normal curves to the FFT results. For the gerbil, the two peaks were located at 362 nm and 493 nm. <b>(D)</b> The same as in (B) showed in log scale.</p

Spectral series and spectral sensitivity obtained by using the AC Constant-Response Method.

<p><b>(A)</b> ERG responses obtained from a light-adapted mouse. ERG responses were driven by flashes of monochromatic equal quanta lights of different wavelength. Note responses to wavelengths in the UV and green ranges. <b>(B)</b> Mean spectral sensitivity for mice. Filled circles and bars represent means and standard deviations for n = 3 animals. <b>(C)</b> and <b>(D)</b> are the mean spectral sensitivities obtained for rats (n = 3) and gerbils (n = 3), respectively. Spectral sensitivity curves for mice and rats were obtained at 4 nm intervals while curves for gerbils were obtained at 12 nm intervals.</p