Abstract
Purpose::
To characterize the temporal frequency response of the rat ERG in terms of the underlying neuronal components.
Methods::
Adult Long-Evans rats were anesthetized with ketamine and xylazine. Light stimuli were delivered by pulse-width modulated LEDs with a peak wavelength of 505 nm. Luminance was modulated sinusoidally at temporal frequencies from 2 to 30 Hz, with a mean luminance of 350 cd/m2 and a contrast of 90%. In addition, contrast-response functions were measured at contrasts ranging from 5 to 90% at temporal frequencies of 6 and 20 Hz. The amplitudes of the ERG response harmonics were derived from power spectral densities using Matlab.
Results::
The fundamental response (F1) to sinewave flicker exhibited a low-pass pattern in the frequency range from 2 to 30 Hz, but the second harmonic (F2) showed two regions. For low temporal frequencies (below 10 Hz), the F2 component was small and decreased with increasing stimulus frequency. On the other hand, F2 made a substantial contribution to the flicker ERG responses at higher temporal frequencies. The properties and neuronal origin of these two components of the ERG responses were investigated with stimuli at temporal frequencies of 6 and 20 Hz. At 6 Hz, the contrast-response function for F1 was well-fit by a straight line, whereas at 20 Hz, the F1 response elicited at various contrast modulations could best be described by an exponential function, indicating a compressive contrast-response relationship. At both 6 and 20 Hz, the F1 responses were significantly reduced by the intravitreal injection of L-AP4, whereas the application of PDA or CNQX slightly enhanced the F1 responses. These results indicate that ON bipolar cells are likely to be the primary source of the ERG responses at both low and high temporal frequencies.
Conclusions::
Two components were observed in the temporal frequency response of the rat ERG: a relatively linear component at low temporal frequencies, and a nonlinear component at high temporal frequencies. This difference may reflect the intrinsic dichotomy of sustained and transient ON-bipolar cells in mammalian retina.
Keywords: electroretinography: non-clinical • bipolar cells • retinal connections, networks, circuitry