Rod- and cone-isolated flicker electroretinograms and their response summation characteristics

Abstract

AbstractThis study defined the amplitude and phase characteristics of rod- and cone-isolated flicker electroretinograms (ERGs) and determined how these responses summate to generate the nonreceptor-specific ERG. Full-field ERGs were obtained from six normally sighted subjects (age 26 to 44 years) using a four-primary LED-based photostimulator and standard recording techniques. The four primaries were either modulated sinusoidally in phase to achieve simultaneous rod and cone activation (ERGR+C; nonreceptor-specific) or in different phases to achieve rod-isolated (ERGR) and cone-isolated (ERGC) responses by means of triple silent substitution. ERGs were measured at two mean luminance levels (2.4 and 24 cd/m2), two contrasts (20 and 40%), and four temporal frequencies (2–15 Hz). Fundamental amplitude and phase for each condition were derived by Fourier analysis. Response amplitude and phase depended on the stimulus conditions (frequency, mean luminance, and contrast), however, for all conditions: 1) response phase decreased monotonically as stimulus frequency increased; 2) response amplitude tended to decrease monotonically as stimulus frequency increased, with the exception of the 24 cd/m2, 40% contrast ERGR+Cthat was sharply V-shaped; 3) ERGRphase was delayed (32 to 210 deg) relative to the ERGCphase; 4) ERGRamplitude was typically equal to or lower than the ERGCamplitude, with the exception of the 2.4 cd/m2, 40% contrast condition; and 5) the pattern of ERGR+Cresponses could be accounted for by a vector summation model of the rod and cone pathway signals. The results show that the ERGR+Camplitude and phase can be predicted from ERGRand ERGCamplitude and phase. For conditions that elicit ERGRand ERGCresponses that have approximately equal amplitude and opposite phase, there is strong destructive interference between the rod and cone responses that attenuates the ERGR+C. Conditions that elicit equal amplitude and opposite phase rod and cone responses may be particularly useful for evaluating rod–cone interactions.