Electroretinograms and circadian rhythms in green sunfish

Dear Sir, The article by McMahon and Barlow (1992) in this issue seeks to reinterpret some results reported in a prior publication in this journal (Dearry and Barlow, 1987). McMahon and Barlow (1992) suggest that a "fast" response observed in the previous electrical recordings originates from eye movement, and that this potential was incorrectly interpreted as the b-wave of the electroretinogram (ERG) in the prior publication. In response, this letter addresses several pertinent points: (I) observation of ostensibly movement-related artifacts during the previous recording sessions; (2) ERG characteristics; (3) effects of optic nerve section; and (4) circadian rhythm. (I) During the original experiments in 1985, occasional responses obtained from green sunfish maintained in constant darkness evidenced a fast negative deflection, possibly related to eye movement. Of more than 100 fish used in these experiments, only 8 exhibited this type of fast negative response. Furthermore, responses from each of these eight fish were recorded every half hour during a 12-48-h period (i.e., 24-96 responses per fish), and no indication of this type of fast potential was found in more than three responses from a given fish. Other responses recorded from these fish exhibited normal positive b-waves. Thus, these fast negative responses were atypical. This type of response was most often observed during generation of an intensity-response function and was rarely observed at the lower intensities used in circadian experiments. It should be emphasized that such responses were not used to assess ERG amplitude or rhythm. They were not included in the original publication since this type of response, whatever its origin, was thought to be artifactual, was not an issue in its own right at the time, and other responses better demonstrated the difference between subjective night vs. subjective day. In appearance, these responses exhibited a time course and waveform similar to those shown in the right-hand panels of Fig. 1 of McMahon and Barlow (1992). Thus, as in the latter, b-waves in Dearry and Barlow (1987) were characterized by positive, upward deflections; conceivably movement-related responses were downward and negative. The occurrence of this negative response and its potential relationship to movement are not in dispute. However, the presence of this fast negative response in recordings having the same polarity as those reported by Dearry and Barlow (1987) is inconsistent with the idea proposed by McMahon and Barlow (t992) that the original authors misidentified the ERG b-wave. (2) McMahon and Barlow (1992) state that, " . . . the intensity response function of the fast component reported by Dearry and Barlow (1987) differs from that of the b-wave." In the curarized fish used by McMahon and Barlow (1992), a 10-fold increase in illumination intensity (I) elicited an approximately fourfold increase in response amplitude (V). In the report by Dearry and Barlow (1987; Fig. 6 therein), a