Dynamic properties of visual evoked potentials in the tectum of cartilaginous and bony fishes, with neuroethological implications.

We have extended the study of Bullock ('84), who reported on visually evoked potentials (VEP) in the tectum of 10 species of elasmobranchs, by adding further stimulus regimes, multichannel recording, and additional taxa, particularly addressing the question of flicker fusion frequency by electrophysiological signs in central processing centers. Using principally the guitarfish, Platyrhinoidis and Rhinobatos, and the bass, Paralabrax, with some additional data from 32 other species, the findings support the following conclusions: 1. Latency of the first main VEP peak, a sharp surface negativity, to a diffuse white flash of submaximal intensity while the eye is moderately light adapted varies from less than 20 ms in some teleosts to greater than 120 ms in agnathans, holocephalans, and some rays. Among the elasmobranchs tested, the sharks are generally faster than the rays. Among the teleosts tested, some species are at least three times slower than others. There is little overlap between the fastest elasmobranchs and the slowest teleosts. 2. After the first VEP peak, later components are more diverse than the classic descriptions of one late surface-negative hump; they may include also sharp peaks, slow humps, and oscillatory waves extending out to greater than 1 s postflash. These are highly labile, variable and similar to OFF responses after a long light pulse. All these components occur already in the retina, whether the optic nerve is intact or cut. Many records do not show the late components; in the same preparation, some tectal loci may and others may not. 3. Ongoing activity (the micro-EEG, over all frequency bands) is depressed between early and late waves after a flash as well as during a long light pulse. 4. Repeated flashes above a few per second do not so much cause fatigue of the VEPs as reduce or prevent them by a sustained inhibition; large late waves are released as a rebound excitation any time the train of flashes stops or is delayed or sufficiently weakened. 5. Repeated flashes depress first the early waves; later waves follow 1:1 up to an upper following frequency (UFF) of approximately 13 Hz in the guitarfishes at optimal intensity and light adaptation (15-17 degrees C). A transition zone of gradual fusion from 15 to 30 Hz is marked by sputtering or irregular sharp VEPs; above a lower fusion frequency (LFF) of 30-40 Hz, the flash train becomes equivalent to steady light.(ABSTRACT TRUNCATED AT 400 WORDS)