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 · Unformatted text preview: 4 . D e m o n i a c : o n e w h o i s p o s s e s s e d b y a d e m o n o r d e m o n s . G . A l d o u s H u x l e y : " T h e D ...

Figure 1 shows ERG responses from different species. These responses are to bright light stimuli applied in the dark-adapted state. The ERG of the turtle eye (Fig. 1A) as elicited by a long (900ms) step of light, shows an a-wave and b-wave complex separated from the d-wave which is generated at stimulus offset. A bright light stimulus of 40sec duration is used to record the ERG of the bullfrog in figure 1B (Oakley, 1977). The a-wave and b-wave are followed by the slow corneal-positive c-wave. After termination of the stimulus, a d-wave develops. The ERG responses of the rabbit (Fig. 1C) and the human (Fig. 1D) are elicited by fast bright flashes (50 or 100ms in duration) and therefore, only the a-wave and b-wave are seen. In the human response (Fig. 1D), fast oscillations can also be seen on the ascending limb of the b-wave. These ERGs in the different species clearly differ in amplitude and pattern. Some of this variability is due to species differences, particularly, the relative densities of rods and cones, while technical factors such as duration and intensity of photostimulation and method of recording also affect the waveform. Nevertheless, ERG responses of turtle, bullfrog, rabbit and human (Fig. 1), in addition to those recorded from other vertebrate species, are characterized by the basic features of a negative a-wave followed by a positive b-wave.


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