Electrical potentials were recorded in cats from the cornea (ERG) and the cortical surface (VEPs) to visual stimulation centered on the area centralis. Stimuli consisted of: (i) brief flashes of light, (ii) onset-offset luminance modulation of a uniform field; (iii) vertical sinusoidal gratings of various spatial frequencies reversed in contrast at various temporal frequencies (patterned stimuli). Patterned stimuli had the same field size and mean luminance as the uniform field. ERGs were recorded before and after intravitreal injection of two drugs: (i) 2-amino-4-phosphono butyric acid (APB), a glutamate analogue, to block the synapse between photoreceptors and ON-bipolar cells and therefore cell activity in the ON channel; (ii) tetrodotoxin (TTX) to block the spike activity in the retina. Before drug application, flash ERGs consisted typically of a large b-wave. Onset-offset ERGs consisted of two main components: A positive wave at onset and a negative one at offset of the stimulus. Pattern ERGs (PERGs) had a waveform approximately sinusoidal with periodicity corresponding to the second harmonic of the stimulus frequency. Pattern-VEPs had the same waveform as PERGs but a larger amplitude. One hour after intravitreal injection of APB (100 μl 0.1 M) both the flash-ERG b-wave and the positive wave in response to stimulus onset of onset-offset ERG disappeared. The responses to pattern stimuli both at retinal (PERGs) and cortical (VEPs) levels were present and maintained a typical waveform but they were, on average, halved in amplitude. Five minutes after intravitreal injection of TTX (20-40 μl 0.1 M) the activity along the optic nerve was blocked (the activity recorded at lateral geniculate level was suppressed). The onset-offset ERG was completely unaffected. The PERG was still present but the amplitude of response was reduced with respect to the control condition. These results suggest that in the cat: (i) it is possible to dissociate pharmacologically the ERG in response to luminance stimuli from the PERG; (ii) ON channel participates in building up the PERG; (iii) spike activity is necessary, but not sufficient, for PERG generation.
|Original language||English (US)|
|Number of pages||6|
|Journal||Clinical Vision Sciences|
|State||Published - Jan 1 1990|
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