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A.-L. Wurschum, J. Classen, T. Meigen; Non-Invasively Introduced Longterm-Potentiation of VEP Responses Without PERG Changes. Invest. Ophthalmol. Vis. Sci. 2007;48(13):2343.
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Recent research has shown that cortical longterm-potentiation (LTP), introduced non-invasively by rapid visual stimulation with sine wave-gratings (photic tetanus) is specific to spatial-frequency (McNair et al. 2006). The aim of this study was to assure that LTP-like effects are not due to adaptation on a retinal level.
19 visually normal subjects were examined. We simultaneously recorded pattern electroretinograms (PERGs) using DTL electrodes and visual evoked potentials (VEPs) using 3 occipital positions (Oz, 3.5 cm lateral to the left and right of Oz). All recordings were preformed monocularly from the right eye. During the 2 pre-tetanic blocks horizontal sinusoidal gratings of 2 different spacial frequencies (1cpd, 5cpd) were presented for 33ms with randomly jittered inter-stimulus intervals of 1000-1500ms. During the photic tetanus, sinusoidal gratings with a spatial frequency of either 1cpd or 5cpd were presented with a temporal frequency of 8.6 Hz for 2 minutes. Two post-tetanic blocks consisting of the same stimuli as the pre-tetanic blocks followed the photic tetanus. In order to quantify systematic effects of the photic tetanus we subtracted PERGs and VEPs of the pre-tetanic blocks from the post-tetanic block. In order to evaluate, whether these effects exceeded normal fluctuations a similar comparison was preformed for both pre-tetanic and both post-tetanic blocks. Statistical significance was evaluated by "projection onto centroid difference vectors" (Achim, 2001).
No significant differences in waveform of the PERG could be shown, when pre- and post-tetanic blocks were compared. We did though find significant differences (p<5%) in grand mean VEP waveform between the pre-tetanic and the post-tetanic blocks in both tetanized and non-tetanized sinusoidal gratings in the group tetanized with 5cpd gratings. Significant differences could be shown between both pre-tetanic blocks for both sinusoidal gratings.
We showed that LTP modulations of VEP responses are not due to adaptive processes in the retina. The significant changes in VEP waveform between the two pre-tetanic blocks indicate that even the presentation of non-tetanic stimuli can affect responses of subsequent VEP recordings. Our data suggest that a different experimental design is required to properly separate possible spatial frequency specific effects of tetanic stimulation on VEP responses from more general adaptive modulations.
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