The latter is observed for both waves ( P 100 or N 2 ) irrespective of electrode position. In contrast, the timing of the SO VEP responses shortens as the contrast level increases. Previous VEP studies have shown that FO stimuli are strongly affected by contrast adaptation and saturation phenomenon. We believe the latter timing increment could be indic- ative of an adaptation or saturation response. In response to FO stimuli, the timing of the VEP P 100 and N 2 waves initially shortens as the contrast level increases from 3 to 10% and then gradually increases thereafter. However the stimulus type does determine the sign of this contrast–timing relationship. This relationship remains irrespective of wave ( P 100 or N 2 ) or electrode position ( O Z or O L ). Regarding the VEP signal, there is a clear relationship (more pronounced with SO stimuli) between the contrast level and the time taken to process the stimulus. electrode position), the method used to measure the cortical processing time (i.e., VEP or RTs) or the VEP wave considered ( P 100 or N 2 waves). However, contrast will impact differently the retino-cortical timing depending on the type of stimulus, the cortical area sampled (e.g. The latter was also found to be contrast dependent. As expected, our results confirm that more complex stimuli require more cortical processing time. For each type of stimuli, the contrast level was varied from 3 to 100%. To achieve our goal, we compared the retino-cortical processing time estimated with VEP ( P 100 and N 2 waves) and the eye-hand RTs measurements evoked to checkerboard pattern reversal stimuli, FO and SO motion reversal stimuli. The purpose of this study was to examine if more complex visual stimuli required more time to be processed and whether this increase was equally reflected in VEP and RT measurements. However, for contrast differences above 50%, the VEP timing differences are not significantly different from 0 while they still remain significant for the RT measurements. Regression analysis reveals that all three slopes decrease progressively as the contrast increase, the slope being steeper with the RT measures, especially at high contrast. This concept is graphi- cally represented in Figure 8 where the timing difference (SO–FO: ordinate) obtained with VEP ( P 100 and N 2 ) and RT measurements is plotted against the contrast level (abscissa). The results presented above clearly suggest that, as the contrast level decreases, SO stimuli progressively require more cortical time than FO stimuli to be processed. Significance levels are indicated in the figures (* p 0.00625, with Bonferroni correction). Student t -tests (Pairwise comparisons) were employed for comparison between subgroups with a Bonferroni correction (0.007 (0.05/8)).
The methods of analysis included a two way repeated measure of analysis (ANOVA test) for latency and contrast for both FO and SO responses. Statistical analysis was performed on average group data. Applying the latter criteria eliminated less than 10% of all RT measurements from the final analysis.
Finally in order to eliminate responses that could have resulted from anticipation or contaminated by the fatigue of the subject, RTs faster than 140 ms or slower than 800 ms were not included in the analysis.
amplitude largest at either side compared to central lead). From here on, the term ’’lateral- ity effect’’ refers to the difference between the responses recorded at O z and O L (i.e. to calculate the P 1 and N 2 peaks and was referred to as O L.
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