| Abstract |
PURPOSE: Visual Evoked Potentials (VEPs) provide important diagnostic information about functional vision by studying the latency and amplitude of specific peaks of the signal which correspond to neural responses. Previous neurological and psychophysical research has demonstrated that the processing of the contrast occurs at the early stages of the visual pathways. Moreover, recent studies have shown that under threat conditions subcortical processes, such as the pupillary light reflex are inhibited, due to the activation of the amygdaloid nucleus. This effect is more pronounced at higher light intensities. The present study consists of two parts. The purpose of the first part was to study the effect of contrast on Visual Evoked Potentials (VEPs) and in particular on the latency and amplitude of the response P100. In the second part the latency characteristics of the VEPs were studies under threat conditions, while the amygdaloid nucleus was active. METHODOLOGY: For the first part of the experiment 14 healthy volunteers participated, aged 2430 yrs (mean 27.2 ± 2 yrs), with refractive errors corrected. VEPs were elicited either using on/off vertically oriented gratings [at a rate of 2 Hz with square wave modulation, having a spatial frequency of 4 c/deg] or using checkerboards (angular substance was 60 and 15 arcmin at 150 cm distance) reversing at 2Hz. A range of contrasts was studied, 14 levels between 100% - 2% for the gratings and 100% and 10% for checkerboards. Stimuli were displayed on a SONY F-520 GDM monitor by means of a VSG 2/5 card (CRS) subtending a circular field of 7 degrees with a constant mane luminance of 30 cd/m2. Subjects viewed the stimulus monocularly (dominant eye open) with natural pupils. VEP recordings represent the average of 128 repetitions. For the second part VEP were elicited using the onset/offset gratings with the stimulus parameters being the same to the first part of the experiment. Only two contrast levels, 8% and 12% were used. Ten out of the 14 volunteers from the first group participated, two weeks following the first examination. Three conditions were studied, baseline, safe and threat. In order to employ a threat condition a devise which could produce electric shock through electrodes attached to the wrist of the volunteers was used (Philip Harris). The purpose of this device was only to threaten the volunteers and not to electrocute them actually. Before the recordings the volunteers were asked to fill a STAIT Trait and STAI State Questionnaires, in order to evaluate their anxiety. For the statistical analysis we used repeated measures ANOVA with psychological condition (Threat and Safe) and contrast (8% and 12%) being the within-subject factors. Similar analysis was applied for the P100 amplitude. Finally, we used Pearson correlation coefficient to relate the anxiety score of each volunteer with the change of P100 latency under safe and threat conditions. RESULTS: Analyzing the measurements we found a strong effect of contrast in latencies: increasing contrast generates shorter P100 latencies. On the contrary, the contrast did not show any consistent effect on the P100 amplitude. Using the average of the absolute values from all the volunteers we observed that P100 latency was differentially affected either side of 10% contrast, with the effect being more pronounced for contrasts > 10%. The recordings of Pattern Reversal VEPs showed strong correlation between latency and target contrast for target size 15 arcmin and 60 arcmin and the same correlation existed for amplitude and contrast. Although the target size affected latency of P100 we found no correlation with response amplitude. As for the second part of the study we were able to confirm our assumptions concerning the decrease of latency in threat conditions and higher stimulus contrast, but we found no change of amplitude with these factors. We also demonstrated a strong correlation between anxiety and P100 latency. We observed that low anxiety accelerated visual processing of contrast, while high anxiety decelerated it. We were not able to demonstrate a statistical correlation between amplitude and target contrast but this was expected since VEP amplitude shows huge between-subject variability. CONCLUSIONS: The change in slope in latency contrast plot is likely due to higher sensitivity of Magno neurons in comparison to the Parvo neurons as has already been demonstrated by neurophysiological and psychophysical studies. Using average values from the latency-contrast responses from all the volunteers, we were able to reveal that for the specific stimulus conditions Magno neurons show ~ 2.5 times higher contrast gain than Parvo neurons. Regarding the behavior of VEPs under threat conditions our results are important because they hint that fast visual processing is a mechanism that enables a person with low or normal anxiety to perceive faster an external stimulus in conditions where he/she needs high demands of attention or alertness. This mechanism does not seem to operate so effectively in persons with high anxiety. This implies two things: that this ineffectiveness is primal or that it forms an adaptive reaction of the organism. Moreover, the correlation of VEP latency with anxiety may contribute to the understanding of the variability of the VEP latency in normal persons, an important finding that has not been demonstrated in the past.
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Μελέτη Οπτικών Προκλητών Δυναμικών σε Συνθήκες Απειλής
