Biomedical subjects

Alessandra Martins

Publications and source records attributed to Alessandra Martins.

3 recordsLinked to original sources

PubMed2005

Effect of fixation tasks on multifocal visual evoked potentials.

PURPOSE: This study investigated the effects of cognitive influence on the multifocal visual evoked potential (mVEP) at different levels of eccentricity. Three different foveal fixation conditions were utilized involving varying levels of task complexity. A more complex visual fixation task has been known to suppress peripheral signals in subjective testing. METHODS: Twenty normal subjects had monocular mVEPs recorded using the AccuMap objective perimeter. This allowed simultaneous stimulation of 58 segments of the visual field to an eccentricity of 24 degrees. The mVEP was recorded using three different fixation conditions in random order. During task 1 the subject passively viewed the central fixation area. For task 2 alternating numbers were displayed within the fixation area; the subject on viewing the number '3' in the central fixation area indicated recognition by pressing a button. Throughout task 3, numbers were displayed as in task 2. The subject had the cognitive task of summating all the numbers. RESULTS: Analysis revealed that the increased attention and concentration demanded by tasks 2 and 3 in comparison with task 1 resulted in significantly enhanced central amplitudes of 9.41% (Mann-Whitney P = 0.0002) and 13.45% (P = 0.0002), respectively. These amplitudes became reduced in the periphery and approached those of task 1, resulting in no significant difference between the three tasks. Latencies demonstrated no significant difference between each task nor at any eccentricity (P > 0.05). As the complexity of each task increased the amount of alpha rhythm was significantly reduced. CONCLUSIONS: Our findings indicate that task 1 required a minimal demand of cognition and was associated with the greatest amount of alpha rhythm. It was also the most difficult to perform because of loss of interest. The other two tasks required a greater demand of higher order cognitive skills resulting in significantly enhanced amplitudes centrally and the attenuation of alpha rhythm. Therefore, amplitudes are increased around the area of attention.

Adult↗

PubMed2004

Effect of check size and stimulation rate on blue-yellow multifocal visual evoked potentials.

BACKGROUND: To determine the effect of different stimulus frame rates and check sizes on blue-yellow multifocal visual evoked potentials (mVEP). METHODS: Subjects were examined at the Save Sight Institute at the University Sydney. Experiment 1 involved five adult subjects who underwent binocular stimulation by the Accumap multifocal objective perimeter. The eyes were stimulated with a cortically scaled dartboard pattern consisting of isoluminant blue and yellow checks. These were arranged in three concentric rings extending to an eccentricity of 26 degrees in the visual field. The stimulus pattern was driven by binary sequences resulting in pseudorandom binary exchange of two opposite checkerboard patterns at each of the 32 sites in the visual field. The mVEP were recorded at two different rates of display of the pattern stimulus. In experiment 2, mVEP were tested on 10 normal subjects. Each of the 36 stimulation sites contained a checkerboard pattern of 20, 30, 42 or 56 checks/site, the stimulation pattern was displayed at the optimum rate found in experiment 1. The size of the checks was inversely proportional to the number of checks per site. RESULTS: In experiment 1, the slow frame rate significantly increased the average amplitude throughout the field tested by 50 +/- 10.1% (P = 0.001). Latency was significantly shortened by 6.3% (P < 0.01). In experiment 2, the average amplitude peaked at 30 checks per segment; however, this was only calculated to be significantly different from the smallest check size (F(crit range 4,27) = 0.09 P < 0.05, anova, Tukey's T method). A similar difference was found in ring 1 (F(crit range 4,27) = 0.09, P < 0.05, anova, Tukey's T method). In ring 2, however, there was also a significant difference between 56 checks and 20, 30 and 42 (F(crit range 4,27) = 0.09, anova, P < 0.05). Altering the check sizes did not significantly affect the amplitudes in ring 3. The latencies were not significantly modified by altering check size at any eccentricity. CONCLUSIONS: These findings suggest that slowing the stimulation rate and displaying 30 checks per stimulation segment optimizes the blue-yellow mVEP stimulus.

Adult↗

PubMed2003

Effect of pupil size on multifocal pattern visual evoked potentials.

The purpose of this study was to investigate the influence of pupil diameter on the amplitude and latency of multifocal visual evoked potentials (mfVEP). The multifocal objective perimeter (Accumap; Objectivision) was used to stimulate the visual field at 56 sites extending to 32 degrees using a pseudo-random pattern stimulus. The mfVEP were recorded using bipolar occipital electrodes, 7 min/eye. Ten normal subjects were recruited from the community and one eye was randomly selected for testing. The mfVEP were recorded at four different pupil diameters (2 mm, 4 mm, 6 mm, 8 mm), obtained by applying tropicamide (0.5%) or pilocarpine (2%) in different dilutions. Appropriate refractive correction was provided to overcome cycloplegia and achieve a visual acuity of 6/7.5 or better. Analysis revealed that at most pupil diameters the normalized full field amplitude did not show significant variation, except at the most miotic pupil diameter (2 mm), where the amplitude became reduced, based on 2-way anova and Tukey's T method. There was, however, significant correlation between latency and pupil area (correlation coefficient: upper field -0.63, lower field -0.76). The results suggest that even in the presence of mydriatics or miotics, the mfVEP test can be used to assess diseases that affect amplitude, provided near correction is used. The interpretation of latency, however, must be made with caution, as a borderline conduction defect with a dilated pupil may appear normal.

Adult↗