Changes of Visual Evoked Potentials in Patients with Thyroid Dysfunction

The visual evoked potentials technique records the cerebral bioelectric activity generated by light stimulation. Evoked potentials is a good method of assessing the electrical response of the brain to different stimuli and has been used extensively in the study of brain disturbances. We registered VEP on 18 hyperthyroid and 18 hypothyroid patients, without other endocrine, metabolic, ophthalmologic or neurological illnesses, the 2 groups having practically identical ages, but sex repartition, illness age and treatment duration being statistically different (p[0.001). Hormonal, radiological, biochemical dosages and physiological paraclinic determinations were performed on all 36 subjects, through classical methods, and VEP recordings, with a 4 channel polygraph through pattern reversal stimulation (1 Hz), measuring all parameters (latency, amplitude, duration, surface, steepness etc.) of N75 and P100 waves. Between groups, the recorded hormone levels and other recordings showed great differences, very highly statistically significant (p[0.001), only arterial pressures being just statistically significant (p[0.05). The Pearson�s r correlation coefficients indicated extremely high correlations (r=0.90-0.98) between all parameters. Between the 2 groups, hypothyroid patients had latencies of P100 wave higher with 17.88% (p=0.0028) and of the N75 wave with 9.98% (p=0.016), and the duration of the N75 wave lower with 26.37% (p=0.026), the other parameters of VEP waves modifying up to 52.35%, not statistically sgnificant due to high standard deviations. The duration of N75 correlates with the majority of humoral-paraclinical parameters, indicating a functional interdependency between them and the function of the cortex. The ratio of the amplitudes and of the areas of P100/N75 indicate a severe inhibition to hypothyroid patients, a precisely quantified reduction of the cerebral activity.

Amplitude of the transient visual evoked potential (tVEP) as a function of achromatic and chromatic contrast: Contribution of different visual pathways

We investigated how the stimulation mode influences transient visual evoked potentials (tVEP) amplitude as a function of contrast of achromatic and isoluminant chromatic gratings. The chromatic stimulation probed only responses to the red-green axis. Visual stimuli were monocularly presented in a 5° diameter circle, achromatic and chromatic horizontal gratings, 1 Hz pattern reversal stimulation, and achromatic and chromatic gratings, 300 ms onset per 700 ms offset stimulation. For the achromatic pattern reversal stimulation, a double slope function describes how the P100 amplitude varied as a function of log contrast which had a limb at low-to-medium contrasts and another limb at high contrasts. For the achromatic onset/offset stimulation, C2 amplitude saturated at the highest contrast tested and a single straight line described how it changed along most of the contrast range. Both presentation modes for chromatic gratings resulted in amplitude versus log contrast relations which were well described by single straight lines along most of the contrast range. The results may be interpreted as if at 2 cpd, achromatic pattern reversal stimulation evoked the activity of at least two visual pathways with high and low contrast sensitivity, respectively, while achromatic onset/offset stimulation favored the activity of a pathway with high contrast sensitivity. The neural activity in the M pathway is the best candidate to be the high contrast mechanism detected with pattern reversal and pattern onset/offset VEPs. The activity of color opponent pathways such as the P and K pathways either combined or in isolation seems to be responsible for VEPs obtained with isoluminant chromatic gratings at both presentation modes. When the amplitudes of chromatic VEPs were plotted in the same contrast scale as used for achromatic VEPs, chromatic contrast thresholds had similar values to those of the achromatic mechanism with high contrast sensitivity.