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PubMed2006

Visual evoked potentials modulation during direct current cortical polarization.

Transcranial direct current stimulation (tDCS) at low intensity induces changes in cortical excitability that persist after polarization ends. The effects of anodal and cathodal polarization remain controversial. We studied changes in visual evoked potentials (VEPs) during and after anodal and cathodal tDCS by applying, in healthy volunteers, 1 mA polarization through surface electrodes placed over the occipital scalp (polarizing) and over the anterior or posterior neck-base (reference). We compared tDCS applied at two durations, 3 and 10 min and both polarities. We assessed VEP-P100 latencies and amplitudes in response to pattern-reversal checkerboard stimuli before, during, and after polarization. Anodal polarization reduced VEP-P100 amplitude whereas cathodal polarization significantly increased amplitude but both polarities left latency statistically unchanged. These changes persisted for some minutes after polarization ended depending on the duration of tDCS and on the contrast level of visual stimuli. tDCS-induced changes in VEPs seem to depend on the duration of polarization and type of visual stimuli used. The effects induced on visual cortical neurones during polarization are more consistent than the aftereffects. Studying these changes during polarization may therefore improve our understanding of these phenomena.

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PubMed2006

Vep latency: sex and head size.

OBJECTIVE: To investigate whether differences in visual evoked potential (VEP) latencies in a large sample of healthy subjects are influenced by different head size or sex or both. METHODS: Black-and-white pattern-reversal checkerboard VEPs at a frequency of 2c/deg. were recorded in a group of 54 normal subjects of both sexes (age 30.15+/-9.12 years). P100 latency was measured in all subjects and the data were analyzed in the whole sample and in a selected subgroup of subjects of both sexes with comparable head size. RESULTS: In the study group overall, the P100 latency was slightly shorter in females than males and this small difference reached only weak statistical significance (P<0.04) whereas head size differed significantly between sexes (females<males) (P<0.001). No difference was found in the P100 latency in the subgroup of the two sexes with a comparable range of head size. CONCLUSIONS: These findings suggest that VEP latency is relatively constant in a sample of healthy subjects. The slight sex difference in P100 latencies observed in a normal sample is mainly related to the slightly smaller average head size in females than in males. SIGNIFICANCE: Head size, not sex, should be considered for VEP latency normative studies.

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