Paired-pulse flash-visual evoked potentials: New methods revive an old test

Impaired Visual Inhibition in Amnestic Mild Cognitive Impairment

Objective.The pathophysiology of amnestic mild cognitive impairment (aMCI) and Alzheimer disease (AD) is still a matter of debate. Visual system might be precociously altered, especially for its cholinergic connections. We thus studied patients with aMCI compared to AD with paired-pulse flash-visual evoked potentials (paired-F-VEPs), a putative marker of cholinergic function. Methods. We enrolled 12 adult patients with aMCI and 12 with AD. 14 normal age- and sex-matched subjects acted as controls (HS). Stimuli were single flashes, with interspersed random flash pairs at critical interstimulus intervals (ISIs, 16.5 to 125 ms) with closed eyes. The "single" (unconditioned) F-VEP was split into a "main complex" (50 to 200 ms after the flash) and a "late response" (200 to 400 ms). As for paired stimulation, the "test" F-VEP emerged from electronic subtraction of the "single" F-VEP from the "paired"-F-VEP. Results. In the single F-VEP, P2 latency was prolonged in patients (aMCI and AD) compared to HS (p < .05). As to the paired F-VEPs, in aMCI the "late response" normal inhibition was abolished at ISIs 50-62.5 ms (p ≤ .016), compared to AD and controls. No changes were detected for the "main complex". Conclusions. Paired-F-VEPs demonstrate a defective neural inhibition in the visual system of patients with aMCI at critical intervals. It may represent a compensatory mechanism against neuronal loss, the failure of which may be involved in AD development. Paired-F-VEPs may warrant inclusion in future preclinical/clinical studies, to evaluate its potential role in the pathophysiology and management of aMCI.

Multimodal integration and modulation of visual and somatosensory inputs on the corticospinal excitability

OBJECTIVE

Corticospinal excitability may be affected by various sensory inputs under physiological conditions. In this study, we aimed to investigate the corticospinal excitability by using multimodal conditioning paradigms of combined somatosensory electrical and visual stimulation to understand the sensory-motor integration.

METHODS

We examined motor evoked potentials (MEP) obtained by using transcranial magnetic stimulation (TMS) that were conditioned by using a single goggle-light-emitting diode (LED) stimulation, peripheral nerve electrical stimulation (short latency afferent inhibition protocol), or a combination of both (goggle-LED+electrical stimulation) at different interstimulus intervals (ISIs) in 14 healthy volunteers.

RESULTS

We found MEP inhibition at ISIs of 50-60 ms using the conditioned goggle-LED stimulation. The combined goggle-LED stimulation at a 60 ms ISI resulted in an additional inhibition to the electrical stimulation.

CONCLUSIONS

Visual inputs cause significant modulatory effects on the corticospinal excitability. Combined visual and somatosensory stimuli integrate probably via different neural circuits and/or interneuron populations. To our knowledge, multimodal integration of visual and somatosensory inputs by using TMS-short latency inhibition protocol have been evaluated via electrophysiological methods for the first time in this study.

Flash-evoked high-frequency EEG oscillations in photosensitive epilepsies

OBJECTIVE

To determine the feasibility of measuring scalp-recorded, flash-evoked, high-frequency EEG oscillations (F-HFOs) using a relatively simple technique. Furthermore, to assess whether F-HFOs are enhanced in photosensitive epileptic patients and if they might be proposed as a putative non-provocative biomarker of photosensitivity.

METHODS

We studied 19 photosensitive patients with idiopathic generalized epilepsy, and 22 controls matched for demographic features. We extracted F-HFOs from the broadband scalp flash-visual evoked potential (b F-VEP) through appropriate filtering. We measured F-HFO amplitude, number and latency. Also, we carried out a time-frequency domain spectral F-HFO analysis. Inter-group statistics was performed. Within-groups, F-HFO features were correlated to the b F-VEP.

RESULTS

The N3-N3^I wave of the b F-VEP was significantly (p = 0.01) larger in patients compared to controls. The same was true for the inter-group F-HFO amplitude (p = 0.01). F-HFOs showed two main spectral peaks (∼88 and ∼125 Hz), whose power was greater (p = 0.001) in patients than in controls. The ∼88 Hz peak power exceeded the upper normal range in 15/19 patients. Patients showed a significant (p = 0.04) correlation between the ∼88 Hz peak power and the size of the N3-N3^I wave.

SIGNIFICANCE

A simplified F-HFO measurement proved feasible. In patients, F-HFOs were enhanced in terms of both size and spectral power, suggesting a role in the generation of the photoparoxysmal response. Some spectral features of the F-HFOs may be proposed as a putative non-provocative marker of epileptic photosensitivity.

Reliability of the Flash Visual Evoked Potential P2: Double-Stimulation Study

The flash visual evoked potential P2 (FVEP-P2) has been identified as a potentially useful clinical, diagnostic tool for Alzheimer's dementia (AD) and mild cognitive impairment (MCIa) due to its association with cholinergic functioning in the brain. The FVEP-P2 is the second positive component of the VEP waveform elicited by a single strobe flash. Despite finding a selective delay in the latency of the FVEP-P2 in AD and MCIa groups, adequate levels of sensitivity and specificity have not been achieved due to natural group differences and inter-individual variability. In response, Fix and colleagues introduced a novel, double-stimulation paradigm that contained two strobe flashes (i.e., stimulations). The first stimulation served as a visual challenge while the second stimulation produced the recorded FVEP-P2 component. The results of that investigation indicated that the latency of the FVEP-P2 could be used to reliably discriminate between aMCI and healthy controls when the ISI of the double-stimulation condition was 100 ms or higher. Unfortunately, very little is known regarding the psychometric properties of the FVEP-P2 when produced by a double-stimulation condition. Consequently, we assessed the test-retest reliability of the FVEP-P2 latency produced by a single- and twelve double-stimulation conditions in a sample of young, healthy individuals (N = 20). Results indicated that while the FVEP-P2 latencies produced by the single- and double-stimulation paradigm were reliable, the intra-individual variability continued to be too high for the FVEP-P2 latency to be used clinically. Methods of reducing the intra-individual variability are discussed, including the use of monochromatic light.

Change in flash visual evoked potentials in New Zealand albino rabbits after sub-tenon's anesthesia

CONTEXT

The occurrence of amaurosis during ophthalmic anesthesia is well known. The reason for this manifestation has not been studied.

PURPOSE

To investigate the effect of sub-tenon's anesthesia on visual conduction in rabbit eyes.

METHODS

Fifteen right eyes of 15 New Zealand albino rabbits were included. 2% lidocaine hydrochloride and 0.75% bupivacaine hydrochloride (1 ml, 1:1 mixture) was injected in the sub-tenon's space of 8 eyes while the control group (n = 7) was injected with 1 ml physiological saline. Flash visual evoked potentials (FVEP) were performed with Roland reti-scan system before and, 5 min, 15 min, and 5 days after injection. The natural pupillary diameter and minimal pupillary diameter with light reflex were recorded.

RESULTS

In the anesthesia group, N1 latency, P1 latency, and P1 amplitude were 17.13 ± 1.13 ms, 28.25 ± 1.83 ms, 13.45 ± 4.36 μv respectively before injection; 21.75 ± 3.06 ms, 29.63 ± 2.67 ms, 7.24 ± 4.64 μv at 5 min after injection; 22.25 ± 1.39 ms, 29.50 ± 2.51 ms, 7.54 ± 4.47 μv at 15 min after injection, and, 17.75 ± 0.71 ms, 28.13 ± 2.42 ms, 13.17 ± 4.08 μv 5 days after injection. When compared with baseline, N1 latency at 5 min and 15 min after injection showed prolongation (p = 0.019 and p = 0.001, respectively). Likewise, P1 amplitude decreased at 5 min and 15 min after injection (p < 0.001, p < 0.001, respectively). Both N1 latency and P1 amplitude recovered 5 days after the injection. Pupillary light reflex (PLR) constriction amplitude was 35.42% and 0.00% before and at 5 min after injection (p = 0.012). After 5 days it recovered to 33.33%. The FVEP and PLR constriction amplitude did not change significantly after injection in the control group.

DISCUSSION

Sub-tenon's anesthesia was associated with changes in the FVEP and pupullary light reflex in rabbit eyes in our study.

CONCLUSIONS

The data from this study suggested that sub-tenon's anesthesia could reversibly block visual conduction in rabbit's eyes.

Bilateral tDCS on Primary Motor Cortex: Effects on Fast Arm Reaching Tasks

Background

The effects produced by transcranial direct current stimulation (tDCS) applied to the motor system have been widely studied in the past, chiefly focused on primary motor cortex (M1) excitability. However, the effects on functional tasks are less well documented.

Objective

This study aims to evaluate the effect of tDCS-M1 on goal-oriented actions (i.e., arm-reaching movements; ARM), in a reaction-time protocol.

Methods

13 healthy subjects executed dominant ARM as fast as possible to one of two targets in front of them while surface EMG was recorded. Participants performed three different sessions. In each session they first executed ARM (Pre), then received tDCS, and finally executed Post, similar to Pre. Subjects received three different types of tDCS, one per session: In one session the anode was on right-M1 (AR), and the cathode on the left-M1 (CL), thus termed AR-CL; AL-CR reversed the montage; and Sham session was applied likewise. Real stimulation was 1mA-10min while subjects at rest. Three different variables and their coefficients of variation (CV) were analyzed: Premotor times (PMT), reaction-times (RT) and movement-times (MT).

Results

triceps-PMT were significantly increased at Post-Sham, suggesting fatigue. Results obtained with real tDCS were not different depending on the montage used, in both cases PMT were significantly reduced in all recorded muscles. RT and MT did not change for real or sham stimulation. RT-CV and PMT-CV were reduced after all stimulation protocols.

Conclusion

tDCS reduces premotor time and fatigability during the execution of fast motor tasks. Possible underlying mechanisms are discussed.

Transcranial Direct Current Stimulation Effects on Single and Paired Flash Visual Evoked Potentials

Transcranial direct current stimulation (tDCS) applied over the occipital cortex has a controversial effect on the visual cortex excitability. Paired flash visual evoked potentials (paired F-VEPs) offer a unique method to express neural inhibition within the visual system. However, no studies have explored the effects of tDCS on F-VEPs in humans. The aim of this study was to evaluate the changes of single- and paired-F-VEPs during and after tDCS in healthy humans. Twenty-six healthy volunteers participated. F-VEPs were recorded from occipital electrodes with closed eyes. Stimuli were single flashes, intermingled to flash pairs at the interstimulus interval of 125, 62.5, 50, 33.3, 16.6, and 11.1 ms (internal frequency of 8, 16, 20, 30, 60, and 90 Hz). The single F-VEP was split into a "main complex" and a "late response." As to paired stimuli, the "test" F-VEP emerged from electronic subtraction of the single-F-VEP to the paired-F-VEP. In experiment 1, the return electrode was located on the scalp and we studied changes in F-VEPs after anodal, cathodal (1 mA, 15 min) and sham stimulation. A second experiment was performed in which F-VEPs were recorded before, during and after tDCS stimulation (anodal and cathodal) with the return electrode on the neck. F-VEPs recorded in experiment 1 did not detect any significant change after tDCS. In experiment 2 anodal polarization significantly increased the P2 latency (P = .031) and reduced the amplitude of the "late response" of the single F-VEP (P = .008). As for the paired F-VEPs, no significant changes were detected. In conclusion, low-intensity anodal tDCS has weak inhibitory aftereffects on the single F-VEP and no effects on the paired F-VEPs. Further methodological studies are needed to improve polarization efficacy.

Interaction between visual and motor cortex: a transcranial magnetic stimulation study

The major link between the visual and motor systems is via the dorsal stream pathways from visual to parietal and frontal areas of the cortex. Although the pathway appears to be indirect, there is evidence that visual input can reach the motor cortex at relatively short latency. To shed some light on its neural basis, we studied the visuomotor interaction using paired transcranial magnetic stimulation (TMS). Motor-evoked potentials (MEPs) were recorded from the right first dorsal interosseous in sixteen healthy volunteers. A conditioning stimulus (CS) was applied over the phosphene hotspot of the visual cortex, followed by a test stimulus over the left primary motor cortex (M1) with a random interstimulus interval (ISI) in range 12-40 ms. The effects of paired stimulation were retested during visual and auditory reaction-time tasks (RT). Finally, we measured the effects of a CS on short-interval intracortical inhibition (SICI). At rest, a CS over the occiput significantly (P < 0.001) suppressed test MEPs with an ISI in the range 18-40 ms. In the visual RT, inhibition with an ISI of 40 ms (but not 18 ms) was replaced by a time-specific facilitation (P < 0.001), whereas, in the auditory RT, the CS no longer had any effect on MEPs. Finally, an occipital CS facilitated SICI with an ISI of 40 ms (P < 0.01). We conclude that it is possible to study separate functional connections from visual to motor cortices using paired-TMS with an ISI in the range 18-40 ms. The connections are inhibitory at rest and possibly mediated by inhibitory interneurones in the motor cortex. The effect with an ISI of 40 ms reverses into facilitation during a visuomotor RT but not an audiomotor RT. This suggests that it plays a role in visuomotor integration.