Recording Of Visual Evoked Responses Research Articles

Functional development of the visual system in human fetus using magnetoencephalography

The development of the human brain in utero is normally regarded as a dynamic process involving mainly structural and quantitative changes in neurons and their distribution. However, it is generally accepted that a parallel development of functional specialization occurs in certain areas of the brain, especially in the primary cortex. Nearly all knowledge of functional fetal brain development has been obtained from various animal studies rather than human studies. These studies show that the primary sensory areas like auditory, visual, and somatosensory cortex show a basic function similar to that of a fully developed brain. It has been specifically shown that the visual system develops during fetal life and becomes functional before birth. Several studies have demonstrated the feasibility of using visual evoked response (VER) recordings on preterm human infants to follow the functional development of the visual system. With the advent of the noninvasive technique of magnetoencephalography (MEG), human fetal VER recordings are now possible thus providing the opportunity to track its functional development with gestation. We present and discuss the results of VER recordings in human fetuses starting at 28 weeks of gestation performed using a 151-channel MEG system.

The light-flash-evoked response as a possible indicator of increased intracranial pressure in hydrocephalus

Surgical treatment of infantile hydrocephalus by shunt implantation may result in suboptimal intracranial pressure. Major neurological impairments and death are usually prevented by shunt treatment, but minor sequelae may persist or develop. The introduction of adjustable shunts has improved the possibilities of optimizing shunt function and minimizing the risk of such impairments. However, it is still impossible to determine the intracranial pressure without invasive measurements. Clinical findings and procedures such as computed tomography (CT) are not always enough to allow a conclusion as to whether a child's signs and symptoms are the result of suboptimal intracranial pressure (shunt dysfunction) or are of another etiology. With the aim of reducing the number of invasive pressure measurements and CT scans, we investigated the effect of increased intracranial pressure on the visual evoked response (VER). Binocular light flash stimuli of supramaximal intensity were used and VER recordings were performed from Oz and Cz. The VER results from a group of 31 infants and children with hydrocephalus and 2 children with pseudotumor cerebri were compared with responses from a control group of 35 healthy children. The results show that a subpotential, P' (P-prime), usually just preceding P1 (P100), had an increased latency ( > 96 ms) in all hydrocephalic children before surgery. The P' latency in this group was usually even above 110 ms. The latencies of other VER potentials were also increased but not as consistently as P'. After surgical intervention the VER latencies decreased and usually normalized. The P' latency in four children in the control group was just above the borderline latency, but was less than 110 ms.(ABSTRACT TRUNCATED AT 250 WORDS)

The auditory P300 event—related potential: An objective marker of the encephalopathy of chronic liver disease

Recently many variants of electroencephalogram-evoked responses have been studied as potential diagnostic aids in the detection and evaluation of hepatic encephalopathy. This study assesses the value of the auditory P300 event-related potential--a slow component of the auditory evoked response--as a tool in this field. Twenty-one nonencephalopathic and 12 encephalopathic (grade 1/2) cirrhotic patients and 26 controls were assessed clinically and psychometrically. Electroencephalogram spectral analysis and visual evoked response recordings were also conducted. An auditory P300 wave was elicited using the standard two-tone discrimination paradigm. The latency and amplitude of this wave were measured. The latency of the P300 was found to be significantly increased in the encephalopathic patients compared with both nonencephalopathic cirrhotic and control groups (p less than 0.05). Amplitude of the wave was decreased in both nonencephalopathic and encephalopathic patients, but this was not statistically significant. This study suggests that the latency of the P300 is a good marker of grades 1 and 2 clinical hepatic encephalopathy. The delays in the P300 latency may indicate that encephalopathic patients have a deterioration of their stimulus evaluation abilities.

An evaluation of different techniques for removing eye-blink artifact from visual evoked response recordings.

ABSTRACTThis investigation compared four methods used to control eye‐blink artifact in visual evoked response (VER) recordings using the augmenting‐reducing paradigm: rejecting trials containing blinks, presenting the stimuli through closed eyelids, and two different techniques for subtracting a proportion of the averaged electro‐oculogram (FOG) from the averaged VER. The two subtraction procedures differed in the way in which they estimated the amount of EOG to be subtracted and in the case with which they could be applied. Twenty normal, female subjects were exposed to a series of four different intensity light flashes twice with eyes open and once with eyes closed. Blinking was monitored by recording the EOG and EEG was recorded from three midline electrodes. The results indicated that the two subtraction techniques were very similar. They effectively removed ocular artifact and produced corrected VERs that showed high retest stability. Presenting the flashes through closed lids yielded idiosyncratic data, and rejecting trials contaminated by blinks was found to have limited utility with the VER paradigm used in the present study.

Effect of pupillary dilatation in flash VER testing.

Light flash transient visual evoked response (VER) testing is often a part of the perioperative evaluation of eyes with opaque media, and pupillary size in these patients may not be knowable or may be inadvisable to alter. We therefore evaluated transient light flash VER responses before and after pupillary dilation in normal controls and in patients with media opacities to evaluate the effects of pupil size on the amplitude and latency of the recorded VER. Monocular testing was performed bilaterally with a Grass PS-22 photostimulator at the S1 and S16 settings, with monopolar recordings 1 cm above the inion. One hundred twenty-eight 200-msec samplings were recorded and averaged on a Nicolet CA-1000 clinical averager. Latencies were often shorter with the stronger light stimulus, but pupillary dilation did not produce a clear trend toward shorter latencies. Overall, pupillary dilatation did not significantly increase recorded amplitudes, nor did iris color appear to be a factor. These results suggest that, under our testing conditions, pupillary dilatation is not necessary for obtaining reliable transient flash VER recordings.

Visual evoked potentials in the evaluation of "cortical blindness" in children.

Visual evoked responses (VERs) to brief light flashes were recorded from occipital regions in a group of 30 "cortically blind" children aged 4 months to 15 years and were compared with those of 31 children of similar age range who had the same type of central nervous system diseases but without signs or symptoms of blindness. The VERs were analyzed for amplitude, number of peaks, and morphology, following a method used previously by other authors. All the responses had some degree of abnormality, but there were no significant differences between the two groups. The findings suggest that recording of VERs is not always an accurate method for diagnosis of cortical blindness in children since the presence of abnormal responses is not incompatible with normal vision. Only 1 patient with coritcal blindness showed no response.