E. W. Snyder

Life-span alterations in visually evoked potentials and inhibitory function.

Visually evoked potentials (VEPs) elicited by patterned and unpatterned flashes were recorded from 211 healthy males aged 4-90 years. A measure of similarity between the two kinds of VEPs was obtained by correlating the digital values comprising the two waveforms. Across the life-span, correlations followed a U-shaped curve; patterned and unpatterned flash VEPs were most alike for the youngest and oldest subjects. This age effect, localized to scalp areas overlying visual cortex, is compatible with a concept of reduced inhibitory functioning within the visual systems of the young and the old. At central scalp, patterned and unpatterned flash VEP waveforms were more effectively differentiated by the right hemisphere. This observation agrees that the right hemisphere specializes in analyses of spatial material.

PATTERN REVERSAL EVOKED POTENTIAL AMPLITUDES: LIFE SPAN CHANGES

Pattern reversal evoked potentials (PREPs) were recorded from people whose ages ranged from 4 to 90 years. Dramatic reductions in PREP amplitudes occurred between childhood and adolescence. These changes were most evident in females. Following adolescence there were no significant changes in amplitudes, even to old age. Latencies, on the other hand, have been shown to change most dramatically between adulthood and old age. PREP amplitudes and latencies, therefore, appear to provide different and unique information regarding development and aging. One cannot ignore PREP amplitudes without sacrificing information regarding early development.

The Evoked Response as a Measure of Cerebral Dysfunction

It has been scarcely a generation since Dawson (1947,1954) described and developed the technique for summing and averaging the brain’s electrical response to repeated stimuli. Since then, there has been an avalanche of research including many aspects of behavior in species ranging from worms to man. What was first thought to be a fairly simple response has cone to be seen as a complex pattern of as many as 15 identifiable wave components or reproducible patterns of polarity, amplitude and duration that lasts for hundreds of milliseconds. Recently great strides have been made in understanding the neurogenesis of these components, thereby strengthening both the clinical and research applications of the technique.

Age differences in augmenting/reducing of occipital visually evoked potentials

Abstract Visually evoked potentials (VEPs) to 3 intensities of flash were recorded from 220 males aged 4–90 years. Amplitude-intensity slope measures of occipital VEPs revealed that most children were reducers, i.e., the amplitude of a middle latency wave attenuated in response to increased flash intensity. The tendency to ‘reduce’ VEP amplitude was strongest in the youngest children then rapidly decreased in strength with age so that by mean age 14 years they were predominantly augmenters, as were older individuals. Two groups of 15 children were studied further: those with the most negative slopes, reducers, and those with the most positive slopes, augmenters. Reducers had significantly larger VEPs than augmenters for dim flash conditions, thus supporting the theory that reducing is a CNS protective mechanism. However, reducers required significantly brighter flashes than augmenters for visual threshold determinations, suggesting that the tendency to augment or reduce sensory stimulation may be a basic perceptual phenomenon that governs the registration of even very weak stimuli. Analyses of single trial potentials (STPs) from which VEPs were averaged indicated that augmentation and reduction of VEP amplitude were more closely related to increased and decreased variability among STP wave forms than to intensity related changes in STP amplitudes. Individuals cannot be accurately classified as augmenters and reducers unless the area from which VEPs are recorded is specified since for some subjects, especially children, VEPs can augment at central scalp while reducing at occipital scalp.

Visual evoked potentials in monkeys

Visual evoked potentials (VEPs) were recorded from 2 cortical sites in stump-tailed macaques. VEPs recorded from striate cortex were basically consistent between animals (especially at low light intensity), remained remarkably stable over time, and compared favorably to VEPs reported by other investigators. We concluded that the VEP recorded from the striate cortex of day-active monkeys consists of 5 major peaks within the first 250 msec. The potentials recorded from post-central gyrus were simpler and more individualized and did not show intensity-related latency changes or increases in inter-subject variability. However, amplitudes of potentials recorded from both electrode placements increased with light intensity apparently reflecting the amplitude of individual potentials rather than the variability of these potentials from which the average VEPs were derived.

Phencyclidine-induced alterations of rat electrophysiology

Phencyclidine (PCP) at high doses causes both excitation and depression in the rat. The visual evoked potential (VEP) was measured in rats following PCP administration in doses ranging from 1 mg/kg to 56 mg/kg. Consistent lengthening of VEP latencies suggests that PCP has an unusual inhibitory effect on visual function in the presence of the excitatory signs of bilaterally synchronous cortical spiking. The epileptogenic properties of PCP are quite evident in rats.

Sudden toxicity of methadone in monkeys: Behavioral and electrophysiological evidence

A sudden and potentially lethal toxic reaction to a previously well-tolerated maintenance dose of methadone occurred in 4 of 6 monkeys. The reaction was characterized by gross behavioral and respiratory depression and a marked attenuation of both early and late components of the visual evoked response with an increase in most latencies. The nature of the evoked response alteration suggests a widespread central nervous system depressant effect of the drug during toxicity. Concomitant with the toxic reactions were dramatic increases in plasma methadone concentrations. Therefore the observed changes in sensitivity to methadone would appear to be the consequence of a sudden shift in pharmacokinetics resulting in toxic plasma concentrations.

Naloxone-induced epileptogenesis has brain-site specificity in rats☆

Naloxone injected into the lateral septum caused intense electrocortical seizures which did not occur following callosal or periaqueductal gray (PAG) injections. Conversely, injections into periaqueductal gray caused a characteristic and pronounced backward circling with only moderate electrocortical changes. Intracerebral injection of naloxone had effects, including epileptogenesis, which appeared to be site-specific. These effects are in some ways similar to the effects of enkephalin and morphine.

Methadone-induced changes in the visual evoked response recorded from multiple sites in the cat brain.

Visual evoked responses (VERs) and EEG were recorded following the i.p. administration of five doses of methadone (0.5–4 mg/kg) to 12 adult cats, which were implanted with cortical and subcortical electrodes. Additional cats, subjected to the same drug regimen, were used to evaluate plasma methadone concentrations. Doses of methadone that produced plasma concentrations between 80 and 190 ng/ml differentially affected VERs recorded from cortical and subcortical sites. Of the subcortical structures evaluated, the limbic system, specifically the hippocampus, was the most sensitive to the effects of the drug. These effects appeared to be primarily depressant. Responses recorded from the reticular formation and centromedian were affected only by the highest dose of methadone, while VERs recorded from cortical sites were reliably altered following the two highest doses and appeared to reflect both excitation and depression. Behavioral changes, however, were clearly evidenced in some cats at lower doses of methadone. Therefore, the data suggest (1) that those structures evaluated electrophysiologically did not reflect the full force of the drugs action as evidenced by its effect on behavior, (2) that cortical and subcortical recording sites have differential sensitivities, and (3) that one clearly defined, principal site of action of methadone is absent in the cat.

Naloxone-induced augmentation of the photically evoked afterdischarge in conscious rats ☆

Naloxone, at subconvulsive dose levels, from 1 to 15 mg/kg were administered to conscious rats. Significant increases in photically evoked afterdischarge occurrence were seen at naloxone dose levels above 5 mg/kg with no clinical evidence of seizure activity being observed. Typically photically evoked afterdischarge augmentation is only observed following the administration of convulsive drugs.