The photically evoked afterdischarge: Current concepts and potential applications

Primate somatosensory cortical neurons are entrained to both spontaneous and peripherally evoked spindle oscillations

Recurrent thalamocortical circuits produce a number of rhythms critical to brain function. In slow-wave sleep, spindles (7-16 Hz) are a prominent spontaneous oscillation generated by thalamic circuits and triggered by cortical slow waves. In wakefulness and under anesthesia, brief peripheral sensory stimuli can evoke 10-Hz reverberations due potentially to similar thalamic mechanisms. Functionally, sleep spindles and peripherally evoked spindles may play a role in memory consolidation and perception, respectively. Yet, rarely have the circuits involved in these two rhythms been compared in the same animals and never in primates. Here, we investigated the entrainment of primary somatosensory cortex (S1) neurons to both rhythms in ketamine-sedated macaques. First, we compared spontaneous spindles in sedation and natural sleep to validate the model. Then, we quantified entrainment with spike-field coherence and phase-locking statistics. We found that S1 neurons entrained to spontaneous sleep spindles were also entrained to the evoked spindles, although entrainment strength and phase systematically differed. Our results indicate that the spindle oscillations triggered by top-down spontaneous cortical activity and bottom-up peripheral input share a common cortical substrate.NEW & NOTEWORTHY Brief sensory stimuli evoke 10-Hz oscillations in thalamocortical neuronal activity and in perceptual thresholds. The mechanisms underlying this evoked rhythm are not well understood but are thought to be similar to those generating sleep spindles. We directly compared the entrainment of cortical neurons to both spontaneous spindles and peripherally evoked oscillations in sedated monkeys. We found that the entrainment strengths to each rhythm were positively correlated, although with differing entrainment phases, implying involvement of similar networks.

Baclofen alters flash-evoked potentials in Long–Evans rats

This experiment examined the effects of the GABA-B agonist baclofen on flash-evoked potentials (FEPs) recorded from both the visual cortex (VC) and superior colliculus (SC) of chronically implanted male Long-Evans rats. FEPs were recorded at 5, 25, 45, and 65 min following intraperitoneal injections of saline, and of 1.25, 2.5, 5.0, and 10.0 mg/kg baclofen on separate days. In the VC, the amplitude of components P(23), P(37), N(55), N(150), and P(242) increased, while the amplitude of components N(31) and P(48) decreased following baclofen administration. P(88) was unchanged. In the SC, components P(28), N(49), N(55), and N(59) were reduced in amplitude, while P(39) was unaffected by baclofen. These effects on amplitudes were dose- and time-dependent. Many peak latencies in the VC and SC were altered by baclofen, although there was no obvious pattern of change, with some decreasing, a few increasing, and others unchanged. Body temperature was recorded in a separate group of animals, with both the 5.0 and 10.0 mg/kg doses of baclofen producing significant hypothermia. The 10.0 mg/kg dose of baclofen resulted in a significant decrease in movement during the recording sessions, but not in subsequent open field observations. The results show the involvement of GABA-B receptors in the production/modulation of the various components of FEPs.

Effects of ethanol on flash-evoked potentials of rats: lack of antagonism by naltrexone

The present study examined the effects of ethanol and naltrexone hydrochloride (a nonselective opiate receptor antagonist) on flash-evoked potentials recorded from both the visual cortex (VC) and the superior colliculus (SC) of chronically implanted hooded rats. There were four treatment conditions administered on separate days: Either saline or naltrexone (10 mg/kg; volume of 1.0 ml/kg) was given 10 min before either saline or ethanol (2.0 g/kg; 20% ethanol solution in a volume of 1.26 ml/100 g). Evoked potentials were recorded 15 min after the intraperitoneal injections were completed. Animals were tested at 23.1 degrees C room temperature. In the VC, ethanol significantly decreased the amplitude of components N1, P3, and N3, whereas it increased the amplitude of P2. Components P1 and N2 were unaffected by ethanol treatment. The SC components P3 and N4 were reduced in amplitude by ethanol, but component P1 was not altered. Latencies of all components in both structures were increased by ethanol. Naltrexone alone did not significantly affect the potentials, nor did naltrexone pretreatment significantly alter the effects of ethanol on the potentials. Naltrexone produced a modest hypothermia of about 0.25 degrees C, whereas ethanol resulted in hypothermia of about 1.0 degrees C. Ethanol, either alone or in combination with naltrexone, significantly reduced body movement during the evoked-potential recording sessions. The results indicate that endogenous opioid systems do not play a major role in the acute effects of ethanol on flash-evoked potentials recorded from primary areas of the visual system.

Within-session changes in peak N160 amplitude of flash evoked potentials in rats

The negative peak occurring approximately 160 ms after stimulation (peak N160) of flash evoked potentials (FEPs) of rats changes with repeated testing. Habituation, sensitization, and arousal have all been invoked to explain these changes, but few studies have directly tested these explanations. We examined within-session changes in peak N160 amplitude with repeated testing, and the modulatory effects of stimulus intensity and auditory white noise. Peak N160 amplitude increased with daily testing (between-session changes), and was larger at greater stimulus intensities. In contrast, peak N160 amplitude underwent within-session increases on early days and within-session decreases on later days. The within-session changes were not affected by stimulus intensity. In rats previously tested in a quiet environment, exposure to acoustic white noise increased motor activity and transiently decreased peak N160 amplitude, which then increased and subsequently decreased with continued photic and acoustic stimulation. Repeated testing in the presence of noise did not alter the within-session changes in peak N160 amplitude. Heart rate showed both within- and between-session decreases, but was unaffected by noise. The data suggest that the within-session changes in peak N160 amplitude may reflect a habituation-like response to the test environment.

The effects of low-pass filtering on the flash visual evoked potential of the albino rat

Flash visual evoked potentials (FVEPs) were recorded from the rat in order to determine the effects of low-pass filtering on the wave form. The low-frequency (high pass) filter remained fixed at 3.2 Hz while the setting of the high-frequency (low-pass) filter was progressively raised from 32 Hz to 3.2 kHz. The amplitude of the primary cortical potential (P30) steadily increased while its latency decreased until asymptotic values were recorded with a low-pass cut-off of 320 Hz. Thereafter, there was little additional change in wave form. It is concluded that a bandpass of 3.2-320 Hz is optimal to record the primary cortical response of the FVEP, and this is consistent with the theory that the P30 potential is generated by comparatively slow post-synaptic activity. In a second experiment the effects of low-pass filtering were examined on the later and more labile secondary components of the FVEP wave form. These were found to be less responsive to low-pass filtering than the early components and assumed a near optimal configuration when the high-frequency cut-off was raised to 80 Hz. The high-frequency filter setting which is most appropriate to record the primary component of the FVEP therefore appears to be more than adequate also to record the secondary responses. It is also shown that the same principles of low-pass filtering on the FVEP will apply irrespective of whether the subject is awake or anaesthetised.

Rat flash-evoked potential peak N160 amplitude: modulation by relative flash intensity

The flash-evoked potential (FEP) of rats has a large negative peak (N160) approximately 160 ms following stimulation. This peak has been reported to be modulated by the subject's state of behavioral arousal and influenced by several test parameters. These experiments examined the influences of repeated testing, the number of stimuli/session, interactions of ambient illumination and flash intensity, and the effect of pupillary dilation on the development and amplitude of peak N160. The amplitude of peak N160 increased with daily testing and reached an asymptotic amplitude by about day 10. This amplitude was affected by the intensity of the flash stimulus relative to the ambient illumination (RFI) and appeared to reach a "ceiling" amplitude at greater than 50 dB RFI. The number of stimuli/session and dilation of the subject's pupils did not have a large influence on the growth or asymptotic level of peak N160 amplitude. The data are consistent with the hypothesis that the growth of peak N160 may represent a sensitization-like phenomenon.

Effect of acute respiratory acidosis on multimodality sensory evoked potentials of Long-Evans rats

Auditory, visual and somatosensory evoked potentials (EPs), recorded epidurally from 31 chronically implanted male Long-Evans rats, were studied to examine the pattern of sensory effects caused by hypercapnia. Recordings were obtained before exposures, 10-20 min after the beginning of exposure to CO2 in synthetic air, and 30 min after the end of exposure. Previous recordings revealed no substantial effects of the extended recording period itself. Blood pH during an average exposure of 18.8% CO2 was about 7.1. During this level of CO2 exposure the somatosensory response was almost completely abolished, but the latencies of early detectable components were not affected. In contrast, the latencies of all brainstem auditory evoked response components and the 1-5 interwave time increased, whereas amplitudes were only slightly affected. Amplitudes and latencies of early and late components of the flash EP were decreased and lengthened, but the after-discharge components appeared to be most sensitive to CO2. Concentration-response relationships were examined by exposure of rats to 8 and 16% CO2. The most sensitive EP parameter was average amplitude of the late somatosensory EP components. These results suggest that EPs might be useful for assessing acute metabolic disturbances as well as more commonly assessed neurologic disorders.

Effects of ketamine, naloxone, and physostigmine on flash evoked potentials in rat superior colliculus

Flash evoked potentials were recorded from the superior colliculus of chronically implanted hooded rats at 5 and 20 min following IP injections of saline, ketamine (75 mg/kg), naloxone (10 mg/kg), or physostigmine (0.4 mg/kg) on separate days. Components in an early positive complex were unaffected by ketamine and naloxone, but were reduced in amplitude by physostigmine. A positive spike emerged from the middle of a later negative wave following ketamine administration, but the amplitude of the negative wave was unaltered by naloxone or physostigmine. A succeeding positive component was enhanced by both ketamine and physostigmine. Physostigmine produced the most consistent alterations in latency, with most components increasing in latency. Naloxone pretreatment did not alter ketamine's influence on evoked potential amplitudes. Pretreatment with physostigmine briefly decreased the amplitude of the ketamine-induced positive spike, augmented the amplitude of the succeeding positive component, and also increased most peak latencies. Ketamine, naloxone and physostigmine all produced approximately equivalent hypothermia. Physostigmine, but not naloxone, pretreatment augmented the ketamine-induced hypothermia. The body temperature data suggest that some of the observed latency alterations are secondary to hypothermia. The amplitude data indicate that ketamine and physostigmine produce a combination of similar, distinct, and antagonistic effects on evoked potentials.

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.

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.

Thalamocortical coupling and component properties of visually evoked afterdischarge

Dorsal lateral geniculate nucleus (dLGN) multiple-unit activity (MUA) and single visual evoked responses (VER) followed by afterdischarge (AD) were examined in lightly restrained albino rats. It was found that VER AD spike components are initially quite prominent, exceeding at times the amplitude of the broad surface negative wave components. As VER AD progresses toward termination spike amplitude systematically declines. Wave amplitude, however, remains relatively stable until spike components disappear. Averaged dLGN MUA and VER AD responses to photic stimuli suggest the existence of a precise time-locked relationship between peak dLGN MUA and VER AD spike components. It is proposed that thalamic and cortical mechanisms sustaining VER AD bursting might be operative during sensory information ;processing, serving to sustain neural activity functionally related to stimulus input.

Inappropriate use of albino animals as models in research

Sensory-neural, biochemical-metabolic, and physiological anomalies occur in albino mammals. There are ontogenic and biochemical parallels between the senses, peripheral nervous system, endocrine glands, metabolism, and melanin pigmentation. All albino mammals examined have abnormal optic systems. Many drugs cannot be adequately evaluated in an albino model because of melanin's ability to bind and interact with some chemicals. There is evidence that a general reduction in melanin pigment is correlated with a paucity of amino acids necessary for normal chemical function of the brain. There is a high probability that enzyme levels indicative of metabolic performance are deficient in the liver and kidneys oif albinos. Congenital defects are associated with hypopigmentation in animal models and human syndromes. Melanin is found in abundance in the eye, inner ear, and midbrain where neural impulses are initiated indicating a possible role as an electrophysiologic mechanism. Microwave irradiation differentially affects albino and pigmented animals. Implications of these observations and other reports of anomalies associated with hypopigmentation suggest caution in the use of albino and other hypomelanotic animals as normal models in biological research.

Photically evoked potentials and afterpotentials recorded from the visual cortex of the unanesthetized hedgehog

Evoked potentials to visual stimuli (VEP) were recorded from the visual cortex of the unanesthetized hedgehog (Erinaceus europaeus), a primitive placental mammal with relatively little differentiation of cortex and thalamus. The VEPs consisted of several distinct positive and negative voltage deflections. Reproducibility of the response was high, as indicated from the small intrasession and intersession response variability. Rhythmic afterpotentials (AP), previously reported for higher mammals, were readily elicited. They had a lower frequency (3/sec) than APs observed in other mammals. The use of the hedgehog in electrophysiological and psychophysiological research is suggested because data obtained from this primitive placental mammal may shed light on CNS functions of higher mammals, as well as mammalian forms 'lower' on the evolutionary scale.