Component-specific effects of physostigmine on the cat visual evoked potential

Expression of early growth responsive gene-1 in the visual cortex of monocular form deprivation amblyopic kittens

PURPOSE

The present study compared the expression of early growth responsive gene-1 (Egr-1) in visual cortex between amblyopia kittens and normal kittens, and to explore the role of Egr-1 in the pathogenesis of amblyopia.

METHODS

A total of 20 healthy kittens were randomly divided into deprivation group and control group with 10 kittens in each group. Raised in natural light, and covered the right eye of the deprived kittens with a black opaque covering cloth. Pattern visual evoked potentials (PVEP) were measured before and at the 1st, 3rd and 5th week after covering in all kittens. After the last PVEP test, all kittens were killed. The expression of Egr-1 in the visual cortex of the two groups was compared by immunohistochemistry and in situ hybridization.

RESULTS

PVEP detection showed that at the age of 6 and 8 weeks, the P100 wave latency in the right eye of deprivation group was higher than that in the left eye of deprivation group (P < 0.05) and the right eye of control group (P < 0.05), while the amplitude decreased (P < 0.05). The number of positive cells (P < 0.05) and mean optical density (P < 0.05) of Egr-1 protein expression in visual cortex of 8-week-old deprivation group were lower than those of normal group, as well as the number (P < 0.05) and mean optical density of Egr-1 mRNA-positive cells (P < 0.05).

CONCLUSIONS

Monocular form deprivation amblyopia can lead to the decrease of Egr-1 protein and mRNA expression in visual cortex, and then promote the occurrence and development of amblyopia.

C57BL/6J and DBA/2J strains present opposite sex differences in flash visual evoked potential latency: A possible confusing factor in gender studies on neurological diseases' transgenic models

The cholinergic neurotransmitter system in the brain is crucial in processing information related to cognitive, behavioral, and motor functions. A cholinergic dysfunction has been correctly described as one of the primary causes of neurodegenerative diseases. Differences in levels of acetylcholine or expression and function of receptors in selected brain areas have been indicated as one of the causes of sexual dimorphism in neurotransmission. However, variability in results among studies based on different mice strains could affect conclusions on this topic. Visual evoked potentials (VEPs) of male and female DBA/2J and C57BL/6J mice, which are two of the most common strains backgrounds in use for developing transgenic mice models of neurological diseases, have been studied. Effects induced by a single low dose of physostigmine have also been performed to evaluate the cholinergic system involvement. VEPs responses to luminous stimuli in C57BL/6J mice have shown a consistently lower latency than in DBA/2J, confirming the previous observation of strain differences in cholinergic function. Interestingly, strains present an opposite-sex difference in VEP latency not apparently related to sensitivity to physostigmine. These findings point at paying extreme attention to the choice of the genetic background of the animal model, especially in those basic and pre-clinical experiments that involve visual functioning.

Different spatial frequency bands selectively signal for natural image statistics in the early visual system

Early visual evoked potentials (VEPs) measured in humans have recently been observed to be modulated by the image statistics of natural scene imagery. Specifically, the early VEP is dominated by a strong positivity when participants view minimally complex natural scene imagery, with the magnitude of that component being modulated by luminance contrast differences across spatial frequency (i.e., the slope of the amplitude spectrum). For scenes high in structural complexity, the early VEP is dominated by a prominent negativity that exhibits little dependency on luminance contrast. However, since natural scene imagery is broad band in terms of spatial frequency, it is not known whether the above-mentioned modulation results from a complex interaction within or between the early neural processes tuned to different bands of spatial frequency. Here, we sought to address this question by measuring early VEPs (specifically, the C1, P1, and N1 components) while human participants viewed natural scene imagery that was filtered to contain specific bands of spatial frequency information. The results show that the C1 component is largely unmodulated by the luminance statistics of natural scene imagery (being only measurable when such stimuli were made to contain high spatial frequencies). The P1 and N1, on the other hand, were observed to exhibit strong spatial frequency-dependent modulation to the luminance statistics of natural scene imagery. The results therefore suggest that the dependency of early VEPs on natural image statistics results from an interaction between the early neural processes tuned to different bands of spatial frequency.

From spatial frequency contrast to edge preponderance: the differential modulation of early visual evoked potentials by natural scene stimuli

The contrast response function of early visual evoked potentials elicited by sinusoidal gratings is known to exhibit characteristic potentials closely associated with the processes of parvocellular and magnocellular pathways. Specifically, the N1 component has been linked with parvocellular processes, while the P1 component has been linked with magnocellular processes. However, little is known regarding the response properties of the N1 and P1 components during the processing and encoding of complex (i.e., broadband) stimuli such as natural scenes. Here, we examine how established physical characteristics of natural scene imagery modulate the N1 and P1 components in humans by providing a systematic investigation of component modulation as visual stimuli are gradually built up from simple sinusoidal gratings to highly complex natural scene imagery. The results suggest that the relative dominance in signal output of the N1 and P1 components is dependent on spatial frequency (SF) luminance contrast for simple stimuli up to natural scene imagery possessing few edges. However, such a dependency shifts to a dominant N1 signal for natural scenes possessing abundant edge content and operates independently of SF luminance contrast.

Effects of repetitive TMS on visually evoked potentials and EEG in the anaesthetized cat: dependence on stimulus frequency and train duration

Repetitive transcranial magnetic stimulation (rTMS) has been shown to alter cortical excitability that lasts beyond the duration of rTMS application itself. High-frequency rTMS leads primarily to facilitation, whereas low-frequency rTMS leads to inhibition of the treated cortex. However, the contribution of rTMS train duration is less clear. In this study, we investigated the effects of nine different rTMS protocols, including low and high frequencies, as well as short and long applications (1, 3 and 10 Hz applied for 1, 5 and 20 min), on visual cortex excitability in anaesthetized and paralysed cats by means of visual evoked potential (VEP) and electroencephalography (EEG) recordings. Our results show that 10 Hz rTMS applied for 1 and 5 min significantly enhanced early VEP amplitudes, while 1 and 3 Hz rTMS applied for 5 and 20 min significantly reduced them. No significant changes were found after 1 and 3 Hz rTMS applied for only 1 min, and 10 Hz rTMS applied for 20 min. EEG activity was only transiently (<20 s) affected, with increased delta activity after 1 and 3 Hz rTMS applied for 1 or 5 min. These findings indicate that the effects of rTMS on cortical excitability depend on the combination of stimulus frequency and duration (or total number of stimuli): short high-frequency trains seem to be more effective than longer trains, and low-frequency rTMS requires longer applications. Changes in the spectral composition of the EEG were not correlated to changes in VEP size.

From neuroscience to application in neuropharmacology: A generation of progress in electrophysiology

A continuum from neuronal cellular/subcellular properties to system processes appears to exist in many instances and to allow privileged approaches in neuroscience and neuropharmacology research. Brain signals and the cholinergic and GABAergic systems, in vivo and in vitro evidence from studies on the retina, or the "gamma band" oscillations in neuron membrane potential/spiking rate and neuronal assemblies are examples in this respect. However, spontaneous and stimulus-event-related signals at any location and time point reflect brain state conditions that depend on neuromodulation, neurotransmitter interaction, hormones (e.g., glucocorticois, ACTH, estrogens) and neuroendocrine interaction at different levels of complexity, as well as on the spontaneous or experimentally-induced changes in metabolism (e.g., glucose, ammonia), blood flow, pO2, pCO2, acid/base balance, K activity, etc., that occur locally or systemically. Any of these factors can account for individual differences and/or changes over time that often are (or need to be) neglected in pharmaco-EEG studies or are dealt with statistically and by controlling the experimental conditions. As a result, the electrophysiological effects of neuroactive drugs are to an extent non-specific and require adequate modeling and precise correlation with independent parameters (e.g., drug kinetics, vigilance, hormonal profile or metabolic status, etc.) to avoid biased results in otherwise controlled studies.

Studies of human visual pathophysiology with visual evoked potentials

Visual evoked potentials (VEPs) offer reproducible and quantitative data on the function of the visual pathways and the visual cortex. Pattern reversal VEPs to full-field stimulation are best suited to evaluate anterior visual pathways while hemi-field stimulation is most effective in the assessment of post-chiasmal function. However, visual information is processed simultaneously via multiple parallel channels and each channel constitutes a set of sequential processes. We outline the major parallel pathways of the visual system from the retina to the primary visual cortex and higher visual areas via lateral geniculate nucleus that receive visual input. There is no best method of stimulus selection, rather visual stimuli and VEPs' recording should be tailored to answer specific clinical and/or research questions. Newly developed techniques that can assess the functions of extrastriate as well as striate cortices are discussed. Finally, an algorithm of sequential steps to evaluate the various levels of visual processing is proposed and its clinical use revisited.

Stimulus frequency dependence of the central and peripheral somatosensory evoked activity in rats treated with various pesticides

Rats were treated with a combination of insecticide agents in different timing schemes. In acute administration, 1/5 LD50 of the three insecticides: dimethoate, propoxur and cypermethrin, or their combination, was given once by gavage. In the developmental model, female rats received oral doses of 1/25 LD50 of the above insecticides in combination in three timing schemes including pregnancy and lactation. Responses in the somatosensory cortex and in the tail nerve, evoked by peripheral electric stimulation, were recorded in acute preparation under urethane anesthesia. It was tested whether the parameters of the cortical and peripheral evoked response are dependent on the frequency and whether this dependence is different in control and treated animals. The latency increase of the cortical responses with increasing stimulation frequency was significantly stronger in rats treated acutely with cypermethrin and the combination, and in rats receiving the combination during both intra- and extrauterine development. On the duration, the effects were less clear. Frequency dependent increase of the tail nerve action potential latency was significantly intensified by cypermethrin, and the amplitude decrease, by cypermethrin and dimethoate. Fatigue of this response during a stimulation series was also altered by the insecticides. Frequency dependence and fatigue possibly reflect the actual state of the nervous system and may have the potency to be developed to functional biomarkers.

Mapping distributed sources of cortical rhythms in mild Alzheimer's disease. A multicentric EEG study

The study aimed at mapping (i) the distributed electroencephalographic (EEG) sources specific for mild Alzheimer's disease (AD) compared to vascular dementia (VaD) or normal elderly people (Nold) and (ii) the distributed EEG sources sensitive to the mild AD at different stages of severity. Resting EEG (10-20 electrode montage) was recorded from 48 mild AD, 20 VaD, and 38 Nold subjects. Both AD and VaD patients had 24-17 of mini mental state examination (MMSE). EEG rhythms were delta (2-4 Hz), theta (4-8 Hz), alpha 1 (8-10.5 Hz), alpha 2 (10.5-13 Hz), beta 1 (13-20 Hz), and beta 2 (20-30 Hz). Cortical EEG sources were modeled by low resolution brain electromagnetic tomography (LORETA). Regarding issue i, there was a decline of central, parietal, temporal, and limbic alpha 1 (low alpha) sources specific for mild AD group with respect to Nold and VaD groups. Furthermore, occipital alpha 1 sources showed a strong decline in mild AD compared to VaD group. Finally, distributed theta sources were largely abnormal in VaD but not in mild AD group. Regarding issue ii, there was a lower power of occipital alpha 1 sources in mild AD subgroup having more severe disease. Compared to previous field studies, this was the first investigation that illustrated the power spectrum profiles at the level of cortical (macroregions) EEG sources in mild AD patients having different severity of the disease with respect to VaD and normal subjects. Future studies should evaluate the clinical usefulness of this approach in early differential diagnosis, disease staging, and therapy monitoring.

Nicotine alters flash-evoked potentials in Long–Evans rats

This experiment examined the effects of nicotine on flash-evoked potentials (FEPs) recorded from both the visual cortex (VC) and the superior colliculus (SC) of chronically implanted male Long-Evans rats. FEPs were recorded at 5, 20, 40, and 60 min following subcutaneous injections of saline, and of 0.4, 0.7, and 1.0 mg/kg nicotine on separate days. In the VC, the amplitude of components N(39), N(53), N(67), and P(88) increased, while the amplitude of components N(30) and P(235) decreased following nicotine administration. P(22), P(47), and N(153) were unchanged. In the SC, components P(27), N(48), and N(53) were reduced in amplitude, while P(37) and N(57) were unaffected by nicotine. Many peak latencies in the VC and SC were increased by nicotine, often at all three doses. However, effects of nicotine on FEPs were both dose- and time-dependent. When body temperature was recorded 65 min after drug administration, significant hypothermia was found with both the 0.7- and 1.0-mg/kg nicotine doses. The 1.0-mg/kg dose of nicotine resulted in a significant increase in movement during the recording sessions, but not in subsequent open-field observations. The results demonstrate that nicotinic acetylcholine receptors (nAChRs) play a differential role in the production/modulation of the various components of FEPs.

Stimulus-specific oscillatory responses of the brain: a time/frequency-related coding process

OBJECTIVES

To review the coherent, rhythmic oscillations above approximately 20 Hz that occur in response to sensory inputs in the firing rate and membrane or local field potentials of distributed neuron aggregates of CNS layered structures.

RESULTS

Oscillatory activity at approximately 20-80 Hz occurs in response to either olfactory, auditory and visual (contrast) stimuli; oscillations at frequencies centered on 100-120 Hz or 600 Hz are recorded, respectively, from the visual system (luminance stimulation) and from the somatosensory cortex. Experimental evidence suggests sources/mechanisms of generation that depend on inhibitory interneurons and pyramidal cells and are partially independent from those of conventional (broadband) evoked responses. In the olfactory and visual systems, the oscillatory responses reflect the global stimulus properties. A time/phase correlation between firing rate, spiking coincidence and oscillatory field responses has been documented. The oscillatory responses are postsynaptic both in cortex and in precortical structures (e.g. retina; LGN). Evidence indicates intracortical and thalamocortical interacting mechanisms of regulation as well as GABAergic and cholinergic modulation. In the visual cortex the oscillatory responses are driven by oscillations in the synaptic input. Oscillatory potentials are dependent on resonance phenomena and produce narrow-band synchronization of activated neurons. They may have a role in the 'binding' of separate neuronal aggregates into sensory units.

CONCLUSIONS

Oscillatory responses contribute as a time/frequency coding mechanism to pacing neurons selectively for the physical properties of stimulus and are involved in sensory information processing.

Cholinergic modulation, visual function and Alzheimer's dementia

Electrophysiological evidence at a cellular level and in vivo macroelectrode recordings converge in indicating a degree of specificity of acetylcholine action in vision. Acetylcholine (ACh) function is also thought to play a significant role in memory, learning and other cognitive processes. In this respect, ACh action is suggested to serve in both sensory and cognitive processes. The pharmacological blocking of brain muscarinic transmission has been proposed as a model of geriatric memory impairment and Alzheimer's dementia. Visual electrophysiological testing is deemed of diagnostic specificity for this disease. ACh brain neurotransmission, however, mostly contributes to the modulation of nonspecific aspects of cognition, such as arousal or attention. Alzheimer's dementia results from complex neuron alterations [which also affect muscarinic receptors among other (sub)cellular structures] rather than simply reflecting ACh impoverishment. A substantial loss of retinal ganglion cells is documented in patients with Alzheimer's disease and is consistent with electrophysiological observations. However, it is unclear to what extent the dysfunction of the visual system observable in Alzheimer's dementia is qualitatively different from that occurring spontaneously during aging. The dissimilarities between the effect of acute muscarinic blocking (e.g. by scopolamine) and dementia outnumber the similarities. Accordingly, the conventional ACh agonist-antagonist model of dementia now appears questionable, and replacement treatment with compounds enhancing ACh function proved disappointing. It is suggested that (nonspecific) ACh action becomes function-specific, as determined by the architecture of local brain circuits in which it is involved.

Different effects of cholinergic agents on responses recorded from the cat visual cortex and lateral geniculate nucleus dorsalis

We investigated the effect of cholinergic agents on the cat visual evoked potentials (VEPs) recorded from the primary visual cortex (V1) and lateral geniculate nucleus dorsalis (LGNd) to determine on which level of the visual pathway the cholinergic system acts. VEPs to the alternation of 0.1 cycles per degree sinusoidal gratings at 1 and 4 Hz were recorded from N2O-anesthetized cats directly from the surface of V1 and LGNd. The depth of recording in LGNd was determined by the site where the maximal response was obtained by 1 Hz stimulation. VEPs to 4 Hz stimulation, which showed sinusoidal waveforms and were analyzed by fast Fourier transforms, were used as indicators for modulation by cholinergic agents. Physostigmine, an acetylcholinesterase inhibitor, 0.7 mg/kg i.v., suppressed the amplitude of the responses more at V1 (suppression ratio: mean +/- SD, 85.4 +/- 9.3%) than at LGNd (32.4 +/- 30.7%) (P < 0.05). Conversely, scopolamine, a muscarinic receptor blocker, 0.7 mg/kg i.v., increased the amplitude of the responses more at V1 (enhancement ratio: mean +/- SD, 60.3 +/- 22.3%) than at LGNd (-22.2 +/- 22.5%) (P < 0.05). These results indicate that the V1 changes reflect a direct cortical cholinergic effect, probably by modulating the cholinergic projection from the nucleus basalis of Meynert to V1.

Scalp-recorded oscillatory potentials evoked by transient pattern-reversal visual stimulation in man

Replicable oscillatory potentials, time-locked to pattern stimuli (9.0 degrees central; counterphase reversal at 2.13 Hz) were dissociated from conventional, broad-band VEPs recorded in healthy volunteers at occipital scalp locations by high-pass digital filtering at 17.0-20.0 Hz. Nine consecutive wavelets were identified with a 56.4 +/- 8.4 msec mean latency of the first replicable wavelet and mean peak-to-peak amplitude varying between 0.9 and 2.0 muV. The first 2 wavelets had significantly shorter latencies than wave N70 of unfiltered VEP, whereas the last 2 wavelets had longer latencies than N145. Latency and amplitude values varied as a function of contrast and spatial frequency of the stimulus, with shorter latencies and larger amplitudes at 60-90% contrast level and tuning of amplitude at 5.0 c/deg. All wavelets were correlated with wave P100 of unfiltered VEP, while a correlation with N70 of VEP was observed only for those wavelets with latencies in the range of wave P100. Two patients with documented brain lesions involving the visual system are described as examples of oscillatory responses occurring irrespective of filter bandpass and instead of the expected conventional VEP when the generation of these is interfered with by brain pathology. A substantial cortical contribution to the origin of the oscillatory response is conceivable. It is suggested that the oscillatory response to pattern-reversal stimulation reflects events in the visual system that are parallel to, and partly independent of, the conventional VEP, with potential application in research or for clinical purposes.

Short-term changes in the response characteristics of the human visual evoked potential

The present study examined how the response characteristics of the visual evoked potential (VEP) varied during the course of trials using a sinusoidal grating stimulus that reversed contrast in a square-wave manner. To accomplish this, amplitude and phase values were derived in short segments during the course of continuous stimulation for three subjects. When stimulus spatial frequencies of 0.77 or 1.55 c/deg were used, VEP amplitude remained at a stable value throughout the trial. At 3.1 c/deg, 6-12 sec were required for VEP amplitude to increase to a stable value, which was on average 204% greater than the value noted during the first few seconds of the trial. At 6.2 and 12.4 c/deg, VEP amplitude changes were more complex, first increasing and then decreasing substantially, to levels that were on average 63.8% and 38% of the peak reached earlier in the trial. In all cases, VEP phase decreased during the trial. The magnitude of this decrease ranged up to 50 deg, corresponding to an approx. 10.5 msec delay for the 6.65 Hz stimulation rate used. Prior exposure to an adapting grating diminished the changes in VEP amplitude and advanced the phase changes. Therefore, these changes appear to represent a form of contrast adaptation that is restricted to responses to high spatial frequencies. In addition, the present results provide evidence against a fundamental assumption of signal averaging--that an invariant stimulus will evoke an invariant response.