Nonlinear analysis of visual evoked potentials elicited by color stimulation

The relationship between chromatically modulated stimuli and visual evoked potentials (VEPs) was considered. VEPs of normal subjects elicited by chromatically modulated stimuli were measured under several color adaptations, and their binary kernels were estimated. Up to the second-order, binary kernels obtained from VEPs were so characteristic that the VEP-chromatic modulation system showed second-order nonlinearity. First-order binary kernels depended on the color of the stimulus and adaptation, whereas second-order kernels showed almost no difference. This result indicates that the waveforms of first-order binary kernels reflect perceived color (hue). This supports the suggestion that kernels of VEPs include color responses, and could be used as a probe with which to examine the color visual system.     

The effect of stimulus sequence on the waveform of the cortical event-related potential

The waveform of the cortical event-related potential is extremely sensitive to variations in the sequence of stimuli preceding the eliciting event. The waveform changes were manifested primarily in the amplitudes of the negative component of the potential that peaked at 200 milliseconds, the positive component that peaked at 300 milliseconds, and the slow-wave components. A quantitative model was developed relating the waveform changes to changes in event expectancy. Expectancy is assumed to depend on a decaying memory for events within the prior sequence, the specific structure of the sequence, and the global probability of event occurrence. For stimuli relevant to the task, the less expected the stimulus the larger the amplitudes of late components of the event-related potentials.     

Lateral inhibitory interactions in the intermediate layers of the monkey superior colliculus

The intermediate layers of the monkey superior colliculus (SC) contain neurons the discharges of which are modulated by visual fixation and saccadic eye movements. Fixation neurons, located in the rostral pole of the SC, discharge action potentials tonically during visual fixation and pause for most saccades. Saccade neurons, located throughout the remainder of the intermediate layers of the SC, discharge action potentials for saccades to a restricted region of the visual field. We defined the fixation zone as that region of the rostral SC containing fixation neurons and the saccade zone as the remainder of the SC. It recently has been hypothesized that a network of local inhibitory interneurons may help shape the reciprocal discharge pattern of fixation and saccade neurons. To test this hypothesis, we combined extracellular recording and microstimulation techniques in awake monkeys trained to perform oculomotor paradigms that enabled us to classify collicular fixation and saccade neurons. Microstimulation was used to electrically activate the fixation and saccade zones of the ipsilateral and contralateral SC to test for inhibitory and excitatory inputs onto fixation and saccade neurons. Saccade neurons were inhibited at short latencies following electrical stimulation of either the ipsilateral (1-5 ms) or contralateral (2-7 ms) fixation or saccade zones. Fixation neurons were inhibited 1-4 ms after electrical stimulation of the ipsilateral saccade zone. Stimulation of the contralateral saccade zone led to much weaker inhibition of fixation neurons. Stimulation of the contralateral fixation zone led to short-latency (1-2 ms) excitation of fixation neurons. Only a small percentage of saccade and fixation neurons were activated by the electrical stimulation (latency: 0.5-2.0 ms). These responses were confirmed as either orthodromic or antidromic responses using collision testing. The results suggest that a local network of inhibitory interneurons may help shape not only the reciprocal discharge pattern of fixation and saccade neurons but also permit lateral interactions between all regions of the ipsilateral and contralateral SC. These interactions therefore may be critical for maintaining stable visual fixation, suppressing unwanted saccades, and initiating saccadic eye movements to targets of interest.     

Coupling potentials in CA1 neurons during calcium-free-induced field burst activity

Small amplitude depolarizations (fast prepotentials, spikelets) recorded in mammalian neurons are thought to represent either dendritic action potentials or presynaptic action potentials attenuated by gap junctions. We have used whole-cell recordings in an in vitro calcium-free model of epilepsy to record spikelets from CA1 neurons of the rat hippocampus. It was found that spikelet appearance was closely correlated with the occurrence of dye coupling between pyramidal neurons, indicating that both phenomena share a common substrate. Spikelets were characterized according to waveform (amplitude and shape) and temporal occurrence. Spikelet amplitudes were found to be invariant with neuronal membrane potential, and their pattern of occurrence was indistinguishable from patterns of action potential firing in these cells. Voltage and current recordings revealed a spikelet waveform that was usually biphasic, comprised of a rapid depolarization followed by a slower hyperpolarization. Numerical differentiation of spike bursts resulted in waveforms similar to recorded spikelet sequences, while numerical integration of spikelets yielded waveforms that were indistinguishable from action potentials. Modification of spikelet waveforms by the potassium channel blocker tetraethylammonium chloride suggests that spikelets may arise from both resistive and capacitive transmission of presynaptic action potentials. Intracellular alkalinization and acidification brought about by perfusion with NH4Cl caused changes in spikelet frequency, consistent with reported alterations of field burst activity in this model of epilepsy. These results suggest that spikelets result from gap junctional communication, and may be important determinants of neuronal activity during seizure-like activity.     

Contribution of single-unit spike waveform changes to temperature-induced alterations in hippocampal population spikes

Brain temperature changes accompany exploratory behavior and profoundly affect field potential amplitudes recorded in hippocampus. The waveform alterations in fascia dentata include a reduction in population spike area, which might be explained by fewer granule cells firing in response to a given stimulus or by an alteration in the size or shape of the individual action potentials. This study was designed to assess these alternate possibilities. In experiment 1, changes in the shape and firing rates of single cells recorded in the fascia dentata of awake rats were compared with changes in the population spike before and after a bout of activity. Single-unit amplitudes were significantly reduced following exploration, and there was a small (< 3%) change in unit spike-width. These changes, however, were insufficient to account, in a linear fashion, for the entire decline in the population spike. In experiment 2, radiant heat was used to manipulate brain temperature in anesthetized rats. As in the first experiment, the magnitude of change in the extracellular units was much smaller than the change in population spike amplitude. The spontaneous firing rates of the cells were also modified by brain temperature changes. In experiment 3, the polysynaptic, contralateral commissural response (which covaries with changes in the ipsilateral population spike at a fixed temperature) was measured as a function of either exploratory behavior or radiant heat. The relationship between the ipsilateral population spike and corresponding polysynaptic commissural response was altered following exploration and passive warming in a manner consistent with a reduction in net granule cell output, reduced transmission efficacy through the polysynaptic circuit, or a combination of these. Taken together these data suggest that at least two factors contribute to temperature-dependent changes in the perforant path-evoked population spikes recorded in the fascia dentata: changes in the size of individual action potentials and alterations in discharge of action potentials in response to a given stimulus.     

Age differences in visual evoked potential estimates on interhemishperic transfer.

Twenty-six younger (ages 18–36 years) and 19 older (ages 60–88 years) healthy right-handed men and women were tested for interhemispheric transfer by using visual evoked potentials lo laterally presented checkerboards. Interhemispheric transfer time (IHTT) was estimated by subtracting latencies for both P100 and N160 peaks of the waveform contralateral to the stimulus from the waveform ipsilateral to the stimulus for homologous sites. The quality of interhemispheric transfer was estimated by comparing peak-to-peak amplitudes for homologous sites. IHTT did not change across age, but there was a suppression of the waveform over the indirectly stimulated hemisphere in the older participants. The significance of this finding for age-related changes in functions mediated by the corpus callosum is discussed. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Effects of temporal integration on the shape of visual backward masking functions.

Many studies of cognition and perception use a visual mask to explore the dynamics of information processing of a target. Especially important in these applications is the time between the target and mask stimuli. A plot of some measure of target visibility against stimulus onset asynchrony is called a masking function, which can sometimes be monotonic increasing but other times is U-shaped. Theories of backward masking have long hypothesized that temporal integration of the target and mask influences properties of masking but have not connected the influence of integration with the shape of the masking function. With two experiments that vary the spatial properties of the target and mask, the authors provide evidence that temporal integration of the stimuli plays a critical role in determining the shape of the masking function. The resulting data both challenge current theories of backward masking and indicate what changes to the theories are needed to account for the new data. The authors further discuss the implication of the findings for uses of backward masking to explore other aspects of cognition. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

The effect of motion on pattern-onset visual evoked potentials in adults and children

Visual evoked potentials can be elicited by a variety of visual stimuli, including pattern-onset and motion-onset. It may be desirable to combine pattern-onset with motion-onset stimuli, for example, to make a direct comparison between optokinetic nystagmus and visual evoked potential acuity thresholds. Both procedures employ grating stimuli; however, the gratings must be moving to produce optokinetic nystagmus. We compared pattern-onset visual evoked potentials with both a static and a moving pattern to investigate the effect of motion on the pattern-onset visual evoked potential waveform. Visual evoked potential recordings were made from 10 adults (aged 20-37 years) and 10 children (aged 5-7 years) with the active electrode at Oz. Stimuli consisted of onset of high-contrast vertical bars of three sizes (12', 30' and 60') both with and without motion (3 cycles/s). In a subgroup of subjects, visual evoked potentials were recorded to motion onset of constantly present gratings. Motion of the pattern had no significant effect on any of the latency components of the visual evoked potential waveform in adults or children. The amplitude of the C2-C3 component was significantly increased (p < 0.001) in adults. The motion appears to add a late negative component to the visual evoked potential similar to that produced by the motion-only stimulus. The latency of the early components of the pattern-onset visual evoked potential was unaffected by the presence of motion. Therefore, pattern-onset visual evoked potentials with moving gratings could be used to estimate visual acuity, and direct comparisons could be made between visual evoked potential and optokinetic nystagmus acuity thresholds with the use of the same stimulus parameters.     

Brain evoked potential in schizophrenic patients

The authors reviewed the literature on evoked potentials in mentally ill patients, with particular emphasis on schizphreniacs. The commonly observed abnormalities were as follows: 1) higher SEPs amplitudes with less waveshape variability during first 100 ms in non-depressed chronic, paranoid or undifferentiated patients with florid psychotic symptoms; normal SEPs amplitudes in acute or latent schizophrenics and in chronic depressed schizophrenics but without florid psychotic symptoms; 2) reduced SEPs and VEPs amplitude recovery and faster latency recovery; 3) reduced AEPs amplitude and latency; 4) greater VEPs waveshape variability and tendency to be "reducers" in hallucinating patients; reduced amplitude and latency recovery; prolonged latencies in patients with positive family history (schizophrenia or affective disorders in close relatives); prolonged N2 latency in motor responses to "easy" and "difficult" stimuli; reduced activity of "late potentials"; 5) greater waveshape variability in all modalities in chronic schizophrenics, abnormal P300 (reduced amplitude, lack of P300 or negative "effect of uncertainty") and abnormal CNV (less "readiness" potential, prolonged negativity with motor responses).     

Pain-related cerebral potentials in response to acute painful electrical stimulation in sheep

OBJECTIVE: To examine associations between late event-related cerebral potential amplitudes and behavioural responses to noxious electrical stimulation as an indicator of acute pain in sheep. DESIGN: Analysis of variance for the effects of stimulus intensity on the behaviour and event-related cerebral potential variables. PROCEDURE: Ninety-six brief constant current electrical pulse trains were presented to the front left leg of eight sheep at four intensities (2.5 to 10 mA) in a randomised order. An event-related cerebral potential and a graded flinch response were recorded for each stimulus and the 24 event-related cerebral potentials at each intensity were averaged to produce a mean waveform. Various components of this waveform were analysed and changes in these measures and the sheep's flinch response, as stimulus intensity increased, were determined. RESULTS: Both the flinch response and some event-related cerebral potential components, that is, peak amplitudes 114 [N1], 187 [P1], 318 [P2] and 230 [Pm] ms after stimulus onset, were significantly affected as stimulus intensity increased. CONCLUSION: These corresponding behavioural and event-related cerebral potential changes indicate the usefulness of using changes in the event-related cerebral potential to measure acute pain in sheep.     

Macular pigment and the colour-specificity of visual evoked potentials

A modified Moreland anomaloscope was used to examine two subjects, one with dense macular pigment, the other with relatively light pigmentation. Chromatic visual evoked potentials (VEPs) were elicited from these two subjects using coarse, isoluminant gratings of different sizes. Colour-specificity was verified by comparing chromatic onset VEPs (reflecting sustained activity) with chromatic reversal VEPs (reflecting mainly transient activity) and also by Fourier analysis (colour-specific responses are dominated by the fundamental, transient activity by the second harmonic). Achromatic (transient-type) intrusions, produced by large blue-green gratings could be related to the extent of subject-specific macular pigmentation.     

Effect of phenactil (chlorpromazine) on cortical visual evoked potentials in clinically healthy subjects with normal and abnormal electroencephalograms

The results of a study of averaged cortical visual evoked potentials in healthy subjects with abnormal EEG records and in subjects with normal EEG are presented. The investigations were performed at rest and after activation with chlorpromazine. The potentials were recorded from typical sites in the visual cortex (O2--CZ and O1--CZ). Stimulation with flashes at 1 c/sec., 40 msec. duration, 2000 lux light intensity at a distance of 60 cm from the eyes was applied. The pupils were dilated with homatropine. The mean amplitudes of potential components measured by the method peak-to-peak, the mean latency time, and the mean duration of components were measured. It was found that at rest the inter-group differences included an amplitude rise at the time of change of the specific part into non-specific and the change of the evoked potential to the after-discharge, and the duration of positive components in the non-specific part prevailed over the negative ones. Chlorpromazine changed somewhat the amplitude of components (fall in the specific part and rise in the non-spefific one) but the differences were statistically not significant. Similar changes were observed in the latency time of various components (prolongation of latency time of late components was statistically not significant). The results suggest differences in the reactivity of cerebral structures of these groups at rest as well as after chlorpromazine activation.     

Emotional scene content drives the saccade generation system reflexively.

The authors assessed whether parafoveal perception of emotional content influences saccade programming. In Experiment 1, paired emotional and neutral scenes were presented to parafoveal vision. Participants performed voluntary saccades toward either of the scenes according to an imperative signal (color cue). Saccadic reaction times were faster when the cue pointed toward the emotional picture rather than toward the neutral picture. Experiment 2 replicated these findings with a reflexive saccade task, in which abrupt luminosity changes were used as exogenous saccade cues. In Experiment 3, participants performed vertical reflexive saccades that were orthogonal to the emotional–neutral picture locations. Saccade endpoints and trajectories deviated away from the visual field in which the emotional scenes were presented. Experiment 4 showed that computationally modeled visual saliency does not vary as a function of scene content and that inversion abolishes the rapid orienting toward the emotional scenes. Visual confounds cannot thus explain the results. The authors conclude that early saccade target selection and execution processes are automatically influenced by emotional picture content. This reveals processing of meaningful scene content prior to overt attention to the stimulus. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Effect of phenytoin and carbamazepine on evoked potentials in patients with newly diagnosed epilepsy. Part I. Visual evoked potentials

Pattern-reversal visual evoked potentials (VEPs) were recorded in 64 patients with newly diagnosed epilepsy. Before starting medication the patients with partial and primary generalized epilepsy, had prolonged latencies of the VEPs component P100, as compared with controls. VEPs were repeated after 3 months in 43 patients with focal epilepsy, during carbamazepine (22 cases) or phenytoin (21 cases) treatment. The plasma concentration of the drugs were within therapeutic levels. Carbamazepine but not phenytoin, was associated with prolongation of the P100 peak latency and induced increase of its amplitude, as compared with the baseline condition. The VEPs abnormality was most pronounced in patients whose seizures were poorly controlled. We conclude, that administration of carbamazepine or phenytoin, at therapeutic serum level, have minimal effect on the VEPs.     

A comparison of the electrophysiological effects of two organophosphates, mipafox and ecothiopate, on mouse limb muscles

Adult male albino mice were given single subcutaneous injections of either mipafox (110 mumol/kg) or ecothiopate (0.5 mumol/kg), two organophosphorus compounds (OPs). Acetylcholinesterase activity was measured in the soleus (slow-twitch) and extensor digitorum longus (EDL; fast-twitch) muscles. At 7 and 28 days after dosing, in vitro electrophysiological measurements were carried out in the soleus and EDL. Action potentials and end-plate potentials were evoked at 30 Hz and recorded intracellularly from single muscle fibers. The amplitudes, time course, and latencies of these potentials were measured and the variability (jitter) of latencies was calculated. Recordings after mipafox were also made with 3-Hz stimulation. Acetylcholinesterase activity was inhibited by mipafox (65% in the soleus; 76% in the EDL) and ecothiopate (59% in the soleus; 42% in the EDL). Mipafox and ecothiopate both increased postjunctional (muscle action potential) jitter in the soleus and EDL at 7 days after dosing. Organophosphates caused an increase in end-plate potential amplitudes in the soleus. Mipafox caused an increase in prejunctional (end-plate potential) jitter at 28 days after dosing in both muscles. A single dose of ecothiopate also caused an increase in prejunctional jitter at 28 days in the soleus. The OP-induced increase in jitter was different at different frequencies of stimulation. The results show that there are electrophysiological changes in both muscles after administration of organophosphorus compounds. The slow-twitch soleus appears more sensitive to prejunctional changes caused by OPs than the fast-twitch EDL.     

Visual response latencies in temporal lobe structures as a function of stimulus information load.

In a monkey performing a visual delayed matching-to-sample task, units and visual evoked potentials (VEPs) were sampled from the inferior bank of the superior temporal sulcus (STS; Areas TEa and IPa), the hippocampus, and the presubiculum. VEP latencies indicated that flash information—signaling the imminent presentation of a color sample to be retained—reached the presubiculum and the hippocampus substantially earlier than the STS. In contrast, color sample VEP latencies did not differ between sites, arriving at all sites appreciably later than flash VEPs. Unit data indicated generally excitatory responses to both stimuli at all sites and net inhibition during the interstimulus interval separating flash from sample. As with VEPs, unit latencies to flash were shorter than to sample stimuli. The alerting flash data imply activation of the hippocampus occurring before activation of the STS cortex, whereas the coincident arrival of color sample information suggests temporal synchronization between these structures. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Inverse relationship between the size of pattern reversal visual evoked potentials from the left brain and the degree of left-hand preference in left-handed normal subjects: importance of the left brain

The relation of the degree of left-hand preference to pattern reversal visual evoked potentials (VEPs) from right and left brain was studied in male left-handers. The degree of the left-hand preference was assessed by the Waterloo Handedness Questionnaire. Visual stimuli consisted of black and white checkerboard patterns generated on a TV screen. VEPs were simultaneously recorded from occipital leads of the right and left hemispheres. The degree of left-hand preference was found to be inversely and significantly related to size of VEPs only from left brain, not from right brain. That is, the conduction time, amplitude, duration, and area of N1-P1 waves linearly decreased as the degree of left-hand preference increased. These results were in accord with the testosterone hypothesis of cerebral lateralization, but not compatible with the right shift theory of handedness. It was concluded that visuomotor control by the left brain would be the main biological correlate of left-hand preference with regard to sensorimotor and cognitive functions.     

Visual evoked potentials are affected by trunk rotation in neglect patients

Steady-state visual evoked potentials (VEPs) were recorded in four patients with unilateral visuo-spatial neglect, stimulating either the left or the right hemifield. In the standard condition (head and body oriented straight ahead towards the stimulus) the left hemifield VEP was delayed. When the body was turned to the left, however, the two hemifield latencies were comparable. These results were confirmed with the transient VEP technique. No effect of trunk rotation was observed in a group of patients with left brain damage and without neglect. The results imply that the sensory afferents from neck muscles might restore the altered occipital activity and suggest that the same conditions which modulate neglect modulate VEPs latencies.     

Relation of albinism and drugs to the visual evoked potential of the mouse.

The individual and combined influences of pentobarbital and chlorprothixene on the early components of the cortical visual evoked potential (VEP) were examined in 2 experiments with a total of 31 C57BL/6 mice. Pentobarbital produced a large increase in latency, and chlorprothixene resulted in a smaller latency increase. When these agents were combined, their effects on the VEP latency were antagonistic. The effects of pentobarbital on VEP amplitude varied as a function of flash intensity, and this barbiturate anesthetic also produced qualitative VEP waveform changes. When the congenic albino (c/c) C57BL/6 was compared with the black (+/+) C57BL/6 S, pentobarbital anesthesia produced a decrease of latency with increasing stimulus intensity only in the c/c genotype, whereas VEP amplitudes were similarly affected by pentobarbital in both genotypes. (20 ref) (PsycINFO Database Record (c) 2010 APA, all rights reserved)