Duplication of Evoked Potential Waveform by Curve of Probability of Firing of a Single Cell

Neural correlates of correct and errant attentional selection revealed through N2pc and frontal eye field activity

The goal of this study was to obtain a better understanding of the physiological basis of errors of visual search. Previous research has shown that search errors occur when visual neurons in the frontal eye field (FEF) treat distractors as if they were targets. We replicated this finding during an inefficient form search and extended it by measuring simultaneously a macaque homologue of an event-related potential indexing the allocation of covert attention known as the m-N2pc. Based on recent work, we expected errors of selection in FEF to propagate to areas of extrastriate cortex responsible for allocating attention and implicated in the generation of the m-N2pc. Consistent with this prediction, we discovered that when FEF neurons selected a distractor instead of the search target, the m-N2pc shifted in the same, incorrect direction prior to the erroneous saccade. This suggests that such errors are due to a systematic misorienting of attention from the initial stages of visual processing. Our analyses also revealed distinct neural correlates of false alarms and guesses. These results demonstrate that errant gaze shifts during visual search arise from errant attentional processing.

A brief introduction to the use of event-related potentials in studies of perception and attention

Because of the precise temporal resolution of electrophysiological recordings, the event-related potential (ERP) technique has proven particularly valuable for testing theories of perception and attention. Here, I provide a brief tutorial on the ERP technique for consumers of such research and those considering the use of human electrophysiology in their own work. My discussion begins with the basics regarding what brain activity ERPs measure and why they are well suited to reveal critical aspects of perceptual processing, attentional selection, and cognition, which are unobservable with behavioral methods alone. I then review a number of important methodological issues and often-forgotten facts that should be considered when evaluating or planning ERP experiments.

From synchronous neuronal discharges to subjective awareness?

For practical clinical purposes, as well as because of their deep philosophical implications, it becomes increasingly important to be aware of contemporary studies of the brain mechanisms that generate subjective experiences. Current research has progressed to the point where plausible theoretical proposals can be made about the neurophysiological and neurochemical processes which mediate perception and sustain subjective awareness. An adequate theory of consciousness must describe how information about the environment is encoded by the exogenous system, how memories are stored in the endogenous system and released appropriately for the present circumstances, how the exogenous and endogenous systems interact to produce perception, and explain how consciousness arises from that interaction. Evidence assembled from a variety of neuroscience areas, together with the invariant reversible electrophysiological changes observed with loss and return of consciousness in anesthesia as well as distinctive quantitative electroencephalographic profiles of various psychiatric disorders, provides an empirical foundation for this theory of consciousness. This evidence suggests the need for a paradigm shift to explain how the brain accomplishes the transformation from synchronous and distributed neuronal discharges to seamless global subjective awareness. This chapter undertakes to provide a detailed description and explanation of these complex processes by experimental evidence marshaled from a wide variety of sources.

Shared and private variability in the auditory cortex

The high variability of cortical sensory responses is often assumed to impose a major constraint on efficient computation. In the auditory cortex, however, response variability can be very low. We have used in vivo whole cell patch-clamp methods to study the trial-to-trial variability of the subthreshold fluctuations in membrane potential underlying tone-evoked responses in the auditory cortex of anesthetized rats. Using methods adapted from classical quantal analysis, we partitioned this subthreshold variability into a private component (which includes synaptic, thermal, and other sources local to the recorded cell) and a shared component arising from network interactions. Here we report that this private component is remarkably small, usually about 1-3 mV, as quantified by the variance divided by the mean of the ensemble of tone-evoked response heights. The shared component can be much larger, and shows more heterogeneity across the population, ranging from about 0 to 10 mV. The remarkable fact that, at least 5 synapses from the auditory periphery, this variability remains so small raises the possibility that the intervening neural circuitry is organized so as to prevent private noise from accumulating as neural signals propagate to the cortex.

Genetics of human prefrontal function

Evolution of the prefrontal cortex was an essential precursor to civilization. During the past decade, it became increasingly obvious that human prefrontal function is under substantial genetic control. In particular, heritability studies of frontal lobe-related neuropsychological function, electrophysiology and neuroimaging have greatly improved our insight. Moreover, the first genes that are relevant for prefrontal function such as catechol-O-methyltransferase (COMT) are currently discovered. In this review, we summarize the present knowledge on the genetics of human prefrontal function. For historical reasons, we discuss the genetics of prefrontal function within the broader concept of general cognitive ability (intelligence). Special emphasis is also given to methodological concerns that need to be addressed when conducting research on the genetics of prefrontal function in humans.

Excitatory effects of fentanyl upon the rat electroencephalogram and auditory-evoked potential responses during anaesthesia

BACKGROUND AND OBJECTIVE

Previous studies have shown existence of inconsistent data concerning the use of auditory-evoked potential (AEP) and electroencephalogram (EEG) changes to measure the depth of anaesthesia in regimens involving the use of opioids. The present studies characterize the effects of fentanyl on those responses in rats.

METHODS

The effects of a bolus of fentanyl (6-10 microg kg(-1) intravenously) alone or following naloxone (100 microg kg(-1) intravenously) were examined using brain responses in rats during light anaesthesia with either propofol (20-30 mg kg(-1) h(-1)) or isoflurane (0.8%). Electrophysiological data were recorded using silver ball electrodes. The rats' tracheas were intubated and a femoral artery cannula was inserted to monitor blood pressure. Body temperature, respiratory and pulse rate, and pedal withdrawal data were also collected. Parameters measured before and following administration of naloxone and fentanyl or of fentanyl alone were compared using repeated-measures ANOVA.

RESULTS

Fentanyl significantly increased the latency of the major peak from the AEP during propofol and isoflurane anaesthesia (F = 13.2 and 13.5, respectively; P < 0.05) and the amplitude differential between two waveform complexes, and the second differential index (F = 28.3 and 57.2, respectively; P < 0.01). The spectral edge frequency and median frequency from the EEG tended to increase. These effects were abolished by the prior administration of naloxone.

CONCLUSIONS

These excitatory effects were inconsistent with the classical concept of brain activity depression indicating a deepening of anaesthesia.

Effects of different propofol infusion rates on EEG activity and AEP responses in rats

Parameters calculated from the auditory-evoked potential (AEP) recorded over the auditory cortex and from the electroencephalogram (EEG) recorded over the near vertex were compared in rats at three different infusion rates of propofol (62.5, 35 and 25 mg/kg/h). Depth of anaesthesia was assessed clinically using the strength of the pedal withdrawal reflex. Well-defined AEP responses were consistently obtained. As the propofol concentration was reduced, peak latencies decreased and peak to peak amplitudes increased. Amplitude and latency values were closely associated with the strength of the pedal withdrawal responses. Parameters calculated from the EEG showed no significant change as the propofol concentration was reduced. Periods of burst suppression became more frequent as the propofol infusion rate was increased. The study showed some of the difficulties that may be encountered when using EEG as a tool to assess depth of anaesthesia during propofol infusion. The AEP showed dose dependent changes in rats at different infusion rates of propofol. However, large variability between animals limits the use of this technique for monitoring depth of anaesthesia.

Quantitative EEG and the Frye and Daubert standards of admissibility

The 70-year-old Frye standards of "general acceptance" were replaced by the Supreme Court's 1993 Daubert criteria of the scientific method, which established the standards for admissibility of evidence in Federal Court. The four Daubert criteria were: 1- Hypothesis testing, 2- Estimates of error rates, 3- Peer reviewed publication and 4- General acceptance (Daubert v. Merrell Dow Pharmaceuticals, 61 U.S.LW 4805 (U.S. June 29, 1993)). The present paper starts with the Daubert four factors and then matches them, step by step, to the scientific peer reviewed literature of quantitative EEG (QEEG) in relation to different clinical evaluations. This process shows how the peer reviewed science of the Digital EEG and the Quantitative EEG (QEEG) meet all of the Daubert standards of scientific knowledge. Furthermore, the science and technical aspects of QEEG in measuring the effects of neurological and psychiatric dysfunction also match the recent Supreme Court standards of "technical" and "other specialized" knowledge (General Electric Co v. Joiner, 1997, Kumho Tire Company, Ltd. v. Carmichael, 1999). Finally, it is shown that QEEG scientific knowledge and QEEG "technical" and "other specialized" knowledge meet the trilogy standards of the Supreme Court rulings in support of QEEG's admissibility as a clinically valid method in the evaluation of the nature and extent of neurological and psychiatric disorders.

The neurophysics of consciousness

Consciousness combines information about attributes of the present multimodal sensory environment with relevant elements of the past. Information from each modality is continuously fractionated into distinct features, processed locally by different brain regions relatively specialized for extracting these disparate components and globally by interactions among these regions. Information is represented by levels of synchronization within neuronal populations and of coherence among multiple brain regions that deviate from random fluctuations. Significant deviations constitute local and global negative entropy, or information. Local field potentials reflect the degree of synchronization among the neurons of the local ensembles. Large-scale integration, or 'binding', is proposed to involve oscillations of local field potentials that play an important role in facilitating synchronization and coherence, assessed by neuronal coincidence detectors, and parsed into perceptual frames by cortico-thalamo-cortical loops. The most probable baseline levels of local synchrony, coherent interactions among brain regions, and frame durations have been quantitatively described in large studies of their age-appropriate normative distributions and are considered as an approximation to a conscious 'ground state'. The level of consciousness during anesthesia can be accurately predicted by the magnitude and direction of reversible multivariate deviations from this ground state. An invariant set of changes takes place during anesthesia, independent of the particular anesthetic agent. Evidence from a variety of neuroscience areas supporting these propositions, together with the invariant reversible electrophysiological changes observed with loss and return of consciousness, are used to provide a foundation for this theory of consciousness. This paper illustrates the increasingly recognized need to consider global as well as local processes in the search for better explanations of how the brain accomplishes the transformation from synchronous and distributed neuronal discharges to seamless global subjective awareness.

Timing and connectivity in the human somatosensory cortex from single trial mass electrical activity

Parallel-distributed processing is ubiquitous in the brain but often ignored by experimental designs and methods of analysis, which presuppose sequential and stereotypical brain activations. We introduce here a methodology that can effectively deal with sequential and distributed activity. Regional brain activations elicited by electrical median nerve stimulation are identified in tomographic estimates extracted from single trial magnetoencephalographic signals. Habituation is identified in both primary somatosensory cortex (SI) and secondary somatosensory cortex (SII), often interrupted by resurgence of strong activations. Pattern analysis is used to identify single trials with homogeneous regional brain activations. Common activity patterns with well-defined connectivity are identified within each homogeneous group of single trials across the subjects studied. On the contralateral side one encounters distinct sets of single trials following identical stimuli. We observe in one set of trials sequential activation from SI to SII and insula with onset of SII at 60 msec, whereas in the other set simultaneous early co-activations of the same two areas.

A field theory of consciousness

This article summarizes a variety of current as well as previous research in support of a new theory of consciousness. Evidence has been steadily accumulating that information about a stimulus complex is distributed to many neuronal populations dispersed throughout the brain and is represented by the departure from randomness of the temporal pattern of neural discharges within these large ensembles. Zero phase lag synchronization occurs between discharges of neurons in different brain regions and is enhanced by presentation of stimuli. This evidence further suggests that spatiotemporal patterns of coherence, which have been identified by spatial principal component analysis, may encode a multidimensional representation of a present or past event. How such distributed information is integrated into a holistic precept constitutes the binding problem. How a precept defined by a spatial distribution of nonrandomness can be subjectively experienced constitutes the problem of consciousness. Explanations based on a discrete connectionistic network cannot be reconciled with the relevant facts. Evidence is presented herein of invariant features of brain electrical activity found to change reversibly with loss and return of consciousness in a study of 176 patients anesthetized during surgical procedures. A review of relevant research areas, as well as the anesthesia data, leads to a postulation that consciousness is a property of quantum-like processes, within a brain field resonating within a core of structures, which may be the neural substrate of consciousness. This core includes regions of the prefrontal cortex, the frontal cortex, the pre- and paracentral cortex, thalamus, limbic system, and basal ganglia.

Estimating cortical activity from VEPS with the shrinking ellipsoid inverse

An iterative inverse method using Tikhonov regularization (the shrinking ellipsoid method) previously tested in a model system is used to invert the sequence of bioelectric scalp fields evoked by the onset of a checkerboard pattern in either the right or left lower hemifield. The shrinking ellipsoid method is modified from its original description to accommodate simultaneously inverting a sequence of thirteen VEP scalp fields measured from 65 to 125 ms after stimulus onset. This allows the evoked cortical activity to be tracked in 5-ms intervals without distortion due to occasional VEP scalp fields in the sequence that have too low a signal-to-noise ratio to be reliably inverted in isolation. A new method is described to identify the surface of the cortex from MRI data. This is required to implement the shrinking ellipsoid inverse. Results from two subjects studied in detail are presented. The earliest cortical activity occurs either in area MT (the middle temporal area) or simultaneously in MT and striate cortex (V1). However when it does occur in both areas, the activity in V1 is relatively weak and quickly subsides. Seventy-five ms after stimulus onset activity is seen mainly near MT corresponding to a region identified from PET studies as one that subserves motion processing. Activity moves to V1 by 90-100 ms after stimulus onset. Near 120 ms after stimulus onset, cortical activity returns to the region near MT. Virtually all activity identified in this time epoch occurs in the cortical hemisphere contralateral to the location of the stimulus in the visual field.

Changes in early acoustic-evoked potentials by mildly arousing priming stimuli in carp (Cyprinus carpio)

1. Averaged acoustic-evoked potential (AEPs) in the medulla and midbrain were recorded, as were changes in heart rate, indicating arousal, to a previous non-acoustic priming stimulus. 2. Useful AEP measures were amplitude of the early biphasic wave (less than 10 msec) in medulla and amplitude and duration of this wave in midbrain. 3. There was a negative regression of heart rate and medullary AEP amplitude especially evident for a 2 sec light stimulus. Decreased AEP amplitude in both regions was induced by water movement and an increase in midbrain AEP duration by the tactile stimulus. 4. Arousal effects even on these early AEP measures are specific to the form of arousing stimulus.

Applied psychophysiology and biofeedback of event-related potentials (brain waves): historical perspective, review, future directions

This paper reviews the efforts of workers in the 1960s-1980s to demonstrate voluntary control of exogenously evoked (event-related) potentials in visual, somatic sensory, and auditory systems in rats, cats, and humans. The first part of the paper reviews the conceptual foundation and development of the work--it actually arose from traditional sensory coding and neural correlates of behavior studies. The second part summarizes recent applications of the method in the area of pain control. In reviewing these matters, the major effort is directed at revealing how the ideas unfolded in very human, day-to-day, anecdotal terms. There is not much of an attempt to formally review the literature, which is cited for consultation elsewhere. In the same spirit, many possible future experiments are suggested by way of elucidating the key remaining questions in the area.

Centrifugal regulation of neuronal activity in the olfactory bulb of the waking rabbit as revealed by reversible cryogenic blockade

The influences of centrifugal projections to the olfactory bulb were examined on the bulbar EEG and mitral-tufted cell activity in waking rabbits. Each of 6 rabbits was implanted, under surgical anesthesia, with fine wire electrodes for recording of the EEG and mitral-tufted cell unit activity and for stimulating the lateral olfactory tract. Two cooling probes, for reversible cryogenic blockade, were implanted on either side of the left olfactory peduncle. Records of EEG and unit activity were taken for 200 s before, during and after cooling of the probes to 3 degrees centigrade. Antidromic evoked potentials were used to assess the efficacy of the blockade. During the cryogenic blockade bursts of EEG activity, evoked in the bulb by inspiration through the nose, were augmented in amplitude and reduced in frequency. Mitral-tufted cell unit activity was reduced in rate but was more highly correlated with the phase and amplitude of the EEG bursts. Analysis of individual EEG bursts revealed that the variance in frequency of bulbar activity was significantly reduced in the isolated state. The data demonstrate that oscillatory bursting activity in the olfactory bulb is intrinsically maintained within a relatively fixed frequency range during receptor input and does not depend on centrifugal projections for its electrogenesis. Changes in EEG frequency, amplitude and correlation with unit activity support the hypothesis that centrifugal projections act in part to inhibit mitral-tufted cell output by direct excitation of granule cells. These findings are supported by a theoretical model in which distributed feedback to the granule cells from more central olfactory structures acts to regulate the coherency of bulbar activity.

Properties of the evoked potential generators: current source-density analysis of visually evoked potentials in the cat cortex

The depth profiles of visually evoked field potentials were recorded in areas 17 and 18 of the cat visual cortex. For comparison, potential profiles evoked by electrical stimulation of the primary afferents and of the nonspecific reticular system were also recorded. From these profiles the current source-density (CSD) distributions were calculated using the one-dimensional CSD method. CSD distributions evoked by the different types of stimuli differ in their amplitudes and time courses by approximately one and two orders of magnitude. Qualitatively, however, they are very similar. Thus, the CSDs can be interpreted as reflecting the same basic pattern of excitatory synaptic activations. This pattern consists of early activation components in the input layers, followed by excitatory synaptic activations in layer III, then in layer II, and in layer V. The basic pattern of cortical activation was found to be modulated by specific features of the visual stimuli. Modulations reflecting contour-versus contrast-contents as well as those reflecting characteristic features of moving patterns have been identified. Most of the CSD components of the cortical activation sequence were obtained from regions extending well beyond the cellular receptive fields in visual cortex. Thus, they reflect nonretinotopic activities. Parameters other than specific features of the visual stimuli have profound influence on cortical CSDs. Nonspecific parameters which have been considered are the general state of cortical excitability, the temporal interactions of successive activities (which are predominantly facilitatory), and the lateral interactions of simultaneous activations from different regions of the visual field (predominantly inhibitory).

Occipital and inferotemporal responses to visual signals in the monkey

This study analyzes cellular and field-potential responses in striate and inferotemporal cortex to visual stimuli in monkeys performing a memory task (delayed matching-to-sample). Each trial was initiated by a brief alerting diffuse flash preceding presentation of the memorandum (sample); the latter was a lighted circle (red or green, 1.5 s) to be retained by the animal during a subsequent delay for correct behavioral response (color match). The alerting flash evoked distinct excitatory cell responses and field potentials in the occipital cortex; those two orders of phenomena were broadly related to each other in temporal terms. By contrast, most cells in the inferotemporal region were inhibited by the flash, although the local evoked field potential had a configuration similar to that of the occipital potential. In each region, the sample stimuli elicited excitatory unit responses which summed to a unimodal distribution with an initial component roughly corresponding in time course to the local field potential. Although the shortest response latencies were found in occipital cortex, considerable temporal overlap of the sample-related activities in the two cortices was observed. The finding that most inferotemporal cells, unlike occipital cells, treated only the sample with excitatory response indicates that the inferotemporal cortex is selectively attuned to visual detail. However, the largely simultaneous activation of both cortical regions following the onset of the sample suggests that discriminative visual information is processed by hierarchic interactions of the two cortices through their reciprocal connections.

IQ revisited in the time domain: a critique or irresponsible brainwaves

In 1969 two authors proposed that human visual evoked potentials could be predictors of psychometrically scored "intelligence." This interpretation of their data was subsequently invalidated. In 1982 two other authors made a similar claim for auditory evoked potentials; their published methodology and general theoretical background in sensory neurophysiology suggest even less likelihood of predictive validation.

Conditioning of single units in visual association cortex: cell-specific behavior within a small population

These experiments examine the interrelationships between the activity of adjacent neurons during learning. Does learning depend on coherent behavior in a population of neurons or does it depend on particular neurons engaging in a particular activity at specific times? A second purpose was to examine specificity in response modification as a function of reinforcement contingency. Cells from visual association cortex of locally anesthetized, paralyzed cats and rabbits were studied with extracellular microelectrodes capable of recording single and multiunit activity, as well as local field potentials. Multiunit records were fractionated by amplitude "windows" discrimination. Pavlovian discriminative conditioning procedures were used to evaluate selective plasticity. Cells that were activated by at least two different visual stimuli were selected. Only one of the effective stimuli was paired with foot-shock (reinforcement). Of the 180 cells or cell clusters studied, 27% exhibited conditioned modification to the reinforced stimulus (CS+) and 19% changed their response pattern to the unreinforced stimulus (CS-). None of the well isolated cells showed conditioning to both CS+ and CS-. Thus, cellular plasticity was specific to reinforcement contingency. These results provide a first demonstration of reinforcement-dependent functional distinctiveness at the neuronal level. Some cells showed no alteration of response pattern despite a most prolonged conditioning procedure. Neighboring cells, responsive to the same stimuli, revealed increases or decreases in firing rate, selective changes in the latency or amplitude of single response peaks, or the appearance of one or more new peaks as a function of conditioning. Rarely did adjacent cells show the same type of alteration when alteration occurred; there was no general tendency toward coherent firing patterns as conditioning proceeded.

Reference-free identification of components of checkerboard-evoked multichannel potential fields

A method is proposed to determine components of evoked scalp potentials, in terms of times of occurrence (latency) and location on the scalp (topography). The scalp field distributions were evoked by checkerboard reversal and were recorded simultaneously in 47 channels. Component latencies are defined as times of maximal values of the electrical power of the evoked field (a measure of the amount of field relief); this measurement is independent of the choice of the reference electrode. In 10 subjects, two evoked components were found consistently: at 100 and at 140 msec. Plots of scalp locations of the extreme field values (i.e., reference-free data) at the occurrence times of the components showed occipitally positive and anteriorly negative extreme values at 100 msec, and vice versa at 140 msec. The occipital extreme values were surrounded by steep field gradients suggesting occipital generator processes. The polarity reversal of the evoked field distributions between 100 and 140 msec was a quick, jump-like location change of the extreme values in the field. The locations of the extreme field values were stable for long periods around peak times of the power curve. During these periods, the shape of the field remained constant (assessed by the average standard deviation of voltages per electrode between successive field distributions), suggesting also a stable localization of the generating process in depth. The field distributions tended to be concentric around the extreme field values. The major characteristics of the observed scalp fields showed no wave fronts, and no continuous 'traveling' of extreme values over larger distances.

Acute FeCl3-induced epileptogenic foci in cats: electrophysiological analyses

Fifteen cats were rendered acutely epileptic via transpial iontophoresis of ferric ions. Extracellular electrical activity of individual cortical neurons was temporally correlated with ECoG epileptic spikes. Each unit's activity was characterized as excited, inhibited, or unaffected. The spatial coordinates of each neuron were logged, thereby generating a 3-dimensional activity map. Discharge patterns were further classified as bursting or non-bursting. A significantly increased population of inhibited cells was detected at radial distances greater than 3 mm from the iontophoretic site. This increase was at the expense of "unaffected-bursting" cells. Pathological discharge patterns, including units which fired in long-first-interval bursts, were common in the focus. A system was designed to electronically calculate single-unit firing probability before, during and after surface epileptiform spikes. An 8-12 Hz oscillation in firing probability level, time-locked to the epileptic spike, was observed in most excited-bursting units. Many of these findings in the ferric-induced epileptic focus accurately model the electrophysiological characteristics previously reported for human epileptic foci examined at neurosurgery.

The human magnetoencephalogram: some EEG and related correlations

Simultaneous magnetoencephalographic (MEG) and electroencephalographic (EEG) data were recorded from six normal adult subjects. MEG signal strength and EEG voltage level appear to be linearly correlated. Spectral analysis suggested that the MEG and EEG data were produced by similar but non-identical generator systems. A vertex region magnetic averaged evoked response to flash was recorded in one (of four) subjects, consisting of a waveform similar to but out of phase with the simultaneous EEG averaged evoked response, such that cortical negativity was correlated with a magnetic field directed into the scalp. Eye movement artifact, which can seriously compromise EEG recordings, does not appear to be a major problem in MEG recordings.

Evoked potential-unit relationships in behaving cats

Adult cats were implanted with a movable microelectrode, and trained to respond differentially to two different frequencies of light flicker. Unit responses and evoked potentials were recorded along trajectories in the visual cortex and lateral geniculate nucleus. Although strong correlations are shown to exist between certain components of the evoked potential and peaks in the poststimulus histogram, it is demonstrated that it is impossible to specify a causal, predictive relationship.

A late component of flash-evoked potentials in the cat's optic chiasma and superior colliculus: its appearance due to background illumination

Flash-evoked potentials (FEPs) in the cat's optic chiasma and superior colliculus were recorded under the following two conditions: complete darkness and background illumination. Special attention was paid to a specific late component of FEP'S and comparison in the behavior of the late component was made between the two conditions. It was found that a late component of FEPs in the optic chiasma appeared in the presence of background illumination while it was not observed under the condition of complete darkness. A corresponding late component of FEPs was detected in the superior colliculus. The late component might be supposed to be driven from a class of W-cells in the cat's retina.

Intensity coding in the frog retina. Quantitative relations between impulse and graded activity

The impulse discharge of single on-off neurons and a graded field potential, the proximal negative response (PNR), were simultaneously recorded with an extracellular microelectrode in the inner frog retina. Normalized amplitude-intensity functions for the on-response of the PNR and the neuron's post-stimulus time histogram (PSTH) were nearly coincident and typically showed a dynamic range spanning approximately 2 log units of intensity. Thus a nearly linear relation is found between the amplitude of the PNR and the neuron's PSTH. A neuron's PSTH amplitude and maximum instantaneous frequency of discharge were usually highly correlated, but occasional marked disparities indicate that temporal jitter of the first spike latency is an additional, relatively independent variable influencing PSTH amplitude. It typically changes by a factor of 20-30 over the intensity range. These and other findings have implications for the functional significance of the PNR and the PSTH, for a possible linear link between amacrine and on-off ganglion cells, and for a mechanism of intensity coding in which temporal jitter of latency exerts a major role.

Statistical features of brain responses under conditions of attention and habituation

This research is concerned with the variance of average evoked responses (AER) under conditions of habituation and that of attentive viewing. The major findings suggest that habituation is the result of increased activity in central sensory systems but an activity that is less organized than that resulting from attentive-state stimulation. Data are also presented which assess certain broad assumptions common to AER research.