Direct evidence for local oscillatory current sources and intracortical phase gradients in turtle visual cortex.

Visual stimuli induce oscillations in the membrane potential of neurons in cortices of several species. In turtle, these oscillations take the form of linear and circular traveling waves. Such waves may be a consequence of a pacemaker that emits periodic pulses of excitation that propagate across a network of excitable neuronal tissue or may result from continuous and possibly reconfigurable phase shifts along a network with multiple weakly coupled neuronal oscillators. As a means to resolve the origin of wave propagation in turtle visual cortex, we performed simultaneous measurements of the local field potential at a series of depths throughout this cortex. Measurements along a single radial penetration revealed the presence of broadband current sources, with a center frequency near 20 Hz (gamma band), that were activated by visual stimulation. The spectral coherence between sources at two well-separated loci along a rostral-caudal axis revealed the presence of systematic timing differences between localized cortical oscillators. These multiple oscillating current sources and their timing differences in a tangential plane are interpreted as the neuronal activity that underlies the wave motion revealed in previous imaging studies. The present data provide direct evidence for the inference from imaging of bidirectional wave motion that the stimulus-induced electrical waves in turtle visual cortex correspond to phase shifts in a network of coupled neuronal oscillators.

Anatomical connections of auditory and lateral line areas of the dorsal telencephalon (Dm) in the osteoglossomorph teleost, Gnathonemus petersii.

Local field potentials evoked either by auditory or by mechanosensory (water displacement) lateral line stimuli were recorded in sensory subregions of the telencephalic nucleus dorsalis pars medialis (Dm) in the weakly electric fish Gnathonemus petersii. The neural tracer Neurobiotin was injected into these two physiologically defined subregions. A reciprocal connection between the two subregions of Dm, as well as cell bodies and terminals in other telencephalic regions, whose distribution was somewhat different for the two injection types, were found. The course of labeled fibers outside the telencephalon was similar after injections in both Dm regions. Fibers were seen running through the lateral forebrain bundle (lfb) to the ventral surface area of the brain within the diencephalic preglomerular region (PGv). Within a narrow streak along the ventral side of the brain densely arranged cell bodies were labeled. The locations of labeled cells within PGv were indistinguishable after tracer was injected into either acoustical or lateral line areas of Dm. Only after injection into the mechanosensory Dm region labeled cell bodies were found in the anterior preglomerular nucleus (PGa), in addition. When crystals of the fluorescent tracer DiI were inserted in the ventral part of PGv, a path of labeled fibers similar to that after telencephalic injections was found. Labeled terminals, but no cell bodies, were located both in the acoustical and in the mechanosensory regions of Dm as well as in several other telencephalic areas. Even though sensory regions in Dm that process acoustical and mechanical stimuli are segregated and unimodal, they both receive input from neurons of PGv. The specificity of the mechanosensory region of Dm might originate from the additional input from PGa and from other endbrain areas.

Sensory processing in the pallium of a mormyrid fish.

To investigate the functional organization of higher brain levels in fish we test the hypothesis that the dorsal gray mantle of the telencephalon of a mormyrid fish has discrete receptive areas for several sensory modalities. Multiunit and compound field potentials evoked by auditory, visual, electrosensory, and water displacement stimuli in this weakly electric fish are recorded with multiple semimicroelectrodes placed in many tracks and depths in or near telencephalic area dorsalis pars medialis (Dm). Most responsive loci are unimodal; some respond to two or more modalities. Each modality dominates a circumscribed area, chiefly separate. Auditory and electrical responses cluster in the dorsal 500 micrometer of rostral and caudolateral Dm, respectively. Two auditory subdivisions underline specialization of this sense. Mechanoreception occupies a caudal area overlapping electroreception but centered 500 micrometer deeper. Visual responses scatter widely through ventral areas. Auditory, electrosensory, and mechanosensory responses are dominated by a negative wave within the first 50 msec, followed by 15-55 Hz oscillations and a slow positive wave with multiunit spikes lasting from 200 to 500 msec. Stimuli can induce shifts in coherence of certain frequency bands between neighboring loci. Every electric organ discharge command is followed within 3 msec by a large, mainly negative but generally biphasic, widespread corollary discharge. At certain loci large, slow ("deltaF") waves usually precede transient shifts in electric organ discharge rate. Sensory-evoked potentials in this fish pallium may be more segregated than in elasmobranchs and anurans and have some surprising similarities to those in mammals.

Voltage-sensitive dyes for monitoring multineuronal activity in the intact central nervous system.

Optical monitoring of activity provides new kinds of information about brain function. Two examples are discussed in this article. First, the spike activity of many individual neurons in small ganglia can be determined. Second, the spatiotemporal characteristics of coherent activity in the brain can be directly measured. This article discusses both general characteristics of optical measurements (sources of noise) as well as more methodological aspects related to voltage-sensitive dye measurements from the nervous system.

Visual stimuli induce waves of electrical activity in turtle cortex.

The computations involved in the processing of a visual scene invariably involve the interactions among neurons throughout all of visual cortex. One hypothesis is that the timing of neuronal activity, as well as the amplitude of activity, provides a means to encode features of objects. The experimental data from studies on cat [Gray, C. M., Konig, P., Engel, A. K. & Singer, W. (1989) Nature (London) 338, 334-337] support a view in which only synchronous (no phase lags) activity carries information about the visual scene. In contrast, theoretical studies suggest, on the one hand, the utility of multiple phases within a population of neurons as a means to encode independent visual features and, on the other hand, the likely existence of timing differences solely on the basis of network dynamics. Here we use widefield imaging in conjunction with voltage-sensitive dyes to record electrical activity from the virtually intact, unanesthetized turtle brain. Our data consist of single-trial measurements. We analyze our data in the frequency domain to isolate coherent events that lie in different frequency bands. Low frequency oscillations (<5 Hz) are seen in both ongoing activity and activity induced by visual stimuli. These oscillations propagate parallel to the afferent input. Higher frequency activity, with spectral peaks near 10 and 20 Hz, is seen solely in response to stimulation. This activity consists of plane waves and spiral-like waves, as well as more complex patterns. The plane waves have an average phase gradient of approximately pi/2 radians/mm and propagate orthogonally to the low frequency waves. Our results show that large-scale differences in neuronal timing are present and persistent during visual processing.

Visual motion induces synchronous oscillations in turtle visual cortex.

In mammalian brains, multielectrode recordings during sensory stimulation have revealed oscillations in different cortical areas that are transiently synchronous. These synchronizations have been hypothesized to support integration of sensory information or represent the operation of attentional mechanisms, but their stimulus requirements and prevalence are still unclear. Here I report an analogous synchronization in a reptilian cortex induced by moving visual stimuli. The synchronization, as measured by the coherence function, applies to spindle-like 20-Hz oscillations recorded with multiple electrodes implanted in the dorsal cortex and the dorsal ventricular ridge of the pond turtle. Additionally, widespread increases in coherence are observed in the 1- to 2-Hz band, and widespread decreases in coherence are seen in the 10- and 30- to 45-Hz bands. The 20-Hz oscillations induced by the moving bar or more natural stimuli are nonstationary and can be sustained for seconds. Early reptile studies may have interpreted similar spindles as electroencephalogram correlates of arousal; however, the absence of these spindles during arousing stimuli in the dark suggests a more specific role in visual processing. Thus, visually induced synchronous oscillations are not unique to the mammalian cortex but also occur in the visual area of the primitive three-layered cortex of reptiles.

Event-related potentials to omitted visual stimuli in a reptile.

Visual omitted stimulus potentials (OSPs) were recorded from awake pond turtles with arrays of 3-20 electrodes in the dorsal cortex (DC), dorsal ventricular ridge (DVR) and optic tectum. Since they are generally longer in duration than the interstimulus interval (ISI), the standard experiment is a short conditioning train of regular light or dark flashes (1-20 Hz) whose termination elicits the OSP. Tectal surface OSPs after trains > 7 Hz have 2 major positive peaks, P120-140 and P220-250 after the due-time of the first omission; after < 7 Hz down to the minimum of 1.5 Hz only the slower peak appears. Some deep tectal loci also have one to three 100 msec wide negative waves peaking at variable times from 200 to 1300 msec. Forebrain OSPs in DC and DVR are approximately 30 msec later and often include induced 17-25 Hz oscillations, not phase-locked and attenuated in averages. Both tectal and forebrain OSP main waves tend toward a constant latency after the due-time, over a wide range of ISIs, as though the system expects a stimulus on schedule. Jitter of ISI around the mean does not greatly reduce the OSP. At all loci higher conditioning rates cause the amplitudes of the steady state response (SSR) VEPs to decline and of the OSPs to increase. Some similarities and correlations of regional amplitude fluctuations between OSPs and VEPs are noted. The OSP dynamics are consistent with the hypothesis of a postinhibitory rebound of temporally specific VEP components increasingly inhibited with higher stimulation rates; much of this response is retinal but each higher brain level further modulates. OSPs in this reptile are similar to those known in fish and to the "high frequency" type in humans, quite distinct in properties from the "low frequency" OSPs. It will be important to look at the high frequency type in laboratory mammals to determine whether they are present in the midbrain and retina, as in fish and reptiles.

Plurality of viual mismatch potentials in a reptile.

Abstract Studies with auditory stimuli have established in humans that a mismatch potential (MMP) is elicited whenever a deviant stimulus is substituted for a standard stimulus in a train of monotonous standard stimuli presented at rates > 0.25 Hz. The MMP in humans is localized in the auditory cortex and is known as mismatch negativily, from its polarity in scalp recordings. It is hypothesized to reflect the operation of sensory memory and to be a necessary component of the auditory orienting response. To examine the generality of MMPs we used a visual mismatch paradigm with pond turtles (Pseudemys scripta) while recording with electrode arrays (200 µm spacing) from near surface and deep visual projection areas within the forebrain and optic tectum. Standard stimuli were 10-sec trains of diffused strobe flashes presented at rates of 1-6 Hz against backgrounds of 2-11 lux. Deviant stimuli were brighter or dimmer flashes that followed the last standard flash. MMps were separated from visual evoked potentials by subtracting the response to the last standard flash of the train from the response to the same flash (bright or dim) when delivered as a deviant. Comparisons were also made with evoked potentials to isolated bright or dim flashes, that is, equal in frequency (1 per 12 sec) to the deviants but without intervening standard flashes. At tectal loci bright and dim deviants elicited net positivities that reached statistical significance in the period between 141 ± 8 and 184 ± 12 msec after the deviant stimulus (mean ± SEM). Earlier components in the tectal responses correlated with the intensity of the stimulus rather than its deviance. In the case of the bright deviants the early waves (P50-P75) were larger in amplitude. Forebrain recordings showed a similar although broader period of net positivity, associated with deviance, between 129 ± 8 and 195 ± 12 msec. Deviants, delivered as isolated flash responses evoked larger early components (100-140 msec). In separate experiments with cortical epipial electrodes, a condition somewhat more comparable to scalp recording, MMPs were similar in latency but had a negative polarity. Regression analyses revealed a relationship between the amplitude (base-to-peak) of the MMF' and the degree to which the standard response had declined with repeated stimulation. Rate decrement, as measured by the isolated (long ISI) flash response minus the last standard response, was a significant predictor of MMP amplitudes (r(2) = 0.37, tectum; r(2) = 0.31, forebrain), whereas standard response amplitudes alone were not (r(2) = 0.09; r(2) = 0.06). MMPs are present in nonmammals plurally, that is, at different levels of the visual system, at least as early as the tectum. The existence of subcortical MMPs caution against assigning a primary or exclusive role to those recorded from the cortex.

Barbiturate sensitive components of visual ERPs in a reptile.

The selective effects of methohexital anesthesia were used to differentiate components of visual event-related brain potentials (ERPs) in pond turtles (Pseudemys scripta). Tectal and forebrain omitted stimulus potentials (OSPs) were found to be particularly sensitive to the barbiturate; they are reversibly abolished while the large early wave of the forebrain flash visual evoked potential (VEP; 110-120 ms) is reduced by only 27 +/- 11% and that of the tectal VEP (55-65 ms) is increased by 40 +/- 12%. Concurrent with the decline of the OSP is the loss of late slow wave components (ca. greater than 125 ms) of forebrain and tectal VEPs and the appearance of irregularities in the responses of 5 Hz repetitive flashing. The barbiturate effects on the VEP and recovery cycles are remarkably similar to those reported in mammals.

Event-related potentials in the retina and optic tectum of fish.

1. Compound field potentials were recorded with up to 18 microelectrodes in comb, brush, or spear arrays on and in the optic tectum and with suction electrodes from the distal stump of the cut optic nerve and from the optic nerve head in the opened eye in elasmobranchs and teleosts. Diffuse light flashes of different durations and submaximal intensities were delivered in trains with regular or irregular interstimulus intervals (ISI). 2. Event-related potentials (ERPs) are visible in single trials and begin at 50-200 ms after an "oddball" flash, especially one that is slightly weaker, briefer, or delayed by as little as 6% of ISI, compared with the more frequent stimulus. ERPs to the opposite condition are not of the same form or size. 3. One or more stimuli were omitted from a train or the train terminated after various conditioning times. Deflections occur beyond the expected visual-evoked potentials (VEPs) to the last flash and are called omitted-stimulus potentials (OSPs). They occur on schedule--approximately 100 ms after the next flash would be due--almost independent of intensity, duration, or conditioning time. They are considered to be ERPs without any necessary implication or denial of a temporally specific expectation. 4. Three components of OSP occur alone or in combination: an initial fast peak, a slow wave, and an oscillatory spindle up to ls or more in duration. This resembles the OFF response to steady light. 5. All these components are already present in the retina with optic nerve cut. 6. The same mean ISI with a high proportion of jitter gives OSPs with only slightly longer latencies and smaller amplitudes; the OSP acts as though the retina makes an integrated prediction of ISI, intensity, and duration. 7. During a conditioning train the equilibrium between excitation and inhibition after each flash changes according to frequency, intensity, duration, and conditioning time; the VEP reflects this in a shape unique to the ISI; inhibition increases rapidly after each flash and then decays slowly according to the recent mean ISI. This allows rebound disinhibition after missing, weak, or delayed flashes (OSP or ERP) or causes an altered VEP after a longer or stronger oddball. 8. It seems unlikely that the OSP or oddball ERP in fish tectum is equivalent to mammalian ERPs under the same regime or signals higher cognitive events, because they are already present in the retina, require flash frequencies greater than 1 Hz, and grow with frequency up to and beyond flicker fusion.(ABSTRACT TRUNCATED AT 400 WORDS)

The fiber composition of the abdominal vagus of the rat.

The present study provides a LM and EM inventory of the fibers of the rat abdominal vagus, including dorsal and ventral trunks and the five primary branches. Whole mounts (n = 15) were prepared to characterize the branching patterns. A set of EM samples consisting of both trunks and all branches (i.e. dorsal and ventral gastric, dorsal and accessory celiac, and hepatic) were then obtained from each of six additional animals. A complete cross-sectional montage (x 10000) was prepared from each sample. All axons were counted, and greater than 10% of them were evaluated morphometrically. The means of unmyelinated axon diameters for each of the five branches were similar (0.75-0.83 microns). However, the shapes of the fiber size distributions, as summarized by their skew coefficients, revealed that the two gastric branches differed significantly from the two celiac branches; furthermore, the hepatic size distribution differed from all others. Most of the myelinated fibers (85%) in all branches were less than 2.6 microns in diameter and had sheath widths between 0.1 and 0.5 micron. The gastric branches, however, also contained a few larger myelinated fibers with sheath widths as great as 0.85 micron. Whole mounts revealed fibers which were not of supradiaphragmatic origin within all five vagal branches; these adventitial bundles were traced along the perineurium between adjacent branches. The sum of the fibers in the five branches (26930) was 21% more than the number counted in the parent trunks (22272); this excess probably reflects the adventitial fiber content. The whole mounts also showed that a large and regularly positioned paraganglion was associated with the dorsal branches. The structural profiles observed (i.e. unmyelinated and myelinated fibers size distributions, presence of extrinsic fascicles, glomus tissue content, etc.) differentiate the vagal branches into three morphologically distinct sets: a gastric pair, a celiac pair, and a hepatic branch. The fiber counts, when considered with observations of the numbers of efferents and adventitial fibers in the nerve, suggest that the percentage of efferent fibers is much higher than in all the widely accepted estimates found in the literature: efferent fibers may represent over a quarter of the total number of fibers.

A light and electron microscopic examination of the vagal hepatic branch of the rat.

The rat's vagal hepatic branch and associated tissues were studied using light and electron microscopy. Whole mounts, serial sections, and vascular endocasts were used to characterize the tissue from the anterior vagal trunk to the porta hepatis. Fiber number and caliber as well as intraneural organization were analyzed from complete cross-sectional electron micrographic montages of the hepatic branch sampled at its point of separation from the anterior vagal trunk. The hepatic branch ramified from the anterior vagus in one (in 47% of the specimens), two (in 37%) or three (in 16%) bundles. The single bundled hepatic branch contained 2887 +/- 287 unmyelinated fibers, and their size distribution, with a mean diameter of 0.66 +/- 0.02 micron, was Gaussian. Myelinated fibers numbered only 21 +/- 4 per branch and had a complex size distribution ranging from 0.5 to 1.8 micron with a mean of 1.2 +/- 0.03 micron. Forty four +/- 6% of the myelinated fibers were found in a single "subfascicle" in the dorso-medial pole of the nerve. Whole mounts at this level revealed that a distinct bundle, here designated an extrinsic "hepato-gastric bundle", occurred within the hepatic branch and linked the omental hepatic branch and the distal anterior gastric branch, apparently without central vagal connections. In the lesser omentum, between the esophagus and the hepatic artery proper, the hepatic branch formed a plexus which was characterized by numerous nerve divisions, anastomoses and large paraganglia (196-463 glomus cells per paraganglion). This plexiform segment ended with the recombining of the hepatic branch into 5-7 bundles which variously ascended in the porta, descended on the hepatic artery proper, or traversed the portal vein. Through its omental course, the hepatic branch traveled in close apposition to the hepato-esophageal artery and the corresponding vein as well as a prominent lymphatic vessel with associated hemolymph nodes.

Gold thioglucose selectively damages dorsal vagal nuclei.

To characterize the lesion produced in the medulla oblongata by gold thioglucose (GTG), the present experiment quantified the medullary damage in C57B1 mice that had become obese after treatment with 800 mg/kg of GTG at 30 days of age. At the rostrocaudal level of the area postrema, the neurotoxin destroyed up to 75% of the neurons in the medial cell column of the dorsal motor nucleus of the vagus (DMX), while sparing the lateral pole of the nucleus. GTG also produced significant tissue loss in the central and commissural subnuclei of the nucleus of the solitary tract (NST). In contrast, the GTG lesion did not affect cell number in the hypoglossal nucleus or reduce the volume of the area postrema. Additional observations indicated that at 48-72 h after GTG administration the affected regions of the medulla already show advanced necrosis including cell loss and gliosis; and when the relative contributions of hypothalamic, DMX, and NST damage to the obesity that develops are evaluated statistically with partial correlational analysis, it appears the the obesity primarily correlates with the hypothalamic lesion produced by GTG.

Changes in socio-emotional behavior under imipramine treatment in normal and amygdalo-hypothalamic dogs.

The effects of imipramine on learned social responses were examined in ten dogs with dorsomedial amygdalar lesions and/or lateral hypothalamic lesions. Six of the ten dogs were also tested preoperatively. The social responses were instrumentally conditioned using social interaction with the experimenter as reinforcement (petting and verbal reassurance). In the non-lesioned dogs imipramine treatment produced a dose-dependent deterioration of performance during drug administration followed by a long-term amelioration of performance. In the lesioned dogs imipramine produced various changes in performance depending on the pretreatment level of responding. When the pretreatment level of performance was high drug administration resulted in a long-term deterioration, and when performance was poor imipramine produced a continuous and long-term increase. It is suggested that imipramine facilitates the recovery of performance, but suppresses well-performed responses.

Organization and distribution of the rat subdiaphragmatic vagus and associated paraganglia.

This experiment analyzed the organization of the rat abdominal vagus. To spare delicate tissues and preserve positional information, untrimmed blocks of the subdiaphragmatic viscera (N = 22) were fixed, impregnated by using a pyridine-silver protocol, and double embedded. Each block was sectioned transversely at 7 micron, and a section every 70 micron from the diaphragm to the cardia was analyzed. The features of the section were traced and digitized for computer reconstruction. Included in the measurements were sizes and locations of bundles, fascicles, and paraganglia. The anterior and posterior vagi were consistently distinctive in size, distribution, cross-sectional shape, and paraganglionic content. In the most common pattern (41% of animals), the anterior trunk coursed longitudinally on the ventral surface of the esophagus, giving off at successively more distal levels the hepatic branch, the accessory coeliac branch and then the bundles of the anterior gastric branch. The posterior trunk separated into a coeliac branch and a posterior gastric branch, each consisting of numerous bundles, in the most distal quarter of the esophagus. Fifty-nine percent of all animals exhibited one or more significant variations in vagal organization (e.g., double primary trunks--41%, supernumerary branches--18%, or atypical branching sequences--9%). Four to 14 vagal paraganglia (mean = 8 +/- 1; equivalent to 32/rat, corrected for sampling) were found in each animal, and no branch was consistently devoid of paraganglia. Ninety-four percent of the paraganglia were located at nerve branch points. Some of the larger paraganglia contained at their central poles one to six neurons with soma diameters ranging from 14 to 22 micron.

Anatomical considerations for surgery of the rat abdominal vagus: distribution, paraganglia and regeneration.

In order to provide a detailed surgical anatomy of the rat abdominal vagus, we examined pyridine silver-stained tissue from one group of normal animals and a second group that survived 9 months after vagotomy. In the normal sample, as has been established for man, there was considerable variability in the levels at which each of the vagal branches separated from the main trunks. Contrary to reports from dissection studies, most of the branches were not single fiber bundles but rather consisted of two or more separate bundles. At the extreme, the posterior gastric and coeliac branches each consisted of as many as 15 individual bundles. Even the main trunks of the subdiaphragmatic vagus were occasionally observed to have multiple components (anterior trunk, 13% of the cases; posterior, 25%). In addition to the classically recognized hepatic, anterior gastric, coeliac, and posterior gastric branches, we also observed an accessory coeliac branch of the anterior trunk in all animals. This accessory coeliac division originated just caudal to the hepatic branching and extended first laterally and then dorsally while running caudally to exit from the esophagus just before the separation of the coeliac branch from the posterior trunk. The vagi were observed to contain paraganglia consisting of islands of glomus cells, neurons, and extensive capillary beds, all situated within the perineurium. The paraganglia occurred in greatest frequency at the sites where the hepatic and coeliac branches divide from their respective trunks. Paraganglia were also observed peripherally within vagal branches; there they were most numerous within the coeliac branch and least numerous in the accessory coeliac. Other studies yielded evidence that regeneration had occurred after complete vagotomy. First, stumps of the branches distal to the resection scar contained axons. Central to the scar, axons grew out in all directions from the neuroma; some of them appeared to cross the scar and to reinnervate the distal stumps. Secondly, 30% of the animals in which regeneration was thought to be possible increased their insulin secretion in response to electrical stimulation of the cervical vagus. The implications of the above findings for experiments that involve manipulation or recording of the vagus are discussed.