A single trial analysis of hippocampal theta frequency during nonsteady wheel running in rats

Stimulation of spinal cord according to recorded theta hippocampal rhythm during rat move on treadmill

OBJECTIVES

Several studies have revealed that after spinal cord injury (SCI), in acute and sub-acute phase the spinal cord neurons below the injury are alive and could stimulate by use of electrical pulses. Spinal cord electrical stimulation could generate movement for paralyzed limbs and is a rehabilitation strategy for paralyzed patients. An innovative idea for controlling spinal cord electrical stimulation onset time is presented in current study.

METHODS

In our method, the time of applying electrical pulse on the spinal cord is according to rat behavioral movement and two movements behaviors are recognized only based on rat EEG theta rhythm on the treadmill line. Briefly, 5 rats were placed on the treadmill and the animals experienced zero or 12 m/min speeds.

RESULTS

These speeds were recognized based on EEG signals and off-line periodogram analysis. Finally, the electrical stimulation pulses had been applied to the spinal cord if the results of the EEG analysis had detected running behavior.

CONCLUSIONS

These findings may guide future research in utilizing theta rhythms for the recognition of animal motor behavior and designing electrical stimulation systems based on it.

Effects of visual inputs on neural dynamics for coding of location and running speed in medial entorhinal cortex

Neuronal representations of spatial location and movement speed in the medial entorhinal cortex during the ‘active’ theta state of the brain are important for memory-guided navigation and rely on visual inputs. However, little is known about how visual inputs change neural dynamics as a function of running speed and time. By manipulating visual inputs in mice, we demonstrate that changes in spatial stability of grid cell firing correlate with changes in a proposed speed signal by local field potential theta frequency. In contrast, visual inputs do not alter the running speed-dependent gain in neuronal firing rates. Moreover, we provide evidence that sensory inputs other than visual inputs can support grid cell firing, though less accurately, in complete darkness. Finally, changes in spatial accuracy of grid cell firing on a 10 s time scale suggest that grid cell firing is a function of velocity signals integrated over past time.

Speed modulation of hippocampal theta frequency and amplitude predicts water maze learning

Theta oscillations in the hippocampus have many behavioral correlates, with the magnitude and vigor of ongoing movement being the most salient. Many consider correlates of locomotion with hippocampal theta to be a confound in delineating theta contributions to cognitive processes. Theory and empirical experiments suggest theta-movement relationships are important if spatial navigation is to support higher cognitive processes. In the current study, we tested if variations in speed modulation of hippocampal theta can predict spatial learning rates in the water maze. Using multi-step regression, we find that the magnitude and robustness of hippocampal theta frequency versus speed scaling can predict water maze learning rates. Using a generalized linear model, we also demonstrate that speed and water maze learning are the best predictors of hippocampal theta frequency and amplitude. Our findings suggest movement-speed correlations with hippocampal theta frequency may be actively used in spatial learning.

Voluntary exercise enhances hippocampal theta rhythm and cognition in the rat

Regular exercise promotes learning and memory functions. Theta activity is known to relate to various cognitive functions. An increase in theta power may be related to higher cognitive functioning and learning functions. However, evidence is lacking to directly confirm that exercise training can increase the theta activity and promote various cognitive functions simultaneously. We hypothesize that long-term voluntary exercise increases the activity of hippocampal theta rhythm and enhances memory behavior. We used the voluntary wheel running model and a training period of 8 weeks. We started the training when the rats were 12 weeks old. Before and after intervention, we performed a 24 -h electrophysiological recording and 8-arm radial maze test to analyze the hippocampal theta rhythm in awake stage, and spatial memory functions. We discovered that middle to high range frequency (6.5-12 Hz) of theta power was increased after exercise intervention. In addition, the working memory error of 8-arm radial maze test in the exercise group decreased significantly after the 8 weeks of treatment, and these reductions were negatively correlated with hippocampal theta activity. Our results demonstrate that 8-weeks voluntary exercise increases both hippocampal theta amplitude and spatial memory in the rats.

Spatial encoding in primate hippocampus during free navigation

The hippocampus comprises two neural signals-place cells and θ oscillations-that contribute to facets of spatial navigation. Although their complementary relationship has been well established in rodents, their respective contributions in the primate brain during free navigation remains unclear. Here, we recorded neural activity in the hippocampus of freely moving marmosets as they naturally explored a spatial environment to more explicitly investigate this issue. We report place cells in marmoset hippocampus during free navigation that exhibit remarkable parallels to analogous neurons in other mammalian species. Although θ oscillations were prevalent in the marmoset hippocampus, the patterns of activity were notably different than in other taxa. This local field potential oscillation occurred in short bouts (approximately .4 s)-rather than continuously-and was neither significantly modulated by locomotion nor consistently coupled to place-cell activity. These findings suggest that the relationship between place-cell activity and θ oscillations in primate hippocampus during free navigation differs substantially from rodents and paint an intriguing comparative picture regarding the neural basis of spatial navigation across mammals.

A theta rhythm in macaque visual cortex and its attentional modulation

Theta rhythms, ≈3–8 Hz, have been found in many different parts of the brain. They are predominant in the rodent hippocampus, yet have also been described in the neocortex, primarily in frontal and parietal areas in relation to executive functions. Here, we show a ≈4-Hz theta rhythm in awake macaque monkey area V4 and primary visual cortex. This theta rhythm was spatially coextensive with visually induced gamma-band activity, and gamma power was modulated by theta phase. The strength of theta and of theta-rhythmic gamma modulation was markedly reduced by selective attention. Theta rhythmicity has been observed in microsaccade sequences, and microsaccades influence early visual activity. Yet, removing (the effects of) microsaccades did not influence the results.

Theta rhythms govern rodent sniffing and whisking, and human language processing. Human psychophysics suggests a role for theta also in visual attention. However, little is known about theta in visual areas and its attentional modulation. We used electrocorticography (ECoG) to record local field potentials (LFPs) simultaneously from areas V1, V2, V4, and TEO of two macaque monkeys performing a selective visual attention task. We found a ≈4-Hz theta rhythm within both the V1–V2 and the V4–TEO region, and theta synchronization between them, with a predominantly feedforward directed influence. ECoG coverage of large parts of these regions revealed a surprising spatial correspondence between theta and visually induced gamma. Furthermore, gamma power was modulated with theta phase. Selective attention to the respective visual stimulus strongly reduced these theta-rhythmic processes, leading to an unusually strong attention effect for V1. Microsaccades (MSs) were partly locked to theta. However, neuronal theta rhythms tended to be even more pronounced for epochs devoid of MSs. Thus, we find an MS-independent theta rhythm specific to visually driven parts of V1–V2, which rhythmically modulates local gamma and entrains V4–TEO, and which is strongly reduced by attention. We propose that the less theta-rhythmic and thereby more continuous processing of the attended stimulus serves the exploitation of this behaviorally most relevant information. The theta-rhythmic and thereby intermittent processing of the unattended stimulus likely reflects the ecologically important exploration of less relevant sources of information.

Differential response of hippocampal and prefrontal oscillations to systemic LPS application

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1 mg/kg LPS i.p. injection robustly suppresses theta frequency in hippocampus.

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LPS administration augments delta frequency in hippocampus but not mPFC.

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LPS injection triggers hippocampal spike-wave discharges.

The early electrophysiological phenomena linked to systemic inflammation are largely underexplored. We developed here local field analyses to detect prodromal oscillatory abnormalities. We identified early band-specific patterns in local field potential recorded from freely-moving rats injected intraperitoneally with lipopolysaccharide (LPS, 1 mg/kg). Theta frequency was significantly reduced and this effect was not related to the decreased locomotion of the animal. Furthermore, LPS-induced alterations show a region-specific response when compared between the hippocampal region and medial prefrontal cortex. Delta mean frequency increased in the hippocampal region but not in the prefrontal cortex. We explored also the hypothesis that systemic inflammation increases the propensity of abnormally synchronized brain activity. Our data indicate that the LPS-evoked alteration of delta and theta frequency parameters reflects the formation of abnormal synchronization in similar frequency ranges. The onset of abnormal brain activity was indicated by spike-wave discharges in the range of 1–10 Hz with three main frequency domains. Importantly, the occurrence of spike-wave discharges was observed in the hippocampus but not in the cortex. In summary, the hippocampal theta rhythm is an accurate indicator of the oscillatory changes evoked by LPS application. The findings offer clear patterns of altered brain function that will facilitate mechanistic investigations of brain dysfunction and delirium occurring during sepsis.

Automatic Detection of Highly Organized Theta Oscillations in the Murine EEG

Theta activity is generated in the septohippocampal system and can be recorded using deep intrahippocampal electrodes and implantable electroencephalography (EEG) radiotelemetry or tether system approaches. Pharmacologically, hippocampal theta is heterogeneous (see dualistic theory) and can be differentiated into type I and type II theta. These individual EEG subtypes are related to specific cognitive and behavioral states, such as arousal, exploration, learning and memory, higher integrative functions, etc. In neurodegenerative diseases such as Alzheimer's, structural and functional alterations of the septohippocampal system can result in impaired theta activity/oscillations. A standard quantitative analysis of the hippocampal EEG includes a Fast-Fourier-Transformation (FFT)-based frequency analysis. However, this procedure does not provide details about theta activity in general and highly-organized theta oscillations in particular. In order to obtain detailed information on highly-organized theta oscillations in the hippocampus, we have developed a new analytical approach. This approach allows for time- and cost-effective quantification of the duration of highly-organized theta oscillations and their frequency characteristics.

Medial septal GABAergic projection neurons promote object exploration behavior and type 2 theta rhythm

Exploratory drive is one of the most fundamental emotions, of all organisms, that are evoked by novelty stimulation. Exploratory behavior plays a fundamental role in motivation, learning, and well-being of organisms. Diverse exploratory behaviors have been described, although their heterogeneity is not certain because of the lack of solid experimental evidence for their distinction. Here we present results demonstrating that different neural mechanisms underlie different exploratory behaviors. Localized Cav3.1 knockdown in the medial septum (MS) selectively enhanced object exploration, whereas the null mutant (KO) mice showed enhanced-object exploration as well as open-field exploration. In MS knockdown mice, only type 2 hippocampal theta rhythm was enhanced, whereas both type 1 and type 2 theta rhythm were enhanced in KO mice. This selective effect was accompanied by markedly increased excitability of septo-hippocampal GABAergic projection neurons in the MS lacking T-type Ca(2+) channels. Furthermore, optogenetic activation of the septo-hippocampal GABAergic pathway in WT mice also selectively enhanced object exploration behavior and type 2 theta rhythm, whereas inhibition of the same pathway decreased the behavior and the rhythm. These findings define object exploration distinguished from open-field exploration and reveal a critical role of T-type Ca(2+) channels in the medial septal GABAergic projection neurons in this behavior.

Ovarian cycle-linked plasticity of δ-GABAA receptor subunits in hippocampal interneurons affects γ oscillations in vivo

GABA_A receptors containing δ subunits (δ-GABA_ARs) are GABA-gated ion channels with extra- and perisynaptic localization, strong sensitivity to neurosteroids (NS), and a high degree of plasticity. In selective brain regions they are expressed on specific principal cells and interneurons (INs), and generate a tonic conductance that controls neuronal excitability and oscillations. Plasticity of δ-GABA_ARs in principal cells has been described during states of altered NS synthesis including acute stress, puberty, ovarian cycle, pregnancy and the postpartum period, with direct consequences on neuronal excitability and network dynamics. The defining network events implicated in cognitive function, memory formation and encoding are γ oscillations (30–120 Hz), a well-timed loop of excitation and inhibition between principal cells and PV-expressing INs (PV + INs). The δ-GABA_ARs of INs can modify γ oscillations, and a lower expression of δ-GABA_ARs on INs during pregnancy alters γ frequency recorded in vitro. The ovarian cycle is another physiological event with large fluctuations in NS levels and δ-GABA_ARs. Stages of the cycle are paralleled by swings in memory performance, cognitive function, and mood in both humans and rodents. Here we show δ-GABA_ARs changes during the mouse ovarian cycle in hippocampal cell types, with enhanced expression during diestrus in principal cells and specific INs. The plasticity of δ-GABA_ARs on PV-INs decreases the magnitude of γ oscillations continuously recorded in area CA1 throughout several days in vivo during diestrus and increases it during estrus. Such recurring changes in γ magnitude were not observed in non-cycling wild-type (WT) females, cycling females lacking δ-GABA_ARs only on PV-INs (PV-Gabrd^-/-), and in male mice during a time course equivalent to the ovarian cycle. Our findings may explain the impaired memory and cognitive performance experienced by women with premenstrual syndrome (PMS) or premenstrual dysphoric disorder (PMDD).

Voluntary and involuntary running in the rat show different patterns of theta rhythm, physical activity, and heart rate

Involuntarily exercising rats undergo more physical and mental stress than voluntarily exercising rats; however, these findings still lack electrophysiological evidence. Many studies have reported that theta rhythm appears when there is mental stress and that it is affected by emotional status. Thus we hypothesized that the differences between voluntary and involuntary movement should also exist in the hippocampal theta rhythm. Using the wheel and treadmill exercise models as voluntary and involuntary exercise models, respectively, this study wirelessly recorded the hippocampal electroencephalogram, electrocardiogram, and three-dimensional accelerations of young male rats. Treadmill and wheel exercise produced different theta patterns in the rats before and during running. Even though the waking baselines for the two exercise types were recorded in different environments, there did not exist any significant difference after distinguishing the rats' sleep/wake status. When the same movement-related parameters are considered, the treadmill running group showed more changes in their theta frequency (4-12 Hz), in their theta power between 9.5-12 Hz, and in their heart rate than the wheel running group. A positive correlation between the changes in high-frequency (9.5-12 Hz) theta power and heart rate was identified. Our results reveal various voluntary and involuntary changes in hippocampal theta rhythm as well as divergences in heart rate and high-frequency theta activity that may represent the effects of an additional emotional state or the sensory interaction during involuntary running by rats.

Evidence for a medial prefrontal cortex-hippocampal axis associated with heart rate control in conscious humans

Cardiovascular arousal correlates to activity within the medial prefrontal cortex (MPFC). Additional evidence provides anatomical and functional links between the MPFC and hippocampus (HC). This study tested the hypothesis that the MPFC and HC form a sub-network associated with rapid heart rate (HR) responses to volitional effort. Primary analyses were performed on 29 individuals (18 males) ranging from 21 to 80 years of age, who produced a HR response >3bpm to an isometric handgrip (IHG) task. HR and cortical activity were recorded using functional magnetic resonance imaging with blood oxygen level-dependent contrast. The average change in HR from baseline was 6bpm ±2. Activity in the MPFC and left HC was reduced relative to baseline in all subjects when correlated with the HR time course. Measures of connectivity demonstrated that the MPFC engaged in significantly stronger functional connectivity to the left HC during a 40% IHG task. Effective connectivity revealed a directionality of influence from the MPFC to the left HC. A second group (n=15) of individuals without a HR response (~1bpm) to IHG were studied post-hoc and these individuals showed no deactivation in either the MPFC or left HC. These results suggest the presence of a MPFC-HC axis that participates in the neurally-mediated HR response to exercise.

Running speed alters the frequency of hippocampal gamma oscillations

Successful spatial navigation is thought to employ a combination of at least two strategies: the following of landmark cues and path integration. Path integration requires that the brain use the speed and direction of movement in a meaningful way to continuously compute the position of the animal. Indeed, the running speed of rats modulates both the firing rate of neurons and the spectral properties of low frequency, theta oscillations seen in the local field potential (LFP) of the hippocampus, a region important for spatial memory formation. Higher frequency, gamma-band LFP oscillations are usually associated with decision-making, increased attention, and improved reaction times. Here, we show that increased running speed is accompanied by large, systematic increases in the frequency of hippocampal CA1 network oscillations spanning the entire gamma range (30-120 Hz) and beyond. These speed-dependent changes in frequency are seen on both linear tracks and two-dimensional platforms, and are thus independent of the behavioral task. Synchrony between anatomically distant CA1 regions also shifts to higher gamma frequencies as running speed increases. The changes in frequency are strongly correlated with changes in the firing rates of individual interneurons, consistent with models of gamma generation. Our results suggest that as a rat runs faster, there are faster gamma frequency transitions between sequential place cell-assemblies. This may help to preserve the spatial specificity of place cells and spatial memories at vastly different running speeds.

Passive rotation-induced theta rhythm and orientation homeostasis response

Rhythmic oscillatory activities at the theta frequency (3-12 Hz) have long attracted attention, as they have been implicated in diverse brain functions. There are two kinds of hippocampal theta rhythms: Type 1 is an atropine-resistant noncholinergic theta rhythm, and Type 2 is an atropine-sensitive cholinergic theta rhythm. However, it has not yet been determined whether the theta rhythm generated during passive whole-body rotation is of Type 1 or 2. To clarify this issue, we investigated passive whole-body rotation-induced theta rhythm using C57BL/6J normal and atropine-treated mice. The results demonstrated that atropine [50 mg/kg, intraperitoneally (i.p.)], a cholinergic antagonist, abolished the theta rhythm generated during passive whole-body rotation. Therefore, the passive whole-body rotation-induced theta rhythm is an atropine-sensitive Type 2 theta rhythm. In addition, we found that blocking cholinergic receptors using atropine resulted in the loss of the orientation homeostasis response, which is a circling behavior in the direction opposite to that of the rotating circular treadmill that is generated to maintain a constant orientation. These results suggest that atropine-sensitive Type 2 theta rhythm can be generated by a passive rotation-induced vestibular sensory signal and may be necessary for spatial orientation homeostasis response behavior.

The interrelationship between movement and cognition: θ rhythm and the P300 event-related potential

The relationship among brain electrophysiological activity, motor activity, and cognition has been a matter of great interest. For example, it has been discussed whether hippocampal theta rhythm reflects motor activity or cognitive activity, whereas it is widely accepted that the P300 event-related potential (ERP) reflects cognitive processes such as updating working memory. Here, we investigated the interrelationships among motor activity, hippocampal theta rhythm, and hippocampal P300 ERP using electrophysiological and behavioral data recorded from rats performing an auditory discrimination task (i.e., the auditory oddball paradigm) in a chamber with and without a running-wheel. We found that the hippocampal theta rhythm generated during locomotion codes information about self-motion, and event-related increases in hippocampal theta rhythm observed when rats performed the auditory discrimination cognitive task reflect a change in motor behavior after learning the cognitive task. Interestingly, the hippocampal P300 ERP occurred coincidently with increases in the power and frequency of hippocampal theta rhythm. In addition, we found that changes in theta rhythm observed during spontaneous wheel running without performing a cognitive task as well as when performing the cognitive task are associated with changes in delta- and gamma-band EEG activities. These major findings are discussed with respect to current hypotheses regarding P300 ERP and theta-, delta-, and gamma-band EEG activities in brain functions.

Phospholipase C beta 4 in the medial septum controls cholinergic theta oscillations and anxiety behaviors

Anxiety is among the most prevalent and costly diseases of the CNS, but its underlying mechanisms are not fully understood. Although attenuated theta rhythms have been observed in human subjects with increased anxiety, no study has been done on the possible physiological link between these two manifestations. We found that the mutant mouse for phospholipase C beta 4 (PLC-beta 4(-/-)) showed attenuated theta rhythm and increased anxiety, presenting the first animal model for the human condition. PLC-beta 4 is abundantly expressed in the medial septum, a region implicated in anxiety behavior. RNA interference-mediated PLC-beta 4 knockdown in the medial septum produced a phenotype similar to that of PLC-beta 4(-/-) mice. Furthermore, increasing cholinergic signaling by administering an acetylcholinesterase inhibitor cured the anomalies in both cholinergic theta rhythm and anxiety behavior observed in PLC-beta 4(-/-) mice. These findings suggest that (1) PLC-beta 4 in the medial septum is involved in controlling cholinergic theta oscillation and (2) cholinergic theta rhythm plays a critical role in suppressing anxiety. We propose that defining the cholinergic theta rhythm profile may provide guidance in subtyping anxiety disorders in humans for more effective diagnosis and treatments.

Decline in hippocampal theta activity during cessation of locomotor approach sequences: amplitude leads frequency and relates to instrumental behavior

Hippocampal theta frequency and amplitude decrease as locomotor approach slows and the goal is reached. This study compared the declines of these theta parameters and related them to behavioral events. Theta activity was recorded with bipolar electrodes spanning cornu Ammon, sector 1 or cornu Ammon, sectors 2/3 cell layers of the dorsal hippocampus in 12 rats trained to approach and depress a treadle which exposed a milk dipper. Behavioral events were identified using a video capture system (20-ms sampling) synchronized to the hippocampal recording system (10-ms sampling). Peri-event averages of theta activity were made around the initial paw contact with the treadle, the presentation of the dipper, and the first lick at the dipper. Phase relationships between averaged hippocampal slow wave activity and behavioral events occasionally were found but they were inconsistent. In averages of both amplitude and frequency, times of minimum were less variable around paw contact indicating that compared with reward presentation and consummatory behavior, it more closely related to the processes determining the declines. Theta amplitude declined more rapidly than frequency and reached an earlier minimum in averages around initial paw contact and dipper presentation. Mean amplitude minimum occurred after the paw contact at 159 ms but the decline of frequency continued into the licking bout with its minimum occurring at 343 ms. The findings indicate that during the termination of approach locomotion, the amplitude of hippocampal theta activity is closely related to specific expected sensorimotor events.

Human neocortical oscillations exhibit theta phase differences between encoding and retrieval

We analyzed intracranial brain activity recorded from human participants during the performance of a working-memory task. We show that 6-13 Hz activity exhibits consistent phase across trials following experimental stimuli, and that this phase significantly differs between study and test stimuli. These findings suggest that oscillatory phase reflects the encoding-retrieval state of neural networks, supporting predictions of recent models of memory.[Hasselmo, M.E., Wyble, B.P., and Bodelon, C., 2002. A proposed function for hippocampal theta rhythm: Separate phases of encoding and retrieval enhance reversal of prior learning. Neural Comput. 14 793-817.; Judge, S.J., Hasselmo, M.E., 2004. Theta rhythmic stimulation of stratum lacunosum-moleculare in rat hippocampus contributes to associative LTP at a phase offset in stratum radiatum. J. Neurophys. 92 1516-1624.].

Genetic dissection of theta rhythm heterogeneity in mice

Rhythmic oscillatory activities at the theta frequency (4-12 Hz) in the hippocampus have long-attracted attention because they have been implicated in diverse brain functions, including spatial cognition. Although studies based on pharmacology and lesion experiments suggested heterogeneity of these rhythms and their behavioral correlates, controversies are abundant on these issues. Here we show that mice harboring a phospholipase C (PLC)-beta1(-/-) mutation (PLC-beta1(-/-) mice) lack one subset of theta rhythms normally observed during urethane anesthesia, alert immobility, and passive whole-body rotation. In contrast, the other subset of theta rhythms observed during walking or running was intact in these mutant mice. PLC-beta1(-/-) mice also have somewhat disrupted theta activity during paradoxical sleep but do have an atropine-resistant component of theta rhythm. In addition, carbachol-induced oscillations were obliterated in hippocampal slices of PLC-beta1(-/-) mice. Interestingly, PLC-beta1(-/-) mice showed deficits in a hidden platform version of the Morris water maze yet performed well in motor coordination tests and a visual platform version of the Morris water maze. The results genetically define the existence of at least two subtypes of theta rhythms and reveal their association with different behaviors.

Hippocampal theta activity and behavioral sequences in a reward-directed approach locomotor task

Hippocampal rhythmic slow wave activity (theta) has been implicated in the processing of stimuli associated with movement. This study determined whether the theta rhythm showed phase relationships or changes in amplitude and frequency with the onset of stimuli and behavioral sequences in a skilled locomotor approach task. Rats with bipolar electrodes spanning CA1 approached a stall, turned to enter it, approached and depressed a treadle, waited 1.35 s, and approached a milk reward located forward either to the right or to the left. Auditory cues indicated the location of the reward during the waiting period and at the reward onset. A video capture system (20-ms sampling) was synchronized to the hippocampal recording system (10-ms sampling). Behavioral events identified by motion analysis were used to generate averages of hippocampal slow wave activity, theta peak amplitudes, and intervals between peaks. Theta activity at 8-10 Hz was almost continuous during the behavioral sequences. Phase relations with stimuli or movement onsets occurred infrequently and were not consistent across the four subjects. Theta peak amplitude and frequency decreased as the rat slowed locomotion in the stall and reached the treadle. Onset of locomotion directed to a reward location occurred on a positive peak of averaged theta activity. When locomotion had short latencies, increases in theta frequency appeared after the onset but, when it had longer latencies, frequency increases appeared 200 ms before onset. The results indicate that the execution of instrumental movement modulates both theta amplitude and frequency, and that the preparation for locomotion modulates theta frequency.

Level of arousal during the small irregular activity state in the rat hippocampal EEG

The sleeping rat cycles between two well-characterized hippocampal physiological states, large irregular activity (LIA) during slow-wave sleep (SWS) and theta activity during rapid-eye-movement sleep (REM). A third, less well-characterized electroencephalographic (EEG) state, termed "small irregular activity" (SIA), has been reported to occur when an animal is startled out of sleep without moving and during active waking when it abruptly freezes. We recently found that the hippocampal population activity of a spontaneous sleep state whose EEG resembles SIA reflects the rat's current location in space, suggesting that it is also a state of heightened arousal. To test whether this spontaneous SIA state corresponds to the SIA state reported in the literature and to compare the level of arousal during SIA to the other well-characterized physiological states, we recorded unit activity from ensembles of hippocampal CA1 pyramidal cells, EEG from the hippocampus and the neocortex, and electromyography (EMG) from the dorsal neck musculature in rats presented with auditory stimuli while foraging for randomly scattered food pellets and while sleeping. Auditory stimuli presented during sleep reliably induced SIA episodes very similar to spontaneous SIA in hippocampal and neocortical EEG amplitudes and power spectra, EMG amplitude, and CA1 population activity. Both spontaneous and elicited SIA exhibited neocortical desynchronization, and both had EMG amplitude comparable to that of waking LIA. We conclude based on this and other evidence that spontaneous SIA and elicited SIA correspond to a single state and that the level of arousal in SIA is higher than in the well-characterized sleep states but lower than the active theta state.

Investigation on acetylcholine, aspartate, glutamate and GABA extracellular levels from ventral hippocampus during repeated exploratory activity in the rat

The extracellular levels of aspartate, glutamate, gamma-aminobutyric acid (GABA), and acetylcholine (ACh) were investigated by microdialysis, coupled with HPLC, in the ventral hippocampus of rats during two 30-min exploration periods. Motor activity was monitored. During exploration I, an increase in motor activity associated with a 315% increase in aspartate, 181% in glutamate, and 264% in ACh levels, occurred during the first 10 min. The increase in GABA level reached a maximum of 257% during the second 10 min. The neurotransmitter levels returned to basal values within 40 min. During exploration II, 1 h later, a smaller increase in neurotransmitter levels and motor activity was observed. In both explorations, the increase in neurotransmitter levels was completely abolished by 1 and 3 microM TTX. A statistically significant relationship was found between neurotransmitter extracellular levels and motor activity, for aspartate and glutamate in exploration I, and for ACh in exploration I and II. In conclusion, exploratory activity is associated with or depends on the activation of neuronal systems in the ventral hippocampus releasing aspartate, glutamate, GABA, and ACh. The activation is dampened by habituation.

A unifying theory on the relationship between spike trains, EEG, and ERP based on the noise shaping/predictive neural coding hypothesis

Cracking the neural code has long been a central issue in neuroscience. However, it has been proved difficult because there logically exist an infinite number of other models and interpretations that could account for the same data and phenomena (i.e. the problem of underdetermination). Therefore, I suggest that applying biologically realistic multiple constraints from ion-channel level to system level (e.g. cognitive neuroscience and human brain disorders) can only solve the problem of underdetermination. Here I have explored whether the noise shaping/predictive neural coding hypothesis can provide a unified view on following realistic multiple constraints: (1) cortical gain control mechanisms in vivo; (2) the relationships between acetylcholine, nicotine, dopamine, calcium-activated potassium ion-channel, and cognitive functions; (3) oscillations and synchrony; (4) why should spontaneous activity be irregular; (5) whether the cortical neurons in vivo are coincidence detectors or integrators; and (6) the causal relationship between theta oscillation, gamma band fluctuation, and P3 (or P300) ERP responses. Finally, recent experimental results supporting the unified view shall be discussed.

Adaptation in spiking neurons based on the noise shaping neural coding hypothesis

Shin, Koch and Douglas [Shin, J., Koch, C., & Douglas, R. (1999). Adaptive neural coding dependent on the time-varying statistics of the somatic input current. Neural Computation, 11, 1983-2003] proposed an adaptive neural coding model that makes spiking neurons adapt its input/output relation to the stimulus statistics. In a surprisingly precise manner, the adaptive neural coding model has been supported by recent experiments. However, the previous report has two problems: (a) although the adaptive neural coding model was developed based on the noise shaping neural coding hypothesis, their connection was not explained clearly in the previous report; and (b) the previous model did not suggest a biologically plausible method to estimate the stimulus mean and variance from spike-evoked intracellular calcium concentration. In this paper, I present how the noise shaping neural coding hypothesis produced such a precise model without any available experimental data at that time. Moreover, I propose a computational model for a biologically plausible signal statistics extraction from spike-evoked intracellular calcium concentration. An asymmetry in contrast adaptation time between increasing and decreasing variance, observed in biological experiments, is explained using the signal statistics extraction method. In addition, a new perspective on the relationship between the spike train of spiking neurons and EEG (or local field potential (LFP)) is suggested based on the noise shaping neural coding hypothesis.