Ponto-geniculo-occipital waves or PGO waves are distinctive wave forms of propagating activity between three key brain regions: the

pons The pons (from Latin , "bridge") is part of the brainstem that in humans and other bipeds lies inferior to the midbrain, superior to the medulla oblongata and anterior to the cerebellum. The pons is also called the pons Varolii ("bridge of V ...

lateral geniculate nucleus In neuroanatomy, the lateral geniculate nucleus (LGN; also called the lateral geniculate body or lateral geniculate complex) is a structure in the thalamus and a key component of the mammalian visual pathway. It is a small, ovoid, ventral projec ...

, and

occipital lobe The occipital lobe is one of the four major lobes of the cerebral cortex in the brain of mammals. The name derives from its position at the back of the head, from the Latin ''ob'', "behind", and ''caput'', "head". The occipital lobe is the vi ...

; specifically, they are phasic field potentials. These waves can be recorded from any of these three structures during and immediately before

REM sleep Rapid eye movement sleep (REM sleep or REMS) is a unique phase of sleep in mammals and birds, characterized by random rapid movement of the eyes, accompanied by low muscle tone throughout the body, and the propensity of the sleeper to dream ...

. The waves begin as electrical pulses from the pons, then move to the lateral geniculate nucleus residing in the

thalamus The thalamus (from Greek θάλαμος, "chamber") is a large mass of gray matter located in the dorsal part of the diencephalon (a division of the forebrain). Nerve fibers project out of the thalamus to the cerebral cortex in all directions, ...

, and end in the primary visual cortex of the occipital lobe. The appearances of these waves are most prominent in the period right before REM sleep, albeit they have been recorded during wakefulness as well. They are theorized to be intricately involved with eye movement of both wake and sleep cycles in many different animals.

Discovery

The discovery of PGO waves goes back to 1959, when three French scientists released their scientific article of their study of these waves in animal test subjects.Jouvet, M., Michel, F., and Courjon, J. 1959. L'activite electrique du rhinencephale au cours dusommeil chez le chat. C.R. Soc. Biol. 153:101–105. Although at this time, they did not have a specific name for this neurological phenomenon. It was not until the published work of Brooks and Bizzi that these waves became known as PGO waves.Brooks, D.C., and Bizzi, E. 1963. Brain stem electrical activity during deep sleep. Arch. Ital. Biol.101:648–665. Their research focused on the propagation of these waves in cats, noticing that these field potentials started in the pons, propagating down to the lateral geniculate nucleus and the occipital lobe. Other studies with these waves have been done on rats as well. Scientists tried to discern whether the rats had PGO waves, but learned that they are present only in the pons, and wave propagation does not excite any neurons in the lateral geniculate nucleus.Stern, W.C., Forbes, W.B., and Morgane, P.J. 1974. Absence of ponto-geniculo-occipital (PGO) spikes in rats. Physiol. Behav. 12:293–295. As a result of this study, PGO waves are known as P waves in rodents. PGO waves have been studied mostly through cat and rodent animal models. Despite the focus of the research, PGO waves have been found to exist in other mammalian species including humans and nonhuman primates, such as the macaque and baboon.Datta S. 1997. Cellular basis of pontine ponto-geniculo-occipital wave generation and modulation. Cellular and Molecular Neurobiology 17:341–65

Detection

In the original experiments, PGO waves (or P waves in rodent models) are found by placing electrodes inside the brain, next to either the pons, lateral geniculate nuclei, or occipital lobe. Along with

electroencephalography Electroencephalography (EEG) is a method to record an electrogram of the spontaneous electrical activity of the brain. The biosignals detected by EEG have been shown to represent the postsynaptic potentials of pyramidal neurons in the neocortex ...

(EEG) recording techniques, scientists are also able to show the correlation between other brain waves associated with REM sleep and PGO waves. Although scientists know they exist, PGO waves have not been detected in healthy humans due to the ethical concerns about accessing these areas where the readings need to be taken from. However, advances in

deep brain stimulation Deep brain stimulation (DBS) is a neurosurgical procedure involving the placement of a medical device called a neurostimulator, which sends electrical impulses, through implanted electrodes, to specific targets in the brain (the brain nucle ...

has made it possible to put electrodes inside the brains of humans with different pathologies and make EEG recordings of different nuclei. Due to the similarities with the animal models, we can infer that PGO waves are happening at the same frequency in human EEGs.Fernández-Mendoza J., Lozano B., Seijo F., Fernández-González F., Vela-Bueno A. 2006. Subthalamic nucleus activity during human REM sleep: the PGO-like waves. J Sleep Res 2006;15:243. Thus, scientists can infer that PGO waves exist in humans.

Mechanism for generation and propagation

The neurophysiological studies on PGO waves conclude that the generation of these waves resides in a collection of neurons located in the pons, regardless of species research is done on. From this point, the

neuron A neuron, neurone, or nerve cell is an electrically excitable cell that communicates with other cells via specialized connections called synapses. The neuron is the main component of nervous tissue in all animals except sponges and placozoa. ...

s branch out in a network that leads the phasic electrical signal toward the lateral geniculate nucleus and the occipital lobe. Within this network, there are two types of neuronal groups: executive neurons and modulatory neurons.

Executive neurons

These neurons are the ones that help to generate and propagate the PGO waves throughout the brain. One research paper further breaks down this "class" of neurons into two subsets: triggering neurons and transfer neurons. All of these neurons are located in the peribrachial area, which is a group of neurons surrounding the superior cerebellar penduncle.

Triggering neurons

These neurons are located in the caudolateral region of the peribrachial area. These neurons actively fire during non-REM (NREM) sleep. The most recorded activity of the neurons is during the N3 stage of NREM, also known as the

slow-wave sleep cycle Slow-wave sleep (SWS), often referred to as deep sleep, consists of stage three of non-rapid eye movement sleep. It usually lasts between 70 and 90 minutes and takes place during the first hours of the night. Initially, SWS consisted of both St ...

. These same neurons are also active during REM sleep, but at a greatly reduced amplitude than NREM sleep.

Transfer neurons

The neuronal cells that allow for the transfer of PGO waves from the pons to the other parts of the brain reside on the rostral portion of the peribrachial area. This grouping of cells fire in precisely two modes. The first mode is burst firing through low-threshold Calcium (Ca²⁺) ion channels. The other mode is a repetitive tonic firing through Sodium (Na⁺) dependent ion channels. During the times when triggering neurons are firing, these cells receive those signals and begin increasing their firing. This, in turn, allows the wave to go out to the other portions of the brain.

Modulatory neurons

As the executive neurons are firing, the spread of the wave is controlled by both excitatory and inhibitory inputs. These inputs come from the modulatory neurons, which help to regulate and control the amplitude and frequency of the wave. The following types of cells play a huge part in this control process.

Aminergic neurons

Aminergic neurons are neurons that use monoamines as a

neurotransmitter A neurotransmitter is a signaling molecule secreted by a neuron to affect another cell across a synapse. The cell receiving the signal, any main body part or target cell, may be another neuron, but could also be a gland or muscle cell. Neurotr ...

. This class of neurotransmitters is what keeps PGO wave amplitudes at very low levels during periods of a mammal being awake. The three specific aminergic neurotransmitters are

serotonin Serotonin () or 5-hydroxytryptamine (5-HT) is a monoamine neurotransmitter. Its biological function is complex and multifaceted, modulating mood, cognition, reward, learning, memory, and numerous physiological processes such as vomiting and vas ...

dopamine Dopamine (DA, a contraction of 3,4-dihydroxyphenethylamine) is a neuromodulatory molecule that plays several important roles in cells. It is an organic chemical of the catecholamine and phenethylamine families. Dopamine constitutes about 80% o ...

and

norepinephrine Norepinephrine (NE), also called noradrenaline (NA) or noradrenalin, is an organic chemical in the catecholamine family that functions in the brain and body as both a hormone and neurotransmitter. The name "noradrenaline" (from Latin '' ad'', ...

Cholinergic neurons

Cholinergic neurons A cholinergic neuron is a nerve cell which mainly uses the neurotransmitter acetylcholine (ACh) to send its messages. Many neurological systems are cholinergic. Cholinergic neurons provide the primary source of acetylcholine to the cerebral cortex, ...

are neurons that use

acetylcholine Acetylcholine (ACh) is an organic chemical that functions in the brain and body of many types of animals (including humans) as a neurotransmitter. Its name is derived from its chemical structure: it is an ester of acetic acid and choline. Parts ...

as a neurotransmitter. Through different studies, these types of neurons have been proven to promote PGO wave generation, thus being an excitatory neuromodulator for triggering neurons.Steriade, M., Datta, S., Pare, D., Oakson, G., and Currodossi, R. (1990a). Neuronal activities in brain-stem cholinergic nuclei related to tonic activation processes in thalamortical systems. J. Heurosci. 10:2541–2559.

Nitroxergic neurons

Nitroxergic neurons use

nitric oxide Nitric oxide (nitrogen oxide or nitrogen monoxide) is a colorless gas with the formula . It is one of the principal oxides of nitrogen. Nitric oxide is a free radical: it has an unpaired electron, which is sometimes denoted by a dot in its ch ...

(NO) as a neurotransmitter. In theory, the increase of nitric oxide is seen as an excitatory neuromodulator in PGO wave generation. This stems from animal testing that has shown increases in PGO waves as nitric oxide levels were increased in the pons.

GABA-ergic neurons

GABA-ergic neurons use gamma-aminobutyric acid (GABA) as a neurotransmitter. These neurons are theorized to be inhibitory to aminergic neurons, and thus inhibitory to PGO wave propagation.

Vestibular nuclei

The neurons within the

vestibular nuclei The vestibular nuclei (VN) are the cranial nuclei for the vestibular nerve located in the brainstem. In Terminologia Anatomica they are grouped in both the pons and the medulla in the brainstem. Structure Path The fibers of the vestibular nerv ...

region of the brain have been shown to provide excitatory bouts of PGO wave generation when stimulated.Morrison, A.R., and Pompeiano, O. (1966). Vestibular influences during sleep. IV. Functional relations between the vestibular nsleep. Arch. Ital. Biol. 104:425–458. The tests showed that, while the vestibular nuclei aided in creating PGO waves, the excitation of this area of the brain was in no way needed for PGO wave formation.

Amygdala

The neurons within the amygdala region of the brain have also been shown to provide excitatory bouts of PGO wave generation when electrically stimulated.

Suprachiasmatic nuclei

The neurons within the suprachiasmatic nuclei region of the brain help to regulate REM sleep. The REM sleep cycle length causes the frequency of PGO waves to be phase locked.

Auditory stimulation

The use of auditory stimulation has been shown to increase PGO waves during waking and sleeping cycles with neurons associated with transfers of auditory information.Callaway C.W., Lydic R., Baghdoyan H.A., Hobson J.A. 1987. Pontogeniculoocipital Waves – Spontaneous Visual-System ACtivity During Rapid Eye-Movement Sleep. Cellular and Molecular Neurobiology 7:105–49 Even while the subject is awake and in total darkness, the amplitude of PGO waves increases by auditory stimulation. Another study also found that auditory stimulation increased the amplitude of PGO waves in slow-wave sleep and REM sleep and did not reduce the amplitude of the waves with repeated auditory stimulation.Bowker, R.M., and Morrison, A.R. 1977. The PGO spikes: An indicator of hyperalertness. In Sleep Research, (W.P. Koella and P. Levin, Eds.), Karger, Basel, pp. 23–77. From this research, scientists can theorize that PGO wave generation from auditory stimulation contains a positive-feedback mechanism that can be excited by evoked PGO waves.

Basal ganglia

The

basal ganglia The basal ganglia (BG), or basal nuclei, are a group of subcortical nuclei, of varied origin, in the brains of vertebrates. In humans, and some primates, there are some differences, mainly in the division of the globus pallidus into an exter ...

are a group of nuclei in the brains of vertebrates, situated at the base of the forebrain and strongly connected with the cerebral cortex, thalamus and pons. The basal ganglia are associated with a variety of functions, including arousal, motor control and learning. The main components of the basal ganglia are the striatum, pallidum, substantia nigra, and

subthalamic nucleus The subthalamic nucleus (STN) is a small lens-shaped nucleus in the brain where it is, from a functional point of view, part of the basal ganglia system. In terms of anatomy, it is the major part of the subthalamus. As suggested by its name, the s ...

(or subthalamus). This latter, glutamatergic nucleus is reciprocally connected with the PGO-transferring nuclei of the

. In humans, subthalamic PGO-like waves, that resemble the PGO waves typically recorded in cats, can be recorded during pre-REM and REM sleep.Fernández-Mendoza J., Lozano B., Seijo F., Santamarta-Liébana E., Ramos-Platón M.J., Vela-Bueno A., Fernández-González F. 2009. Evidence of subthalamic pgo-like waves during rem sleep in humans: a deep brain polysomnographic study. SLEEP 32(9):1117–26. This suggests that the subthalamus may play an active role in an ascending activating network implicated in the rostral transmission of PGO waves during REM sleep in humans.

REM sleep

PGO waves are an integral part of rapid eye movement (REM) sleep. As stated earlier, the density of the PGO waves coincides with the amount of eye movement measured in REM sleep. This has led some researchers to further theorize about the usefulness of PGO waves for dreaming. One key use of REM sleep is for the brain to process and store information from the previous day. In a sense, the brain is learning by establishing new neuronal connections for things that have been learned. Neurophysiological studies have indicated a relationship between increased P-wave density during post-training REM sleep and learning performance. Basically, the abundance of PGO waves translates into longer periods of REM sleep, which thereby allows the brain to have longer periods where neuronal connections are formed. The importance of PGO waves during REM sleep also aids the idea of PGO waves as a signal that a person is dreaming. Since dreaming occurs during REM sleep, the PGO waves are theorized to be the signals that make the brain start to recount the experiences from the previous day. This, in turn, allows us to "see" our dreams since our visual sense is quickly going through the information it has stored. For more information of the importance of PGO waves during REM sleep, please refer to

Activation synthesis theory The activation-synthesis hypothesis, proposed by Harvard University psychiatrists John Allan Hobson and Robert McCarley, is a neurobiological theory of dreams first published in the American Journal of Psychiatry in December 1977. The differenc ...

. Another area of potential research interest involves PGO waves during

lucid dreaming A lucid dream is a type of dream in which the dreamer becomes aware that they are dreaming while dreaming. During a lucid dream, the dreamer may gain some amount of control over the dream characters, narrative, or environment; however, this is n ...

active imagination An active imagination is a conscious method of experimentation. It employs creative imagination as an organ for "perceiving outside your mental boxes." For the first hundred years of active imagination, it was applied primarily by individuals for ...

and

hallucination A hallucination is a perception in the absence of an external stimulus that has the qualities of a real perception. Hallucinations are vivid, substantial, and are perceived to be located in external objective space. Hallucination is a combinati ...

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References

External links

{{DEFAULTSORT:Pgo Waves Dream Neuroscience of sleep Electroencephalography Electrophysiology