1. Introduction

2. Methods to study animals sleep and dreams

The standard method used to study sleep is called polysomnography. It uses electrode patches placed on specific parts of the animal's head and body to record electrical activity on a polygraph (resulting in readable data that looks like scribbled lines).

It records simultaneously the electrical activity of the brain, the eye movements, the muscles tone, the heart activity and the respiration of the animal. The electroencephalogram (EEG) is a measurement of bioelectric brain activity, by the mean of electrodes placed on the scalp. When looking at EEG levels, sleep researchers can clearly see distinctions between the different stages of sleep, as well as when the initial sleep onset actually occurs. The electro-oculography (or EOG) is the standard method used to measure eye movements during sleep. The front of the eye (the cornea) is electrically positive compared to the back (the retina). Therefore, when the eye moves the change in voltage can be recorded by electrodes placed on the sleeper’s face and recorded on the polygraph. The muscle activity is measured by a method called the electromyogram (or EMG). Muscles emit electrical potentials when they move, and that electricity is also detected by electrodes and recorded on the polygraph. EMG electrodes are usually placed over the animal’s chin muscles. Similarly, the electrocardiogram (ECG) records from the body surface and registers the differences in electrical potential generated by the heart.

These systems involve either tethered systems that are restrictive and heavy on the animal or wireless systems using transponders that are large relative to the animal and surgically invasive for implantation, thus natural behavior/activity might be altered. But a study (Zielinski, Mark R., et al, 2013) using a novel telemetric system to measure polysomnography biopotentials in freely moving mice showed good results in analysis of sleep architecture.

Fig1

The mouse head cap containing EEG/EMG electrodes is connected to a telemetry transponder (4) encapsulated in a protective covering (2) by short cables contained in a lightweight protective sheath (1). This system is counterbalanced (3), rotates on an O-ring that swivels 360 degrees and slides horizontally between two contained ends, and possesses an additional swivel allowing the maximal range of movement within the mouse housing (5).

3. When do dreams happen during sleep?

3.1 Two states of sleep in mammals

This type of studies - made since 1953 on humans (Aserinski, Kleitman and Dement, USA) and since 1959 on cats (Jouvet, France) – have proved that there is two distinct stages of sleep: the slow sleep or nREM sleep (non Rapid Eyes Movement) and the paradoxical sleep or REM sleep, defined by their electrophysiological characteristics.

Table 1: Physiological characteristics of the awake, nREM and REM sleep states

These phases alternate cyclically in a highly structured pattern throughout the night. Those cycles last about 90 min in human and 28 min in cats. Jouvet (1994) has proved the existence of a duty ratio, calculated by dividing the average duration of sleep cycles by the average duration of REM sleep episodes. It is close to 4 in the vast majority of species (except rabbits and monkeys).

3.2 Dreams and REM sleep

At the end of the nineteenth century, a scientist first made the assumption that rapid eye phenomena were due to feelings and representations of dreams (Manaseina, 1899). Then, studies found that by waking his sleeping human patients during REM sleep and interviewing them, they remember their dreams much clearly. So scientists made the hypothesis that most dreams occur during REM sleep (Dement, William, and Nathaniel Kleitman, 1957).

However, the forced awakening of patients during nREM sleep gives equivocal results. Memorizing dreams is not demonstrated but in 8 to 30% of the cases, elementary mental activity defined as "dream" was found. (Foulkes, 1990)

In animals, it has been experimented on cats by abolishing muscle atony in REM sleep and then observing the behavior of the animal when sleeping. Under these conditions, a cat exhibits during REM sleep a behavior typical of game or hunting and behaviors of aggression or defence that can be related to dreams.

Fig2

Jouvet’ experiences has proved that The destruction of the locus coeruleus alpha prevents muscle atonia observed during REM sleep. In these animals, the cortical synchronization, rapid eye movements and the activity of the pons attest to a REM sleep. However animals simultaneously present motor behavior instead of the normal atony. The behaviors observed are characteristic of the species: predatory attack (like round back, erection of hairs), fear, grooming,exploration. All of these behaviors are never referred to an element of the environment and the animal is largely insensitive to any external stimulus. The animals"live" their dream.

3.3 Pontic origin of the REM sleep

Michel Jouvet (1961), French researcher, found out the origin of the REM sleep by experimenting on “pontic cats”,it mean cats whose brain stem (cerebral trunk) are connected to electrodes.

Fig 3

If you remove the part of the brain located forward of the pons in a cat, we still observe periodic symptoms of REM sleep. Such animals can survive for several months if the following conditions are ensured artificially: thermal balance, nutritional and hydromineral supply. The animal is then subject, with clockwork regularity, to periods of postural weakness. These periods of atony are accompanied by rapid eye movements. These periods of atony are accompanied by rapid eye movements, in relationship with an activity ponto geniculo occipital (PGO). We can conclude that the REM activity arises from the pons. The REM basic mechanisms are responsible for two functions which are complementary to each other. First, they involve an endogenous system of excitations of the brain through the PGO activity. This stimulation causes the excitation of the sensory systems (mostly visual) and motor systems (pyramidal neurons of the motor area). Thus descending signals will respond to these stimuli and go to the spinal cord to trigger actions and behaviors. This is to prevent the motor activity that a second mechanism must arise: a powerful descending inhibition, motor neurons of the spinal cord. Thus, the dreamer is paralyzed and cannot move. Also, the topography of neurons (cholinergic likely) that constitute the endogenous generator of the dream of PGO activity was well defined. It is located in the pontine reticular formation. We also know the paths that lead the PGO activity in the nuclei eye motors (where it triggers the rapid eye movements.

4. What animals dream?

4.1 Problem in the definition of sleep states

4.2 Sleep states in warm blooded vertebrates

4.2.1 Mammals

4.2.2 Birds

4.3 Sleep like state in invertebrates

4.4 Activity and inactivity in cold blooded vertebrates

4.4.1 Fishes

4.4.2 Amphibians

4.4.3 Reptiles

5. Factors influencing the amount and rhythmicity of REM sleep

6. Phylogeny of sleep states

7. Dreams content in animals

8. Roles of dreaming for animals