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
3.2 Dreams and REM sleep
3.3 Pontic origin of the REM sleep
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