Do dogs ’dream’ of electric spindles? Evidence for shared mechanisms of sleep-dependent learning between man and dog

Interest in the dog brain has been growing considerably since brain scans provided first glimpses from inside the head of man’s best friend. However, many presumably fundamental principles of brain function, known from work with other species, are yet to be confirmed and properly characterized. Researchers at the Eötvös Loránd University and Hungarian Academy of Sciences in Budapest, Hungary, were now able to show that in dogs sleep-dependent learning is aided by oscillations analogous of ’sleep spindles’. The name ’spindle’ refers to their appearance, a train of rhythmic waves, rising and falling in amplitude as they unfold on the EEG recording screen. Sleep spindles mostly show during the so called ’slow wave’ sleep and were long known to support memory and sleep-stability in humans and rats. Their role in the dog, however, was never studied and different ideas about the characteristics of dogs’ spindles compete in the literature. Analyzing EEG data from a learning-experiment, the scientists found that sleep spindles in dogs are more similar to the human variant than previously thought and are similarly associated with learning. They also reveal sex differences characterized, as in humans, by more spindles and increased learning in females.

Pictures by Zsófia Bognár and Enikő Kubinyi.

Sleep spindles are brief (0.5 – 5 seconds) episodes of brain activity, generated in the thalamus. The thalamus is the brain’s major filter and is poetically known as „the gate to consciousness”. In EEG they show up as trains of relatively fast, symmetrical waves, nested in the slower oscillations of non-REM or ’slow wave’ sleep. In humans, their rate of occurrence has been found to predict sleep dependent learning, general intelligence, sleep stability, and (healthy) aging. Their occurrence and characteristics also differ markedly between the sexes.

„Much of what we know about spindles comes from studies with humans and rodents, but just how universal they are is not a trivial question. Spindles do not occur in birds, although the relevant thalamic structures are present.” says first author, PhD student Ivaylo Iotchev. Co-author Dr. Anna Kis adds „In mammals, spindles have been reported for cats, dogs, sloths, opossums, rats, mice and humans, but only for the last three species do we have, to date, systematic quantifications and associations with function”. For the latter—humans and rodents—estimates of internal frequency (how many waves per second define a spindle) overlap strongly, around 9-16 Hz (cycles/second). In other animals estimates are much lower, often below 10 Hz.

In the dog different researchers have reported markedly diverging estimates of spindle frequency: from as low as 2-5 Hz to the more recently proposed 5-12 Hz or 12-16 Hz. „Using automated detection we tested three hypotheses about the internal frequency of dog spindles validating each for its ability to reproduce relationships between spindle occurrence and learning, well-documented in the literature on humans and rodents” explains co-author Dr. Gilles van Luijtelaar, expert on thalamic oscillations. Dr. Kis adds: „To this end we used a data set from a previous learning experiment in which dogs’ EEG was collected from each of three sessions, one for habituation to the electrode set-up, one in which the dogs had to perform familiar commands (control condition) and one in which they had to learn new words for the old commands (learning condition).”

Illustration by first author.

Principal investigator Dr. Enikő Kubinyi sums up the major findings: ”We were particularly excited to discover that learning success was best predicted by the occurrence of those events which most resemble human spindles and, as in humans, they occur more frequently in females. The female dogs also seem to be the better learners according to our behavioral data”.

Example of a sleep spindle in the dog (marked with a red square, upper right corner). Below: visualization of the main findings. Images by first author. Photograph of sleeping dog in the upper left corner by Enikő Kubinyi.

The study is part of a larger project, evaluating dogs as models of cognitive aging, financed by the European Research Council. „These results open up great possibilities for our team, because in the human literature spindle occurrence, amplitude, duration and frequency have been shown in many studies to predict and distinguish healthy and abnormal aging.” concludes Iotchev. „In addition EEG is a measurement technique familiar to veterinarians, as well as medical doctors. Albeit not used in large scale in the veterinary praxis at the moment, its potential utility will increase as we proceed to better identify and measure neural signatures like the sleep spindle.”

The study was published in the open access Scientific Reports from the publishers of Nature: http://rdcu.be/wEuM

 

 

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