How are age and sex reflected in a sleeping dog’s brain?

Through the „eye” of EEG (electroencephalography), a commonly used neuroimaging technique, the activity of different brain areas or networks becomes visible as repetitive, rhythmic waves of varying shape, size, duration and frequency. In a sea of otherwise large and slow (less than 4 waves per second) waves that characterize the EEG signal during the deepest phase of sleep in mammals (called non-REM), sleep spindles emerge as short (ca. 0.5-5 seconds) and fast (> 9 waves/second) trains of smaller waves. These bursts originate in the thalamus, the brain’s „gate-keeper”, where sensory information is filtered or even halted on its way to „higher” brain areas. The occurrence of a sleep spindle is associated with the „closing of the gates” which is believed to support both sleep stability and memory consolidation. This has been investigated by countless works in humans, rodents and even cats, but sleep spindles were mostly neglected in dogs.

Aging follows a different course with regard to slow and fast spindles in the dog – reports a new study published in Scientific Reports from Nature Research, conducted by researchers at the Eötvös Loránd University and the Hungarian Academy of Sciences. Slow and central spindles appear to follow similar trends to what has been observed in humans: older dogs have lower spindle amplitudes and occurrence. However, dogs’ frontal fast spindles continue to increase in density which surprisingly mimics changes characterizing spindle development in humans from childhood to adolescence.

Dr. Enikő Kubinyi, who leads the Senior Family Dog Project at the Eötvös Loránd University, funded by the European Research Council, notes „The mapping of where aging processes in dogs and humans converge and diverge is an important step in fully understanding dog’s potential as a model of cognitive aging. The effects of age partly overlap with what we know from humans, in particularly, older dogs have lower spindle amplitudes and occurrence is also lower for centrally recorded spindles. However, we were surprised to find that older dogs display a higher occurrence of frontally detected fast spindles, which seems unique to dogs.”

„Most of what we know about sleep spindles is derived from the human literature” thus Dr. Anna Kis, a co-author who pioneered a surge of canine EEG studies in Budapest, „our group first confirmed that a higher incidence of spindle-like bursts in sleeping dogs predict better performance on a novel task, back in 2017.

It was the first time that in dogs sleep spindles were directly examined, rather than just mentioned in footnotes.”

More than 150 pet dogs participated in the study where they had three hours time to sleep on a mattress with their owner. EEG data were obtained with surface attached electrodes in the same way as is done with humans, thus the procedure was fully non-invasive. All dogs except eight slept during the three hours. Sometimes the owners also fall asleep, although they were not tested.

 „We developed an automatic detection method which works in the dog,” says corresponding author Ivaylo Iotchev, „and this time we wanted to examine a large sample, to see how age, sex and reproductive status influence spindle properties, like their occurrence, amplitude and oscillating frequency; that is how many spindles there are in a recording, how big they are, and how many waves/second are observed within a spindle. The human literature is rich with descriptions of the hormonal and age-related impacts on spindle activity. If we could confirm that these findings generalize to dog sleep spindles as well, this would strengthen the argument that dogs are good model animals for understanding human evolution, aging and brain function. Such arguments are so far mostly based on behavior, whereas the dog brain is still a terra incognita compared to the brains of other model animals like rats and mice.”

The new findings indeed lend further support to the assumption, that dogs possess human-like sleep spindles. Similarly to humans, on an electrode placed centrally on the head we observe more spindles with a ’fast’ frequency, than on an electrode placed directly over the frontal cortex. This localized difference in spindle frequency is a strong argument, that a distinction between ’slow’ and ’fast’ spindles, well established in humans, is justified in dogs as well. „This is important,” said Dr. Borbála Turcsán, „because fast and slow spindles are associated with different functions in humans. Notably the learning of motor sequences has been quite reliably linked to fast spindles in the past, while slow spindles are associated with verbal learning and intelligence.” Fast spindles showed human-like differences between the sexes, with female dogs displaying more, larger and faster fast spindles, a difference that was particularly expressed in intact animals. According to Iotchev „the fact that both the sex and reproductive status of a dog influenced the features of its spindles, is the strongest argument we can make in a non-invasive study for the role of sexual hormones on spindle expression, which in turn is a particularly strong argument for shared mechanisms in both species underlying the generation and modulation of spindles.”

“Surface EEG recordings will not allow us to understand in all detail what causes a higher incidence of fast spindles in older dogs, especially since it was not observed in any other species,” so Iotchev, “it is interesting to note, however, that in humans fast spindle occurrence keeps rising during adolescence. In rats, on the other hand, a human-like decline in older individuals was not confirmed, yet.”