Chronic recording of electrical activity from the brain of unrestrained crayfish: The basal, unstimulated activity

Sleep Phases in Crayfish: Relationship Between Brain Electrical Activity and Autonomic Variables

In vertebrates like mammals and birds, two types of sleep have been identified: rapid eye movement and non-rapid eye movement sleep. Each one is associated with specific electroencephalogram patterns and is accompanied by variations in cardiac and respiratory frequencies. Sleep has been demonstrated only in a handful of invertebrates, and evidence for different sleep stages remains elusive. Previous results show that crayfish sleeps while lying on one side on the surface of the water, but it is not known if this animal has sleep phases. Heart rate and respiratory frequency are modified by diverse changes in the crayfish environment during wakefulness, and previously, we showed that variations in these variables are present during sleep despite that there are no autonomic anatomical structures described in this animal. Here, we conducted experiments to search for sleep phases in crayfish and the relationships between sleep and cardiorespiratory activity. We used the wavelet transform, grouping analysis with k-means clustering, and principal component analysis, to analyze brain and cardiorespiratory electrical activity. Our results show that (a) crayfish can sleep lying on one side or when it is motionless and (b) the depth of sleep (measured as the power of electroencephalographic activity) changes over time and is accompanied by oscillations in cardiorespiratory signal amplitude and power. Finally, we propose that in crayfish there are at least three phases of sleep.

Slow waves during sleep in crayfish. Origin and spread

Previous results show that when unrestrained crayfish sleep, the electrical activity of the brain changes from multiple spikes (frequencies above 300 Hz) on a flat baseline to continuous slow waves at a frequency of 15–20 Hz. To study the temporal organization of such activity, we developed a tethered crayfish preparation that allows us to place electrodes on visually identified regions of the brain. Recording the electrical activity of different brain areas shows that when the animal is active (awake), slow waves are present only in the central complex. However, simultaneously with the animal becoming limp (sleeping), slow waves spread first to deuto- and then to protocerebrum, suggesting that the central complex of the crayfish brain acts as the sleep generator.

Slow waves during sleep in crayfish: A time–frequency analysis

NREM phases of sleep in vertebrates are characterized by slow waves. Crayfish also sleeps while lying on one side on the surface of the water. At this time the numerous spikes on an almost flat base line generated by the brain when alert are replaced by slow waves of 15-20 Hz. In this work, we conducted experiments to determine the temporal relationship between the lying on one side position and the brain slow waves. We videotaped chronically implanted animals to detect their body position and simultaneously recorded their brain electrical activity. To analyze brain electrical activity, we developed a wavelet based method and correlated the results with body position. Among results are: (a) during sleep signals in the frequency range 30-45 Hz show a large decrease in power; (b) sleep slow waves are generated 1-2 min after the animal lies on one side and are maintained throughout the whole period in such position. We conclude that the strong correlation between brain slow waves and lying on one side position further indicates periods of true sleep in these animals.

Slow wave sleep in crayfish

Clear evidence of sleep in invertebrates is still meager. Defined as a distinct state of reduced activity, arousability, attention, and initiative, it is well established in mammals, birds, reptiles, and teleosts. It is commonly defined by additional electroencephalographic criteria that are only well established in mammals and to some extent in birds. Sleep states similar to those in mammals, except for electrical criteria, seem to occur in some invertebrates, based on behavior and some physiological observations. Currently the most compelling evidence for sleep in invertebrates (evidence that meets most standard criteria for sleep) has been obtained in the fruit fly Drosophila melanogaster. However, in mammals, sleep is also characterized by a brain state different from that at rest but awake. The electrophysiological slow wave criterion for this state is not seen in Drosophila or in honey bees. Here, we show that, in crayfish, a behavioral state with elevated threshold for vibratory stimulation is accompanied by a distinctive form of slow wave electrical activity of the brain, quite different from that during waking rest. Therefore, crayfish can attain a sleep state comparable to that of mammals.

Event-related potentials in an invertebrate: crayfish emit ‘omitted stimulus potentials’

Electrical signs of neural activity correlated with stimuli or states include a subclass called event-related potentials. These overlap with, but can often be distinguished from, simple stimulus-bound evoked potentials by their greater dependence on endogenous (internal state) factors. Studied mainly in humans, where they are commonly associated with cognition, they are considered to represent objective signs of moderately high-level brain processing.

We tested the hypothesis that invertebrates lack such signs by looking in the crayfish Procambarus clarkii for a class of OFF-effects shown in humans to index expectancy. Disproving the hypothesis, we find, using chronic, implanted preparations, that a good omitted stimulus potential is reliably present. The system learns in a few cycles of a regularly repeated light flash to expect one on schedule. Omitted stimulus potentials are found in the protocerebrum, the circumesophageal connective and in the optic tract – perhaps arising in the retina, as in vertebrates. These potentials can be very local and can include loci with and without direct visual evoked potentials in response to each flash. In some loci, the omitted stimulus potential has a slow wave component, in others only a spike burst. Omitted stimulus potentials are more endogenous than visual evoked potentials, with little dependence on flash or ambient light intensity or on train duration. They vary little in size at different times of the day, but abruptly fail to appear if the ambient light is cut off. They can occur during walking, eating or the maintained defense posture but are diminished by ‘distraction’ and are often absent from an inert crayfish until it is aroused.

We consider this form of apparent expectation of a learned rhythm (a property that makes it ‘cognitive’ in current usage), to be one of low level, even though some properties suggest endogenous factors. The flashes in a train have an inhibitory effect on a circuit that quickly ‘learns’ the stimulus interval so that the omitted stimulus potential, ready to happen after the learned interval, is prevented by each flash, until released by a missing stimulus.

Evoked potentials elicited by natural stimuli in the brain of unanesthetized crayfish

Experiments were conducted to test some characteristics of vision by crayfish underwater and in air, and determine possible motion reactions elicited in response to naturalistic or quasi-ethological visual stimuli. Chronically implanted electrodes on the brain were used to record visually evoked potentials in response to moving bars at different speeds or to fish of different sizes. Electroretinograms were also recorded to detect when an object or a shadow appeared in the crayfish visual field. Ongoing brain activity is mild under basal conditions, but increases in RMS by approximately 6% in response to bar passage and 12 to 53% in response to fish motionless or swimming in front of the crayfish. When crayfish are free to move, fish swimming in front of them elicit intense brain activity, together with displacement toward them and an attempt to grab them. Visual evoked potentials are elicited by moving objects as small as 1 degree at a distance of 30 cm in air as well as underwater. None of the stimuli used induced evident behavioral responses under our conditions. We conclude that vision-action activities can be divided into (a) vision of irrelevant objects with short lasting electrical activity and no motion in response to it; (b) vision of mildly interesting objects with long-lasting electrical effects, but no motion in response to it; and (c) vision of relevant objects with appropriate motion reaction.