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The brain processes weak visual stimuli better in the morning and evening than at noon
FRANKFURT. In the pre-industrial age, twilight was a dangerous time for humans since they were at risk of encountering nocturnal predators. Anyone still able to recognise things despite the weak light was at a clear evolutionary advantage. As neuroscientists at Goethe University Frankfurt have now discovered, the human brain prepares for dawn and dusk by shutting down resting activity in the visual cortex at these times so that weak visual stimuli do not disappear in the brain’s background noise.
The transition from night to day, light and dark, has a greater influence on perception that we realise. The time of day has a particularly significant impact on the quality of visual signals around us. In the course of evolution, our visual system has adapted perfectly to light conditions during the day. It has, however, also developed a strategy for twilight: Evidently it allows our inner clock to predict these periods and prepare our visual system for times when the quality of visual signals deteriorates.
“Whilst the cogs of our inner clock have already been studied in depth, it was not known to date which mechanism optimises visual perception at times when poor signal quality can be expected,” explains Dr. Christian Kell from the Brain Imaging Center of Goethe University Frankfurt. That is why Lorenzo Cordani, his doctoral researcher, examined how 14 healthy test persons reacted to visual stimuli at six different times of the day in the framework of a complex fMRI study.
The main idea of the study was to relate the perception of sensory signals to the brain’s resting activity. There is namely a certain “background noise” in the brain even in the complete absence of external stimuli. The international team led by Christian Kell, Lorenzo Cordani and Joerg Stehle was able to show that the body independently downregulates resting activity in the sensory areas during dawn and dusk. The more resting activity was reduced, the better the test persons were able to perceive weak visual signals when measured afterwards.
This means that humans can perceive weak visual stimuli during dawn and dusk better than at other times of the day. In other words: During twilight, the signal-to-noise ratio in the sensory areas of the brain improves. Since resting activity during twilight decreases not only in the visual but also in the auditory and somatosensory regions of the brain, the researchers assume that perception sharpens not only in the visual system. An earlier study already showed that weak auditory stimuli during twilight were perceived better. The mechanism now discovered, which was published in the latest issue of Nature Communications, could therefore represent a key evolutionary advantage that ensured survival in the pre-industrial era.
Publication: Lorenzo Cordani, Enzo Tagliazucchi, Céline Vetter, Christian Hassemer, Till Roenneberg, Jörg H. Stehle, Christian A. Kell: Endogenous Modulation of Human Visual Cortex Activity Improves Perception at Twilight, in: Nature Communications http://dx.doi.org/10.1038/s41467-018-03660-8
Information: Privatdozent Dr. Christian Kell, Brain Imaging Center der Goethe-Universität, Cognitive Neuroscience Group, Fachbereich Medizin, Campus Niederrad, Tel: (069) 6301 5739; firstname.lastname@example.org.
New insights in the animals´ extraordinary evolutionary history
FRANKFURT. For the first time, scientists of the German Senckenberg Biodiversity and Climate Research Center, Goethe University and the University of Lund in Sweden have deciphered the complete genome of the blue whale and three other rorquals. These insights now allow tracking the evolutionary history of the worlds’ largest animal and its relatives in unprecedented detail. Surprisingly, the genomes show that rorquals have been hybridizing during their evolutionary history. In addition, rorquals seem to have separated into different species in the absence of geographical barriers. This phenomenon, called sympatric speciation, is very rare in animals. The study has just been published in "Science Advances".
Blue whales are the giants of the sea. With up to 30 meters (100 feet) long and weighing up to 175 tons, they are the largest animals that ever evolved on earth; larger even than dinosaurs. Short of becoming extinct due to whaling by the end of the 80s, currently the populations of the gentle giants are slowly recovering. Now new research highlights that the evolution of these extraordinary animals and other rorquals was also anything but ordinary.
A research team led by Professor Axel Janke, evolutionary geneticist at the Senckenberg Biodiversity and Climate Research Center and Goethe University, has found that the rorquals, including the blue whale, mated across emerging species boundaries. “Speciation under gene flow is rare. Usually, species are assumed to be reproductively isolated because geographical or genetic barriers inhibits genetic exchange. Apparently however, this does not apply to whales”, explains Fritjof Lammers, co-lead-author of the study, Senckenberg Biodiversity and Climate Research Centre
Teaming up with cetacean specialist Professor Ulfur Arnason at University of Lund, Sweden, Lammers and his colleagues are the first to have sequenced the complete genome of the blue whale and other rorquals, including the humpback and the gray whale. For these migratory whales, geographical barriers do not exist in the vastness of the ocean, instead some rorquals differentiated by inhabiting different ecological niches. Cross-genome analyses now indicate that there are apparently no genetic barriers between species and that there has been gene flow among different rorqual species in the past.
This is confirmed by spotting hybrids between fin and blue whales still to date, which have been witnessed and genetically studied by Professor Arnason. However, the researchers could not detect traces of recent liaisons between the two species in their genomes. This is probably because whale genomes are currently known only from one or two individuals.
To track down the rorquals’ evolution, the scientists have applied so-called evolutionary network analyses. "In these analyses, speciation is not considered as a bifurcating phylogenetic tree as Darwin has envisioned it, but as an interwoven network. This allows us to discover hidden genetic signals, that otherwise would have stayed undetected", says Janke.
Overall, the research also shows that the relationships among the rorqual species are more complicated than hitherto thought. So far, the humpback whale has been seen as an outsider among the rorquals because of its enormous fins. The genome reveals that this classification does match the evolutionary signals. The same is true for the gray whale, which was believed to be evolutionarily distinct from rorquals due to its appearance. Genomic analyses show however that gray whales are nested within rorquals. Gray whales just happened to occupy a new ecological niche by feeding on crustaceans in coastal oceanic waters.
"Our research highlights the enormous potential of genome sequencing to better understand biological processes and the fundamentals of biodiversity. It even reveals how population sizes of whales have changed during the last million years", summarizes Janke. Janke is one of the leading researchers at the Hessian LOEWE Research Centre for Translational Biodiversity Genomics (LOEWE-TBG). Launched in January 2018, LOEWE-TBG is set to systematically analyze complete genomes or all active genes. The research center is envisaged to do basic research with a strong emphasis on transferring knowledge to benefit the study of natural products and protect biodiversity.
To study and understand nature with its limitless diversity of living creatures and to preserve and manage it in a sustainable fashion as the basis of life for future generations – this has been the goal of the Senckenberg Gesellschaft für Naturforschung (Senckenberg Nature Research Society) for 200 years. This integrative “geobiodiversity research” and the dissemination of research and science are among Senckenberg’s main tasks. Three nature museums in Frankfurt, Görlitz and Dresden display the diversity of life and the earth’s development over millions of years. The Senckenberg Nature Research Society is a member of the Leibniz Association. The Senckenberg Nature Museum in Frankfurt am Main is supported by the City of Frankfurt am Main as well as numerous other partners. Additional information can be found at www.senckenberg.de