Our pupil moves to the rhythm of the environment

May 08, 2020

When we find something particularly beautiful or impressive, we literally get big eyes: Our pupils dilate. The pupil controls how much light enters the eye and falls on the retina. When there is a lot of light, the pupil contracts; when there is little light, it opens again. Neuroscientists from the German Primate Center (DPZ) - Leibniz Institute for Primate Research and the European Neuroscience Institute Göttingen have now found out in a study of humans and rhesus monkeys that the movement of the pupil is not only reflexively controlled by the amount of incident light, but unconsciously also by our mind. Thus, the pupil can follow rhythms that arise in the environment. In this way, the opening of the pupil is optimally adapted to our environment which enhances perception (Journal of Neuroscience).

Sensory impressions from our environment are often rhythmic, not only when we hear, but also when we see. For example, the blue light of a passing ambulance flashes about 120 times per minute. We also react unconsciously to visual events in our environment, which can be registered by our senses as regular patterns. From these patterns our brain can, for example, deduce when the next flash of blue light will hit the eye and prepare itself for it.

An important factor in vision is the adjustment of the pupil diameter. Smaller pupils provide a sharper image, while larger pupils allow more light to reach the retina, making it more likely that even weak stimuli will be processed at all. Pupil diameter is controlled by the pupillary reflex, which automatically, i.e. without our knowledge or intention, adjusts the pupil muscles to the incidence of light. But not all relevant environmental information is contained in the amount of incident light alone. Computations are therefore required in the brain that go beyond the capabilities of a reflex to take into account all available information. The aim of this study, funded by the German Research Foundation (DFG), was to find out whether and to what extent pupil dynamics are controlled fully automatically or whether they are also influenced by more complex rhythms in the environment.

For the investigations, pupil movements of two male rhesus monkeys (Macaca mulatta) and several test subjects of both sexes were measured using a high-speed video camera, while the subjects were shown sequences of images of human faces at a temporal frequency of two hertz. A dark background was shown between the images. The alternation of background and image causes the pupil to dilate and contract in rhythm with the images. During the experiments, the order of the images was manipulated - they were grouped in pairs so that a particular image always followed a particular other image. Thus, there are two rhythms to which the pupil reacts: a fast one (two hertz), which results from the alternation of image and background, and one at half that pace (one hertz), which results from the arrangement of the images as pairs. The sequence of the pairs is not given by the light itself, and therefore requires an additional computation of environmental rhythms in the brain. Since the luminance of the faces in all pictures as well as the dark background in the "pauses" remained unchanged, but the arrangement of the pictures varied, conclusions could be drawn about the influence of this additional computation on pupil dynamics.

In addition to the structured sequence, randomly arranged images with the same frequency (two hertz) were shown. A comparison of the results between structured and unstructured image sequences at the same image frequency shows that in both species studied the pupil follows not only the light-related rhythm of the images, but also the more complex rhythm of the pairs. Pupil movement in a slow (one hertz) rhythm keeps the pupil open longer, as if a pair should not be interrupted by the closing of the pupil. This allows more light to reach the retina. "The additional information contained in the environment thus complements the information already reaching the retina via the incident light," says Caspar Schwiedrzik, head of the junior research group "Perception and Plasticity". Furthermore, the study was able to show that this contributes to an improvement in perception, even if the test subjects are not aware that there is a rhythm in the environment. "Pupil control is therefore not purely reflexive, but is also influenced by our unconscious thoughts," adds Schwiedrzik.
-end-


Deutsches Primatenzentrum (DPZ)/German Primate Center

Related Brain Articles from Brightsurf:

Glioblastoma nanomedicine crosses into brain in mice, eradicates recurring brain cancer
A new synthetic protein nanoparticle capable of slipping past the nearly impermeable blood-brain barrier in mice could deliver cancer-killing drugs directly to malignant brain tumors, new research from the University of Michigan shows.

Children with asymptomatic brain bleeds as newborns show normal brain development at age 2
A study by UNC researchers finds that neurodevelopmental scores and gray matter volumes at age two years did not differ between children who had MRI-confirmed asymptomatic subdural hemorrhages when they were neonates, compared to children with no history of subdural hemorrhage.

New model of human brain 'conversations' could inform research on brain disease, cognition
A team of Indiana University neuroscientists has built a new model of human brain networks that sheds light on how the brain functions.

Human brain size gene triggers bigger brain in monkeys
Dresden and Japanese researchers show that a human-specific gene causes a larger neocortex in the common marmoset, a non-human primate.

Unique insight into development of the human brain: Model of the early embryonic brain
Stem cell researchers from the University of Copenhagen have designed a model of an early embryonic brain.

An optical brain-to-brain interface supports information exchange for locomotion control
Chinese researchers established an optical BtBI that supports rapid information transmission for precise locomotion control, thus providing a proof-of-principle demonstration of fast BtBI for real-time behavioral control.

Transplanting human nerve cells into a mouse brain reveals how they wire into brain circuits
A team of researchers led by Pierre Vanderhaeghen and Vincent Bonin (VIB-KU Leuven, Université libre de Bruxelles and NERF) showed how human nerve cells can develop at their own pace, and form highly precise connections with the surrounding mouse brain cells.

Brain scans reveal how the human brain compensates when one hemisphere is removed
Researchers studying six adults who had one of their brain hemispheres removed during childhood to reduce epileptic seizures found that the remaining half of the brain formed unusually strong connections between different functional brain networks, which potentially help the body to function as if the brain were intact.

Alcohol byproduct contributes to brain chemistry changes in specific brain regions
Study of mouse models provides clear implications for new targets to treat alcohol use disorder and fetal alcohol syndrome.

Scientists predict the areas of the brain to stimulate transitions between different brain states
Using a computer model of the brain, Gustavo Deco, director of the Center for Brain and Cognition, and Josephine Cruzat, a member of his team, together with a group of international collaborators, have developed an innovative method published in Proceedings of the National Academy of Sciences on Sept.

Read More: Brain News and Brain Current Events
Brightsurf.com is a participant in the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for sites to earn advertising fees by advertising and linking to Amazon.com.