British scientists have found that the reaction of the pupil to the change in brightness of the light can occur without external stimulation. To do this, they conducted three experiments, during which they measured the size of the pupil of the participants, who needed to remember the direction of the lines either on a dark or light circle. Depending on which circle you had to remember, the size of the pupils changed, just like when the external lighting changed. The work is published in the journal Proceedings of the National Academy of Sciences.
Two separate muscles are responsible for the expansion and narrowing of the pupils: the dilator and the sphincter. Their work is determined by how much light gets to the retina: in bright light, the sphincter narrows the pupil within the first few seconds, and when darkened, the dilator dilates it – but in a few minutes. Sensory stimulation (or rather, changes in lighting), however, is not necessary for changing the shape of the pupil. Thus, some psychoactive or medical drugs can expand them, and a change in the pupil accompanies a number of psycho-emotional conditions.
Scientists led by Anna Nobre from the University of Oxford decided to test whether it is possible to regulate the size of pupils due to information that is stored in working memory: in other words, will the size of the pupils change if a person imagines a bright object. To do this, they conducted a series of experiments, which involved 25 people. The size of the pupils was assessed with the help of an IT tracker.
During the first experiment, participants were shown images of two circles – dark and light grey – and asked to remember the direction of the lines on them. After that, a sound was turned on that informed the participants which of the two laps they should remember. After that, a green circle of different brightness appeared on the screen and participants had to answer whether the brightness of the circle corresponding to the one they remembered. The authors specify that the different brightness of the green circle did not cause a physiological reaction of the pupil in itself.
The change in the size of the pupil was evaluated during all stages of the experiment, starting with sound stimulation, but only for those times when the brightness of the green circle was correct. Analysis of the data showed that participants’ pupils actually expand or narrow depending on which circle they needed to remember – dark or light grey.
The second experiment was exactly the same as the first, but this time instead of green circles of different brightness, the scientists used circles that were half or full painted: this was done to check whether the pupil’s reaction would continue even without the use of relative brightness in the response. Scientists were able to reproduce the results of the previous experiment: pupils also expanded when participants needed to remember the dark circle and narrowed down when they needed to remember the light one.
Finally, in the last experiment, the scientists decided to see if such a change in the pupil would be dynamic – that is, change as the working memory is updated. To do this, participants were taught to associate a dark and grey circle with a time period of either one or three seconds: respectively, information about which circle to remember, updated over time. It turned out that the size of the pupil in different time segments depends on what circle the participants expect – dark or light.
Successful a job often requires storing information in work memory, from where it can be effectively extracted and used. In this study, such information was the brightness of the visually presented object, and in order to complete the task, it had to be extracted, which, apparently, and affected change the size of the pupil. The authors have thus been able to show that such a physiological reaction is controlled not only by upward processes, i.e. external stimulation, but can also occur beyond the account of the processes of descending.
Changing the size of the pupil is often used in the diagnosis of traumatic brain injuries: the usual narrowing in bright light as a result of the injury is disturbed. Two years ago, the developers introduced an application that can detect a traumatic brain injury by the reaction of the pupil using a smartphone with the analogue VR headset.