The brain modulates visual signals according to internal states, as revealed by a new study by LMU neuroscientist Laura Busse.
What we see is not simply a neural representation of the light pattern in the eye, but an interpretation of this image, to which our needs and expectations contribute. These factors are shaped by previous experiences and also depend on internal states such as our behavioral activity and our vigilance or attention, often known collectively as “arousal.” In a new study recently published in the journal PLoS biology, researchers from LMU, the University of Freiburg and the Bernstein Center for Computational Neuroscience analyzed neuronal activity in the visual thalamus. This is an area of the brain that receives visual signals directly from the eye through the optic nerve before processing and transmitting them. More specifically, they investigated the dorsal lateral geniculate nucleus of the thalamus (dLGN), the main interface for visual signals from the retina to the visual cortex.
What happens in the thalamus?
“It has long been known that neurons in the dLGN, like neurons in other thalamic nuclei, exhibit conspicuous patterns of activity related to arousal,” explains LMU Professor Laura Busse, principal investigator of the study and soon-to-be researcher. principal in the SyNergy Cluster of Excellence. In particular, two state-dependent firing modes have been described: burst firing, which tends to occur during states of low arousal and behavioral inactivity; and tonic triggering, which is observed during vigilance states. “This led to the hypothesis that thalamic nuclei use burst and tonic firing modes to dynamically control or modify the flow of information to and between cortical areas according to the arousal state of the animal.” However, this proposal has not yet been tested experimentally. “The neural mechanisms by which arousal influences the processing of visual information remain largely unexplained.”
In the new study, Busse’s team directly compared the activity measured in the thalamus with the degree of arousal. “Arousal is reflected in the diameter of the pupil of mammals; larger pupils indicate a state of excitement,” explains the neurobiologist. In this way, we can make inferences about an animal’s state of arousal from changes in the size of its pupils. Joint investigation of thalamic and pupillary activity helped Busse’s team gain clear insight into the connection between thalamic activity and the degree of arousal.
Sensory impressions are modulated.
“We found that during certain phases of pupil dilation and constriction, different patterns of neuronal activation occur,” says Professor Christian Leibold of the University of Freiburg and the Bernstein Center for Computational Neuroscience, one of the lead authors of the study. “Electrical activity in the visual thalamus combines with pupil dynamics for periods of time ranging from seconds to several minutes.” This modulation of neural activity patterns during arousal was robust: it did not depend on other factors, such as what the animal was seeing or whether it was moving, sitting still, or moving its eyes.
“Therefore, our research shows that fundamental visual information is transmitted with differential encoding to “higher” areas of the brain, such as the visual cortex, in different phases of arousal,” summarizes neurobiologist Laura Busse. This provides a first mechanistic explanation of how visual perception can be influenced by changes in arousal state. “Our results support the hypothesis that excitation-dependent modulation is not a singular process, but probably an interaction of changes taking place on multiple time scales.”