Our senses influence each other. Experiments with mice show why blind people hear and feel better
In the end everything is one. The objects in our perception – respectively. their mental and neuronal representations – appear as units acting on different senses and yet always remain the originators of perception. I can see, hear, feel, smell and in special situations also taste a person, and thus experience characteristics of this person.
But in the beginning, everything seems to be separate. I can only hear the music on the radio, only see the landscape outside the train window, only smell the silence in the subway. Each sensory modality is apparently recorded and processed separately. Only the finished pictures, noises, smells and foot impressions are connected to representations in a way that is still puzzling and not the subject here – presumably via the synchronous and fast firing of widely interconnected neurons from the involved brain areas.
In these areas, however, each sensory impression coming from the sensory organs is first evaluated completely for itself. Therefore there is a visual cortex at the rear end of the cerebrum, and completely separated from it a auditory cortex on the sleep lobe, and again completely elsewhere, in the parietal lobe behind the central furrow, the sensory cortex.
But this supposed separation exchanges. Interactions between the senses occur very early in processing. There are direct anatomical connections between the three cortical fields mentioned above, through which seeing, hearing and feeling can influence each other long before their result becomes conscious.
Apparently without mutual contact: primary sensory fields of the cerebral cortex. Image: K. Lehmann
In principle this has been known for a long time. It is common knowledge that blind people can hear better than sighted people. However, general knowledge needs scientific examination (which turned out positively years ago – and by the way, blind people can also smell better), and besides, this is just the beginning of the story.
It starts with the obvious question: Why? The cortex of the blind works more efficiently because it is challenged more? Or does auditory processing also resort to the visual cortex, which has become useless??
"Cross-modal plasticity"
Naturally: both. The improved processing within the spared bark areas will be "intramodal plasticity" and has been demonstrated many times in the auditory cortex – for example, with evidence of more efficient processing of sounds. At the same time, the brain does not leave areas unused. When the visual cortex no longer receives information from the eyes, it looks for other tasks to do. The result is then called "cross-modal plasticity". Especially in people who became blind at an early age, the visual cortex is therefore very active, is used for Braille reading and is also used for auditory tasks.
The reverse is also true. An early study of people born deaf used the EEG to detect significantly stronger responses of the visual cortex to stimuli in the outer visual field, i.e., a form of intramodal plasticity. And cross-modal plasticity was z.B. in deaf cats, which develop abnormally good visual abilities and use different parts of the unemployed cortex for this purpose.
Changes within minutes
Similar compensatory processes have recently been demonstrated in mice by colleagues around the PhD students Manuel Teichert and Marcel Isstas at the Institute for General Zoology and Animal Physiology in Jena, where I also work. And – what makes it even more fascinating – not in animals that were deprived of a sense for a long time. But within a few days. Yes, the first changes occur within minutes in deaf cats.
However, mice are not humans. Although our research focuses on the visual system, mice can actually do without their eyes quite well. Their spatial resolution is about one hundredth of ours. If the horizon were divided into more than about 200 black-and-white stripes, it would be a uniform gray for a mouse. Therefore, vision is at the bottom of the hierarchy of senses in the mouse. If you make them blind, nothing changes for hearing and feeling.
If you are a mouse, this box is gray for you (provided it is displayed about 15cm wide, and you sit comfortably about 80cm in front of the screen). image: K. Lehmann
Mice can hear up to the ultrasonic range, where they prefer to communicate, and with their whiskers (vibrissae) they actively and precisely scan the near environment. The loss of one of these senses poses real challenges for mice.
This shows up in brain and behavior. And they do it very quickly and even at a mature age. Many works on the adaptations to the loss of a sense are dedicated to individuals who were born deaf or blind or who became deaf or blind shortly afterwards. In such cases, the brain has adjusted to the new situation during maturation.
In the studies of Manuel Teichert and colleagues, however, adult mice were made deaf. Within minutes, activity in the visual cortex increased, and when the colleagues physiologically measured the animals’ visual acuity, it improved markedly by over ten percent. It was not even necessary to damage the animals’ hearing: a kind of earplug had the same effect. Mice need rest to see better.
How can such a seemingly basic ability as visual acuity change so quickly?? The solution lies in the already mentioned connections between the sensory cortices. The nerve pathway from the auditory to the visual cortex ends preferentially on inhibitory neurons. If the cortex is no longer stimulated, it also inhibits the visual cortex less strongly. In this way, the visual cortex increases its activity in minutes and improves processing.
The brain then builds up this immediate effect in the long term. In another study just published, in which I was also involved, we looked at what happens to vision over a week after loss of a major sense. In this case we cut off the whiskers of the animals and shaved them every day. The longer time frame made it possible to measure visual acuity behaviorally, i.e. in a test in which the animals had to distinguish stripes from gray. Within a few days after losing their whiskers, they were about 40% more visible than before.
Even slight noise noticeably worsens visual performance
Can this now be applied to humans?? Possibly yes, at least partially. Japanese researchers recently put their subjects in front of a kind of eye test and determined the number of correct answers in silence compared to female noise of different (but always mild) loudness (45 to 65dB). Even the quiet noise was enough to measurably worsen performance by about ten percent.
And colleagues from the Jenens Clinic for Neurology under Otto Witte recently showed that the ability to feel differences with the fingertips immediately improves when we close our eyes. And this regardless of whether it is light or dark. This means that even in the dark, when we can’t see anything anyway, we improve our sense of touch by closing our eyes. With simultaneous functional magnetic resonance imaging, the research group also confirmed that the influence of the visual cortex on the cortex decreases when the eyelids are closed.
With regard to listening, we know this from everyday experience and use it regularly: when we close our eyes to listen to the music better. This is not a blob of affectation, and it is not, as the mice show us, solely for the sake of improved concentration. But we actually perceive the music more accurately, perhaps even louder. Long before the brain recognizes the melody, namely already at the first processing of the sounds, the sight has interfered with the hearing, as long as the eyes are open.