The brain's data compression mechanisms: Neurons subtract images and use the differences

December 17, 2013
Data compression in the brain: When the primary visual cortex processes sequences of complete images and images with missing elements – here vertical contours – it “subtracts” the images from each other. Thereby, the brain computes the differences between the images. Under certain circumstances the neurons forward these image differences (bottom) rather than the entire image information (upper left). Credit: © RUB, Bild: Jancke

that's the information volume transmitted every second with every quick eye movement from the eye to the cerebrum. Researchers from the Ruhr-Universität Bochum (RUB) and the University of Osnabrück describe the way those data are processed by the primary visual cortex, the entry point for the visual information into the brain, in the journal Cerebral Cortex. Deploying novel optical imaging methods, they demonstrated that the brain does not always transmit the entire image information. Rather, it uses the differences between current and previously viewed images.

Efficient reduction of data volumes

Researchers have hitherto assumed that information supplied by the sense of sight was transmitted almost in its entirety from its entry point to higher brain areas, across which visual sensation is generated. "It was therefore a surprise to discover that the data volumes are considerably reduced as early as in the , the bottleneck leading to the cerebrum," says PD Dr Dirk Jancke from the Institute for Neural Computation at the Ruhr-Universität. "We intuitively assume that our visual system generates a continuous stream of images, just like a video camera. However, we have now demonstrated that the visual cortex suppresses redundant information and saves energy by frequently forwarding image differences."

Plus or minus: the brain's two coding strategies

The researchers recorded the neurons' responses to natural image sequences, for example vegetation landscapes or buildings. They created two versions of the images: a complete one and one in which they had systematically removed certain elements, specifically vertical or horizontal contours. If the time elapsing between the individual images was short, i.e. 30 milliseconds, the neurons represented complete image information. That changed when the time elapsing in the sequences was longer than 100 milliseconds. Now, the neurons represented only those elements that were new or missing, namely image differences. "When we analyse a scene, the eyes perform very fast miniature movements in order to register the fine details," explains Nora Nortmann, postgraduate student at the Institute of Cognitive Science at the University of Osnabrück and the RUB work group Optical Imaging. The information regarding those details are forwarded completely and immediately by the primary . "If, on the other hand, the time elapsing between the gaze changes is longer, the cortex codes only those aspects in the images that have changed," continues Nora Nortmann. Thus, certain image sections stand out and interesting spots are easier to detect, as the researchers speculate.

"Our brain is permanently looking into the future"

This study illustrates how activities of visual neurons are influenced by past events. "The neurons build up a short-term memory that incorporates constant input," explains Dirk Jancke. However, if something changes abruptly in the perceived image, the brain generates a kind of error message on the basis of the past images. Those signals do not reflect the current input, but the way the current input deviates from the expectations. Researchers have hitherto postulated that this so-called predictive coding only takes place in higher brain areas. "We demonstrated that the principle applies for earlier phases of cortical processing, too," concludes Jancke. "Our brain is permanently looking into the future and comparing current input with the expectations that arose based on past situations."

Observing brain activities in millisecond range

In order to monitor the dynamics of neuronal activities in the in the millisecond range, the scientists used voltage-dependent dyes. Those substances fluoresce when neurons receive electrical impulses and become active. Thanks to a high-resolution camera system and the subsequent computer-aided analysis, the neuronal activity can be measured across a surface of several square millimetres. The result is a temporally and spatially precise film of transmission processes within neuronal networks.

Explore further: New theory explains how critical periods are triggered during development of the nervous system

More information: N. Nortmann, S. Rekauzke, S. Onat, P. König, D. Jancke (2013): Primary visual cortex represents the difference between past and present, Cerebral Cortex, DOI: 10.1093/cercor/bht318

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5 comments

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axemaster
not rated yet Dec 17, 2013
Hi Physorg, I think your article is broken. Look at the beginning, it starts in the middle of a sentence...
MrVibrating
not rated yet Dec 17, 2013
I've long taken the view that the 'stuff' of the information we process is primarily difference.

I reached this conclusion upon realising that the qualitative equivalencies perceived between audio frequencies in a factor of two relationship mean that all other frequencies are by definition weighted in terms of their comparative inequivalence, or difference.. the equivalencies themselves must pertain to zero informational entropy, thus informational entropy per se must be tied to difference. Suffice to say i believe this principle is more fundamental than audition, and should have corollaries in all other other modalities..
jlevyellow
1 / 5 (1) Dec 17, 2013
Perhaps there is a quantum effect hidden in cognition. "Difference" hints at that phenomenon. All images previously viewed by the organism are available as a superposition and the input view or sound or smell causes a collapse of that superposition, with "differences" being perceived as change from an available image and the external image. This explains why novel external stimuli garner more attention (read: greater energy expenditure) than those that are more similar (or identical) to the internal stimulus.
24volts
1 / 5 (1) Dec 17, 2013
That sounds like our brains are programmed to pick up changes fast to protect against predators too and would also take less brain power to do than using full vision information all the time. Got to give nature credit for being efficient with the system.
Kedas
1 / 5 (1) Dec 18, 2013
Of course we compare (in math subtract) what else are we going to do with it.

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