Visual information from our eyes first enters the brain’s visual cortex at lower areas where elementary image characteristics are encoded. It later reached higher processing levels where objects are identified and recognized. The way in which visual information travels through the brain depends on the direction in which it travels. Information from lower areas is transmitted truthfully to higher level areas, whereas information that is sent back from higher areas to the lower areas strongly depends on the interpretation of the visual world. This is demonstrated by recent research of the Vision & Cognition group, published in the prestigious international journal Neuron on July 5th. Using electrical brain stimulation, the researchers show that signals that are sent back from higher brain area are crucial in recognizing where an object ends and the background begins.
Imagine walking in a forest on a sunny Saturday afternoon. Suddenly, you notice the characteristic sound of an owl. While scanning the trees for the owl’s location, your visual cortex is very active. The visual information that enters the brain through your eyes is processed step-by-step in different brain areas. Lower areas first represent basic image features like orientation and contrast and pass this information along to higher-level areas through ‘feedforward’ connections. These higher-level areas use this information to detect more complex image aspects, like the shape of objects. This complex information is then shared with the lower areas through ‘feedback’ connections. As soon as you spot the owl, the higher areas tell the low areas which ‘lines’ are part of a tree trunk and which are part of the owl. A similar mechanism helps you detect approaching cars in traffic or finding your keys on a cluttered desk.
Feedforward and Feedback
In this study, researchers used electrical stimulation of the visual cortex of rhesus monkeys to show that there is indeed a fundamental difference between the spread of information in feedforward and feedback directions. Researchers stimulated lower brain areas (V1) and higher areas (V4) while the monkeys performed a simple computer task in which they detected a square object against a very similarly looking background. While feedforward stimulation effects were observed in the entire period that the neurons were processing the visual information, feedback stimulation only affected a late phase of processing in which the object is separated from its background. This difference implies that during visual information processing feedforward brain connections pass along information truthfully, whereas feedback connections only start playing a role when complex information has been processed by higher areas and an object has been detected.
he two-way traffic of feedforward and feedback connections appears to be a general principle for information processing in the brain. Electrical stimulation of the cortex might in the future be used to connect a camera to the visual cortex to allow blind people to see again. The newly acquired knowledge the differential functions of the connections between cortical areas is therefore crucial for the development of visual prostheses.
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