Researchers from the Centre for Developmental Neurobiology at King’s College London have shed light on how we perceive and recognise specific visual stimuli.
The first steps in visual processing take place in the retina, which is a thin layer of tissue at the back of the eye containing more than 70 distinct types of nerve cells. Our visual system processes light stimuli to extract important information about the visual world, such as movement, orientation and colour. This information is processed at the cellular level before being transmitted via the optic nerve to the brain, where a visual image is formed.
Although some of the visual circuits underlying this image processing have been identified in previous research, this study is the first to reveal the cellular and molecular building blocks in the retina which enable us to detect oriented visual stimuli, a feature of visual perception described as ‘orientation selectivity’. Orientation selectivity is known to be important for various perceptual processes, including whether something is horizontal or vertical, which in humans is crucial for recognising faces.
Published today in Current Biology, the study shows how ‘orientation selectivity’ is orchestrated in the retina.
The researchers studied larval zebrafish, which has become an organism of choice for many scientists because its biological processes are relatively similar to humans. The development of zebrafish is also very fast, going from fertilised egg to larvae in five days - at which point they have a fully formed visual system, equivalent in overall structure and function to that of humans.
Crucially, larval zebrafish are also transparent, which means scientists can observe development and cellular processes as they happen, without using invasive experimental methods.
The zebrafish were shown videos featuring shapes of different orientation, while at the same time their brain activity was measured. The researchers analysed levels of cellular activity in neurons of the visual system by recording the fluorescence of calcium indicators (i.e. the more fluorescent the indicators were, the higher the level of neural activity). By doing so, it was possible to identify specific neurons in the retina and measure how they are activated by oriented stimuli.
Paride Antinucci, first author
The researchers also found that the structure of these special neurons played a crucial role in this process. The elongated shape of these cells directly determines if they are responding to oriented visual input and along which axis (i.e. vertical or horizontal). This ‘shape-determines-function’ mechanism is an important finding to explain visual processing.
Our new research indicates that specifically shaped cells in the retina are activated when visual stimuli are horizontal, and other cells fire when they are vertical.
Robert Hindges, senior author
This study was funded by the Biotechnology and Biological Sciences Research Council (BBSRC).
Paper reference: ‘Neural mechanisms generating orientation selectivity in the retina’. Antinucci P, Suleyman O, Monfries C, and Hindges R. Current Biology 2016
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