8 July 2005
Retina Adapts By Suppressing The Commonplace
by Kate Melville
The retina actively seeks novel features in the visual environment, dynamically adjusting its processing in order to seek the unusual while ignoring the commonplace, say researchers at Harvard University. Their report on the principle of visual novelty detection appears in the journal Nature.
The study's findings provide evidence that the ultimate goal of the visual system is not simply to construct internally an exact reproduction of the external world. Rather, our optic systems seek to extract the few bits of data that are relevant to behavior from the continuous barrage of raw visual information. The researchers said that this dynamic retinal adaptation provides a means of stripping from the visual stream predictable and therefore less newsworthy signals. "Apparently our thirst for novelty begins in the eye itself," said Harvard professor Markus Meister. "Our eyes report the visual world to the brain, but not very faithfully. Instead, the retina creates a cartoonist's sketch of the visual scene, highlighting key features while suppressing the less interesting regions."
The researchers said that natural environments provided a good example of how this visual pre-processing works. In places such as forests or fields of grass with many vertical elements but only rare horizontal features, the retina adjusts to suppress the routine vertical features while highlighting the singular horizontal elements, they wrote. The researchers examined the retinal ganglion cells, which transport visual images from the eye to the brain. These cells are believed to record local spatial differences and changes over time which scientists had previously interpreted as a form of predictive coding, a strategy shaped by the forces of evolution in adaptation to the average image structure of natural environments. "Yet animals encounter many environments with visual statistics different from this hypothetical 'average' scene," Meister said. "We have found that when this happens, the retina adjusts its processing dynamically. The spatio-temporal receptive fields of retinal ganglion cells change after a few seconds in a new environment. These changes are adaptive, improving predictive coding by enhancing the ability of these receptive fields to pick out unusual features."
Working with salamanders and rabbits, Meister and his co-researchers recorded neural signals from the animals' retinal ganglion cells, testing whether adaptation to a different environment altered the encoding of retinal signals. From the neural responses to novel stimuli, the researchers computed the sensitivity of individual ganglion cells to various scenes. For most cells, sensitivity to a novel scene was greater than sensitivity to control scenes to which the animals had already been exposed, a gap that grew gradually in the seconds after introduction to a new environment. As the adaptation occurred in both salamanders and rabbits, Meister concluded that it typifies retinal function in both amphibians and mammals, animals that differ in physiology but share the challenge of adjusting to variable visual environments.
Source: Media release - Harvard University