Scientists Help Explain Visual System's Remarkable Ability to Recognize Complex Objects

Article ID: 605018

Released: 1-Jul-2013 5:00 PM EDT

Source Newsroom: Salk Institute for Biological Studies

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  • Credit: Image: Courtesy of Salk Institute for Biological Studies

    From left: John Reynolds, Anirvan Nandy and Tatyana Sharpee

Newswise — LA JOLLA, CA----How is it possible for a human eye to figure out letters that are twisted and looped in crazy directions, like those in the little security test internet users are often given on websites?




Their deep and detailed research----involving recordings from hundreds of neurons----may also have future clinical and practical implications, says the study's senior co-authors, Salk neuroscientists Tatyana Sharpee and John Reynolds.

"Understanding how the brain creates a visual image can help humans whose brains are malfunctioning in various different ways----such as people who have lost the ability to see," says Sharpee, an associate professor in the Computational Neurobiology Laboratory. "One way of solving that problem is to figure out how the brain----not the eye, but the cortex---- processes information about the world. If you have that code then you can directly stimulate neurons in the cortex and allow people to see."




Integrating parts into wholes


"Neurons in the visual system are sensitive to regions of space---- they are like little windows into the world," says Reynolds. "In the earliest stages of processing, these windows ----known as receptive fields----are small. They only have access to information within a restricted region of space. Each of these neurons sends brain signals that encode the contents of a little region of space----they respond to tiny, simple elements of an object such as edge oriented in space, or a little patch of color."

Neurons in V4 have a larger receptive field that can also compute more complex shapes such as contours. They accomplishes this by integrating inputs from earlier visual areas in the cortex----that is, areas nearer the retina, which provides the input to the visual system, which have small receptive fields, and sends on that information for higher level processing that allow us to see complex images, such as faces, he says.

Both new studies investigated the issue of translation invariance---- the ability of a neuron to recognize the same stimulus within its receptive field no matter where it is in space, where it happens to fall within the receptive field.




For example, a neuron might respond to a bit of the curve in the number 5 in a CAPTCHA image, no matter how the 5 is situated within its receptive field. Researchers believed that neuronal translation invariance----the ability to recognize any stimulus, no matter where it is in space----increases as an image moves up through the visual processing hierarchy.


A deeper mystery to be solved

The Salk researchers found that neurons that respond to more complicated shapes----like the curve in 5 or in a rock---- demonstrated decreased translation invariance. "They need that complicated curve to be in a more restricted range for them to detect it and understand its meaning," Reynolds says. "Cells that prefer that complex shape don't yet have the capacity to recognize that shape everywhere."





Minjoon Kouh, a former postdoctoral fellow at Salk, participated in the PNAS study. Salk postdoctoral researcher Anirvan Nandy and senior staff scientist Jude Mitchell, of the
Salk Systems Neurobiology Laboratory, were co-authors of the Neuron paper.

Both studies were funded by grants from the National Institutes of Health (R01EY019493), the McKnight Scholarship and the Ray Thomas Edwards and W. M. Keck Foundations. In addition, the PNAS study received a grant from the Searle Funds. The Neuron study was additionally funded by grants from the Alfred P. Sloan Foundation, the National Institutes of Health (EY0113802), the Gatsby Charitable Foundation and the Swartz Foundation, and a Pioneer Fund postdoctoral fellowship.


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