April 12, 2013
Researchers Detail How The Mind’s Visual System Adapts To New Environments
redOrbit Staff & Wire Reports - Your Universe Online
When people escape from the grind of their day-to-day lives and travel to remote, tranquil places, they often claim to “turn off” their brains so that they can soak in the calm, relaxing environment. Despite how restful things might seem, however, new research from the Salk Institute for Biological Studies in California reveals that a person´s mind — and more specifically, their visual system — is working just as hard as ever.“Actually, you've just given your brain a whole new challenge,” Thomas D. Albright, director of the Vision Center Laboratory at of the Salk Institute and an expert in how the human visual system operates, explained in a statement Wednesday.
He added, “You may think you're resting, but your brain is automatically assessing the spatio-temporal properties of this novel environment-what objects are in it, are they moving, and if so, how fast are they moving?”
Our minds only have so many neurons that they can dedicate to the assessment described by Albright, added Sergei Gepshtein, a staff scientist in Salk's Vision Center Laboratory. The visual system has a limited amount of resources and must constantly find a way to use them most efficiently.
The question that Albright, Gepshtein and their colleagues set out to answer is exactly how the brain is able to best allocate those neurological resources — an issue which has produced contradictory results in previous research.
In those past studies, scientists anticipated that prolonged exposure to a novel environment would make a person better at detecting subtle details about that location. However, the results of those studies were all over the grid. Sometimes people reportedly improved at detecting stimuli, sometimes they got worse, and sometimes there was no effect at all, Albright said. There were even times where people improved, but not for the expected stimulus.
The solution, Gepshtein said, came when they looked at the issue of resource allocation from a system´s perspective.
“It's as if the brain's on a budget,” he explained. “If it devotes 70 percent here, then it can only devote 30 percent there. When the adaptation happens, if now you're attuned to high speeds, you'll be able to see faster moving things that you couldn't see before, but as a result of allocating resources to that stimulus, you lose sensitivity to other things, which may or may not be familiar.”
“Simply put, it's a tradeoff: The price of getting better at one thing is getting worse at another,” added Albright.
The Salk Institute researchers reached their conclusion by using an analysis for the brain from a theoretician´s point of view, and by using computations to detail how the visual system accomplishes that adaptation.
Those computations, they explain, are similar to the method signal processing known as Gabor transform. Gabor transform is used to determine the frequency and phase content of local sections of a signal as it changes over time, and was first described by physicist and Nobel Laureate Dennis Gabor.
“In human vision, stimuli are first encoded by neural cells whose response characteristics, called receptive fields, have different sizes,” Gepshtein explained. “The neural cells that have larger receptive fields are sensitive to lower spatial frequencies than the cells that have smaller receptive fields. For this reason, the operations performed by biological vision can be described by a Gabor wavelet transform.”
Essentially, the first stages of the visual process act like a filter, determining which stimuli “get in” and which are blocked, the researchers said. When a person´s environment changes, so does their visual filter, which means that different stimuli are allowed to become visible and formerly visible ones wind up going undetected.
“When you see only small parts of this filter, you find that visual sensitivity sometimes gets better and sometimes worse, creating an apparently paradoxical picture,” Gepshtein said. “But when you see the entire filter, you discover that the pieces — the gains and losses — add up to a coherent pattern.”
A paper detailing their work has been published in a recent edition of the journal Proceedings of the National Academy of Sciences. The research was supported by the National Institutes of Health (NIH) and the UK´s Gatsby Charitable Foundation.