March 30th, 2006

homuncul

NEUROSCIENCE

Your brain cells may “know” more than you let on by your behavior

One way to study associative memory is to train rhesus monkeys to remember arbitrary pairs of symbols. After being shown the first symbol (i.e. dark clouds) they are presented with two symbols, from which they have to pick the one that has been associated with the initial cue (i.e. umbrella). The reward is a sip of their favorite fruit juice.

“We want the monkeys to behave perfectly on these tests, but one of them made a lot of errors,” recalls Albright. “We wondered what happened in the brain when the monkeys made the wrong choice, although they had apparently learned the right pairing of the symbols.”

So, while the monkeys tried to remember the associations and made their error-prone choices, the scientists observed signals from the nerve cells in a special area of the brain called the "inferior temporal cortex" (ITC). This area is known to be critical for visual pattern recognition and for storage of this type of memory.

When Albright and his team analyzed the activity patterns of brain cells in the ITC, they could trace about a quarter of the activity to the monkey’s behavioral choice. But more than 50 percent of active nerve cells belonged to a novel class of nerve cells or neurons, which the researchers believe represents the memory of the correct pairing of cue and associated symbol. Surprisingly, these brain cells kept firing even when the monkeys picked the wrong symbol.

“In this sense, the cells ‘knew’ more than the monkeys let on in their behavior,” says Albright.


Neuron, Oct. 2005
Neural correlates of knowledge: stable representation of stimulus associations across variations in behavioral performance
Behavioral responses to a sensory stimulus are often guided by associative memories. These associations remain intact even when other factors determine behavior. The substrates of associative memory should therefore be identifiable by neuronal responses that are independent of behavioral choices. We tested this hypothesis using a paired-associates task in which monkeys learned arbitrary associations between pairs of visual stimuli. We examined the activity of neurons in inferior temporal cortex as the animals prepared to choose a remembered stimulus from a visual display. The activity of some neurons (22%) depended on the monkey's behavioral choice; but for a novel class of neurons (54%), activity reflected the stimulus that the monkey was instructed to choose, regardless of the behavioral response. These neurons appear to represent memorized stimulus associations that are stable across variations in behavioral performance. In addition, many neurons (74%) were modulated by the spatial arrangement of the stimuli in the display.
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Не имею ничего против данной гипотезы как таковой, но не вижу, чтобы она явно вытекала из data. Результат эксперимента можно трактовать прямо противоположным образом: нанесен укол хеббианизму (активность нейронов не кореллирует с запоминанием!). Если же не бросаться в крайности, очевидно предположить, что нейроны, активность которых не кореллировала с поведением, просто не связаны непосредственно с заданной ассоциацией. Вот и все. По-моему, такой вывод - самое естественное предположение...