Our decisive dilemmas fly in the face of scientific logic
Hesitate, evaluate and decide - the seemingly simple process of making a choice may sometimes take just two or three seconds. But it is a more complicated job for our brains, as well as for the scientists who are interested in finding out how we arrive at a decision.
Nor is it an easy assignment for the fruit fly, an insect that is believed to share some similarities with primates in terms of its decision-making process.
A group of Chinese researchers recently found a two-phase system adopted by the common fruit fly, Drosophila melanogaster, in making value-based decisions. They say it may shed light on similar studies on the human brain, and help with future development of artificial intelligence.
They found that in the face of alternative flight options, the fruit fly first relies on a nonlinear process that involves a neural circuit combining the brain dopamine system and mushroom bodies. Then its brain switches to a simpler, perception-based linear phase called execution that is independent of the neural circuit.
The dopamine system exists in most animals, playing a crucial role in the motivation to acquire a reward or avoid a punishment. Mushroom bodies are a pair of structures in the brain of insects and other arthropods, being essential for olfactory reinforcement learning.
When a fruit fly makes value-based decisions, its dopamine system may send a signal to mushroom and thus form a circuit to generate a decision, according to academician Guo Aike, a co-author of the research report that was published in US journal Science last week.
"We infer that the dopamine-mushroom body circuit is implementing a gating mechanism, which regulates signal transmission," says Guo, a prestigious neurobiologist who works at the Shanghai Institute for Biological Sciences in the Chinese Academy of Sciences.
In simple terms, the dopamine system "filters" signals based on a value it was given by training, and only transmits stronger "winner" signals, which mean rewards to a fruit fly, to the mushroom bodies. The system will ignore other weaker inputs.
More significantly, the researchers discovered that when confronted with a new choice, the fruit fly can reactivate the circuit to balance between maintaining an existing choice and switching to a new decision, Guo says.
In 2001, he reported in Science that during the fruit fly's decision-making process, it tends to make a choice that helps avoid punishment, and that mushroom bodies participate in this process.
The latest research was to further expose the mechanism behind the fruit fly's decision- making process, after five years of concerted effort by Guo and his students.
Zhang Ke, a PhD candidate with the Shanghai institute and first author of the paper, says he and his fellow researchers first trained fruit flies in a flight simulator to associate heat punishment with one of two bars with compound cues - position (upper and lower) and color (blue and green).
After training with one bar, upper and blue for example, flies were confronted with conflicting cues, such as upper-green and lower-blue, and had to decide whether to follow the position or color cue depending on their relative saliency.
The researchers also applied a method to inhibit functions of the dopamine system or the mushroom bodies to see their roles in the process.
"But we still haven't figured out how the two interact with each other specifically, which will probably be our next goal," Zhang says.
For almost a century, the fruit fly has been a common model organism in research topics with implications for primate behaviors. Research has shown that the human brain has a similar circuit controlling decision-making process.
There is a belief that some basic principles during evolution may have been inherited from simple forms to complicated organisms, Guo says.
(China Daily 07/04/2007 page19)
