ISEE Practice Test 2

A Brainless Problem Solver

¹  In a laboratory dish, a bright yellow blob spreads slowly across a

²layer of damp gel. It has no eyes, no nerves, and no brain. It is, in

³fact, a single cell, though one large enough to see without a microscope

⁴and containing many nuclei. Biologists call it Physarum polycephalum,

⁵and over the past few decades this humble slime mold has become an

⁶unlikely celebrity in the study of how living things solve problems.

⁷  Its fame began with a maze. In 2000, a team led by the Japanese

⁸biologist Toshiyuki Nakagaki let pieces of Physarum fill every corridor

⁹of a small maze. The researchers then placed oat flakes, a favorite

¹⁰food, at the maze's two exits. Over the next several hours, the

¹¹organism withdrew from the dead ends and from the longer routes, leaving

¹²a single thick tube that stretched along the shortest path between the

¹³two meals.

¹⁴  How does a creature without a brain manage this? The answer seems to

¹⁵lie in feedback. The slime mold moves by pumping its fluid interior back

¹⁶and forth through a network of tubes. Tubes that carry more flow grow

¹⁷thicker, and thicker tubes carry still more flow, while little-used

¹⁸tubes shrink and disappear. No part of the cell "knows" where the food

¹⁹is; the efficient route simply wins out over the inefficient ones, much

²⁰as a footpath across a lawn deepens wherever the most people choose to

²¹walk.

²²  A later experiment pushed the idea further. In 2010, researchers

²³arranged oat flakes on a dish to match the locations of cities around

²⁴Tokyo and let the slime mold grow among them. The network it built

²⁵looked strikingly similar to the region's actual rail system, which

²⁶engineers had spent years designing. Some scientists now study the

²⁷organism's simple rules in hopes of designing transport and computer

²⁸networks that adjust themselves as conditions change.

²⁹  None of this means that the slime mold is thinking. But it does

³⁰suggest that intelligent-looking behavior does not always require a

³¹brain, and that a few simple rules, repeated across a whole body, can

³²sometimes accomplish what we might expect only a mind to do.