Do goldfish have a positioning system? Scientists have discovered a novel experimental method to unlock the secrets of the human brain's GPS

Goldfish spend most of their time swimming gracefully in glass tanks, but what's going on in their seemingly dazed little heads? Recently, researchers have discovered that they have a sophisticated spatial navigation system that can estimate distances.

Although researchers have previously shown that fish can efficiently navigate through water, how they do it has remained unclear. Understanding how goldfish brain cells do this may help us understand the internal GPS of the human brain, scientists say. "We want to know where these cells appear on the evolutionary tree," said Dr. Adelaide Seabaugh of the University of Oxford, lead author of the latest study.

Writing in the journal Proceedings of the Royal Society B, Seeber and colleagues describe how they created a water tank with 2-centimeter-wide vertical black and white stripes on the walls, connected by similar stripes on the floor.

The team trained nine goldfish to swim along a tank, and when they had covered 70 centimetres, they would return to their starting position if they were waved at.

The team then tested whether the fish could estimate the same distances without the help of gestures.

For six of the goldfish, the team switched the background pattern of the tank wall to 1 cm wide vertical stripes, a 2 cm square grid pattern, and 2 cm wide horizontal stripes aligned with the direction of the fish's travel, and compared these results with the distance they traveled at that time.

For each background pattern, each fish swam 45 times and was recorded in the video.

The team found that when the goldfish were against a background of vertical 2-cm-wide stripes, they travelled an average of 74 cm, moving an average of 17 cm up and down. Similar results were found when the background switched to a checkered pattern. However, when the stripes were vertical but narrow, the fish turned around significantly faster and returned - overestimating the distance they travelled by about 36%.

When horizontal stripes were used, the distances the fish swam varied greatly. “It was completely inconsistent from fish to fish,” says Seebaugh.

The team said the results suggest the goldfish use an "optical flow mechanism" based on the visual density of their environment - in other words, they track how often the vertical pattern switches between black and white to estimate how far they have travelled. When the width of the stripes decreases, the goldfish perceive time as passing faster, and the fish overestimate how far they have swum. Optical flow is the projection of an object's motion in three-dimensional space onto a two-dimensional image plane.

"This is angular motion based on visual features , and mammals, including humans, use different mechanisms for optical flow," said Seebo. "However, the results suggest that the use of distance information based on vision appeared early in our evolution, as it is widespread across different animal groups."

Other mechanisms may also be at play, the team said, noting that goldfish were more accurate in measuring distance when their starting position was closer to the end of the tank, while for some fish the number of their fin flaps correlated with how far they had swum.

Professor Colin Leaver of Durham University, who was not involved in the research, said the study showed that goldfish used optic flow rate at least in part to estimate distance, although other cues may also be used.

"This research is important and novel because, while we already knew that fish respond to geometric information about direction and distance, we didn't know how they estimated distance," he said.

"It is exciting to explore spatial mapping in fishes because navigation evolved earlier in the evolutionary tree than in mammals, and because fish navigation must be more fully coordinated in the vertical dimension than in most mammals."

Note: In sports biomechanics, angular motion is the movement that athletes perform when they turn, flip, twist, pirouette, turn, and swing. In sports, these words all refer to angular motion.

Comprehensive report by Chengshi Interactive reporter Jin Yingying