The Key to the Sixth Sense

Author:  Suvash Shrestha
Institution:  Kathmandu Medical College, Nepal

The lateral line system, the ability to sense the surroundings, could be the sixth sense organ for which we are looking. Professor Leo van Hemmen and his team from the Department of Physics at the Technische University, Muenchen wrote about their progress in decoding the amazing sensory system of fishes and amphibians in Physical Review Letters1,2 August 2009.

The lateral line system of water animals has baffled scientists for many decades. It was a mystery how blind Mexican Cave fish could effortlessly avoid obstacles and how Catfish could follow invisible tracks to their prey until the lateral-line theory was presented. "The lateral-line sense fascinated me from the start because it's fundamentally different from other senses such as vision or hearing, not just at first glance but also the second," van Hemmen says. "It's not just that it describes a different quality of reality, but also that in place of just two eyes or ears this sense is fed by many discrete lateral-line organs – from 180 in the clawed frog to several thousand in a fish. The integration behind it is a tour de force."

As the name suggests, lateral lines are aligned along the left and right sides of the fish's body as well as around its eyes and mouths. These lines contain neuromasts, or gelatinous, flexible, flag-like organs about one tenth of a millimeter long. They lie directly on, or just underneath, animal skin. Neuromasts act as a remote sensing system to detect minor differences in water pressure distribution and water flow. Then, they transmit messages to hair cells, another type of neuronal cell, and, subsequently, to the brain for further processing.

As a physicist, van Hemmen was very keen to discover the mathematical code of this amazingly accurate sensing system. So far, he has discovered a formula that describes the animal's response to a stimulus with satisfactory precision. His calculations match very closely with measurements from the fish's actual nervous system. This formula uses the angle between a fish's axis and the trails left by its prey, which helps the predator continue its chase in the right direction.

Hemmen aims to translate this biological property into technology so that we can have a robot with multiple sensory modalities. This ability could easily help the robot make out its way even in harsh conditions such as deep dark water or volcanoes. This technology could be a revolution in biomimetic engineering because it would eliminate the need for cameras.


1. Sichert AB, Bamler R, Hemmen JLV. Hydrodynamic Object Recognition: When Multipoles Count. Physical Review Letters 2009;102:058104.


2. Franosch JMP, Hagedron HJA, Goulet J, Engelmann J, Hemmen JLV. Wake Tracking and the Detection of Vortex Rings by the Canal Lateral Line of Fish. Physical Review Letters 2009;103:078102.