Sixth Sense in Vertebrates
When we think about senses, we consider one of five possibilities - vision, smell, taste, touch and hearing. Instead, a group of vertebrates communicate through different ‘telepathic’ mechanism that can be described as the sixth sense.
Two weeks back, we stopped by at Fred Hutch Research center and met Matt Arnegard, a very smart post-doctoral researcher working on genomics of stickleback fishes. Matt told us fascinating stories about his past life, when he backpacked in Africa to learn more about a class of fish communicating using ‘sixth sense’. Readers speaking Russian can enjoy Matt’s article on sixth sense here, while others can enjoy his wonderful video -
How does this ‘sixth sense’ work? It is a method of communication between electric fishes, who emit electric pulses from their body. It was found that when two electric fishes emitting pulses of similar frequency came close to each other, one of them raised its frequency and other one lowered its frequency to avoid jamming of their sense of electroreception.
Eigenmannia and other weakly electric fish all use active electrolocation they can locate objects by generating an electric field and detecting distortions in the field caused by interference from the object. Electric fish use the electric organ to create electric fields, and they detect fields using special electroreceptive organs in the skin.
All fish that perform JAR are wave-discharging fish that emit steady quasi- sinusoidal discharges. For the genus Eigenmannia, frequencies range from 240 to 600 Hz. The EOD frequency is very steady, typically with less than 0.3% variation over a 10-minute time span.
If a neighboring electric field is discharging sinusoidally close to the fish’s EOD frequency, then it will cause sensory confusion in the fish, jamming it and preventing it from electrolocating effectively. Eigenmannia typically are within the electric field range of three to five conspecifics at any time. If many fish are located near each other, it would be beneficial for each fish to distinguish between their own signal and those of others; this can be done by increasing the frequency difference between their discharges. Therefore it seems that the function of the JAR to avoid sensory confusion among neighboring fish.
To determine how close the stimulus frequency is to the discharge frequency, the fish compares the two frequencies using its electroreceptive organs, rather than comparing the discharge frequency to an internal pacemaker; in other words, the JAR relies only on sensory information. This was determined experimentally by silencing a fish’s electric organ with curare, and then stimulating the fish with two external frequencies. The JAR response, measured from the electromotor neurons in the spinal cord, depended only on the frequencies of the external stimuli, and not on the frequency of the pacemaker.
For more details, please check jamming avoidance response and neuroethology. Also, Matt has several technical articles on related topics in his website.
