A biomedical scan of a neurogenic field
A neurogenic field is an energy form produced by certain lifeforms, capable of inducing extremely realistic illusions in the minds of others.
The crew of the USS Voyager encountered such a phenomenon twice in their journey through the Delta Quadrant. The first such instance was in 2375, during an encounter with a unique species which existed in a natural state of unconsciousness. While asleep, this species produced a neurogenic field, which they could also induce in others, trapping them in a state of collective unconsciousness, as they did with the Voyager crew. While under the influence of this field, the crew experienced an extremely convincing and collective dream, which they all experienced from their own point-of-view. Commander Chakotay and The Doctor, as the only crewmembers left unaffected, were able to locate the planet on which this species was hiding, and successfully deactivate the transmitter which they were using to broadcast the field, freeing the Voyager crew from the dream. (VOY: "Waking Moments", "Shattered")
The Voyager crew encountered a neurogenic field once again in 2376, this time produced by a large bioplasmic organism, which they dubbed a "telepathic pitcher plant." Generating a neurogenic field, the organism induced illusions in the minds of the crew, making them believe that they had discovered a wormhole back to the Alpha Quadrant when, in actuality, they were flying directly into the creature's digestive tract. The crew was eventually freed from this illusion when Seven of Nine, who was unaffected, was able to pilot Voyager away from the creature, and outside of the influence of its neurogenic field. (VOY: "Bliss")