Neuroscientists offer some surprising results about how the brain finds its way around.
Teleportation turns out to be an easy thing to simulate, neurologically speaking. The rhythmic brain patterns that appear in real-world spatial navigation reappear as subjects navigate virtualized environments on a screen, and, while real-world teleportation is not a thing that exists, in a virtual environment it's just a matter of suddenly dropping a subject in new surroundings. Click.
Thanks to neuroscientists at UC Davis, we now have some idea of what the neurological response is to such an abrupt change. In a study published Thursday in Neuron, the group describes virtualized navigation experiments involving three epilepsy patients who had been previously wired with electrodes inside of their brains for seizure-monitoring purposes. The Davis group was able to "borrow" these electrodes to observe deep-brain neurological activity relating to navigation and memory that would be otherwise impossible to observe with more typical, non-invasive external electrodes.
The neuroscience of spatial navigation is an elusive thing. Observations in rat models have found a rhythmic neural firing deep down within the brain's hippocampus that is clearly part of navigation and related memory functions, but how this firing relates to sensory input has remained mysterious. Based on observing neurological activity from the hippocampus in simulated teleportation events, it appears that these neural oscillations may not have anything to do with sensory input at all, a finding that goes against explanations offered by most neurological models.
"The idea that vestibular/proprioceptive [sensory] input is fundamental to how we code space has dominated in the field of spatial navigation for several decades," Arne Ekstrom, the current study's lead author, told me. "Our results fundamentally challenge this viewpoint and thus require revision of models we have assumed to be correct for quite some time."
The upshot is that navigation persists even when the brain is deprived of new "live" information about its surroundings. That's pretty weird. When all of its inputs go dark, the brain is able to continue on thanks to memory-related signals that allow it to continue updating its position in space. For one thing, the teleportation experiments found that subjects suddenly teleported from place to place within a virtual maze, with a period of sensory darkness in between, could differentiate between small and large distances traveled. Simply: the hippocampal rhythm changed with distance.
"Critically, [teleportation] allowed us to test whether low-frequency oscillations are present when sensory feedback has been removed and explicit motor signals are absent," the paper explains. "If sensorimotor processing is the primary driver of low-frequency hippocampal oscillations, then they should be disrupted or greatly attenuated during virtual teleportation. In contrast, if this oscillatory signal is important for memory-related spatial coding, then low-frequency oscillations should persist during teleportation." The latter of the two possibilities seems to be it.
The experiment went like this. The subjects were each placed within a virtualized maze consisting of four arms, with a virtual store of some sort at the end of every one. They were then instructed to navigate to a particular store. Once they found the store, they were told to enter one of two types of teleporters located at various distances from that store: short- and long-range. Both returned the subject to the center of the maze, following a gap in which the subject just saw a black computer screen.
All the while, the researchers were monitoring the subject's hippocampal activity. Crucially, what they observed was a continuation of neural activity related to navigation as the subjects were faced with the empty screen, e.g. as they were being teleported. To confirm that these teleportation oscillations were indeed related to movement, the neuroscientists employed a control experiment in which subjects were monitored during fake teleportation events, e.g. when they were faced with a black screen outside of the teleporter context. In these cases, hippocampal oscillations diminished as expected.
So, now we're left with a very interesting question. What exactly is encoded in these oscillations? We don't know. Sorting that out will take a lot more work, but for now we can at least be awed once again by the processing powers of the human brain-machine.