How Does Daredevil's Sonar Really Work?
Pop culture is full of superheroes with incredible powers, especially in the summer. But none of these are as amazing as the superpower within your brain. “If we could splice the outer extremity of our optic nerves to our ears, and that of our auditory nerves to our eyes, we should hear the lightning and see the thunder.”-William James, 1892
The latest foray of Marvel Comics into television is the Netflix series, Marvel’s Daredevil (or, if you’re like me, just “Daredevil”).
Figure 1. The Man without Fear.
Daredevil is the story of Matt Murdock, a young boy who saved a pedestrian from being struck by a truck hauling toxic chemicals, but at the cost of being blinded when cannisters on the truck ruptured. Though blinded, his remaining senses were enhanced in a way that provided him with superhuman abilities. Otherwise, Daredevil’s story draws from many superhero tropes, especially the Netflix version, which borrows heavily from Frank Miller’s influence on the comic series. These include the loss of parents, a thirst for justice and some awesome martial arts abilities. In the Netflix series, Daredevil’s main super power appears to be the ability to dish out and to survive brutal beat downs.
Is there any truth in Daredevil’s fantastic sensory abilities?
In the comics, Daredevil’s “radar sense” (actually a form of biological sonar) is portrayed as a compensation for the loss of sight in the accident that took it. And in fact, compensatory changes in brain function can occur in remaining sensory brain areas when a sense is lost. To understand how this can happen, we need to look to one of the brain’s superpowers: neuroplasticity.
In what way are sensations “plastic”? Vision is so important to primates that visual signals are processed by about a third of the entire cortex through myriad brain areas. The first of these areas to analyze vision is in the occipital lobe at the back of your head. Early experiments in the visual system by David Hubel and Torsten Wiesel demonstrated how depriving one eye of sight could rewire regions of the occipital cortex so that it would ignore signals from the closed eye, and would instead respond selectively to the open eye. A normal balance between signals from the two eyes during development sets the stage for binocular vision and a normal sense of depth. But when the process is disrupted, it can produce the condition called “lazy eye” or amblyopia.
Does this neuroplasticity extend to the extreme case of rewiring entire senses, as suggested by William James? When the inputs of one sense are rerouted to the part of the brain providing the real estate to process a different sense, it can reveal the unique roles of nature and nurture in brain development. In animal studies, Mriganka Sur, Doug Frost and others demonstrated that the cerebral cortex can be dramatically remodeled, including rewiring of primary auditory (hearing) or somatosensory (touch) cortices to respond to vision, which isn’t ordinarily processed by those areas.
Figure 2. After "rewiring" the auditory cortex of ferrets by surgically ablating central visual targets, Mriganka Sur and colleagues observed that neurons in the primary auditory cortex could respond to visual activation. Shown here is accumulated spiking activity arranged as a clock, with the direction of motion of a barlike stimulus indicated by the small arrows. Tuning is indicated by the "lobes" of the central plot. Cells were observed that were tuned to the orientation of the bars (A) and in some cases cells possessed a preferred motion (B) as one would find in visual cortex - but this is the primary auditory cortex. Figure from Roe et al., 1992.
So at a very basic level, the brain possesses a degree of plasticity, most evident during early development, that can make fallow cortical areas available to other senses. It’s a tantalizing hint at the truth underlying Daredevil’s unique abilities.
Do blind humans also rewire their cortex? There certainly is something different about the visual cortex in sighted and blind people. In individuals blind from a young age, activity in visual cortex appears to ramp up in response to attentional demands. Congenitally blind people show similar attentional differences in the frontal eye fields, a brain area that would assist with planning eye movements in the sighted. The blind appear to use visual cortex for navigation in ways that the sighted do not.
Interestingly, the use of visual cortex for “visualization” may extend to touch, and studies suggest that this can be learned. Test subjects (blind, and sighted but blindfolded) who learned to visualize a held object had detectable activation of the visual cortex, but those who didn’t visualize the object showed no such activation.
This extends to symbols - Braille readers are disrupted in their ability to read when the visual cortex is blocked by transcranial magnetic stimulation, and can even feel tactile sensations when the occipital cortex is stimulated with TMS.
It may be that the brain may find new uses for these "cross-modal" sensory pathways – rewiring the brain to serve its remaining sensory functions, or revealing existing pathways that are not used as much in the sighted.
Is this sensory rewiring even useful? What sorts of information would be available through reliance on a world forged of sound? Ask a bat. Better yet, refer to the work of Nobuo Suga, who asked them for you. Suga studies echolocation, a special partnership between vocalizations and hearing that is used by bats (an ability also possessed by dolphins) that allows them to construct a rich representation of their environment for navigation and hunting in total darkness.
In the same way the primate brain is tricked out for vision, the auditory system of Parnell’s mustached bat is configured for echolocation. Starting in the cochlea, the snail-like bones within the bat’s inner ears, there’s a resonance for a particular ultrasonic frequency of 61kHz (Hz stands for Hertz, the cycle of a sonic sine wave per second). This frequency (and several harmonics) is emitted by the bat’s vocal cords. The bat is sensitive to the echo of these chirps. In fact, the cortex of its tiny brain is specialized to process the intricate features of the soundscape. Some areas are active during range finding, and one area, called the Doppler shift constant frequency area, is like an auditory fovea – so sensitive that it helps the bat detect wing beats of moths and the texture of surfaces, and to subtly adjust its chirps to optimize the information in the echo. And it does all of this while hurtling through the air in total darkness. Very Daredevilish.
Figure 3. The auditory cortex of the mustached bat. A. The bat's cortex possesses specialized processing domains for biosonar, including range finding and feature detection. One of these, the Doppler Shift Constant Frequency (DSCF) area (shown in Aa), is highly sensitive to the second harmonic of the pulse echo (CF2 in B). From Fitzpatrick at al , 1993.
Some humans have also learned to use a trick similar to that of the bat to navigate. These human echolocation experts can click their tongues and sense the return echo, building a model of the world from sound. It’s unlikely to be as detailed as a bat’s, but it is a remarkable testament to the plasticity of the brain. This human form of echolocation, like the passive navigation responses and response to touch, also appears to reside in the visual cortex.
Figure 4. Bat Echolocation. From Scientific American.
What’s the difference, then, between Daredevil and your blind neighbor? The big difference is that unlike typical human echolocation, Daredevil is picking up ambient vibrations and has a form of super hearing that is well beyond any known sensory enhancement reported by any blind person, including identifying people by their heartbeats, using his senses as a human lie detector, and reading newsprint with his fingers. I guess we have to chalk it all up to the radioactive goo from that truck. As for those who are blind - by now, I hope I’ve convinced you that they don’t need super powers to be amazing.
What's Daredevil’s real secret? Maybe it's that he’s a little less like a devil and a little more like a bat, man.
[Note: Daredevil and all Marvel characters and the distinctive likeness(es) thereof are Trademarks & Copyright © Marvel Characters, Inc. ALL RIGHTS RESERVED.]
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