Researchers are increasingly teasing out evidence that our five basic senses add up to a whole lot more than most of us realize or appreciate. Indeed, by combining and exploiting our basic senses in novel and unexpected ways, humans perceive far more than they can simply see, taste, touch, smell or hear, scientists say.
- A University of California Riverside psychology professor and others have documented the ability of blindfolded volunteers to employ bat-like echolocation to move successfully within unfamiliar spaces and identify shapes simply by the sounds moving around them.
- A Florida-based institute is working on a helmet equipped with cameras, sonar and other equipment that would transmit images and other data to the brain - via the tongue.
- A Canadian researcher says he has empirically tested the presence of a "sixth sense" - the ability to detect change or danger without conscious awareness. At the same time, researchers in St. Louis report possibly finding the part of the brain where the sixth sense resides.
BAT PEOPLE
In his 1974 essay "What Is It Like to Be a Bat?" philosopher Thomas Nagel observed that bat sonar, or echolocation, was so unlike any human sense, so beyond our experience or imagination, that we could never really answer his question.
Nagel was wrong. There is growing evidence that humans possess at least a rudimentary ability to echolocate - that is, to interpret sounds bouncing off surrounding objects to discern one's place.
The phenomenon, of course, is well-established among blind people, but it also has been documented among sighted people, especially after a little training.
"There's nothing categorically different with the brain or sensory perceptions of blind people," said Lawrence D. Rosenblum, a professor of psychology at UC Riverside. "They don't have or develop new mental processes. They simply (echolocate) better by virtue of having more experience."
Rosenblum and others have conducted experiments in which blind and sighted but blindfolded subjects are asked to walk toward a movable wall placed at varying distances from their starting point. The subjects are told to signal when they begin to sense the presence of the wall and to walk as close as possible to the wall without touching it.
Blind participants typically perform well, often detecting the wall several feet before contact and moving within a few inches without touching. Sighted but blindfolded subjects tend to start out shakily but improve with training.
In one experiment, sighted subjects were able to detect walls 36 to 144 inches in front of them simply by listening to sounds bouncing off of the surfaces.
"We are much more sensitive to everyday aspects of acoustics than we assume," said Rosenblum. "Test subjects usually begin by saying there's no way they will be able to do what we ask, but usually they do quite well. We use sound in ways we're not consciously aware of."
In a different set of experiments, Rosenblum asked blindfolded subjects to identify various rooms on campus - a bathroom, the gym, a walk-in closet - by the ambient sound. Or identify the shapes of objects placed in front of an array of speakers emitting white noise. The participants were remarkably accurate.
"It's all about hearing the silent world," said Rosenblum. "Lots of things, like an empty room, don't make a sound, but they do structure it. They give it shape, which people can see without seeing. I have had students listen to sounds broadcast between two boards and be able to tell me whether there was enough space between the boards for them to fit through."
Paul Bach-y-Rita has a favorite saying: "We don't see with our eyes."
What the University of Wisconsin neuroscientist means is that vision is fundamentally the product of the brain: Light enters the eyes, gets converted into electrical signals and sent, via the optic nerve, to the brain's primary visual cortex for interpretation. The eyes are just tools to do the job.
But they are not the only tools. For much of the past 40 years, Bach-y-Rita and others have explored and exploited the brain's amazing plasticity when it comes to sensory input.
Early in his career, Bach-y-Rita designed a glove for leprosy patients who had lost the sense of touch in their hands. The glove was equipped with transducers in the fingertips that transmitted pressure signals to electrodes attached to the forehead. Patients reported being able to distinguish between smooth and rough surfaces with their hands, quickly forgetting where the information was actually coming from.
In the 1980s, Bach-y-Rita attached a microphone to a belt, which vibrated in different ways to reflect various detected sound frequencies. Deaf users said the belt helped them better read lips.
Most of Bach-y-Rita's recent work has focused on using the tongue as a surrogate for eyes. The neuroscientist has tested devices that convert camera images into patterns of electric impulses that can be felt when a flexible cable is pressed against the tongue, which has more tactile nerve endings than any part of the body other than the lips.
The results are far from perfect vision. The electric signals produce only a crude tingling sense of what's being seen. But blindfolded volunteers reportedly have been able to grab objects placed in front of them, catch rolling balls and even recognize large letters on a poster.
"The tongue is an excellent pathway to the brain," said Bach-y-Rita. "It's rich in nerve endings and highly integrated into the brain. And it's the brain, really, that is important. It's the brain that's making sense of the information."
Bach-y-Rita's pioneering work is being carried forward by researchers at the Institute for Human and Machine Cognition in Pensacola, Fla. With funding from the Department of Defense and elsewhere, institute engineers are trying to create wearable devices that would amplify the senses of soldiers and other users.
Marines, for example, might someday wear helmets with cameras providing 360-degree night vision. Ocean divers could wear small sonar systems offering a better "view" into dark or murky waters. In both cases, the visual information would be delivered to the brain through a mouth cable tipped with 144 microelectrodes.
Current technology is still a long way from making those efforts a reality, but Anil Raj, an institute research scientist, said early prototypes have been intriguing. In tests, Raj said blindfolded volunteers (including him) wearing tongue-attached "Brain Ports" have "seen" basic objects.
"I have recognized my hand reaching out to a cup on a table," said Raj. "It's all in gray scales, pretty rough shapes and movement. The brain does a lot of filling in the gaps, making the picture complete, especially when you add other sensory input like touch.
"But we expect to get better with higher resolution imagery."
SIXTH SENSE
People talk about the five basic senses, but clearly there are more. The question, of course, is how many?
Conservative estimates put the number at 10 or so, and include such abilities as sensing temperature and blood pressure. More radical notions add things like joint position, body movement, blood-sugar levels and feelings associated with a full bladder, empty stomach or thirst. Some researchers argue that humans may possess 30 or more senses.
No sense is more mysterious than the so-called sixth: the inexplicable awareness that something is about to happen or something is not quite right.
At one time or another, most people say they have experienced this sensation. It has been suggested that a sixth sense is the sum of the other five senses combined.
Ronald Rensink, a professor of psychology at the University of British Columbia, suggests, however, that it's more likely the sixth sense is a mode of unexplained, unconscious visual perception.
Rensink calls it "mindsight," a still largely unexplained ability to sense a change but not actually see it - at least initially.
In experiments designed to detect mindsight, Rensink has shown volunteers a series of photographic images on a computer screen, each lasting for a quarter of a second, alternating with a brief blank gray screen.
Sometimes the images were unchanged throughout the sequence. Sometimes Rensink inserted a slightly different image. Volunteers were asked to press a button when they had a "feeling" something was different and again when they actually saw or recognized the difference.
Most of the participants pressed the button only when they actually noticed a change, but about one-third reported sensing a difference in the image before they could specifically identify it.
To test whether they were simply guessing, Rensink ran control experiments in which the images remained the same. The same volunteers confidently noted no image change in the control experiments.
Rensink says mindsight is not a simple precursor to normal visual perception because it isn't related to when a person detects a change and when he or she identifies its cause. Sometimes the identification happens simultaneously; sometimes it takes a few moments.
Rensink speculates that what's happening is that some visual clues aren't being processed as vision per se, but rather are triggering other perceptual systems in the brain.
"I would define it as a mode of visual perception, a type of vision that results in a 'gut feeling' or 'sense' of something happening," he said.
"It's not really a sixth sense because you still need light entering the eyes (as stimulus). But what you're getting isn't an image, but a feeling. You can't see it, but you know something has changed."
The phenomenon, said Rensink, suggests that the conscious mind isn't the only thing dictating our perceptions and reactions. Some decisions, it seems, are coming from parts of the brain not directly associated with conscious thought.
"Humans appear to have a highly effective 'pattern-matching' system that is unconscious but highly intelligent," Rensink said. "These unconscious systems often communicate with the conscious mind by such gut feelings.
"The feelings are sometimes incorrect - and so need to be treated cautiously - but they often contain useful information and should be respected. We shouldn't disregard them as 'just a feeling.'
Researchers at Washington University in St. Louis think they may have identified the likely source of that feeling in the brain.
Located near the top of the front lobes and along the walls that divide the left and right hemispheres, the anterior cingulate cortex (ACC) is believed to help mediate between fact-based reasoning and emotional responses, such as love and fear.
In a paper published in 2005, Joshua Brown, a research associate in psychology, and colleagues proposed that the ACC also warns of dangers not yet consciously recognized.
The conclusion is based in part on a series of experiments in which volunteers were given mental tasks while attached to brain-imaging equipment. The tasks, such as quickly pushing correct buttons at specific cues, created mental conflicts.
The scientists noted that, as the level of difficulty and conflict increased, the ACC became more active, appearing to direct responses even when the participants were consciously unaware of the cues.
"Our brains are better at picking up subtle warning signs than we previously thought," Brown said.
"In the past, we found activity in the ACC when people had to make a difficult decision among mutually exclusive options, or after they made a mistake. But now we find that this brain region can actually learn to recognize when you might make a mistake, even before a difficult decision has to be made."
