The basics of echolocation are fairly simple to understand, and it works much like man made sonar technology. Select animals, like bats, shrews, dolphins and whales, produce high frequency sounds that travel outward toward the animal’s immediate environment. Those sound waves echo, or bounce back off objects and structures in the environment, and the echoes are interpreted by the animal into “images” of the surroundings. Here’s a very basic picture of echolocation:

According to Wikipedia, animals that use echolocation have their ears positioned apart, so that the echoes return to each ear at a different loudness and a different time. The animal uses these different cues to formulate the “image” that it can thus interact with. Animal echolocation is very detailed in its resolution, and animals with this ability can detect not only the position of objects in the environment, but size, shape and other details that allow for all sorts of interaction.

I focused on the echolocation process in bats before I examined the phenomenon in humans. Mark Nelson, a neurologist from the University of Illinois, Urbana-Champaign, explores echolocation in mustached bats, a species of microbat (suborder Microchiroptera). Nelson explains that bats can produce sounds at frequencies above 30KHz, well out of the range of normal human hearing. Mustached bats use these sounds during about 80% of the time they are echolocating, classifying their sonar as a “high duty cycle.”

Bats will emit their sonic pulses very rapidly to detect the echo from surrounding objects. When approaching prey, for example, bats will sense that the echoes occur more quickly, which enables them to follow prey accurately.

Of course, not all bats use the same types of sounds to echolocate. The mustached bat is bisonar; that is, it uses two types of sounds: constant frequency (CF) and frequency modulation (FM). The sound begins with an extended CF portion that ends in a downward FM “sweep.” These two parts of the pulse apparently serve distinct purposes. Nelson explains:

The constant frequency portion of a pulse is great for detecting targets and measuring the Doppler shift. The FM portion of a pulse is excellent for honing in on the distance of an object and some of its finer details. This choice between CF and FM may also have to do with the conditions in which the bat hunts. The mustached bat often hunts in vegetation and the CF component of the pulse may help this bat focus in on insects moving within the vegetation.

One explanation of the abilities of this highly-developed system is the fact that mustached bats emit these sounds at the first, second, third, and fourth harmonic frequencies. The second frequency echo, according to Nelson’s account of Nobuo Suga’s research, is excellent for the mustached bat’s perception of the Doppler shift, as there exists a specialized portion of the auditory cortex that deals with the second frequency.

Because bats are mammals, their auditory systems are somewhat similar in structure to those in humans. In the case of echolocation, the echoes travel into the ear, through the cochlea to vibrate the basilar membrane, which is designed to respond best to frequencies between 61.0 and 61.5 KHz, frequencies that fall within the range of the second harmonic frequency from echolocation.

Once the neural messages reach the auditory cortex in the brain, different sections hold specific neuron-types that process the different aspects of the sound and echo. The FM-FM and CF-CF areas process the combinations of emitted sound and echoes of their respective types, and both areas have even more specialized neurons that respond to specific relationships between sound and echo, including velocity and delay. All of these neurons are considered combination-sensitive neurons. In addition to these two areas is the Doppler shifted constant frequency (DSCF) area, which responds only to the echoes of CF sounds. This is the area of the cortex where the echoes of the second harmonic frequency are processed.

So how does this relate to human echolocation? The phenomenon has only been studied since the 1950s, and very few people learn to do it. The process involves perceiving echoes much in the same way that bats and other animals use it, but because humans hear at a comparatively lower frequency level than these other animals, the perception is much less detailed. While there have been some instances of the use of canes to create the sounds, the best human echolocation comes from a system developed by Dan Kish.

Kish, who works for the non-profit World Access for the Blind, developed an echolocation system for humans using tongue clicks, which is much closer, though still very primitive, to the sounds emitted by bats. Kish has taught blind people the system, including teenager Ben Underwood, who we spoke about in class.

Ben was diagnosed with retinal cancer at age three, and has been blind since. After discovering his ability to use clicks to “see” objects around him, Ben worked with Dan Kish to enhance his ability. Now, Ben can play video games, walk down the street, surf, and play foosball using his echolocation skills. The video gives some cool examples of Ben using his clicks.

The neural process is probably not as complex as in bats, where the brain is specifically wired to perceive spatially through sound, but subjects like Ben may give some insight into the adaptability of our brain to almost create its own processes as they are needed.

The creators of the video used computer animation to recreate motion parallax in a process that is called parallax mapping. I’ll admit, I really have no idea exactly how video game designers actually do this, but I do know understand what it does. Mapping enhances the texture of flat images rendered on a computer, like the rocks you see in the video. The movement in the video demonstrates where the details closer to the apparent viewpoint are moving faster than those further away.

Parallax mapping is apparently not used in much 3D rendering for video games anymore, but it is amazing how video game designers use these cues to create the illusion of depth and texture. It enhances the game play, making it more realistic as a player moves through a virtual world.

I found the concept of categorizing color very intriguing. Unlike some of the models we have seen for other senses, like the Henning prism for smell, this one has a good deal of weight based on the empirical evidence. Known as the HSB cone for measuring the hue, saturation, and brightness of a color, it is alternatively known as HSL (with lightness), HSV (value), or even HSI (intensity). For the purposes of our course, we have worked with the HSB cone.

I think this is a great visualization of color variance, and it highlights and explains some of the subtleties of color that are hard to grasp otherwise. There are, however, other graphic representation of these color characteristics, and they follow:

This is a basic HSB circle. As the labels indicate, the three characteristics follow in the same pattern as the cone, with brightness encompassing a separate axis. I see that this is good for a two-dimensional look at color, but I really don’t see the point of using this representation over the cone. Without the context of the cone, I don’t think I would quite understand how brightness related to hue and saturation from this graphic. Graphic designers use this representation in computer programs as a way of choosing specific colors.

This graphic uses luminance rather than brightness, but as I said before, we can understand these two to be the same for our purposes of evaluating these graphics. I really like these separated discs, as if the cone we saw earlier we sliced into pieces. It allows us to see different levels of brightness (or luminance) as we look at saturation and hue. Another interesting feature is the so-called “neutral gray axis,” which I think provides a great visualization of the uniformity of complete saturation.

I have no idea how this represents HSB. Ah, the wonders of Google Images. At any rate, it is amazing to understand colors beyond the simple color wheel model we all learn in 2nd grade art class. The realm of possibilities with color beyond hue means that our perception is quite nuanced. I wonder what these graphics would look like if we could perceive wavelengths beyond those that we can. Exit questions: Would the entire layout break down if this were possible? Would infrared or ultraviolet “colors” completely destroy our theoretical understanding of color in these characteristics?

We examined the tests done through the visual system, from the retina to the cortex, and Mary pointed out that while at birth there are some underdeveloped aspects of the retinal neurons, the supercolliculus, and the lateral geniculate nucleus, the affects of these immaturities are eventually corrected within about 2 to 4 weeks (in macaque monkeys). This means that the “answer” to the visual development question lies in the cortex.

Within the cortex, we looked at V1 and V2, discovering that the neural infrastructure is all there, though a little undeveloped. Since little about the higher aspects of the cortex has been studied, most researchers seem to conclude that there is a combination of natural and experiential reasons for the delayed development of vision in infants. In layman’s terms, the parts are all there, but they need to do some work before they work perfectly together.

I had a passing thought in class that this reminded me of infant bone structure in its delayed development. Upon researching afterward, I found that infant humans actually have more bones than adults, and that these fuse together as human develop. The principle, however, is still the same. The neuron wiring in the cortex is less complex in infants, but the connections branch out over time to provide the more advanced perceptive abilities for adults.

I think it’s very interesting how we still develop some very complex processes after we are born. It makes me think of how fragile those first few weeks, months, and years really are to human lives afterward.

Basically, we can view the same object in a variety of different light settings. Our ability to recognize the object as the same object in environs of various illuminations is due to an adaptation mechanism. This mechanism can require some time to kick in, which explains why we cannot see objects well when we initially enter a darkened movie theater on a sunny day. After our vision adapts, we are able to recognize objects better.

With chromatic adapation and color constancy, we recognize that an apple is red whether the illumination is the white sunlight of midday or the darker light of sunset. Despite this, chromatic adaptation can be manipulated to allow us to “see” colors that aren’t actually there, as demonstrated in the video. The colors presented in the first image condition our eyes to see the colors in the black-and-white image before we “un-adapt” and see the second image for what it is.

Check out some more of these illusions at StareClips.com.

We know from our text that rabbits, like many prey animals, have their eyes place on the lateral sides of their heads, rather than on the front as with predatory animals, including humans.

Rabbits, thus, have very poor depth perception compared to humans. Chapman explains that rabbits see only about 30 degrees with both eyes, meaning that those two wide visual fields on either side of the head are seen by only one eye. This characteristic, as we learned in class, allows rabbits to see a wider range to look for predators, the trade-off being a lack of good depth perception.

There are some ways rabbits try to compensate for this hindered ability to judge distance. Chapman mentions using relative size differences of objects to determine distance. According to the article, rabbits will also bob their heads up and down, in a similar manner to that of birds, in order to judge distance. Objects that appear to change position more are closer; this is called parallax, and rabbits take full advantage of this phenomenon of physics.

Another interesting aspect of rabbit vision is the blind spot in the middle of the visual field, which accounts for some noticeable characteristic of its behavior. According to Chapman, the spot is directly between the eyes and below the chin, and is only about 10 degrees. Still, this is the reason owners of rabbits will observe that their pets will not notice food placed directly in front of them for a little while.

Chapman mentions some other interesting facts about rabbit vision, including their lowered sensitivity to color. He says that there is empirical evidence that rabbits can distinguish between blue and green, but for the most part, the extent of a rabbit’s color vision is largely unknown.

In the Blake and Sekuler text, the authors described floaters as “small opacities that float about in the vitreous,” identifying the opacities as simply “debris”(Blake & Sekuler, 2006). It’s a definition that suffices for our class, but I wanted to know a little more about what constituted this debris.

According to the National Eye Institute, a division of the U.S. National Institute of Health, the debris isn’t dirt or dust, as we normally understand the term. Instead, the NEI explains that “floaters occur when the vitreous, a gel-like substance that fills about 80 percent of the eye and helps it maintain a round shape, slowly shrinks. As the vitreous shrinks, it becomes somewhat stringy, and the strands can cast tiny shadows on the retina.” This is the most common explanation for floaters, and the institute indicates that these are often an annoying result of aging. Floaters in this form begin as fibrils (mini-fibers) of the solid portion of vitreous, called hyaluronic acid. These are also often described as deposits of protein.
Other types of floaters occur from other disturbances of the normal processes in the vitreous. The vitreous can shrink or thicken, causing it to slowly detach itself from the retina. During the process, the retina may be stimulated, causing people to see flashes of light, appropriately called flashers. This often precedes a serious condition known as retinal detachment. Eye-Floaters.com explains that retinal tears, where the retina is literally torn or broken apart, lead to detachment, during which the retina “is lifted or pulled from its normal position.” Increased floaters over a short time indicate possible retinal detachment that can ultimately lead to a partial “curtain” over one’s vision or complete blindness.

There are other causes, the most popularly known resulting from aberrations in fetal development. According to Doctor-Hill.com, the hyaloid artery, which aids in developing the the lens in growing fetuses, disintegrates by the third trimester and can sometimes lead to deposits in the vitreous that are seen as floaters later in life.

Except in cases of retinal detachment, floaters are a harmless, albeit annoying, visual phenomenon. I find that I can (sort of) control the positioning of floaters. Since they sink to the bottom of the eye while the head is in normal position, thanks to gravity, I have found I can move my eyes into different positions and play with the positioning of floaters. Depending on how fast I move my eyes, I can change the speed that the floaters sink. Luckily, my floaters are pretty insignificant, and I really only see them after looking at a lamp or if I really concentrate on looking for them.

LASIK stands for Laser-Assisted in situ Keratomileusis, and it is a type of laser corrective eye surgery for correcting myopia, hyperopia, and astigmatism. Development of the technology began as early as 1950 with the invention of the microkeratome, a small surgical blade that can cut and reshape the cornea. After nearly four decades of research and development, Dr. Gholam A. Penyam received a U.S. patent for LASIK in 1989. The surgery was developed over the next year by Lucio Buratto and Ioannis Pallikaris. Today, there are many variations of the basic LASIK surgery as lasers have become more powerful and more precise.

LASIK surgery consists of cutting a thin flap on the surface of the eye, folding it back to expose the corneal tissue, reforming that tissue to correct the aberration, and repositioning the flap. The patient is awake and mobile during the whole procedure, but they are often put on light sedatives like Valium.

Before the operation, the surgeon uses low-powered lasers to examine the cornea’s thickness and to make a topographical map. The map allows the surgeon to determine the amount of tissue that must be reformed or removed, as well as indicating any irregularities in shape, like those that cause astigmatism.

To create the flap for the first part of the operation, surgeons place a suction ring on the cornea to hold the eye in place. Using either a metal blade microkeratome or a femtosecond laser microkeratome, the flap is cut with a hinge at one end. The flap is carefully pulled back to reveal the middle part of the cornea, the stroma. Here’s what it looks like:

(picture courtesy of www.thecamreport.com)

Next, using a 193 nanometer laser called an excimer laser, parts of the stroma are pinpointed and vaporized. Current technology allows surgeons to track the movement of the eye to help zero in on the exact points on the eye that need reconstructing. It is a very precise process, to be sure.

After the reconstruction, the surgeon carefully repositions the flap, checking to make sure it is on straight and there aren’t any air bubbles. The flap can “stick” naturally to the stroma until it naturally heals. Patients are instructed to take antibiotics and anti-inflammation eye drops for the weeks following surgery.

I thought it was amazing how small the scale is with LASIK surgery. Surgeons are dealing in nano- and micrometers on very delicate tissue. I still marvel at how we have innovated so much so that those who have poor vision from corneal and lens anomalies can see with practically 20/20 vision after LASIK. It leaves me feeling optimistic about the development of even more life-changing medical procedures in the years to come.

Reference: www.wikipedia.org

That Keller, who was both blind and deaf, would seem to value her hearing more than her sight speaks a lot for the sense. I think her point in that statement was that the ability to see, touch, smell, and taste allows us to interact with all of the objects in our lives, but that they don’t connect us to the people in our lives. Almost everything that we see is based on the physical and material properties; hearing the voices of others, however, connects us with the immaterial hearts and minds of other people. The connection we get from this ability to hear the ideas and feelings of others is a large part of what makes us human.

We depend on sound perception for so much of our human interaction. I think about how humans communicated before the written word, and it was completely through verbal communication. Storytelling was a way that cultural and religious values were passed down though generations. Philosophers and orators in antiquity were able to express their views and opinions to large groups of people; in fact, Socrates never wrote down anything, and the only records of his thinking come from the personal accounts of those who heard him speak. Church services from the past and today incorporate sermons and music to express the word of God. We listen to other people’s problems, concerns, feelings, and emotions, and we want them to do the same for us. People can understand so much subjective information from the subtleties of another’s voice, like the tone, speed, and volume.

The understanding of these immaterial aspects of our environment is possible because of our ability to hear. The loss of our sound perception means that one really is cut off from the human aspect of the perceptual world. I think what Helen Keller really wanted people to take away from her statements is that we connect with other human beings best when we listen and hear what everyone has to say.

The truth is, scientists really don’t know much about the sour taste as it is. In 2006, researchers at the Howard Hughes Medical Institute in San Diego were finally able to identify a single, specialized receptor in the tongue for the sour taste. Bitter, sweet, and umami taste receptors had been identified previously by scientists, and they all work similiarly. These three tastes are carried by large molecules, like sucrose, that made finding specialized receptors in the tongue easy to locate. Both the sour and salty tastes work differently, with these perceptions triggered by ions like hydrogen (H+) for sour tastes and sodium (Na+) for salty tastes.

While some scientists believed salty and sour tastes were perceived as a result of these ions passing over a series of cells, much in the way that smell perception passes over the various receptors neurons in the nasal epithelium, Charles Zuker sought a more precise explanation. Zuker was the head of the research team that discovered the specialized sour receptor, along with the other three known taste receptor cells.

Zuker’s team used the method of gene knockout in mice to find the sour receptor. After studying the mouse genome and eliminating unlikely possibilities, the team was able to focus on one specific protein, found in tongue tissue, called PKD2L1. Using a special toxin, the researchers were able to genetically engineer mice without this gene by killing any cells exhibiting the protein. Zuker and his team knew they had identified PKD2L1 as the sour receptors after observing the behavior of these mice. Only humans eat sour foods, and most other animals avoid them, including mice. When the mice with the knocked-out gene were presented with sour food, however, they were not averse to licking the food. More convincingly, probes in these mice’s brains indicated no specific neural activity could be identified while licking sour foods.

Despite this major advance in the understanding of the sour taste, questions still remain. Primarily, there appears no conclusive evolutionary reason for the ability to perceive sour tastes. While bitter can warn about toxic foods and sweet indicates the presence of sugar, an essential nutrient, sour only seems to indicate acidity. Unripe fruit often tastes sour, but Zucker does not believe this completely answers the mystery of sour taste.

Because not much is known yet about sour, my original question was left unanswered. There are obscure references to the question of the “pucker reflex” on a few websites and messageboards. One of the most compelling theories is that we move our mouth much in the same way as we do when we are trying to retrieve saliva. Because of the high acidity of sour foods, this reflex might be our body’s attempt to retrieve more saliva in our mouth to “dilute” the acid.

According to Elliot M. Blass in Developmental Psychobiology, the pucker reflex is one of three taste-related “gusto-facial” reflexes that newborn infants display (the others are the “disgust” reflex from bitter foods and the “smile” reflex from sweet foods). We see the remnants of these reflexes as adults, but the pucker reflex is certainly the hardest to control. Perhaps we react to sour foods as sort of a faux instinct that remains with us even after we have outgrown the others.

Exit question: What are some other possibilities for the purpose of our perception of sour, and what are some more good theories for the presence of the “pucker reflex”?