Stripping Down Science (19 page)

Being called ‘blind as a bat' is usually a term reserved for people who could miss an elephant at five feet, trip over their own toes or even fail to notice their own spectacles sitting on their nose. But it turns out that this is a rather short-sighted insult, because new research reveals that some bat species actually have quite good eyesight.

Bats come in two types:
little
, known as the Microchiroptera, which hunt at night using echolocation and are insectivorous, and
large
, the Megachiroptera or fruit bats (flying foxes), which also forage at night but are sometimes active during the day and at twilight. Traditional bat wisdom, going back over 100 years and based on microscope studies, says that – unlike humans, who have both rods and cones in the retina to enable us to see reasonably in the dark (with the rods) and in colour during the day (with the cones) – both of these bat groups have a retina containing only rods. And since rods produce generally poor visual acuity, and they can't discriminate between different colours, bats have
repeatedly got the gong for being the optically challenged members of the animal world and an ophthalmic insult coined in their honour.

For the night-active and insectivorous Microchiroptera bats, this is probably not an unreasonable conclusion, although having poor vision isn't really a problem when you're blessed with a built-in sonar most submariners would kill to get their hands on. But flying foxes don't use echolocation like this, and they aren't always only active at night either. Researchers have reported that, during the day, they keep a constant eye out for predators, groom and interact with one another, occasionally rearrange their roosting sites to stay out of the sun and, from time to time, younger individuals use the daytime for flight training. Meanwhile, other studies have shown that these animals devote a large region of their brains to visual processing and have forward-facing eyes that fixate on objects of interest.

None of these behaviours seem to square with an animal that is supposed to have limited night-sight and be virtually blind in daylight, which is what prompted German researcher Brigitte Muller and two colleagues
⁶⁴
to blink and then
take a closer look at what was going on in these animals' eyes. They examined specimens from six different species of fruit bats that had been killed for other purposes. To see what was going on in the retina, they used colour-coded antibodies that could distinguish the different chemicals, called opsins, which are contained in rods and cones where they help to turn lightwaves into brainwaves.

Predictably, lots of the cells picked up by the antibodies were long thin rods. In some cases there were nearly one million of these per square millimetre of retinal tissue. But amongst these were cells lighting up that were clearly cones. They were rare, comprising only about one in every 200 rods (0.5%), but sufficiently numerous to mean that these bats can potentially see better than most rats.

The cones the scientists saw were actually made up of two different populations. There were L-cones, which see green, and in three of the bat species, an additional rarer type of cone called an S-cone, which can see blue light. This means that some of the fruit bats studied are neither blind nor colourblind. So next time you need to insult someone's eyesight, consider ‘blind as a rat' as a
more accurate alternative to mocking the humble bat's optic abilities!

FACT BOX

How nocturnal eyes use DNA to make light work of seeing in the dark

Mammals like bats, cats and bushbabies, which are all active at night, face a significant challenge when it comes to making their eyes work, because compared with day-active animals, nocturnal species generally have less than one-millionth of the amount of light to see by. But recently, scientists have shown that these animals solve the problem in a very unusual way: by using the optical properties of their own DNA.

A major anatomical issue that affects the eyes of all of us is that our retina is effectively ‘inside out'. In other words, the energy-hungry rod and cone cells that detect light are positioned closest to the blood vessels at
the back of the eye so that they can pick up sufficient oxygen to meet their needs. But this means that, to reach them, light first has to filter through several layers of supporting cells that are positioned closer to the front of the eye and whose job it is to transmit the information from the rods and cones back to the brain. And because nocturnal animals are working under such low light levels, this can seriously hamper vision.

So what's the solution? By examining rod cells from nocturnal animals under the microscope, Boris Joffe, from the Ludwig-Maximilians University Munich,
⁶⁵
noticed that it's not just the retina that is inside out – the DNA inside these cells is too.

With a few exceptions, every mammalian cell contains a complete copy of the animal's genome, all packed into about two metres of DNA which is itself tightly coiled inside the nucleus, a button-shaped structure at the
heart of the cell. Normally, this nuclear DNA is organised such that the parts of the genome being used by the cell are clustered centrally in the nucleus, and the inactive material (including the so-called junk or non-coding DNA) is kept in a tightly wound form, called heterochromatin, around the outside. But in the rod cells of night-active animals, this situation is reversed. The dense, inactive heterochromatin is in the middle of the nucleus while the active genetic material is arranged around the outside. Daytime-active animals like humans, on the other hand, lack this specialisation.

The big question, of course, is why? It turns out that tightly coiled heterochromatin DNA has a much higher refractive index, meaning that it can focus light more powerfully than the more open structure of the active DNA. This means that, in nocturnal animals, the centres of the rod cells behave like miniature lenses, cutting down scattering and helping to funnel photons (light) into the light-sensitive tips of the rod cells. Without this effect, computer models show, the light paths diverge across
much larger areas of the retina, reducing the likelihood of detection and cutting sensitivity.

Elegant as it is, when first proposed the idea proved so radical that some members of the scientific community took quite a bit of convincing. ‘People laughed at first,' says Joffe. But the argument does make compelling evolutionary sense. All nocturnal mammals have this DNA adaptation in their eyes, which means that it must have arisen early on during mammalian evolution, probably in the region of 100 million years ago.

At this time, mammals were just beginning to appear and most likely adopted a nocturnal lifestyle to enable them to escape the clutches of carnivorous reptiles which, being cold-blooded and reliant on heat from the sun, would have been less active at night. But diurnal (day-active) animals, like humans, have lost the trait again during our evolution because it confers no benefit under the high-light conditions for which our sight is best adapted.

James Joyce once famously described mistakes as ‘portals of discovery', but now scientists have discovered that when it comes to tip-of-the-tongue moments, the claim that you don't make the same mistake twice is a myth. In fact, in this situation, one mistake begets another.

Most people agree that there are very few things more frustrating than the excruciating sensation of knowing you know a word for something but can't for the life of you remember what it actually is. It's like having an intellectual itch you can't scratch and it continues until either you manage to remember the word you wanted, or someone puts you out of your misery and tells you. The sense of relief you experience when they do is so totally overwhelming that you honestly can't believe that you'll ever forget that word again. But then you do, the very next time you want to use it!

Don't believe me? Well, anecdotally, that's what
people – and scientists – are saying (assuming they can find the right words, of course). For many years, scientists had thought that the act of successfully surmounting a tip-of-the-tongue moment should ensure that the lost word would be better remembered in future. Indeed, the American psychologist Edward Thorndike wrote in 1913, ‘When a modifiable connection between a situation and a response is made and is accompanied or followed by a satisfying state of affairs, that connection's strength is increased. When made and accompanied or followed by an annoying state of affairs, its strength is diminished …'

So why do tip-of-the-tongue experiences just keep happening, then? According to Amy Warriner and Karin Humphreys, two researchers at Canada's McMaster University,
⁶⁶
it's because rather than learning
from
our mistakes, in this situation we actually end up learning how better to
make
the mistake itself!

The research duo reached this rather unnerving conclusion when they subjected 30 student volunteers to a barrage of 1500 potential tip-of-the-tongue opportunities. The students were presented
with statements on a computer screen provoking them to recall certain words. These statements were carefully crafted to elicit tip-of-the-tongue experiences, which is best done by focusing on seldom-used words or expressions. As a control, some fake words were included to make sure the volunteers were playing by the rules.

After reading each statement, the participants were told to press one of three buttons: ‘Know', if they knew the word the scientists were after, ‘Don't Know', if they didn't, or ‘TOT' (tip-of-the-tongue) if they knew they knew the word but couldn't articulate it. Whenever they experienced a TOT, the team then let them suffer for either 10 seconds or 30 seconds before telling them the word they were looking for.

Two days later, the students repeated the test using exactly the same statements. If Edward Thorndike was right, the words that had tongue-tied the participants first time round ought to be a doddle a day or so after. But in reality, many of the volunteers experienced TOTs again on the same statements that had tripped them up two days before. Even more shocking was that in cases where previously a student had been made to wait 30 seconds before being told the word
they were looking for, they were 50% more likely to experience a repeat TOT than with words for which they had been made to wait only 10 seconds.

The researchers think that this is because the intellectual exercise associated with trying to retrieve the TOT word actually makes the brain learn to forget it. In effect, the nerve connections that would normally lead to the retrieval of the correct word have been misrouted. And so, by repeatedly activating this aberrant neurological pathway, the brain is making it stronger and therefore more likely to cause the same problem again. ‘Music teachers know this principle,' says Humphreys. ‘They tell you to practise slowly. If you practise fast, you'll just practise your mistakes.'

So what should you do to deal with an aggravating TOT? Don't keep racking your brains for the right answer, say the researchers. Quickly ask a colleague for the missing word, or look it up on the internet. Then repeat it to yourself, out loud or in your head. But don't dwell on it if the answer's not forthcoming. Otherwise, Humphreys points out, ‘You'll keep digging yourself the wrong pathway.'

FACT BOX

Tasty TOT titbit

Tip-of-the-tongue experiences have also been researched recently in volunteers with a rare form of a condition called synaesthesia which causes them to experience specific tastes in their mouths when they are about to say certain words (and not just the names of foods either!). Characteristically, the same words always trigger the same flavour sensations.

This is known as gustatory synaesthesia and scientists think that, in people who have it, there are extra connections between the brain area that processes speech and the brain region that decodes flavours. Consequently, when a word is selected for speech it also causes the brain to fire off a flavour sensation. For these people, conversation can feel like quite a mouthful …

But two UK researchers, Julia Simner from Edinburgh University and UCL scientist Jamie Ward,
⁶⁷
wondered what would happen to these
individuals – who are otherwise neurologically normal – if they were to have a tip-of-the-tongue experience? They asked six gustatory synaesthetes to look at a series of pictures of rare or unusual objects designed to elicit tip-of-the-tongue moments. Intriguingly, whenever this happened in the subjects, although they couldn't say the word they were looking for, they could still ‘taste' the word they wanted. When the study was repeated on the same patients using the same images a year later, many of the subjects had TOTs on the same words as they had before – and reported the same flavour sensation.