Silvery fish & the art of camouflage'
Silvery fish like sardines and Atlantic herring are masters of camouflage. A new study explains how the fish use their silvery skin to stay invisible to predators from nearly every angle.
"What these fish do is get around a fundamental law of reflection," said Nicholas Roberts, a biologist at the University of Bristol and one of the study's authors.
Typically, when light is reflected from different surfaces, any light that comes off the path of that reflection becomes polarised. That's why fishermen wear polarised vests.
Silver fish avoid this problem because of the unique makeup of their skin, which has alternating layers of cytoplasm, as well as two types of guanine crystals (which refract light). The two types of crystals have different refractive indexes that create a unique reflective property."
The polarisation happens over a range of angles instead of one, and the end product of having all the layers together is that it creates a polarisation-neutral reflector," Roberts said.
Over time, the fish have evolved to have the perfect ratio of the two types of guanine, and as a result have a near-constant reflectivity, providing them an invisibility cloak from all angles, Roberts said.
The mechanism is common to all silvery fish, he said. He and his colleagues report their findings in the current issue of the journal Nature Photonics.
In with good and out with bad memories
Most people have a moment or two they would rather not remember. The brain has two opposite ways of dealing with those memories, researchers report in a new study.The first is to simply block out the memory. The second is to recall a substitute memory. Take the case of a fight with a loved one, said Roland Benoit, a cognitive neuroscientist at the Medical Research Council Cognition and Brain Sciences Unit in Cambridge, England.
"You don't want to think about it because you want to just go on with life," Benoit said. Benoit and his colleagues asked study participants to associate the words "beach" and "Africa."Then one group was told to avoid thinking about the associated words altogether. Another group was told to start thinking about the word "snorkel" in association with "beach," rather than "Africa."
The participants were put under a functional MRI scanner, and the researchers found that in the case of memory substitution, the left prefrontal cortex works in conjunction with the hippocampus, an area of the brain connecting to conscious remembering.
But when an unwanted memory is simply suppressed or blocked out, the prefrontal cortex actually inhibits the functioning of the hippocampus.
Healthy individuals probably use both strategies interchangeably, Benoit said, and both techniques appear to be equally effective.
Benoit and his colleagues report their findings in the current issue of the journal Neuron.