AUDITORY SYSTEM THE RUFFLING EFFECT OF RUMBLE

Barely perceptible low-frequency signals nevertheless activate measurable responses in our auditory circuits. Neurobiologists at Ludwig-Maximilians-Universitaet (LMU) in Munich have now characterized the remarkable impact of low-frequency sounds on the inner ear

The human auditory system appears to be poorly adapted to the perception of low-frequency sound waves, as hearing thresholds become markedly higher for frequencies lower than about 250 Hz. Yet sensory cells do react to pressure waves with frequencies below 100 Hz, as revealed by the fact that such signals actually evoke detectable micromechanical responses in nerve cells in the inner ear, as LMU neurobiologists now report in the journal Royal Society Open Science.

Sources of low-frequency signals are a prominent feature of technologically advanced societies like our own. Wind turbines, air-conditioning systems and heat pumps, for instance, can generate such sounds. Hearing thresholds in this region of the acoustic spectrum vary from one person to the next. "But the assumption that the ear is unresponsive to low-frequency sounds because these are seldom consciously perceived is actually quite false. The ear indeed reacts to very low-frequency signals," says Dr. Markus Drexl of LMU. In collaboration with researchers led by Professor Benedikt Grothe (Head of the Division of Neurobiology in LMU's Department of Biology II) and a team based at Munich University Medical Center, Drexl has carried out a laboratory study which shows that low-frequency sounds, though virtually imperceptible, actually have a surprisingly strong effect on sensory cells in the inner ear.

Low-frequency hum stimulates the cochlea The new study is based on data collected from 21 experimental subjects with normal hearing, whose ears were exposed to a 30-Hz tone for 90 seconds at a sound-pressure level equivalent to 80 decibels. To determine how the inner ear responded to the signal, the researchers took advantage of a phenomenon referred to as spontaneous otoacoustic emissions (SOAEs). SOAEs are scarcely perceptible acoustic signals which are produced by the inner ear in the absence of overt stimulation, and can be detected with a sensitive microphone inserted in the ear canal.

"It turns out that low-frequency sounds have a clearly definable modulatory influence on spontaneous otoacoustic emissions," says Drexl. Following exposure to the 30-Hz signal for 90 seconds, the subjects' SOAEs exhibited slow oscillations in frequency and level, which persisted for up to 120 seconds. "Strikingly, the effect of the low-frequency stimulus on the cochlea persists for longer than the duration of the stimulus itself," Drexl points out. Further experiments will probe the possibility that this phenomenon may be linked to noise-induced auditory damage, one of the most common causes of hearing impairment in industrialized countries.

RED EFFECT SPARKS INTEREST IN FEMALE MONKEYS

Recent studies showed that the color red tends increase our attraction toward others, feelings of jealousy, and even reaction times. Now, new research shows that female monkeys also respond to the color red, suggesting that biology, rather than our culture, may play the fundamental role in our "red" reactions

"Previous research shows that the color red in a mating context makes people more attractive, and in the fighting context makes people seem more threatening and angry," explained Benjamin Y. Hayden, a coauthor of the study and professor in brain and cognitive sciences at the University of Rochester.

Hayden, whose research often involves primates, and Andrew J. Elliot, a professor of psychology at Rochester who has published several articles on humans and the red effect and coauthor of the study, sought to uncover what causes humans' response to the color. Is triggered simply by repeated cultural exposures, or if there is a biological basis that may help explain why the color tends to amplify human emotions?

As Hayden put it, "is this just because every year on Valentine's Day we see these red things everywhere and it creates a link for us between the color red and romance, or is it really a fundamental thing rooted in our biology?"

One way to test for biological influence would be to assess reactions in individuals who have not been conditioned to associate the color red with romance, Hayden said. "What if we could test this in someone who is not even human, but was exposed to a lot of the same evolutionary pressures? Well, that would be a monkey," he said. "So, we conducted experiments to see if monkeys would have similar biases as humans, and in a nutshell the answer is, yes, it seems like they do."

The new study, which appears in the journal Evolution and Human Behavior, involved rhesus monkeys (Macaca mulatta) from a free-ranging population of approximately 1000 residing at the Cayo Santiago field site in Puerto Rico. The animals live in naturally formed social groups and are habituated to human observation.

The researchers conducted two trials that measured the amount of time the primates looked at black and white images of the hindquarters of adult monkeys. The stimuli, which included images of both sexes, were surrounded by an "extraneous" color, framed by either red or blue. The researchers also used an image of a common shell found on the island as a control data point.

Hayden noted that a standard measure to gauge interest in those who don't have language -- primates or babies, for example -- is by how long they look at a given object. The longer the gaze indicates a greater amount interest.

In the first trial, the researchers displayed sequential images of male hindquarters surrounded -- in random order -- by frames of red or blue, to adult monkeys of both sexes. They were also presented with the shell image.

The researchers found a significant female bias toward the images of male hindquarters, but only when a red frame surrounded the image. "To our knowledge," the researchers said, "this is the first demonstration of an extraneous color effect in non-human primates."

In a second trial, the researchers displayed images of female hindquarters surrounded, again by either a red or blue frame. Female monkeys did not show a preference for other female hindquarter, regardless of the color of the surrounding frame.

But, surprising to the researchers, male monkeys did not show a preference for the female hindquarters, either, even when surrounded by the color red.

The researchers say additional work is needed to understand why males did not respond to the extraneous colors. One possibility is that the reproductive state of females is reflected in facial color changes rather than changes in the hindquarters. Images of females, which were restricted to the hindquarter region, may have been too limited to elicit male responses.

That female rhesus monkeys' interest in images of the opposite sex appears to be influenced by extraneous color suggests that the "red effect" is not unique to humans. Instead, the researchers argued, it appears to be supported by an "evolved biological mechanism."

Neither males nor females displayed a bias toward the shell image regardless of the color of its frame.

Kelly D. Hughes, a doctoral candidate at the University of Rochester, was lead author of the study. James P. Higham, an assistant professor of anthropology at New York University, and William L. Allen, a post-doctoral fellow in anthropology at the University of Hull, are coauthors.

The Sloan Foundation, NIDA, and two Reach fellowships from the University of Rochester to undergraduate research assistants supported the work. The population of rhesus monkeys at Cayo Santiago is currently supported by the National Center for Research Resources, the Office of Research Infrastructure Programs of the National Institute of Health, and the Medical Science Campus of the University of Puerto Rico.