Pumping up the volume. In the animal kingdom, mammals are known to have extremely sensitive hearing, a faculty associated by researchers with the thousands of tiny hair cells found in the cochlea of the mammalian inner ear. Now ENG Biomedical Engineering Professor David Mountain and one of his students, Domenica Karavitaki (ENG’92,’95), have contributed to a better understanding of the forces at work.

Named for the snail shell it resembles, the cochlea is a fluid-filled tube coiled within the inner ear. It is divided along its length by the basilar membrane, upon which sits the organ of Corti. This structure is composed of several rows of tiny sensory cells called hair cells. Sound vibrations travel down the basilar membrane, stimulating the organ’s hair cells to move with very small (about 10 nanometers, or 1/1000th the length of a hair cell) but very rapid movements.

To capture these nano-sized movements, Karavitaki and Mountain used a strobe light, flashing at rates of 10,000 times a second, a computer-controlled video system mounted on a microscope, and advanced computer vision techniques. The stop-motion images they captured were combined to generate a slow motion animation of the tiny cells in action.

The animation revealed the hair cells contracting and lengthening in response to stimulation, producing increases and decreases in pressure within the organ of Corti. The cells, in effect, were acting as electromechanical fluid pumps, pushing fluid through the tiny channel deep within the inner ear, called the tunnel of Corti. By amplifying the motion of fluid back and forth within the organ of Corti, the researchers surmise, the cells amplify the incoming auditory signal, leading to highly sensitive hearing ability, particularly in the higher frequencies.

Although all animals have a structure similar to the organ of Corti, only mammals have a fluid-filled channel (tunnel of Corti) within it. Mammals also are unique in possessing hair cells within the organ of Corti that can rapidly change their length.

This research was presented at November’s annual meeting of the Acoustical Society of America.

Mapping cities. Worldwide, cities continue to grow -- transforming woodlands, wetlands, and agricultural land into more densely populated urban environments. To better understand the impact of such urbanization, researchers need to be able to clearly identify where cities end and farmland or forests begin. A major step toward clarifying such boundaries has recently been made by Annemarie Schneider (GRS’04), a graduate student working with CAS Professor Curtis Woodcock and Associate Professor Mark Friedl in the department of geography.

The new mapping technique integrates data from three sources to overcome shortcomings inherent in each of the methods. The researchers used specially developed algorithms to integrate satellite data from MODIS (moderate resolution imaging spectrometer), an instrument orbiting Earth onboard NASA’s Terra satellite, with nighttime light data from the Defense Meteorological Satellite Program and population density data.

In the initial phase the researchers focused only on North America, but the work has been extended to assess boundaries between urban and nonurban areas globally. “While urbanization cannot be halted,” say the authors, “identifying and anticipating the location, size, and growth rate of urban areas is an important component to understanding and mitigating many aspects of global population growth, and by extension, global change.”

This work was reported in the December 2003 issue of Photogrammetric Engineering and Remote Sensing. Further information about this project can be found at http://duckwater.bu.edu/urban/.