Our spatial sense of pitch

Across languages and cultures, humans associate high pitches with elevated spatial positions—in the placement of notes on a musical staff, for example, and even in the language: “Low” and “high,” for instance, are used to describe pitch. Cesare Parise , Katharina Knorre, and Marc Ernst at Bielefeld University in Germany now show that our spatial sense of pitch combines two independent contributions. Our head and outer ears act as frequency- and position-dependent filters that strongly affect our ability to localize sound (see the article by Bill Hartmann in Physics Today, November 1999, page 24 ). Analyzing such filters, known as head-related transfer functions, the researchers found that the filters transmit more high-frequency energy for sounds arriving from higher elevations. More than that, tens of thousands of samples the researchers made of environmental sounds—indoors and outdoors, urban and rural—revealed that higher-frequency sounds reaching our ears tend to originate from elevated sources, due perhaps to their inherent distribution (bird songs are at higher frequencies than footsteps) or to frequency-dependent ground adsorption. To tease apart the environmental and physiological effects the researchers sat volunteers in a chair that could tilt sideways, positioned them at different angles, and played sounds through one of several loudspeakers. The participants’ ability to localize white noise was largely independent of body tilt. But the perceived sources for narrowband sounds between 1.4 kHz and 8 kHz were systematically elevated in both the participant’s tilted frame of reference and the laboratory frame. The head- and lab-centered biases were distinctly different, yet correlated: The filtering properties of the ear accentuate, perhaps by evolutionary design, the frequency–elevation mapping already present in natural auditory scenes. (C. V. Parise, K. Knorre, M. O. Ernst, Proc. Natl. Acad. Sci. USA 111, 6104, 2014, doi:10.1073/pnas.1322705111 .)