Correlations between acoustical features of sound (for example, the frequency of a pure tone) and physiological activity in the brain (for example, the number and timing of electrical pulses fired by a neuron) are being investigated with scientific methods that incorporate recent technological advances in the processing of bioelectric, biomagnetic, and biochemical signals. On the rich foundation of basic knowledge accrued since the 1950s about how the brain and ear process acoustical features of simple synthetic stimuli such as pure tones, scientists are analyzing the relationship of acoustical–physiological correlates to perceptual attributes of sounds (for example, the pitch of one's voice). This effort has been accelerated in recent years by rapid developments in human brain imaging.