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Abstract:
The association between sound frequency and spatial elevation is a remarkable example of cross-dimensional sensory mapping. In a wide range of cognitive, perceptual, attentional, and linguistic functions, humans consistently display a positive, sometimes absolute, association between sound frequency and spatial elevation, whereby increasing frequency is mapped to increasing elevation. However, a comprehensive account for the origins of such a pervasive cross-dimensional link is still missing. Here we demonstrate that the frequency-elevation mapping observed in human behaviour is already present in both the statistics of the acoustic stimuli in the environment, and in the filtering properties of the external ear. Specifically, we singled out the effects of head- and world-centred elevation and, through a combined analysis of environmental sounds and anthropometric measures, we show that, (1) in world-centred coordinates, high sounds are statistically more likely to come from higher elevations; moreover, (2) due to the external ear, sounds coming from higher head-centred elevations have more energy at high frequencies. To link these findings to human cognition, in a psychophysical task observers localized narrow band noises with different central frequencies, while head- and world-centred elevations were put into conflict by tilting participants' body. Sound frequency systematically biased localization in both head- and world-centred coordinates in agreement with the mappings measured in the ear and the environment. We argue that, in a shorter timescale, humans learn the statistics of the auditory signals; while, in a longer timescale, evolution might tune the filtering properties of the external ear to the statistics of the acoustic environment.