The sound-free SMARC effect: The spatial-musical association of response codes using only sound imagery.

This study provides clear evidence that the human cognitive system automatically codes sound pitch spatially. The spatial-musical association of response codes (SMARC) effect, in which a high-pitched (low-pitched) tone facilitates an upper (lower) response, is considered to reflect the spatial coding of sound pitch. However, previous studies have not excluded the directional effects of sound localization. Because a high-pitched (low-pitched) tone is automatically misperceived as originating from a spatially high (low) location, the location of a perceived sound source might artificially elicit the SMARC effect. This study challenged this unresolved issue. Participants were trained to associate visual stimuli (novel contoured shapes) with sound pitches (high-pitched or low-pitched pure tones). After training, participants completed a discrimination task in which the vertically aligned keys were associated with the visual stimuli in the absence of sound. Even without sound, the SMARC effect was observed in response to the trained visual stimuli (Experiment 1). However, this sound-free SMARC effect was eliminated when training was omitted (Experiment 2). Therefore, the SMARC effect was observed based solely on the activation of sound imagery that was spatial.