Preperceptual human number sense for sequential sounds, as revealed by mismatch negativity brain response?

Humans and some other species can nonlinguistically operate on the quantities of things or events, including sounds. Whether this ability is restricted to conscious percepts of sounds developing in approximately 200 ms is, however, unclear. To this end, we recorded the mismatch negativity (MMN) brain response, an index of preperceptual auditory change detection, of adult humans who passively listened to rare sequences of four 50-ms tones ("deviants") interspersed among frequently repeated tones ("standards"). Each tone was either 1000 or 1500 Hz in frequency. Deviants differed from standards in a ratio of the tones of the 2 frequencies. MMN was found for deviants by 160 ms from the onset of their largest ratio difference from standards (2:2 vs. 4:0), suggesting some ability of the human brain to operate on the number of sequential sounds of specific frequencies at a preperceptual time scale.