Preattentive and attentive responses to changes in small numerosities of tones in adult humans.

The brain hosts a primitive number sense to non-symbolically represent numerosities of objects or events. Small exact numerosities (~4 or less) can be individuated in parallel. In contrast, large numerosities (more than ~4) can only be approximated. However, whether small numerosities can be approximated without their parallel individuation remains unclear. Parallel individuation is suggested to be an attentive process and numerical approximation an automatic process. We, therefore, tested whether small numerosities can be represented preattentively. We recorded adult humans׳ event-related potentials (ERPs) and behavioral responses to 300-ms sequences of six tones (each of either 440 Hz or 660 Hz in frequency). Mostly, a sequence was of 3 tones of each frequency. Occasionally (P=0.1), the numerosities were 4 and 2 (minor changes) or 5 and 1 (major changes). Mismatch negativity (MMN), but no later attention-related positive-polarity ERPs, was observed to the major but not to the minor changes during a visual non-numerical task. In a following attentive task, behavioral responses even to major changes resulted in a very low hit rates (0.11 for major and 0.023 for minor changes) and yet an above-zero false-alarm rate (0.052). The findings support a view that small numerosities of objects can be automatically approximated independently of their attentive individuation.