Previous studies have shown that numerosity perception is susceptible to adaptation, suggesting that it is a primary visual property (Burr and Ross, 2008). Recent studies on monkeys have revealed a sharp numerosity selectivity for neurons of intraparietal sulcus (IPS), with some tuned to an abstract representation of numerosity independent of the temporal format (simultaneous or sequential) of presentation (Nieder et al. ,2006). Here we exploit adaptation technique to test demonstrate a truly abstract number sense in humans. We demonstrate numerosity adaptation for sequentially presented items both across modalities as well as across presentation format. Adapting to a series of flashes presented at 2 or 8 Hz for about 40 seconds changes the perceived numerosity of visual pulses (range 2-20) presented within a temporal window of 2 seconds: adapting to 2 Hz increased perceived number by 15-20%; adapting to 8 Hz decreased perceived number by a similar amount. For adaptation to occur, the adaptor stimulus had to occupy the same spatial position in external rather than retinal coordinates. Similar adaptation effects were observed for auditory stimuli (adaptation to tone sequences), and also cross-modally: adapting to auditory bursts affected perceived visual numerosity and vice versa, to similar extent as intra-modal adaption. Finally we demonstrated cross-format adaptation: adapting to sequential streams of flashes affect perceived numerosity of spatial arrays and viceversa. All these results point to the existence of a perceptual system that combines visual and auditory information to encode an abstract sense of number, in space and in time.

Numerosity perception; cross-modal perception; adaptation