Number: Still a primary visual feature

Some of the strongest evidence that number is a primary visual feature (like color or contrast) comes from experiments demonstrating visual adaptation to number (e.g., Burr & Ross, 2008), wherein staring at a large number of dots decreases numerosity estimates of subsequent probe displays. Recently, these findings have been challenged by a deflationary account on which these effects reflect spatiotopic attenuation to unchanging information (Yousif et al., 2023). Here, we conduct a crucial comparison of these accounts by testing number adaptation for arrays whose spatial properties constantly change. Centrally fixating observers viewed two large discs subtending 9° that independently and randomly translated on either side (left or right) of the display. During the adaptation phase, varying numbers of dots appeared and faded at changing locations within each continuously-moving disc. After 12 seconds, the dots disappeared and the discs continued moving for an additional 1000ms. Following this delay, a tone signaled the appearance of a new number of probe dots, appearing for 500ms in new locations within each disc; observers judged which disc contained more dots. On critical trials, probe dots were equal in number at the time of the tone. If number is a primary visual feature that can be bound to an object, then subjects should show an adaptation effect (thereby judging the disc previously containing the smaller adapter number as greater). If number adaptation effects are really spatiotopic attenuation to unchanging information, then subjects should not show an adaptation effect. Subjects showed the predicted adaptation effect. This result suggests that number adaptation persists despite drastic changes to spatiotopic and retinotopic position, contra an explanation in which adaptation results from filtering out information that remains unchanged between adapter and probe displays. Our findings re-open the case for genuine number adaptation and numerosity as a primary visual feature more broadly.