Abstract
Introduction Numerosity, the set size of a group of items, helps guide behaviour and decisions. Previous studies have shown that neural populations respond selectively to numerosities. How numerosity is extracted from the visual scene is a longstanding debate, often contrasting low-level visual with high-level cognitive processes. Here, using 7T functional MRI, we investigate how attention influences numerosity selective responses. Method 4 participants took part in three experiments. In all experiments, the stimuli consisted of black and white dots within the same display, and the participants’ attention was focused on either black or white dots while detecting a subtle shape change of the attended dot subset. In Experiment1, the white dots systematically increased from 1 to 7, while the black dots systematically decreased from 26 to 20, keeping the total numerosity constant at 27. Here, we summarized the fMRI signals responding to the attended dot subset’s numerosity using population receptive field modelling (Dumoulin & Wandell, 2008). In experiments2, we used a 2x2 block design to establish the response to the preferred, but unattended numerosities. The subset consisting of 2/3/4 (the preferred numerosities) dots were presented while being attended and unattended, respectively. The total numerosity was fixed at 40 with the non-preferred numerosities varied from 36-38. In Experiment 3 only one non-preferred numerosity of 20 dots was shown, and as a result, the total numerosity varied. The neural responses in experiments 2-3 were analysed using general linear models. Results We found that the numerosity-tuned neural populations respond only when attention is focused on their preferred numerosity, irrespective of the unattended or total numerosities. Without attention, responses to preferred numerosity were inhibited. Conclusions Unlike traditional effects of attention in the visual cortex where attention enhances already existing responses, our results suggest that attention is required to drive numerosity selective responses.