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Yuxuan Cai, Shir Hofstetter, Ben M. Harvey, Serge O. Dumoulin; Attention modulates numerosity responses in human parietal cortex. Journal of Vision 2020;20(11):690. doi: https://doi.org/10.1167/jov.20.11.690.
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© ARVO (1962-2015); The Authors (2016-present)
Numerosity, the set size of a group of items, helps to guide human behavior and decisions. We previously described numerosity-selective neural populations organized in systematic topographic maps in human occipital, parietal and frontal cortices (Harvey et al, 2013, 2017). However, it remains unclear whether numerosity-selective responses reflect stimulus properties only, or reflect which items are attended. Here, we investigate numerosity-selective responses when participants were presented with identical numerosity stimuli but paid attention to different item sub-sets within the stimulus.
We measured responses to systematically varying numerosities using ultra-high field (7T) fMRI. The numerosity stimuli contained black and white dots, presented simultaneously. While the white dots systematically increased from 1 to 7 dots, the black dots systematically decreased from 26 to 20 dots, so the total numerosity remained constant at 27 dots. This stimulus sequence repeated with colors switched, then moved through the numerosity sequence in the opposite order. Subjects attended to either black or white dots, detecting a subtle dot shape change. No numerosity judgments were required. We summarized the fMRI signals using a logarithmic Gaussian function with two parameters, preferred numerosity and tuning width, responding to the attended sub-set’s numerosity. We fit these parameters to predict the measured responses, following a population receptive field (pRF) modeling approach (Dumoulin & Wandell, 2008).
Each cortical location responded specifically to the attended numerosity, yielding very different response time courses to the same stimulus, depending which sub-set was attended. The same cortical location preferred similar numerosities when participants either attended to white or black dots. Numerosity-selective neural populations were topographically organized in both attention states.
Our results suggest that identical numerosity stimuli give rise to different numerosity responses depending on the task the subject performs. Neural responses underlying the perception of numerosity are strongly modulated by attention and task demands.
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