Evidence for a Posterior Parietal Cortex Contribution to Spatial but not Temporal Numerosity Perception.

Posterior parietal cortex (PPC) is thought to encode and represent the number of objects in a visual scene (i.e., numerosity). Whether this representation is shared for simultaneous and sequential stimuli (i.e., mode independency) is debated. We tested the existence of a common neural substrate for the encoding of these modes using fMRI. While both modes elicited overlapping BOLD response in occipital areas, only simultaneous numerosities significantly activated PPC. Unique activation for sequential numerosities was found in bilateral temporal areas. Multivoxel pattern analysis revealed numerosity selectivity in PPC only for simultaneous numerosities and revealed differential encoding of presentation modes. Voxel-wise numerosity tuning functions for simultaneous numerosities in occipital and parietal ROIs revealed increasing numerosity selectivity along an occipito-to-parietal gradient. Our results suggest that the parietal cortex is involved in the extraction of spatial but not temporal numerosity and question the idea of commonly used cortical circuits for a mode-independent numerosity representation.