Numerosity coding in the brain: from early visual processing to abstract representations.

Numerosity estimation refers to the ability to perceive and estimate quantities without explicit counting, a skill crucial for both human and animal cognition. Traditionally, this process has been associated with higher-order cortical regions like the intraparietal sulcus (IPS) and prefrontal cortex (PFC), thought to support abstract number representations. However, noninvasive electrophysiological and neuroimaging studies, along with psychophysical investigations, suggest that early visual areas such as V1 and V2 may play a role in processing visual numerosity, similar to the way low-level visual features are encoded. Recent research shows that the early visual cortex encodes information related to numerosity through the spatial frequency content of visual displays. Numerosity-tuned neurons, however, emerge later in the visual processing hierarchy, where more abstract and location-independent representations are formed in association areas such as the IPS and PFC. This is further supported by single-neuron data revealing spatiotopic integration and tuning curves for specific numerosities within these associative cortical regions. A comprehensive understanding of numerosity processing in the brain requires investigating numerosity presentations across time, space, and sensory modalities, all of which engage association cortices rather than sensory-specific regions.