• Voltage-sensitive dye imaging (VSDI) is a powerful method for measuring neural population responses from the cortex of awake, behaving, subjects. We used VSDI to measure the dynamics of neural population responses in macaque V1 to visual stimuli with a wide range of time courses. We built a simple yet flexible delayed normalization model to capture the dynamics of all these measurements.

• We found that beyond clear nonlinearities for briefly presented visual stimuli, stimulus-evoked VSDI responses are near additive in time, which qualitatively differs from neural dynamics previously measured in human visual cortex using fMRI and electrocorticography (ECoG), which show strong sub-additivity in time.

• To test whether this discrepancy is specific to VSDI, a signal dominated by subthreshold neural activity, we repeated our measurements using a genetically encoded calcium indicator (GCaMP), a signal dominated by spiking activity. We found that GCaMP signals in macaque V1 are also near additive.

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