Adaptive pooling of visual motion signals by the human visual system revealed with a novel multi-element stimulus.

The two-dimensional (2D) trajectory of visual motion is usually not directly available to the visual system. Local one-dimensional (1D) sensors initiate processing but can only restrict the solution to a set of speed and direction combinations consistent with the 2D trajectory. These 1D signals are then integrated across orientation and space to compute 2D signals. Both motion integrations are thought to occur in higher cortical areas, but it remains unclear whether 1D signals are integrated over orientation and space simultaneously (1D pooling process), or instead are integrated locally with the resulting 2D signals then spatially integrated (2D pooling process). From psychophysical responses to novel global-motion stimuli comprised of numerous Gabor (1D) or Plaid (2D) elements, here we show that the human visual system adaptively switches between 1D pooling and 2D pooling depending on the input. When local 2D signals cannot be determined, the visual system shows effective 1D pooling that approximately follows the intersection of constraints rule. On the other hand, when local 2D signals are available, the visual system shows 2D pooling that approximately follows the vector average rule. Spatial motion integration therefore exhibits great flexibility when estimating complex optic flows in natural scenes.