Spatial integration properties in MT neurons affect spatiotemporal motion discrimination

Perception requires integrating noisy dynamic visual information across the visual field to identify relevant stimuli and guide decisions. While temporal integration has been studied extensively in experiments with highly controlled visual stimuli and reverse-correlation techniques, the nonlinear mechanisms underlying spatial integration are often neglected. More concretely, neurons in the Middle Temporal area (MT) respond heterogeneously and nonlinearly to stimuli inside their receptive fields (RFs) [1-3], and these neurophysiological properties could mediate spatial suppression of motion observed in perceptual motion discrimination tasks [4]. However, such a link between neural and behavioral responses is yet to be established. Here, we show that the spatial structure of the stimulus modulates neural responses in MT and impacts perceptual choices.  In particular, we find that monkeys integrate spatial evidence sublinearly in a motion discrimination task (fig. 1a) due to (i) weaker impact of motion further away from the fovea (fig. 1b), and (ii) surround suppression effects causing an attenuation of the responses to motion in the center of the stimulus (fig. 1c).  We then sought to investigate if this spatial-dependent modulation of responses arises at the level of stimulus representation in sensory neurons. We estimate the impact of the spatiotemporal components of the motion field on the instantaneous firing rate of MT neurons using nonlinear regression models (fig. 1d-g). Including nonuniform spatial weighting profiles (fig. 1e-g) significantly enhanced the predictive accuracy of the model in held-out trials ($p = 1.45 × 10^{−18}$, paired T-test, n = 156 units), compared to a similar temporal integration model. Our results revealed contextual modulation at the level of sensory neurons that points towards spatial integration mechanisms taking place at this level of processing. Our findings can be synthesized in a two-stage model of perceptual decision making in which spatial context effects modulate spatial stimulus integration in neurons of the visual cortex, and a decision area supports temporal integration to give rise to perception [5]. Taken together, our work shows how collective neurophysiological properties of sensory neurons shape spatiotemporal perceptual integration.