The attentional modulation of navigational cue integration by cortical neurons

Autonomous navigation is guided by visual information about self-movement. This information is provided by the global pattern of visual motion in optic flow and the relative visual motion of discrete, salient objects. Object motion that is congruent with the surrounding optic flow suggests the presence of an earth-fixed object, whereas non-congruent object motion suggests the presence of an independently moving object. Thus, the processing of these visual cues is critical to their utility for navigation relative to fixed obstacles and animate threats in the environment.;Neurons in the medial superior temporal (MST) area of macaque monkey cerebral cortex respond to both optic flow and object motion, suggesting a possible algorithmic combination of responses to these two visual cues when presented together. Recent evidence shows that MST neuronal responses to optic flow are shaped by the dynamic allocation of attention to visual stimuli. We hypothesize that attentional mechanisms alter MST neuronal response integration of visual stimuli, thus adapting visual motion processing to suit the needs of ongoing behavior. To test this hypothesis, we executed three experiments using single neuron electrophysiology in awake, behaving macaque monkeys trained to maintain centered fixation and view object motion and optic flow stimuli simulating translational self-movement.;In the first experiment, optic flow alone, object motion alone, and sixteen combinations of these stimuli were presented while the monkey maintained centered fixation with no other task to elucidate the mechanism by which MST integrates visual cues. Consideration of single stimulus parameters revealed systematic response relations accounting for as much as half of the variance in the responses to combined stimuli. In general, vigorous responses to both cues yielded sub-additive response interactions for both congruent and non-congruent combination stimuli.;In the second experiment, we delved into the role of attention in MST responses to combined optic flow and object motion stimuli. In this experiment, the animal was trained to use a joystick to steer the simulated heading created by object motion with congruent optic flow field, or to object motion combined with non-congruent optic flow. This study showed that when a behavioral task requires attention to be focused on the motion of the combined optic flow and object motion stimulus, the responses show stronger additive responses with a clear link between response to optic flow alone and to the combined optic flow and object.;In the third experiment, we required the monkey to alternately focus on only one of the two stimuli, optic flow or object motion, in a naturalistic continuous steering task, where the other stimulus was a task irrelevant distractor. This resulted in a direct one to one response relationship between the response to the attended visual cue when presented alone and the response to the same attended visual cue when presented with a salient visual distraction stimulus.;We can conclude that while MST neurons respond to both optic flow and object motion stimuli when presented alone, when presented in combination, attention significantly modulates responses to combined visual motion cues. This supports the notion that MST is involved not only in visual motion processing, but also in the top-down processing of cue discrimination for navigation through the environment.