Trial-to-trial variations in the firing rates of neurons in the middle temporal visual area (MT) are correlated with the behavior of macaque monkeys performing motion detection and motion discrimination tasks. Here we examine how these correlations depend on the direction of motion used for a detection task relative to the preferred direction of the neuron under study. There was a robust correlation between the firing rate of MT neurons and the animal's detection of motion when the direction of that motion was within ∼ 45° of the preferred direction of the neuron. This correlation was undetectable using motions that were ∼ 90° away from the preferred direction, and an inverse correlation between activity and behavior was found for motion in the null direction. Correlation between reaction times and single-cell activity in MT followed a similar pattern. Although motion detection could have been based solely on the activity of neurons preferring the expected direction, these results suggest that it depends on the relative activity of neurons preferring opposite directions of motion. They furthermore show that the subset of neurons used to guide behavior can vary from trial to trial based on task requirements.