Perceptual sensitivity varies from moment-to-moment with fluctuations in the gain of sensory-evoked responses. We previously found that intrinsic traveling waves (iTWs) of activity in marmoset Area MT regulate the gain of evoked responses for targets presented near perceptual threshold and correlate with the monkey’s detection performance (Davis, Muller, et al. Nature 2020; Davis, Benigno, et al. Nat. Communi. 2021). Here we ask whether target detection also depends on the spiking activity of neurons beyond the retinotopic locus of the target (off-target locations). We hypothesized that activity in off-target locations could suppress target-evoked activity and reduce detection performance. If so, then activity in off-target populations should be negatively correlated with detection performance. To test this, we measured single- and multi-unit activity and local field potentials (LFPs) across Area MT using chronically implanted Utah arrays as monkeys performed a visual detection task. We measured spike rates (1) for neurons whose receptive fields encompassed the target location and (2) for near-by neurons whose receptive field centers did not include the target location. We find that detection performance is better predicted from the separation between target-evoked and off-target population activity than from the evoked response alone. We hypothesized that this separation could be driven by the alignment of iTWs with the underlying target and off-target populations to increase gain at target-aligned populations and decrease gain at off-target populations. Consistent with our hypothesis, we find detection performance is predicted by the alignment of relatively depolarized and hyperpolarized iTW phases with target-aligned and off-target populations.