We studied the neural correlates of target omissions in five patients with right hemisphere damage and varying signs of left spatial neglect. Benefiting from the high temporal resolution of magneto-encephalography, we directly compared brain regional synchrony events of detected and omitted left-sided targets. Results showed that before stimulus presentation, a low beta synchronization activity was specifically increased within left frontal areas before pathological response omissions of left-sided targets. In the same pre-stimulus period, there were no such beta oscillations when patients correctly detected the target, or when no target was presented. Our findings emphasize the importance of neural activity during the pre-stimulus period on subsequent stimulus processing, and highlight the consequences of episodic interruptions of large-scale interhemispheric networks on target detection. Furthermore, our data suggest that prefrontal activity is not necessarily beneficial to target detection, but can be detrimental to it.