Theta (3–9 Hz) and gamma (30–100 Hz) oscillations have been observed at different levels along the hierarchy of cortical areas and across a wide set of cognitive tasks. In the visual system, the emergence of both rhythms in primary visual cortex (V1) and mid-level cortical areas V4 has been linked with variations in perceptual reaction times.1–5 Based on analytical methods to infer causality in neural activation patterns, it was concluded that gamma and theta oscillations might both reflect feedforward sensory processing from V1 to V4.6–10 Here, we report on experiments in macaque monkeys in which we experimentally assessed the presence of both oscillations in the neural activity recorded from multi-electrode arrays in V1 and V4 before and after a permanent V1 lesion. With intact cortex, theta and gamma oscillations could be reliably elicited in V1 and V4 when monkeys viewed a visual contour illusion and showed phase-to-amplitude coupling. Laminar analysis in V1 revealed that both theta and gamma oscillations occurred primarily in the supragranular layers, the cortical output compartment of V1. However, there was a clear dissociation between the two rhythms in V4 that became apparent when the major feedforward input to V4 was removed by lesioning V1: although V1 lesioning eliminated V4 theta, it had little effect on V4 gamma power except for delaying its emergence by >100 ms. These findings suggest that theta is more tightly associated with feedforward processing than gamma and pose limits on the proposed role of gamma as a feedforward mechanism. •Visual stimulation elicits theta and gamma oscillations in V1 and V4•Theta and gamma oscillations interact in time•Although lesion of V1 eliminates V4 theta oscillations, gamma oscillations survive•Gamma oscillations still contain stimulus information but emerge delayed without V1 Kienitz et al. show that, upon visual stimulation, V1 and V4 show theta and gamma oscillations, which interacted in terms of phase-to-amplitude coupling. Lesion of V1, the major input source to V4, eliminated V4 theta oscillations. In contrast, V4 gamma oscillations were less affected and still contained stimulus information but emerged delayed (>100 ms).