The causal role of early visual cortex in vividness of visual imagery
While numerous studies have demonstrated objective accuracy in visual imagery tasks to involve the early visual cortex (V1/V2), the role of this region in imagery vividness remains unclear. We addressed this question by combining Transcranial Magnetic Stimulation (TMS) with a cross-adaptation paradigm in which visual adaptation modulates the ability to engage in visual imagery in the adapted part of the visual field. As previously shown, in the control (Vertex) condition, mental imagery accuracy was impaired when the mental image was generated in the adapted region of visual space. This effect was removed by TMS, indicating that the locus of adaptation was the early visual cortex. In contrast, analysis of vividness ratings (collected on trial-by-trial basis) showed no effects of adaptation, indicating that the imagery vividness is somewhat orthogonal to the fidelity of the mental image. The key finding was that TMS impacted vividness ratings on trials when participants performed incorrectly in the imagery task. Thus, the effects of TMS on accuracy and vividness were observed on different trial types: TMS impaired accuracy of trials associated with high baseline performance, but increased vividness on incorrect trials. Overall, our findings suggest that the early visual cortex plays a role in both accuracy and vividness of mental images, yet these processes are dissociable.