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The primary visual cortex (V1) plays an important role in vision and visual perception. Studies in many brain regions demonstrate that opiate abuse can change excitatory and inhibitory neurotransmission. To investigate the effect of chronic morphine exposure on the response modulation of V1 simple and complex neurons, we carried out in vivo extracellular recordings in V1 of morphine- and saline-treated (control) cats. Response modulation was quantified as the ratio of first Fourier components (F1) to the mean response (F0). Compared with saline-treated cats, V1 neurons in morphine-treated cats exhibited weaker response modulation and a longer time course of response. The decrease of response modulation was caused by an increase of F0. Further, morphine re-exposure significantly improved the response properties of V1 neurons in morphine-treated cats. These results suggest that chronic morphine …