Performance on the texture discrimination task improves with practice but was also shown to decrease between closely spaced sessions. Here we explored immediate changes in performance within a single session. We found that, after an initial increase, performance declined with further training within a single session. This deterioration in performance was smaller when the inter-trial interval was longer than 3s. Performance recovered when targets were presented in new locations within the texture stimulus-thereby excluding a general fatigue process or adaptation to the stimulus light-intensity as an explanation for our findings. Further, the complete transfer of deterioration between eyes pointed to cortical origin. Deterioration was also found for task-irrelevant targets, indicating the involvement of a sensory mechanism. Collectively, these findings trace the deterioration of performance in the texture discrimination task, previously observed across several hours, to cortical events occurring during or immediately after stimulus presentation.