Neurons in IT cortex are thought to underlie primate ability to recognize and discriminate among realistic images, but the mechanisms responsible for this process are poorly understood. We propose that IT response similarity is correlated with the discriminability of images, which reflects perceptual or psychological similarity (Allred and Jagadeesh, SFN abstracts, 2002). To test our hypothesis, we manipulated discriminability of realistic image pairs in a delayed-match-to-sample task, (Allred et. al., 2003, VSS abstracts) and measured corresponding changes in neural response similarity. Discrimination performance was manipulated by changing sample duration (SOA) from 0 to 320 ms, and by masking or not masking the sample (Kovacs et. al., 1995; Rolls et. al., 1999). We collected simultaneous behavioral and neural data, and optimized IT selectivity on each day by using two new images (effective and ineffective) and a pair-unique mask. Increasing SOA and removing the mask improved both behavioral and neural p...