The dorsolateral prefrontal cortex (dlPFC) and posterior parietal cortex (PPC) play a critical role in spatial working memory. Activity in these areas has been shown to determine the variability of monkeys’ responses (endpoints of eye movements) in a delayed response task. Here we addressed the role of these areas on categorical judgments based on information retained in working memory. We trained two monkeys in a Match/Nonmatch task, which required them to observe two stimuli presented in sequence with an intervening delay period between them. If the two stimuli were different, the monkeys had to saccade to the location of the second stimulus; if they were the same, they held fixation. Neurophysiological recordings were performed in areas 8a and 46 of the dlPFC and 7a and LIP of the PPC. We collected a total of 210 neurons. We selected neurons with activity during the first stimulus and delay period that was significantly elevated relative to the baseline fixation (n= 53, paired t-test, p< 0.05). We hypothesized that random drifts causing the peak activity of the network to move away from the first stimulus location and towards the location of the second stimulus would result in categorical errors. Indeed, when the first stimulus appeared in a neuron’s preferred location, the neuron showed significantly higher firing rates in correct than in error trials (paired t-test, p= 2.2× 10-4). When the first stimulus appeared at a non-preferred location and the second stimulus at a preferred, activity in error trials was higher than in correct (paired t-test, p= 0.042). The results indicate that the activity of dlPFC and PPC neurons influences categorical judgments of information maintained in working memory, and the magnitude of neuronal firing rate deviations could determine the role of cortical areas in working memory performance.