The problem of neural coding in perceptual decision making revolves around two fundamental questions: (i) How are the neural representations of sensory stimuli related to perception, and (ii) what attributes of these neural responses are relevant for downstream networks, and how do they influence decision making? We studied these two questions by recording neurons in primary somatosensory (S1) and dorsal premotor (DPC) cortex while trained monkeys reported whether the temporal pattern structure of two sequential vibrotactile stimuli (of equal mean frequency) was the same or different. We found that S1 neurons coded the temporal patterns in a literal way and only during the stimulation periods and did not reflect the monkeys' decisions. In contrast, DPC neurons coded the stimulus patterns as broader categories and signaled them during the working memory, comparison, and decision periods. These results show that the initial sensory representation is transformed into an intermediate, more abstract categorical code that combines past and present information to ultimately generate a perceptually informed choice.behaving monkeys | somatosensory cortex | dorsal premotor cortex | pattern discrimination | categorical code F rom the most stereotyped behavior of invertebrates to the most elaborate behavior of primates, a central issue in neurobiology is elucidating how sensory information is represented in neural circuits and how it is used to generate actions. In principle, this process can be understood as a chain of three basic neuronal operations. The representation of the physical/chemical attributes of the environment and the execution of motor commands can be regarded as the end points of this chain of neuronal operations. In the middle of this chain is a crucial processing step in which the sensory representations are analyzed and transformed in such a manner that the nervous system is able to choose the adequate motor action.We have investigated this chain of processes by analyzing the neuronal activity of parietal and frontal cortices in trained monkeys performing a vibrotactile frequency discrimination task (VFDT; reviewed in refs. 1-3). In this task, monkeys compared the frequencies of two vibratory stimuli applied sequentially to the skin of one fingertip and then used their free hand to push one of two response buttons to indicate whether the second stimulus frequency (f2) was lower or higher than the first stimulus frequency (f1). The VFDT, although apparently simple, is designed so that it can only be executed when a minimum number of neuronal operations or cognitive steps are performed: coding f1, holding f1 in working memory, comparing f2 with the memory trace of f1, and, finally, executing a motor response to indicate whether f2 > f1 or f2 < f1. Thus, the VFDT allowed us to investigate a wide range of essential neural processes during perceptual decision making (1-3). However, the VFDT poses a fundamental problem: What is the neural code(s) that an observer might use to decide whether f2 > f1 or f2 < f1?...