COGNITIVE NEUROSCIENCE Classification of extracellularly recorded neurons by their discharge patterns and their correlates with intracellularly identified neuronal types in the frontal cortex of behaving monkeys Satoshi Katai, 1,2,3 Keichiro Kato, 1 Shunpei Unno, 1 Youngnam Kang, 4 Masanori Saruwatari, 1 Naoki Ishikawa, 1 Masato Inoue 1 and Akichika Mikami 1 1 Department of Behavioral and Brain Sciences, Primate Research Institute, Kyoto University, Inuyama, Aichi 484-8506, Japan 2 Department of Neurology and Rheumatology, Shinshu University School of Medicine, Matsumoto, Nagano, Japan 3 Department of Neurology, Kakeyu Hospital, JA Nagano Koseiren Kakeyu-Misayama Rehabilitation Center, Ueda, Nagano, Japan 4 Department of Neuroscience and Oral Physiology, Osaka University Graduate School of Dentistry, Suita, Osaka, Japan Keywords: extracellular recording, fast spiking, frontal cortex, monkey, regular spiking Abstract Neurons in the cerebral cortex are not homogeneous. However, neuronal types have been ignored in most previous work studying neuronal processes in behaving monkeys. We propose a new method to identify neuronal types in extracellular recording studies of behaving monkeys. We classified neurons as either bursting or non-bursting, and then classified the bursting neurons into three types: (i) neurons displaying a burst of many spikes (maximum number of spikes within a burst; NSB max ‡ 8) at a high discharge rate (maximum interspike interval; ISI max < 5 ms); (ii) neurons displaying a burst of fewer spikes (NSB max £ 5) at a high discharge rate (ISI max < 5 ms); and (iii) neurons displaying a burst of a few spikes (NSB max £ 7) at relatively long ISIs (ISI max > 5 ms). We found that the discharge patterns of the four groups corresponded to those of regular spiking (RS), fast spiking (FS), fast rhythmic bursting (FRB) and intrinsic bursting (IB) neurons demonstrated in intracellular recording studies using in vitro slice preparations, respectively. In addition, we examined correlations with the task events for neurons recorded in the frontal eye field and neuronal interactions for pairs of neurons recorded simultaneously from a single electrode. We found that they were substantially different between RS and FS types. These results suggest that neurons in the frontal cortex of behaving monkeys can be classified into four types based on their discharge patterns, and that these four types contribute differentially to cortical operations. Introduction The cerebral cortex contains different types of neurons. Therefore, in order to investigate the neuronal circuits participating in animals’ behavior, identifying the types of neurons present is fundamentally important. Although the majority of extracellular recording studies have ignored neuronal types, a few studies have attempted to identify them based on their spike waveforms. Intracellular recordings in slice preparations have demonstrated that c-aminobutyric acid (GABA)- containing, inhibitory interneurons produce action potentials of a much shorter duration than excitatory, pyramidal neurons (McCormick et al., 1985). Based on this finding, some extracellular recording studies distinguished neuronal types based mainly on the differences in spike width (Wilson et al., 1994; Constantinidis & Goldman-Rakic, 2002; Mitchell et al., 2007; Cohen et al., 2009). However, the identification of neuronal types by their spike width alone has several problems. First, in extracellular recording conditions, the measurement of spike width is often disturbed by background noise near the baseline and the filtering parameters used in the experiment. Second, it has become clear that not all short-duration action potentials are generated by fast spiking (FS) interneurons. Intracellular studies have documented short-duration action potentials in a number of neurons that were subsequently labeled and shown to have a pyramidal morphology (Nowak et al., 2003). Taking another approach, recent intracellular recording studies have revealed that cortical neurons can be classified into four types by their discharge patterns in response to current pulses (Kang & Kayano, 1994; Gray & McCormick, 1996; Nowak et al., 2003; Gonza ´lez- Burgos et al., 2004, 2005a,b). One type generates single spikes and rarely shows burst episodes, which are termed regular spiking (RS) neurons. The other three types generate a burst of spikes and are classified as FS, fast rhythmic bursting (FRB) (or ‘chattering’) and intrinsic bursting (IB) neurons. FS neurons display tonic discharges, at rates as high as 800 Hz, without spike frequency adaptation in response to current pulses. FRB neurons display repetitive burst discharges of 2–5 spikes per burst, and have an intraburst discharge rate as high as 800 Hz, and an interburst interval (IBI) typically ranging from 15 to 50 ms. IB neurons display a low-frequency burst of spikes at the beginning of the current pulse and a transition to tonic Correspondence: Professor A. Mikami, Department of Human Wellbeing, Chubu Gakuin University, Kirigaoka 2-1, Seki, Gifu, 501-3993, Japan. E-mail: mikami@chubu-gu.ac.jp Received 7 November 2009, revised 7 January 2010, accepted 26 January 2010 European Journal of Neuroscience, Vol. 31, pp. 1322–1338, 2010 doi:10.1111/j.1460-9568.2010.07150.x ª The Authors (2010). Journal Compilation ª Federation of European Neuroscience Societies and Blackwell Publishing Ltd European Journal of Neuroscience