Abstract
Experimental studies to understand prefrontal functions started after Jacobsen (1936) first reported that bilateral prefrontal lesions produced a permanent deficit of delayed-response performance in monkeys. However, neurophysiological experiments to understand neural mechanisms of prefrontal functions began in the early 1970s using awake behaving monkeys first by Kubota's group in Japan and Fuster's group in USA. These groups reported many important and basic findings, including the finding of delay-period activity and its characteristics. Although spatial tasks, such as a delayed-response task or a delayed alternation task, were initially used to examine neural mechanisms of prefrontal functions, non-spatial tasks, such as a delayed matching-to-sample task, were also used for these experiments in the 1980s and the importance of delay-period activity was gradually recognized as a key activity to understand prefrontal functions. In the late 1980s, Goldman-Rakic proposed "working memory" as a key concept to combine findings of behavioral and neurophysiological experiments using animals with findings of clinical studies using human frontal patients and to interpret all these findings using one common concept. Her proposal was supported by a series of neurophysiological and behavioral studies conducted by her group. The term working memory became a key word in prefrontal studies in the 1990s, especially in non-invasive brain imaging studies. Although working memory may not be an appropriate concept to interpret functions of whole prefrontal cortex, working memory is a useful concept to understand functions of the lateral prefrontal cortex. Only a small number of groups in the world started neurophysiological studies in the primate prefrontal cortex in the 1970s. Studies of prefrontal functions have now become one of the major fields in neuroscience. The prefrontal cortex is the most important cortical area to understand the human mind. Although a huge volume of new findings regarding prefrontal functions have been accumulated since the 1970s, the prefrontal cortex is still a not-well-understood cortical area. We need further experiments to solve the secrets of prefrontal functions.
