Nerve cells in the monkey's prefrontal cortex and nucleus medialis dorsalis of the thalamus show changes of firing frequency associated with the performance of a delayed response test. Most cells increase firing during the cue presentation period or at the beginning of the ensuing delay; spike discharge highler than that in intertrial periods is present in some cells throughout the delay. These changes are interpreted as suggestive evidence of a role of frontothalamic circuits in the attentive process involved in short-term memory

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Anatomy and ConnectionsThe PFC is the association cortex of the frontal lobe. In Los Angeles, California 90095 primates, it comprises areas 8-13, 24, 32, 46, and 47 according to the cytoarchitectonic map of Brodmann The physiology of the cerebral cortex is organized in (1909), recently updated for the monkey by Petrides and hierarchical manner. At the bottom of the cortical organi-Pandya (Figure 1). Phylogenetically, it is one of the latest zation, sensory and motor areas support specific sencortices to develop, having attained maximum relative sory and motor functions. Progressively higher areas-of growth in the human brain (Brodmann, 1912; Jerison, later phylogenetic and ontogenetic development-support 1994), where it constitutes nearly one-third of the neocorfunctions that are progressively more integrative. The tex. Furthermore, the PFC undergoes late development in prefrontal cortex (PFC) constitutes the highest level of the course of ontogeny. In the human, by myelogenic and the cortical hierarchy dedicated to the representation synaptogenic criteria, the PFC is clearly late-maturing and execution of actions. cortex (Flechsig, 1920; Conel, 1939; Huttenlocher, 1990; The PFC can be subdivided in three major regions: Huttenlocher and Dabholkar, 1997). In the monkey's orbital, medial, and lateral. The orbital and medial re-PFC, myelogenesis also seems to develop late (Gibson, gions are involved in emotional behavior. The lateral 1991). However, the assumption that the synaptic strucregion, which is maximally developed in the human, proture of the PFC lags behind that of other neocortical vides the cognitive support to the temporal organization areas has been challenged with morphometric data of behavior, speech, and reasoning. This function of (Bourgeois et al., 1994). In any case, imaging studies temporal organization is served by several subordinate indicate that, in the human, prefrontal areas do not attain functions that are closely intertwined (e.g., temporal infull maturity until adolescence (Chugani et al., 1987; tegration, working memory, set). Whatever areal special-Paus et al., 1999; Sowell et al. , 1999). This conclusion ization can be discerned in the PFC is not so much is consistent with the behavioral evidence that these attributable to the topographical distribution of those areas are critical for those higher cognitive functions functions as to the nature of the cognitive information that develop late, such as propositional speech and with which they operate. Much of the prevalent confureasoning. sion in the PFC literature derives from two commonThe profuse variety of connections of the PFC is obvierrors. The first is to argue for one particular prefrontal function while opposing or neglecting others that complement it; the second is to localize any of them within a discrete portion of PFC.The functions of the PFC rely closely on its connections with a vast array of other cerebral structures. None of its cognitive functions can be understood if taken out of a broad connectionist context. Any hyp...

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Our thinking on the cortical organization of primate memory is undergoing a copernican change, from a neuropsychology that localizes different memories in different areas to one that views memory as a distributed property of cortical systems. We are shifting our focus from 'systems of memory' to the memory of systems. The same cortical systems that serve us to perceive and move in the world serve us to remember it. Our memories are networks of interconnected cortical neurons, formed by association, that contain our experiences in their connectional structure. Perceptual and motor memory networks are hierarchically organized in post-rolandic and pre-rolandic neocortex, respectively. Recall, recognition and working memory consist largely in their reactivation, also by association.

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The prefrontal cortex is essential for the temporal integration of sensory information in behavioural and linguistic sequences. Such information is commonly encoded in more than one sense modality, notably sight and sound. Connections from sensory cortices to the prefrontal cortex support its integrative function. Here we present the first evidence that prefrontal cortex cells associate visual and auditory stimuli across time. We gave monkeys the task of remembering a tone of a certain pitch for 10 s and then choosing the colour associated with it. In this task, prefrontal cortex cells responded selectively to tones, and most of them also responded to colours according to the task rule. Thus, their reaction to a tone was correlated with their subsequent reaction to the associated colour. This correlation faltered in trials ending in behavioural error. We conclude that prefrontal cortex neurons are part of integrative networks that represent behaviourally meaningful cross-modal associations. The orderly and timely activation of neurons in such networks is crucial for the temporal transfer of information in the structuring of behaviour, reasoning and language.

Converging evidence from humans and nonhuman primates is obliging us to abandon conventional models in favor of a radically different, distributed-network paradigm of cortical memory. Central to the new paradigm is the concept of memory network or cognit--that is, a memory or an item of knowledge defined by a pattern of connections between neuron populations associated by experience. Cognits are hierarchically organized in terms of semantic abstraction and complexity. Complex cognits link neurons in noncontiguous cortical areas of prefrontal and posterior association cortex. Cognits overlap and interconnect profusely, even across hierarchical levels (heterarchically), whereby a neuron can be part of many memory networks and thus many memories or items of knowledge.