ABSTRACT
According to recent proposals, coherent high-frequency activity in cortical neurones may be a specific indicator of cognitive processes in the brain, including those related to the perception of objects, the programming of actions, and the processing of language. One may therefore speculate that cell assemblies involved in higher cortical functions primarily generate high-frequency rhythms. In contrast, it has been argued that such neuronal assemblies may also generate equally strong spectral activity in various frequency bands, high and low, when active. It is proposed here that the controversy can be decided on the basis of knowledge about axonal conduction times inferred from fibre calibres measured in the human cortex. These data suggest that conduction times of most long axons are around 5-10 m/s, and even loops between neurones in distant areas can be travelled in a few hundredths of a second. Reverberation of excitation in assemblies would thus primarily lead to high-frequency cortical activity above 20 Hz. This leads to an explanation of recent findings from large-scale neurophysiological recordings during cognitive processing in humans. KEYWORDS: MEG; EEG; Cell assembly; Axonal conduction time; 30 Hz; 40 Hz; Gamma; Cognition; Language
Cortical neurones usually fire a few spikes per second, that is, with frequencies of a few Hertz (see, for example, Hubel [1988] and Fuster [1995]). However, much faster activity, ranging from tens to hundreds of Hertz, has been found not only in cortical neurones, but in subcortical structures as well (Bressler and Freeman, 1980; Llinás and Ribary, 1993; Nuñez et al., 1992; Singer and Gray, 1995; Steriade, 1993). There are several types of high-frequency activity, including rhythms generated by individual neurones (Llinás et al., 1991; Gray and McCormick, 1996), by subcortico-cortical loops involving many neurones, which force them to fire synchronously (Llinás and Ribary, 1993; Steriade, 1993), and by interactions between excitatory and/or inhibitory cortical cells which also evoke coherent activity in these neurones (Singer and Gray, 1995; Traub et al., 1996). Apparently, several types of rhythms can be distinguished that play different
roles in arousal and attention, perception, and higher cognitive processing (Pulvermüller et al., 1997).
