Hebb's Dream: The Resurgence of Cell Assemblies

of Hebb's theory. Testing this theory would require one and Asif A. Ghazanfar to record from populations of neurons distributed across Department of Neurobiology multiple cortical areas and subcortical nuclei. Using a Duke University Medical Center chronic multiple-electrode recording preparation, Nico-Durham, North Carolina 27710 lelis and coworkers (1995, 1997a) recorded from populations of neurons distributed throughout the trigeminal somatosensory pathway, from the trigeminal brain stem complex and the somatosensory thalamus to the pri-Although Donald Hebb's classic book, The Organization mary somatosensory cortex, in awake, behaving rats. of Behavior (1949), is widely known for its description These authors showed that ensembles of single neurons of a mechanism for synaptic plasticity (the so-called from most of these structures exhibited widespread, Hebbian synapse), it also contains one of the most influ-synchronous oscillatory firing that began during atten-ential proposals on how interactions between large pop-tive immobility and predicted the onset of rhythmic ulations of neurons underlie brain processes. Central to whisker movements. These oscillations were detected Hebb's theory is the concept of the ''cell assembly. .. first in cortex and then spread to subcortical structures. a diffuse structure comprising cells in the cortex and The cortical and subcortical ensembles underlying such diencephalon, capable of acting briefly as a closed sys-rhythmic firing have been found to contain highly distrib-tem, delivering facilitation to other such systems. . .'' (p. xix). In Hebb's view, individual neurons did not work uted representations of tactile information (Ghazanfar in isolation, and consequently, they could not, by them-and Nicolelis, 1997; Nicolelis et al., 1997a, 1997b). Multi-selves, account for any given percept or ability. In fact, variate statistical analysis (e.g., discriminant analysis he suggested that individual neurons could participate and canonical correlation) revealed that in this sensory in different cell assemblies and be involved in multiple system, the precise location of a tactile stimulus could functions and representations. Although the importance only be unambiguously predicted, on a single trial basis, of neural populations in sensorimotor information pro-when population rather than single neural responses cessing had been recognized earlier (Young, 1802; Sher-were taken into account (Nicolelis et al., 1997b). There-rington, 1906), Hebb's work was a landmark because it fore, these results emphasized that the coordinated ac-provided the first elaborated description of mechanisms tivity of large ensembles of neurons, distributed across by which neural populations could underlie a variety of cortical and subcortical structures, may provide the ba-brain functions. sis for the encoding of …