Recent studies using electrocorticographic (ECoG) recordings in humans have shown that functional activation of cortex is associated with an increase in power in the high-gamma frequency range (ϳ60 -200 Hz). Here we investigate the neural correlates of this highgamma activity in local field potential (LFP). Single units and LFP were recorded with microelectrodes from the hand region of macaque secondary somatosensory cortex while vibrotactile stimuli of varying intensities were presented to the hand. We found that high-gamma power in the LFP was strongly correlated with the average firing rate recorded by the microelectrodes, both temporally and on a trial-by-trial basis. In comparison, the correlation between firing rate and low-gamma power (40 -80 Hz) was much smaller. To explore the potential effects of neuronal firing on ECoG, we developed a model to estimate ECoG power generated by different firing patterns of the underlying cortical population and studied how ECoG power varies with changes in firing rate versus the degree of synchronous firing between neurons in the population. Both an increase in firing rate and neuronal synchrony increased high-gamma power in the simulated ECoG data. However, ECoG high-gamma activity was much more sensitive to increases in neuronal synchrony than firing rate.

For over a century neuroscientists have debated the dynamics by which human cortical language networks allow words to be spoken. Although it is widely accepted that Broca's area in the left inferior frontal gyrus plays an important role in this process, it was not possible, until recently, to detail the timing of its recruitment relative to other language areas, nor how it interacts with these areas during word production. Using direct cortical surface recordings in neurosurgical patients, we studied the evolution of activity in cortical neuronal populations, as well as the Granger causal interactions between them. We found that, during the cued production of words, a temporal cascade of neural activity proceeds from sensory representations of words in temporal cortex to their corresponding articulatory gestures in motor cortex. Broca's area mediates this cascade through reciprocal interactions with temporal and frontal motor regions. Contrary to classic notions of the role of Broca's area in speech, while motor cortex is activated during spoken responses, Broca's area is surprisingly silent. Moreover, when novel strings of articulatory gestures must be produced in response to nonword stimuli, neural activity is enhanced in Broca's area, but not in motor cortex. These unique data provide evidence that Broca's area coordinates the transformation of information across large-scale cortical networks involved in spoken word production. In this role, Broca's area formulates an appropriate articulatory code to be implemented by motor cortex.Broca | speech | ECoG S poken word production is fundamental to human communication. Paul Broca was the first to link word production to a cortical region in the posterior inferior frontal gyrus, since referred to as "Broca's area" (1). His iconic findings are among the most influential in the field of cortical specialization, and Broca's area is still considered to be critically involved in speech production (2, 3).The role of Broca's area in production has been extensively studied using paradigms that vary in complexity from single words to full discourse (4, 5). Although these tasks engage multiple different cognitive demands (e.g., phonological, semantic, and syntactic processing), they all share a common set of core operations consisting of retrieving a word's phonological representation, translating it into an articulatory code, and coordinating the fine motor movements of the vocal articulators (6). However, current neuropsychological and neurolinguistic theories still debate the exact role that Broca's area plays in this set of core operations (7-9). Indefrey and Levelt (4) proposed that Broca's area accesses a phonological word representation that is compiled sequentially into segments of syllables (i.e., syllabification). This segmental representation is then forwarded to motor regions where it is transformed into an articulatory (i.e., phonetic) code. Recent models of speech production (9), as well as the dual-stream model of speech processing (10), do not limit the artic...