Functional connections within the human inferior frontal gyrus.
The highly convoluted and cytoarchitectonically diverse inferior frontal gyrus (IFG) of humans is known to be critically involved in a wide range of complex operations including speech and language processing. The neural circuitry that underlies these operations is not fully understood. We hypothesized that this neural circuitry includes functional connections within and between the three major IFG subgyri: the pars orbitalis, pars triangularis, and pars opercularis. To test this hypothesis we employed electrical stimulation tract-tracing techniques in 10 human patients undergoing surgical treatment for intractable epilepsy. The approach involved delivering repeated bipolar electrical stimuli to one site on the IFG while recording the electrical response evoked by that stimulus from a 64-contact grid overlying more distant IFG sites. In all subjects, stimulation of a site on one subgyrus evoked polyphasic potentials at distant sites, either on the same subgyrus or on an adjacent subgyrus. This provided prima facie evidence for a functional connection between the site of stimulation and the sites of the evoked response. The averaged evoked potentials tended to aggregate as response fields. The spatial spread of a response field indicated a divergent projection from the site of stimulation. When two or more sites were stimulated, the resulting evoked potentials exhibited different waveforms while the respective response fields could overlap substantially, suggesting that input from multiple sites converged but by engaging different neural circuits. The earliest deflection in the evoked potential ranged from 2 to 10 msec. No differences were noted between language-dominant and language-nondominant hemispheres.
Greenlee
Jeremy D W
JD
Department of Neurosurgery, University of Iowa, 200 West Hawkins Drive, Iowa City, IA 52242, USA. jeremy-greenlee@uiowa.edu
Oya
Hiroyuki
H
Kawasaki
Hiroto
H
Volkov
Igor O
IO
Severson
Meryl A
MA
Howard
Matthew A
MA
Brugge
John F
JF
D000328:Adult; D001931:Brain Mapping; D004558:Electric Stimulation / Q000379:methods; D004833:Epilepsy, Temporal Lobe / Q000473:pathology / Q000503:physiopathology; D005071:Evoked Potentials / Q000502:physiology* / Q000528:radiation effects; D005260:Female; D005625:Frontal Lobe / Q000473:pathology*; D007839:Functional Laterality; D006801:Humans; D008279:Magnetic Resonance Imaging / Q000379:methods; D008297:Male; D008875:Middle Aged; D009415:Nerve Net / Q000033:anatomy & histology / Q000502:physiology*; D009434:Neural Pathways / Q000503:physiopathology*