50-ms component, which is thought to be generated in the lateral part of the primary
auditory cortex in close proximity to the neighboring secondary areas.
The present paper explores the tonotopic organization of the human auditory cortex
using intracerebrally recorded evoked potentials studied as a function of anatomical
recording site. The sensitivity of a neuronal population to a given frequency was deter-
mined from fluctuations in AEP amplitude between different recording sites in the primary
auditory cortex and surrounding secondary areas like the planum temporale. Indeed, pre-
vious data suggest that the auditory cortex is tonotopically organized for frequency and
that even several different tonotopic maps may be described for different cortical regions.
But is this tonotopic organization the same in both left and right cerebral hemispheres? A
substantial body of data suggests that the perception and processing of pitch is specific to
a single hemisphere,^26 and hemisphere-related differences in the functional organization of
the auditory areas underlying such processing are not inconceivable. The main objective of
this study was to investigate the tonotopic organization of the human auditory cortex in
both cerebral hemispheres.
Patients and methods
Intracerebral AEPs were recorded from 45 adult patients (aged 20–56 years) with medically
intractable partial epileptic seizures of right and left temporal lobe origin (in 31 and 14
patients, respectively). These patients were undergoing stereo-electroencephalographic
(SEEG) evaluation for the surgical relief of their seizures (for a description of this pro-
cedure, see Refs 27 and 28). Recordings were carried out in either the left or the right HG
and right planum temporale, during which time subjects laid comfortably on a bed and
remained vigilant and cooperative. All patients had previously given informed consent to
participate in the study.
The anatomical localization of each implanted electrode was based on a stereotaxic pro-
cedure described elsewhere.29–31The three-dimensional coordinates of each electrode lead
were defined and measured with respect to the temporal branch of the middle cerebral
artery and allowed us to distinguish the primary from the secondary auditory areas in the
different patients.25,32
The visualization of electrode tracks using stereotaxic MRI represents a means of con-
firming the anatomical localization of implanted sites and provides additional information
on the anatomy of the HG in each subject (e.g. the presence of one or two gyri^33 ). Such
information can often be helpful in the interpretation of AEP data. MRI sections were cut
along three spatial planes using bicommissural coordinates. Figure 10.1 shows the position
of one electrode implanted orthogonally in the lateral part of the left HG.
Auditory stimulation
Auditory stimuli were tone bursts (with a 0.3-ms rise and decay time and a total duration
of 30 ms) of different frequencies (ranging from 250 Hz to 4 kHz). These stimuli were gen-
erated using an SM 700 NICOLET auditory stimulator and presented at a rate of 0.7/s via
TDH 39 earphones to the ear contralateral to the implanted site. Stimuli were presented in
random order at an intensity of 70 dB SL (sensation level) for each subject.
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