3 Introduction to Brain Imaging
51
a neurodegenerative disease such as multiple sclerosis [40]. ERPs are particularly
useful in cases when MRI indices of multiple sclerosis are inconclusive [5]. Routine
clinical evaluations can include the measurement of short- and middle-latency
visual, somatosensory, and auditory evoked potentials for the differential diagnosis
of sensory (peripheral) and central nervous system disorders. The measurement of
ERPs is objective in that most clinically useful responses are obligatory and independent of the patient’s compliance. ERPs can be used to assess the neurological
status in patients who are anaesthetised or comatose.
3.1.2.2 Simultaneous fMRI and EEG
Simultaneous recording of brain activity with two neuroimaging modalities can provide information beyond that attainable with each modality alone. The development
of simultaneous MRI/EEG was motivated by clinical interest in mapping sources of
epileptic discharges onto MR images [47]. Simultaneous fMRI/EEG is particularly
attractive because it holds the promise of capitalising on the fine spatial resolution
of fMRI and the fine temporal resolution of EEG. The combination of fMRI and
EEG may allow improved localisation of neural generators identified with EEG,
as well as enhanced temporal resolution of focal activity measured with BOLD
fMRI [60, 66, 67]. Simultaneous recordings are preferable to sequential recordings
in that they allow perfect correspondence of the experimental conditions (acoustic
and visual environment) and cognitive factors (task performance, attention) between
the recordings. This is critical for studying cognitive functions involving learning,
memory, or attention. However, simultaneous recordings are technically challenging
as they essentially involve running two experiments at the same time. The analysis
of simultaneous fMRI/EEG data entails extracting small EEG signals from measurements made in the electromagnetically noisier MRI environment [17, 28, 107], as
well as integrating EEG and fMRI signals with inherently different spatiotemporal
scales [73]. The primary clinical application of simultaneous fMRI/EEG is for the
localisation of epileptic zones [35, 59].
3.1.2.3 Intracranial EEG (iEEG)
EEG can also be obtained intracranially, using strips or grids of electrodes implanted
in the subdural space (electrocorticography – ECog), or using wires of electrodes
inserted into deeper brain structures such as the hippocampus (stereotactic EEG –
sEEG) [93]. Intracranial EEG provides high spatial and high temporal resolution of
neural activity, however only in the brain areas covered by the electrodes. Because
it is an invasive procedure, in humans, iEEG is performed strictly when clinically
indicated. iEEG is performed for presurgical localisation of seizure foci in patients
with medically intractable epilepsy and provides a unique opportunity to study
cognitive functions in vivo and at high spatiotemporal resolution, with the caveats
51
a neurodegenerative disease such as multiple sclerosis [40]. ERPs are particularly
useful in cases when MRI indices of multiple sclerosis are inconclusive [5]. Routine
clinical evaluations can include the measurement of short- and middle-latency
visual, somatosensory, and auditory evoked potentials for the differential diagnosis
of sensory (peripheral) and central nervous system disorders. The measurement of
ERPs is objective in that most clinically useful responses are obligatory and independent of the patient’s compliance. ERPs can be used to assess the neurological
status in patients who are anaesthetised or comatose.
3.1.2.2 Simultaneous fMRI and EEG
Simultaneous recording of brain activity with two neuroimaging modalities can provide information beyond that attainable with each modality alone. The development
of simultaneous MRI/EEG was motivated by clinical interest in mapping sources of
epileptic discharges onto MR images [47]. Simultaneous fMRI/EEG is particularly
attractive because it holds the promise of capitalising on the fine spatial resolution
of fMRI and the fine temporal resolution of EEG. The combination of fMRI and
EEG may allow improved localisation of neural generators identified with EEG,
as well as enhanced temporal resolution of focal activity measured with BOLD
fMRI [60, 66, 67]. Simultaneous recordings are preferable to sequential recordings
in that they allow perfect correspondence of the experimental conditions (acoustic
and visual environment) and cognitive factors (task performance, attention) between
the recordings. This is critical for studying cognitive functions involving learning,
memory, or attention. However, simultaneous recordings are technically challenging
as they essentially involve running two experiments at the same time. The analysis
of simultaneous fMRI/EEG data entails extracting small EEG signals from measurements made in the electromagnetically noisier MRI environment [17, 28, 107], as
well as integrating EEG and fMRI signals with inherently different spatiotemporal
scales [73]. The primary clinical application of simultaneous fMRI/EEG is for the
localisation of epileptic zones [35, 59].
3.1.2.3 Intracranial EEG (iEEG)
EEG can also be obtained intracranially, using strips or grids of electrodes implanted
in the subdural space (electrocorticography – ECog), or using wires of electrodes
inserted into deeper brain structures such as the hippocampus (stereotactic EEG –
sEEG) [93]. Intracranial EEG provides high spatial and high temporal resolution of
neural activity, however only in the brain areas covered by the electrodes. Because
it is an invasive procedure, in humans, iEEG is performed strictly when clinically
indicated. iEEG is performed for presurgical localisation of seizure foci in patients
with medically intractable epilepsy and provides a unique opportunity to study
cognitive functions in vivo and at high spatiotemporal resolution, with the caveats