The relationship between the visually evoked P300 event-related potential and gamma band oscillation in the human medial and basal temporal lobes: An electrocorticographic study
Introduction
Knowledge accumulated from human lesion studies supports the importance of the medial temporal lobe (MTL) for memory and learning. In animal studies, the electrophysiological properties of the MTL's hippocampal structures have been well studied. A high frequency (40–100 Hz) gamma band oscillation (GBO) of the hippocampus has been reported in freely moving rats (Bragin et al., 1995). Together with the hippocampal theta rhythm, the GBO has been suggested to induce long-term potentiation (LTP), which is considered to be a physiological basis for memory (Huerta and Lisman, 1995).
In the primary visual cortex of the cat, coherent neuronal activity in the gamma band has been related to visual feature linking (Eckhorn et al., 1988). Human EEG and ECoG studies revealed the existence of event related GBO (ERGBO) in the human cerebral cortex, which may represent a function of cortical information binding in humans (Pulvermuller et al., 1999, Tallon-Baudry et al., 1997, Tallon-Baudry and Bertrand, 1999, Traub et al., 1996). In our previous study, we reported the presence of continuous GBO (30–150 Hz) in the human MTL (Uchida et al., 2000). However, the electrophysiological properties of the human MTL are still largely unknown.
The P300 event related potential (ERP) is also known to be present in the MTL. Halgren et al. reported that EEG from the human hippocampal formation desynchronizes during attentiveness and movement (Halgren et al., 1978). Halgren's group (Halgren, 1991) reported, based on the topography of scalp-recorded P300 ERPs elicited during an auditory oddball task, that activity generated in the inferior parietal lobe (IPL) is likely to make the major contribution to the scalp-recorded P300. They also reported that large amplitude, polarity—inverting P300-type components were highly localized at the hippocampal contacts of their temporal lobe electrodes. Interestingly, in freely moving rats, Barar (1991) found both the P300 response in the hippocampus and 40 Hz GBO in the same region, and noted their relationships.
Thus, the question arises whether our previously found continuous GBO in the human MTL involves event-related components (Hirai et al., 1999). In the present study, we had epileptic patients with intracranial subdural electrodes attached to the surfaces of the medial and basal temporal lobes (BTL) perform a visual P300 oddball paradigm task, to examine whether event related components are involved in the apparently continuous GBO of the human MTL. We used both averaging techniques and time-frequency analyses; and compared ERPs with ERGBOs to examine both time and frequency domain components of the signals.
Section snippets
Subjects
Subjects were eight patients with intractable epilepsy (four males and four females, average age 30 years, range 24–32 years) who were candidates for neurosurgical operations to treat MTL epilepsy. Epileptic foci were identified in the right hemisphere of six, and in the left hemisphere of two patients. The average Wechsler Adult Intelligence Scale (WAIS) for the group before the operation was scored 92.3. Magnetic resonance imaging (MRI) prior to each operation revealed no significant
ERP and ERGBO
Fig. 2 presents an example averaged signal and time-frequency analysis from subject 2's responses to rare stimuli at the right-6 derivation on the focus side. In that derivation, both P300 ERP and ERGBO were observed. However, while ERGBO overlapped the P300 peak at about 500 ms, it began before the ERP starts and persisted after the P300 component disappeared. Thus, we analyzed the appearance of ERP and ERGBO according to latency, derivation, and frequency (in ERGBO) as follows.
P300 ERPs
To determine
Discussion
In this study, we examined cortical event related oscillations and ERPs in the human MTL and BTL using a visual P300 paradigm. We previously reported the existence of continuous GBO in the MTL (Hirai et al., 1999, Uchida et al., 2000, Uchida et al., 2001) and examined whether the continuous GBO in the MTL could involve event-related components. Our present results revealed some characters of ERGBO as follows. First, ERGBO components were induced more frequently when rare stimuli were indicated
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