Human cortical EEG rhythms during long-term episodic memory task. A high resolution EEG study of the HERA model.

Many recent neuroimaging studies of episodic memory have indicated
an asymmetry in prefrontal involvement, with the left prefrontal
cortex more involved than the right in encoding, the right more than
the left in retrieval (hemispheric encoding and retrieval asymmetry, or
HERA model). In this electroencephalographic (EEG) high-resolution
study, we studied brain rhythmicity during a visual episodic memory
(recognition) task. The theta (4– 6 Hz), alpha (6– 12 Hz) and gamma
(28– 48 Hz) oscillations were investigated during a visuospatial longterm
episodic memory task including an encoding (ENC) and retrieval
(RET) phases. During the ENC phase, 25 figures representing interiors
of buildings (‘‘indoor’’) were randomly intermingled with 25 figures
representing landscapes (‘‘landscapes’’). Subject’s response was given
at left (‘‘indoor’’) or right (‘‘landscapes’’) mouse button. During the
RET phase (1 h later), 25 figures representing previously presented
‘‘indoor’’ pictures (‘‘tests’’) were randomly intermingled with 25
figures representing novel ‘‘indoor’’ (‘‘distractors’’). Again, a mouse
response was required. Theta and alpha EEG results showed no
change of frontal rhythmicity. In contrast, the HERA prediction of
asymmetry was fitted only by EEG gamma responses, but only in the
posterior parietal areas. The ENC phase was associated with gamma
EEG oscillations over left parietal cortex. Afterward, the RET phase
was associated with gamma EEG oscillations predominantly over right
parietal cortex. The predicted HERA asymmetry was thus observed in
an unexpected location. This discrepancy may be due to the
differential sensitivity of neuroimaging methods to selected components
of cognitive processing. The strict relation between gamma
response and perception suggests that retrieval processes of long-term
memory deeply impinged upon sensory representation of the stored
material.