Hippocampal-orbitofrontal connectivity in human: An electrical stimulation study
H. Catenoixa, d,
M. Magninb, d,
M. Guénotc, d,
J. Isnarda, d,
F. Mauguièrea, d,
P. Ryvlina, d, ,
The identification of the pathways involved in seizure propagation remains poorly understood in humans. For instance, the respective role of the orbitofrontal cortex (OFC) and of the commissural pathways in the interhemispheric propagation of mesial temporal lobe seizures (mTLS) is a matter of debate. In order to address this issue, we have directly tested the functional connectivity between the hippocampus and the OFC in 3 epileptic patients undergoing an intra-cranial stereotactic EEG investigation.
Bipolar electrical stimulations, consisting of two series of 25 pulses of 1 ms duration, 0.2 Hz frequency, and 3 mA intensity, were delivered in the hippocampus. Evoked potentials (EPs) were analysed for each series, separately. Grand average of reproducible EPs was then used to calculate latency of the first peak of each individual potential.
Hippocampal stimulations evoked reproducible responses in the OFC in all 3 patients, with a mean latency of the first peak of 222 ms (range: 185–258 ms).
Our data confirm a functionnal connectivity between the hippocampus and the OFC in human.
This connectivity supports the potential role of the OFC in the propagation of mTLS.
Intra-cranial electrical stimulation;
Corresponding author. Address: Hôpital Neurologique, 59 Bd Pinel, Lyon 69003, France. Tel.: +33 4 72 35 79 02; fax: +33 4 72 35 73 97.
Medial temporal lobe
The medial temporal lobe consists of structures that are vital for declarative or long-term memory. Declarative (denotative) or explicit memory is conscious memory divided into semantic memory (facts) and episodic memory (events).:194 Medial temporal lobe structures that are critical for long-term memory include the hippocampus, along with the surrounding hippocampal region consisting of the perirhinal, parahippocampal, and entorhinal neocortical regions.:196 The hippocampus is critical for memory formation, and the surrounding medial temporal cortex is currently theorized to be critical for memory storage.:21 The prefrontal and visual cortices are also involved in explicit memory.:21
Research has shown that lesions in the hippocampus of monkeys results in limited impairment of function, whereas extensive lesions that include the hippocampus and the medial temporal cortex result in severe impairment.
Functionally distinct pathways for auditory processing in the orbitofrontal cortex include a rostral stream associated with phonetic processing, and a more caudal stream terminating just posterior to the orbitofrontal cortex in the periarcuate prefrontal cortex associated with auditory-spatial processing, though these connections share extensive overlap. Both ventral and dorsal visual streams share connections with orbitofrontal cortical areas, including rich projections to and from the superior temporal pole, important for integration of spatial and object processing.