CORTICAL SYNAPTIC ORGANIZATION AND HUMAN INTELLIGENCE

While the biological basis of intelligence has been extensively studied using histology, neuroimaging, genetics, and transcriptomics, the relationship between intelligence and the number of cortical synapses remains largely unexplored.
In this study, we applied Focused Ion Beam/Scanning Electron Microscopy (FIB/SEM) to analyze synaptic organization in the temporal cortex using biopsy samples from patients with temporal lobe epilepsy. The analyzed tissue corresponded to regions with normal EEG activity, resected to provide surgical access to the epileptic focus. We selected individuals with varying Full-Scale Intelligence Quotient (IQ) scores who were seizure-free after surgery. Three FIB/SEM stacks, containing ~240–370 images each, were analyzed per individual using EspINA software.
Surprisingly, in one of the individuals with the highest IQ (115 pre-surgery; 134 post-surgery), we found a stack that exhibited a synaptic density of ~1.2 synapses/µm³. This value is extraordinarily high, significantly exceeding the range previously observed by our group in all human cases and cortical areas to date (from a minimum of 0.23 to a maximum of 0.90 synapses/µm³, including epileptic cases and autopsy controls). These preliminary findings suggest the presence of localized microdomains of high synaptic density potentially associated with high IQ.