| Surprisingly, there is a dearth of information regarding the synaptic properties of interareal corticocortical pathways. Much of our knowledge, theories and concepts of cortical organization are based on studies looking into the mechanisms involved in visual perception, with most relevant data being limited to anatomical descriptions. We have been interested in studying the functional nature of directly projecting corticocortical connections, paying special attention to glutamatergic inputs. In previous studies, corticothalamic pathways prompted a new classification scheme for glutamatergic synapses: drivers and modulators. Drivers define the main receptive field properties, produce large, all-or-none excitatory postsynaptic potentials (EPSPs) with paired-pulse depression (PPD), and they activate only ionotropic glutamate receptors (iGluRs). Modulators alter the probability that certain aspects of driving information will be transferred. They exhibit small, graded EPSPs with paired-pulse facilitation (PPF) and have a metabotropic glutamate receptor (mGluR) component. We now have evidence that this driver/modulator classification holds true for both feedforward and feedback corticocortical inputs, and the finding that many connections are modulatory implies that a full understanding of how information is transmitted between cortical areas will require a more complete functional identification of these pathways. |
