Murray Sherman

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  Thalamus plays a central role in cortical functioning
  Murray Sherman
  Glutamatergic inputs in thalamus and cortex can be classified into two categories: Class 1( driver) and Class 2 (modulator). Following the logic that identifying driver pathways in thalamus and cortex permit insights into information processing leads to the conclusion that there are two types of thalamic relay: first order nuclei like the LGN receive driver input from a subcortical source (i.e., retina), whereas higher order nuclei like the pulvinar relay driver input from layer 5 of one cortical area to another. This thalamic division is also seen in other sensory systems: for the somatosensory system, first order is VPM/L and higher order is POm; and for the auditory system, first order is MGBv and higher order is MGBd. Furthermore, this first and higher order classification extends beyond sensory systems. Indeed, it appears that most of thalamus by volume consists of higher order relays. Many, and perhaps all, direct driver connections between cortical areas are paralleled by an indirect cortico-thalamo-cortical (transthalamic) driver route involving higher order thalamic relays. Such thalamic relays represent a heretofore unappreciated role in cortical functioning, and this assessment challenges and extends conventional views both regarding the role of thalamus and mechanisms of corticocortical communication. Evidence for this transthalamic circuit as well as speculations as to why these two parallel routes exist will be offered.