Organization of the mouse cerebral cortex: a histochemical study using glycogen phosphorylase.

The postmortem phosphorylase a activity in cryostat sections taken from the brains of mice killed without any prior period of maintained anaesthesia ('unanaesthetized' mice) was compared histochemically with the activity in sections taken from mice anaesthetized with pentobarbitone prior to decapitation ('anaesthetized' mice). This study provides evidence for the existence of two separate overlapping modular subdivisions of the somatosensory cortex. The modules observed in the 'unanaesthetized' mice were restricted to layer IV and corresponded to the hollows of the whisker barrels where the thalamic afferents terminate. In contrast, the modules observed in the 'anaesthetized' mice extended from layer I to layer V, and formed a mosaic of cylinders that was out of register with the whisker barrels. These cylinders may correspond to the terminal fields of corticocortical afferents. In a few of the 'unanaesthetized' mice bands of high phosphorylase a activity are evident in the visual areas 17 and 18. This suggests that in the mouse, as in higher mammals, the thalamic input to the visual cortex gives rise to a columnar organization. In the 'anaesthetized' mice the mosaic of modules observed in the somatosensory cortex is also present in the auditory and motor areas. The modules in both groups of mice have similar diameters of between 200 and 350 microns.