The parafascicular nucleus in rats and the centromedian parafascicular complex in primates and other mammals are the so-called posterior intralaminar nuclei (pIL) of the thalamus. Like the ascending reticular activating system and the basal ganglia-thalamocortical circuit, the pIL nuclei are part of the brain arousal systems.To describe and analyze different animal and human studies suggesting that the pIL could also be part of a neuro-physiological subcortical system related to cognitive processes as attention, learning and memory.Both parafascicular nucleus lesion studies in rats and neuropathological and neuroimaging reports in humans, indicate a relationship between the degeneration of pIL neurons and the cognitive deficits observed in learning and memory tasks in animals and also in several human neurological diseases and in consciousness disorders.Considering its neuroanatomical, neurophysiological and functional characteristics, the pIL can be considered excellent candidates for investigating cognitive processes in the field of psychobiology and clinical neurology.