In non-human primates the following structural connections of the posterior cingulate cortex are well documented: As is true in other areas of the posteromedial cortex, the posterior cingulate cortex has no apparent connections to primary sensory or motor areas. Thus, it is unlikely to be involved in low-level sensory or motor processing. While many of the connections in non-human primates may be present in humans, they are less well documented. Studies have shown strong reciprocal connections to medial temporal lobe memory structures, such as the entorhinal cortex and the parahippocampal gyrus, the latter being involved in associative learning and episodic memory. In humans, the PCC is also connected to areas involved in emotion and social behavior, attention (the lateral intraparietal cortex and precuneus), learning and motivation (the anterior and lateral thalamic nucleus, caudate nucleus, orbitofrontal cortex and anterior cingulate cortex). The posterior cingulate cortex is highly connected and one of the most metabolically active regions in the brain, but there is no consensus as to its cognitive role. Cerebral blood flow and metabolic rate in the PCC are approximately 40% higher than average across the brain. The high functional connectivity of the PCC, signifies extensive intrinsic connectivity networks (networks of brain regions involved in a range of tasks that share common spatio-temporal patterns of activity). The posterior cingulate cortex has been linked by lesion studies to spatial memory, configural learning, and maintenance of discriminative avoidance learning. More recently the PCC was shown to display intense activity when autobiographical memories (such as those concerning friends and family) are recalled successfully. In a study involving autobiographical recollection, the caudal part of the left PCC was the only brain structure highly active in all subjects. Furthermore, the PCC does not show this same activation during attempted but unsuccessful retrieval, implying an important role in successful memory retrieval (see below: Alzheimer’s Disease). The posterior cingulate cortex has also been firmly linked to emotional salience. Thus, it has been hypothesized that the emotional importance of autobiographical memories may contribute to the strength and consistency of activity in the PCC upon successful recollection of these memories. The posterior cingulate cortex is significantly bilaterally activated by emotional stimuli, independent of valence (positive or negative). This is in contrast to other structures in the limbic system, such as the amygdala, which are thought to respond disproportionately to negative stimuli, or the left frontal pole, which activated only in response to positive stimuli.