Scientific Understanding of Consciousness
Consciousness as an Emergent Property of Thalamocortical Activity

Parallel Pathways Link Cortex To Subcortical Appendages


A set of parallel, unidirectional chains link the cortex to a set of its appendages, each with a special structure -- the cerebellum, the basal ganglia, and the hippocampus. (Edelman; Universe of Consciousness, 45)

                 Basal Ganglia Parallel Circuit Diagram


Each of the striatatum and cerebellum subcortical structures forms a  nodal point in a major circuit loop than begins with downward projections from the cortex and ends in a route from the thalamus back to the cortex. (Eichenbaum; Neuroscience of Memory, 238)

The fact that different cortical areas project to different regions of the striatum implies that the corticostriatal pathway consists of multiple parallel pathways serving different functions. (Purves; Neuroscience, 419)

Functionally distinct pathways project parallel from the cortex to the striatum. (Purves; Neuroscience, 420)

The underlying structural framework for basal ganglia interactions with the skeletomotor areas of the cerebral cortex is multiple "closed loops." (Houk, Models of Basal Ganglia; Strick,; Basal Ganglia Circuits, 117)

It has been suggested that there at least five pathways, through the basal ganglia, each organized in parallel, and innervating different regions of the thalamus and frontal cortex. (Houk, Models of Basal Ganglia; Jackson & Houghton; Basal Ganglia Model, 342)

The similarity between cerebellar and basal ganglia architectural organizations is striking. Very large numbers a parallel inhibitory loops are funneled back into a relatively small, excitatory hub with widespread projections. (Buzsáki; Rhythms of the Brain, 367)

Cerebellum is organized as multiple parallel loops without interloop communication. (Buzsáki; Rhythms of the Brain, 364)

Research Study — Basal Ganglia loops for Action Selection


Basal Ganglia Inputs and Outputs

Caudate and putamen of the corpus striatum comprise the input zone of the basal ganglia. (Purves; Neuroscience, 417)

The prototypical structure of these five parallel basal ganglia circuits is an origin in the frontal lobes, projection to striatal structures (caudate, putamen, and ventral striatum), connections from striatum to globus pallidus and substantia nigra, projections from these two structures to specific thalamic nuclei, and a final link back to the frontal lobe. (Philip Lieberman; Human Language, 85)

Large dendritic trees of the striatum allow them to integrate inputs from a variety of cortical, thalamic, and brainstem structures. (Purves; Neuroscience, 418)

Globus pallidus and substantia nigra pars reticulata are the main sources of output from the basal ganglia complex. (Purves; Neuroscience, 418)

Nearly all regions of the neocortex project directly to the striatum, making the cerebral cortex the source of the largest input to the basal ganglia, by far. (Purves; Neuroscience, 418)

Of the cortical areas that innervate the striatum, the heaviest projections are from association areas in the frontal and parietal lobes, but substantial contributions also arise from the temporal, insular, and cingulate cortices. (Purves; Neuroscience, 418)

Efferent neurons of the internal globus pallidus and substantia nigra pars reticulata together give rise to the major pathways that link the basal ganglia with upper motor neurons located in the cortex and in the brainstem. (Purves; Neuroscience, 423)

Pathway to the cortex arises primarily in the internal globus pallidus and reaches the motor cortex after a relay in the ventral anterior- and ventral lateral-nuclei of the dorsal thalamus. These thalamic nuclei project directly to motor areas of the cortex, thus completing a vast loop that originates in multiple cortical areas and terminates (after relays in  the basal ganglia and thalamus) back in the motor areas  of the frontal lobe. (Purves; Neuroscience, 423)

Main Output of the Basal Ganglia is  Inhibitory

Because efferent cells  of both the globus pallidus and substantia nigra pars reticulata are GABAergic, the main output of the basal ganglia is  inhibitory. In contrast to the quiescent medium spiny neurons, the neurons in both these output zones have  high levels of spontaneous activity that tend to prevent unwanted movements by tonically inhibiting cells in the superior colliculus and thalamus. (Purves; Neuroscience, 423)

Cerebellum Parallel Pathways

Both the cerebellum and the basal ganglia, are part of a vast loop that receives projections from and sends projections back to the cerebral cortex and brainstem. (Purves; Neuroscience, 435)

The cerebral cortex is by far the largest source of inputs to the cerebellum. The majority originate in the primary motor and premotor cortices of the frontal lobe, the primary and secondary somatosensory cortices of the anterior parietal lobe, and the secondary visual regions of the posterior parietal lobe.  (Purves; Neuroscience, 438)

Cerebellum projects to motor neurons in the cortex via a relay in the thalamus and in the brainstem. (Purves; Neuroscience, 440)

Deep cerebellum nuclei constitute the major input to the ventral lateral nucleus of the thalamus. These fibers leave the cerebellum via the superior cerebellar peduncle and decussate in the mesencephalon. (Afifi; Functional Neuroanatomy, 245)

Motor Neurons in the Superior Colliculus command Eye Movements

Axons from substantia nigra pars reticulata synapse on upper motor neurons in the superior colliculus that command eye movements, without an intervening relay in the thalamus (Purves; Neuroscience, 423)



Return to — Topological Networks

Link to — Consciousness Subject Outline

Further discussion — Covington Theory of Consciousness