The best evidence that the cortex is organised hierarchically comes from the well-established topographical organisation and function of the cerebral cortex.
At the base of this conceptual frame is the separation of cortical areas into primary, secondary and association cortices. The incoming inputs from external sensory signals arrive mostly to the primary sensory somatic, visual and auditory cortices while most cortical motor signals arise from the primary motor cortex. The remaining secondary and association cortices are involved in higher level processing.
This architecture of the CNS suggests that there is a hierarchy of functions with reciprocal connections from the primary sensory cortices progressing to unimodal association and then polymodal association cortices. In parallel, the primary motor cortex is reciprocally connected in a hierarchical way with more frontal areas, premotor areas, caudal prefrontal and rostral prefrontal cortices. At each level up from the primary cortices, the functions become more ‘abstract”, more ‘mental’ and, as implied by this architecture, the distance from the external world (synaptic distance) increases. Motor processing and control proceed from anterior prefrontal, through caudal prefrontal, to premotor cortex. At each level, the processing is informed by the processing at higher levels and by controlling sensory information which, as it moves down the hierarchy, is progressively simpler, more demanding of immediate action and less of temporal integration”. On the other side “Primary sensory areas constitute the bottom stage of the perceptual hierarchy1. This work led to my idea that the cortex is also part of a system of superimposed neural loops.
The broad architecture of the cortex retains the developmental distinction between ventral (motor) and dorsal (sensory) retained from the ventral-motor/dorsal-sensory polarity of the neural tube. The frontal lobes are fundamentally motor and are separated by the central sulcus from the sensory cortex organised in primary sensory cortices for touch (parietal lobe), hearing (temporal lobe) and vision (occipital lobe).
Source: Ian Gibbins
All three primary sensory cortices receive inputs from specific nuclei of the thalamus, which in turn are activated by the sensory pathways of the three primary sensory modalities: vision, touch, and audition.
Adjacent to three primary sensory cortices for each modality are secondary sensory cortices and beyond these are the association cortices.
Similarly in the frontal lobes, located anterior to the central sulcus, the primary motor cortex is where long neurons project to the spinal circuits to generate movements. In the frontal lobes, further anterior to the primary motor cortex, are various components of the frontal association cortex including include premotor, supplementary motor cortex, secondary cortex and different sections of the prefrontal cortex.
Source: Ian Gibbins
These non-primary cortices are also called transmodal cortices because they may receive inputs from different kinds of inputs.
Within the primary somatosensory and motor cortices, there is a strong topological relation between the location on the surface of the body and the corresponding location in the primary cortex. This mapping is called somatotopic organisation with diagrammatic homunculus being graphically represented for the somatosensory and motor primary cortices. Similarly, in the primary visual cortex there is a strong visuotopic (retinotopic) organisation of the visual inputs from the retina, so that every part of the visual field has a corresponding location in the primary visual cortex. In the primary auditory cortex, there is also a tonotopic mapping according to sound frequencies.
Source: Ian Gibbins
These topographical arrangements clearly indicate that the sensory secondary and association cortices are further away from the primary sensory inputs and similarly secondary and association premotor cortices are also further away from the cortical motor output to the spinal cord.
Huntenburg et al (2018)2 proposed that the spatial arrangement of areas along a global gradient between sensorimotor and transmodal regions is a key feature of human cortical organisation. The first dimension of the intrinsic coordinate system can be constructed from the spatial distance along the cortical surface from transmodal regions to primary areas. In the second dimension, each cortical location is described by its relative distance from morphological landmarks in primary sensorimotor areas. It is thus possible to establish a coordinate system based on the intrinsic geometry of the cortex and eventually relate these gradients to specific functional features.
In summary, the extensive mutual connections between higher to lower sensory areas, suggest a clear hierarchical processing within the cortex.
- JM Fuster (2004) Upper processing stages of the perception–action cycle. Trends in Cognitive Science 8, P143-145.
JM Fuster (2003) Cortex and MInd: Unifying Cognition. Oxford University Press. ↩︎ - JM Huntenburg, P-L Bazin, DS Margulies (2018) Large-Scale Gradients in Human Cortical Organization. Trends in Cognitive Sciences 22, P21-P31. ↩︎