The neocortex is part of the human brain’s cerebral cortex where higher cognitive functioning is thought to originate from. Due to the natural grooves and ridges in the brain, the neocortex is comprised of four main lobes with specific functions.
The neocortex comprises the largest part of the cerebral cortex and makes up approximately half the volume of the human brain. It is thought to be responsible for the neuronal computations of attention, thought, perception and episodic memory.
Structural similarity exists across the entire neocortex. However, research shows that specific functions are localized to distinct areas.
This localization has been characterized based on a number of factors, including cytoarchitecture, input and output connectivity, proportion of cell types, modular structure, and micro-circuitry.
Structure of the Neocortex
The human brain differs from brains of other animals and even mammals based on its relatively larger size and predominance of the neocortex. Scientists divide the neocortex into smaller cortical areas that are often referred to as the “organs of the brain”. Some cortical areas have been extremely well described, whereas others pose more puzzling and disagreement over function exists.
The neocortex itself is essentially like a sheet of tissue that varies in thickness but has a very large surface area. It is comprised of six layers of neuronal types: one layer of axon endings and apical dendrites from cortical neurons plus five cell layers. In just one hemisphere of the neocortex, there are an estimated 8 billion neurons.
What are the Layers of the Neocortex?
Roman numerals are used to number the six neocortex layers. In order of superficial to deep, the layers are
- Molecular later: contains a very small number of neurons,
- External granular layer,
- External pyramidal layer,
- Internal granular layer,
- Internal pyramidal layer,
- Fusiform or multiform layer.
Each layer is characterized by varying neuronal shapes, density, sizes, and organization of nerve fibers.
What is the Function of the Neocortex?
The neocortex is comprised of 4 regions based on the patterns of sulci (grooves) and gyri (ridges) in the brain: frontal, parietal, occipital, and temporal lobes. Research on brain function shows that each neocortical layer has specific functions.
The frontal lobes are responsible for the selection and coordination of goal-directed behavior. In this region of the neocortex is the human executive function that manages the intricacies of multiple complex processes such as task switching, reinforcement learning, and decision-making to name a few. Disorders of the frontal lobe include frontotemporal dementia, Parkinson’s disease, and Alzheimer’s disease.
Traditionally thought as the association cortex, the parietal lobe is believed to play a role in decision-making, numerical cognition, processing of sensory information, and spatial awareness.
The occipital lobe is responsible for visual function and is the bulge seen at the back of the brain. It hosts the primary area for visual perception which is closely surrounded by the visual association area.
The temporal lobe houses the hippocampus and the amygdala. Among its functions are to process sensory information and derive language, emotions, and meaningful memories.
Additionally, it is responsible for declarative memory, which is memory that can be spoken aloud (such as learned facts), and is further divided into two subgroups— semantic and episodic memory.
The Neocortex and the Dorsal Cortex
Despite intense research spanning over a century, the evolutionary origin of the neocortex is still unclear. A large amount of research investigating the neocortex’s origin focuses on similarities between the dorsal cortex and the neocortex.
The dorsal cortex makes up a small region of the forebrain and contains a thin layer of pyramidal neurons and inhibitory interneurons.
From the observation of fossil records, it is thought that early mammals had a small neocortex relative to the size of the olfactory bulb and appeared as a small cap at the top of the forebrain. In addition, it was not thought to be more extensive than the dorsal cortex of egg-laying vertebrates.
Early works identified similarities between the patterns of connection between neuronal types in the dorsal cortex of reptiles and the neurons of the neocortex.
The neurons in the dorsal cortex receive visual and sensory inputs from the thalamus on apical dendrites that expand across the cortical surface. Furthermore, each neuron is activated by several inputs.
However, the neocortex has been suggested to receive major activating inputs through a stellate of neurons in layer four. The neurons found in layer four activate the neurons in the neighboring neurons rather than providing a direct output.
Despite there being similarities between the dorsal cortex and the neocortex, many researchers are still unclear on how the neocortex developed over millions of years of evolution, to produce its characteristic structure with more neurons than the early dorsal cortex.