martes, 19 de febrero de 2019

The Neocortex and Sensory Perception

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By Chloe Bennett, B.Sc.Reviewed by Jennifer Logan, MD, MPH

The neocortex comprises the largest part of the cerebral cortex and is the outermost layer that covers the structures of the brain. Comprised of four main lobes with specific functions, the cortex contains an abundance of physical structures that are involved in a range of processes including sensory perception.

What is the Neocortex?

The neocortex is thought to be the most recently evolved area of the brain and is comprised of six layers which vary in density, size, neuronal shapes and the organization of nerve fibers. Furthermore, it has a series of gyri (ridges) and sulci (grooves) that differentiate separate the brain into four main lobes. The deepest groove divides the brain into the right and left hemispheres. The central sulcus and the lateral sulcus split the brain into its four distinct lobes; frontal, parietal, temporal and occipital lobes. The frontal lobe hosts the motor cortex which is involved in planning and execution of movement. The parietal lobe processes signals linked to sensation, and the temporal lobe is responsible for memory, hearing ,and the ability to process language. The final lobe, the occipital lobe, is responsible for the processing of vision and is the bulge seen at the back of the brain.

Sensory Perception

Over the years, researchers have tried to investigate and explain the almost seamless three-dimensional perception of our environment that features color, shape, and movement that results from light entering the eyes. An important factor in this processing is thought to center around the hierarchy of information of the visual system. In this system, a large number of specialized cells pass on information to a smaller number of cells that have their own specialized function. This process is outlined below and begins with the rods and cones in the eyes.

The Retina

Sensory perception begins with the rods and cones which are located in the retina. Rods are attuned to low light conditions, whereas cones are attuned to color and bright light. When light reaches these structures at the back of the eye, they transmit impulses through another layer of the retina followed by a third layer, and subsequently to the neuronal fibers of the optic nerve.

Occipital Lobe and Visual Cortex

The occipital lobe hosts the primary visual cortex, which is responsible for assembling signals received into an image in our conscious awareness. The cortex also transmits signals to neighboring areas that protrude into various other areas. Impulses are thought to travel through several branching systems that further consolidate information between preceding and subsequent layers. This network of lateral associations and hierarchical arrangements enables us to name the image produced and identify whether we have seen it previously.

The signal then arrives at the optic chiasm and branches further. Half of the nerve fiber from the respective retina crosses the midline of the brain and ends in the visual cortex of the opposing hemisphere. This results in signals from the right visual field (picked up by the left half of each eye) being projected to the left visual cortex and the signal from the left visual field (gathered by the right half of each eye) being projected to the right visual cortex.

How Does the Neocortex Produce Images?

The area of the cortex responsible for coordinating a flood of information from the eyes is the primary visual cortex, specifically, layers 2 and 4. Layer 2 and 4 of the primary visual cortex are the granular layers abundant in neurons that make synaptic connections with specific localized regions. Signals originating from the optic nerve are transmitted to the intermediate layers of the cortex through the lateral geniculate. Any cells within this area receive impulses from the right or left eye. There are smaller groups of cells that respond to a particular eye to form a striped pattern in the cortex, also known as ocular dominance columns, and indicate a preference toward which eye they receive a signal from. However, this is not an absolute separation, as research shows there is some flexibility, or plasticity, in the development of this system.

Spatial Perception

In addition to producing images, the neocortex and its associated structures are responsible for coordinating external stimuli to allow us to make sense of visual information and regulate our ability to function within our environment. For example, studies have highlighted cases of patients with a deficit in spatial perception who struggle to pour liquid into a glass; this deficit appears to be linked to the parietal lobe. The right hemisphere of the parietal lobe is associated with the recognition of shapes and textures as well as monitoring the left side of the body and the external environment. From this, we can conclude that a network of systems and structures of the neocortex and the brain are responsible for the coordination of signals associated with sensory and spatial perception.

Sensory perception contains several intricate processes which span across several areas of the neocortex to collectively respond to and make sense of sensory information.