The cerebral cortex is divided into areas concerned with specific functions such as touch, vision, hearing and movement. Cortical neurones are organised as layers of cells according to size and shape; they are also grouped together into functional columns that process particular aspects of their inputs.
Cortical columns process different features of a sensory input simultaneously. For example, visual inputs are simultaneously processed by columns that process shape, colour, movement, light intensity, and depth of the visual field.
Parallel processing is a term used to refer to the simultaneous processing of signals in different ways in adjacent cortical areas.
The primary sensory receiving areas send information to many areas of the association cortex.
The parietal lobe, situated between the primary somatosensory and visual cortices is concerned with spatial awareness - the integration of cutaneous, proprioceptive and visual cues as to the position of objects in near space.
The output of the visual cortex is also used to direct eye movements using areas of the frontal lobe, and to recognise faces, and the emotions expressed by them using areas of the temporal lobe.
Other areas of the temporal lobe are concerned with spatial navigation through complex environments, which utilise all senses - somatosensory, visual, auditory and olfactory.
This chapter attempts to bring together the location and function of neurones concerned with higher functions of the central nervous system, including
Facial recognition (temporal lobe)
Emotion (limbic system)
Spatial Navigation (entorhinal cortex)
Attention (prefrontal cortex)
Cortical neurones degenerate in patients with Alzheimer's disease and other dementias, resulting in disconnection of areas concerned with different cerebral functions. This disintegration of the neural networks of the cerebral hemispheres can be devastating.
Key Words: Cortical layers and columns; parallel processing. Specialised areas for memory (hippocampus) emotion (limbic system) and empathath and compassion (insula).
Image Source: What-When-How.com
Columnar organization of somatosensory cortex. Diagram illustrates that different fingers (D1-D3) are represented on adjoining regions of somatosensory cortex. Within the area represented by each finger, there are alternating columns of neurons that are rapidly adapting (green) and slowly adapting (red). The inputs for each type of receptor for each digit are organized into separate columns. (http://what-when-how.com/neuroscience/the-thalamus-and-cerebral-cortex-integrative-systems-part-3/)
The diagram shows the columnar arrangement of the visual cortex: adjacent columns receive inputs from each eye.
Recordings of Neuronal activity using microelectrodes
The brain is a complex structure whose function is not fully understood. However recordings for neurones have shown that activity in one area of cortex is passed to other areas concerned with different functional analyses of events.
While the cortex is divided usually into six layers (with the principal exception of the hippocampus, which has three), these neurones are arranged in columns of similar function. Each column is a functional unit that contains neurones that discharge in a similar manner and often have similar chemical characteristics (such as the amount of cytochrome oxidase, which varies between adjacent columns).
The somatosensory cortex is divided into strips consisting of neuronal columns, concerned with either rapidly adapting OR slowly adapting skin mechanoreceptor inputs, while others are concerned with proprioceptive inputs.
The principle of PARALLEL PROCESSING is that the analysis of the visual input occurs almost simultaneously in different areas of cortex, each concerned with different features of the image.
Image source: Ganong Medical Physiology 25th Edition and http://accesspharmacy.mhmedical.com/
The diagram shows (left) areas of the occipital lobe concerned with analysing visual inputs, and (right) the medial side of the occipital lobe. Visual areas V1, V2, V3, V3a, V4, V5, V6, V7, V8; LO - lateral occipital area; VP - ventroposterior area.
Areas of the Visual Cortex
The visual primary area in the calcarine (striate) cortex connects to at least five other areas in the occipital lobe, each of which appears to specialise in the analysis of visual input. These areas are called V1-V8, and electrical activity of neueones in each area follows the arrival of impulses in V1.
Some of these areas seem to be concerned primarily with analysing shapes (e.g. into lines and corners), others with colour, others with movement, others with the depth of the visual field, while others connect with the frontal eye fields to direct the eyes on to an object of interest.
The principle of parallel processing is also found in the entorhinal cortex, where columns of neurones are concerned with a cognitive map used for spatial navigation.
The Amygdala is an almond shaped structure within the temporal lobe and closely associated and connected with the hippocampus
Facial Recognition and Emotion
The human face and the emotional messages it can convey provide individuality and a means of non-verbal communication. The visual pathway is naturally the essential input pathway that allows discrimination of individual faces and the emotions expressed on them.
The recognition of faces appears to depend on functions of the temporal lobe, and the emotions conveyed depend on structures within the temporal lobe, such as the amygdala and areas of the limbic system.
The Amygdala are almond shaped structures within the temporal lobes, and contain neurones that are activated by faces and facial expression.
The amygdala are not part of the cerebral cortex, but have inputs from many parts of the cortex, including the sensory and frontal cortices and the hippocampus. It helps to determine mood, and has close links with the autonomic nervous system and limbic system.
Bilateral damage to the amygdala has marked effects on behaviour, featuring a lack of aggression and fear in circumstances that would normally provoke them.
Long Term Potentiation. The diagram shows one mechanism in LTP. The synaptic efficacy is increased by the increased presence of receptor molecules in the post-synaptic membrane.
Memory and Empathy
Short term memory depends on the hippocampus, a specialised area of cerebral cortex with only three layers of neurones, and close links with the entorhinal cortex and the hypothalamus.
At a cellular level the intial memory trace seems to depend on a property of spine synapses on pyramidal neurones whereby repetition of an input increases the degree of excitation in the spine synapse, a process called Long Term Potentiation.
Another small area of cortex is the insula, sometimes descibed as a lobe; it is concerned with nociceptive sensation, autonomic control and higher functions of the cerebrum including behaviour.
The presence of 'mirror neurones' that respond not only to experiences within an individual but also to similar experiences in other members of a social group; their existence suggests the biological basis of empathy and compassion.