The fundamental circularity of being is an age old question of trying to understand the relationship between the brain and the mind. There is definitely a close relationship between the two but what is the nature of that relationship and which one comes first: the mind or the brain? In trying to understand these questions one is reminded that knowledge of the world is subjective. Merleau-Ponty, a French philosopher in the 19th century believed that "the world is inseparable from the subject which is nothing but a projection of the world, and the subject is inseparable from the world, but from a world which the subject itself projects." We create our own representation of the world through experiences that are unique to each of us. There are basic assumptions that have been made in Cognitive Neuroscience: that cognition/mind arises from the particular structures and actions of biological systems or that structures and actions of biological systems arise from cognition/mind processes. Thus is the goal of Cognitive Neuroscience: to understand the mind/brain mapping.
Historical Views
There have been many hypothesis made to explain the relationship between the mind and the brain. Galen in 2AD proposed his Ventricular Hypothesis, suggesting that ventricles (holes in brain) hold cerebral fluid and that there are three chambers: one for vital spirit which is responsible for memory function, one for thinking where estimation takes place and one for sensus communis, responsible for fantasy and imagination. This was the first model of how the brain relates to the mind, which is important because before the model, the mind was thought to be elsewhere (i.e. in the heart or the liver). In 1504 a model was drawn by Gregor Reisch based on evidence and observations.
In 1650 Descartes proposed an adaptation to Galen's model. His interactionist model placed the soul in the head. He believed the pineal gland, a physical organ in the brain, acted as a gate, letting vital fluids flow down tubes to the arms so they can move. His model shows how the substantial (the pineal gland) was involved with the nonsubstantial (soul). This model assumed that input and output of the brain needed to be carried in the same channel. His idea lasted 200 years (till 1800's).
Sir Charles Bell and Francois Magendie mark the beginning of modern neuroscience. They both came up with a model at same time in 1811 that subdivided the brain into two parts, the sensorium and motorium, because they found that input and output needed to be separate. Thus, this became known as the Bell-Magendie Law. Johannes Müller expanded the idea of a division between input and output even further by questioning how information is differentiated between arm and leg inputs, or between visual and auditory inputs if input is the same for all modalities. In 1838 he arrived at his Doctrine of Specific Nerve Energies which stated that it must depend on where in the brain the input signal is processed (sensory experience). Thus it tells us that there are specialized areas in the brain (i.e. function is localized).
In his study of behavior (1887), Franz J. Gall tried to make connections for biology and psychology. He called his findings phrenology . It was a model of radical localization of brain function in which one looks at the bumps on the head to see where specialized areas are located. Basically phrenology said that the cerebral cortex consisted of special functional areas that were responsible different behaviors.
Paul Broca made advances in the 1860's in the studies of localization. He believed language ability to be localized in a restricted brain region. He discovered Broca's area, which is in the left hemisphere in the frontal region of the brain. Controversy still exists today over localization of function.
Many other advances have been made since the 1880's. The beginning of modern biological psychology was by a man named William James. In his "Principles of Psychology" (1890) he labeled consciousness as a property of the nervous system. Karl Lashley's (1890-1958) "search for the engram" assessed the behavioral effects of brain lesions. Donald Hebb (1904-1985) discovered the "Hebbian synapse" and made his hypothesis that explains learning through the strengthening of neurons' connections. In 1980 a model of visual cortex was used to show that low level function is specialized and that high level function is distributed. In 1990 the same model was made much more complex by adding more connections and more areas. There is a big mass area of connections if you look outside visual cortex and their different levels of processing.
To further understand behavior, reductionism, the scientific approach where one analyzes something at a lower or more basic level in hopes of understanding the higher level structure or function, is often used.
Psychology has contributed to cognitive science in helping to explain how bodily mechanisms connect with certain behaviors. Memory, for instance, was thought to be one component, but now we know it consists of many components: declarative (memory for facts), which is mediated by the hippocampus, procedural, (memory for "how to"), which is thought to be mediated by the basal ganglia, and episodic, (memory for past event sequences), which is thought to be mediated by the frontal cortex. More is now understood about the neural networking that is the basis of learning and memory.
Modeling from computer science allow us to gauge the efficacy and biological validity of cognitive theories. This is aided by the size, speed, and memory capabilities of computers.
Neuroscience has contributed to cognitive science through the studies of anatomy and physiology. Anatomy being the study of brain areas and physiology being the study of the response properties of different areas.
One way of studying brain-mind relationships is through MRI's (Magnetic Resonance Imaging). An MRI provides a snapshot of brain tissue at any given time. It measures the change in the magnetic orientation of the nuclei of atoms that make up the tissue. It looks just like a picture. The pros of MRI are its good spatial resolution (very clear detail of tissue) and its non-invasiveness (nothing enters brain tissue). On the other hand, its cons are it offers poor temporal resolution (good details of response over time), no information on function of tissue, and it's expensive. This technique can be used to study structural abnormalities in humans. For example it can show the loss of myelin around groups of axons, which is characteristic of demyelinating diseases, such as multiple sclerosis.
Another technique for studying brain-mind relationships is PET (Positron Emission Tomography). It provides a snapshot of brain activity at a given time. PET measures blood flow by recording emissions from radioactive isotopes such as O15. The levels of radioactivity can then be represented as colors on a picture of the brain. It offers good spatial resolution but poor temporal resolution. It is invasive (enters brain tissue; however, is done with human subjects) and expensive as well. This technique can be used to study function, such as voluntary motor activity or selective attention to somatosensory stimuli.
EEG/ERP (Electroencephalogram /Event-Related Potentials) is another technique that is commonly used. It measures activation of a given brain region by recording its electrical activity. An ERP is a large change in electrical potential in the brain that is elicited by a discrete sensory or motor event. It offers excellent temporal resolution and is non-invasive. Unfortunately it gives poor spatial resolution. This technique is helpful in studying the different stages and kinds of sleep, in distinguishing forms of siezure disorders and with predicting the functional effects of brain injury. ERPs provide a way to assess the hearing pathway and detect hearing impairments early in life. It can also assess brain stem injury or damage produced by tumors or stroke, for example.
Lesions, damaging a specific brain region to study what functions the area is involved in, is yet another technique. The pros of lesions are precise control and good spatial resolution but it can not be done on humans, is very invasive, and offers poor temporal resolution and no recovery from damage done. This technique is used to study how different parts of the brain contribute to sensory, motor, or cognitive processes.
Pharmacology is the use of drugs to turn on or off certain chemical systems in the brain. It gives you precise control, temporary effects, and can be done in humans, but is difficult to interpret the results and is invasive. This technique is used to examine the contributions of chemical systems to behavior.
Single Unit Recording is the use of a microelectrode to record neuronal activity. It offers precise control and excellent temporal and spatial resolution. But like lesions, it cannot be done on humans, is very invasive, and gives a view at a very low level or processing. There are two types of single unit recordings: extra cellular and intra cellular. Extra cellular records from the interstitial fluid surrounding the cell, which is the easier of the two types to use. It records only action potential activity. Intra cellular records from incide the cell. The animal is generally anesthized so that the electrode is not moved about. It records APs (action potentials) and graded potentials (IPSPs and EPSPs).
Immunohistochemistry refers to when the immune system recognizes foreign substances and produces antibodies to detect and kill them. Antibodies are specific only to a given antigen and can be used to label those antigen proteins. It offers precise control and a very specific labeling technique, but it is ultra invasive and thus, cannot be used on humans.
Computer models give a simulation of sensory, motor and cognitive processes on computers. This offers precise control and can model invasive procedures but is not biological and is difficult to generalize on humans.
| Technique | Spatial resolution | Temporal resolution | invasive |
|---|---|---|---|
| Lesion | animal - good / human - poor | N/A | very |
| CT scan | fair | N/A | yes, x-ray |
| MRI | very good | N/A | no? |
| EEG / ERP | poor | excellent (ms) | no |
| MEG | poor, poss. better than ERP | excellent (ms) | no |
| Single Unit Rec. | good | very good | very |
| PET | not as good as CT, better than ERP | OK (min) | yes |
| fMRI | very good | OK, better than PET | no? |
| Pharmacology | good | N/A | yes |
| Computer Modeling | N/A | N/A | no |
The nervous system is divided into two natural subdivisions: a central nervous system (CNS), which consists of the brain and spinal cord, and a peripheral nervous system (PNS), which consists of all the parts outside the bony skull and spinal column. The PNS is divided into two parts: the somatic nervous system and the autonomic nervous system. The autonomic nervous system is composed of the sympathetic division and pare-sympathetic division. It includes pathways that transmit information in both directions between the CNS and various internal organs, which inform the CNS about the surrounding environment and transmits commands from the CNS to the body.
There are different useful terms used in cognitive science to describe orientations for viewing the brain and the body: The sagittal plane bisects the body into right and left halves. The coronal plane divides the body into a front (anterior) and back (posterior). The horizontal plane divides the brain into upper and lower parts. Towards the middle is referred to as medial whereas lateral means toward the side. Rostral is the term for the head end, whereas caudal refers to the tail end. Near the trunk or the center is called proximal and distal means toward the end of the limb. Towards the back is referred to as dorsal and ventral means toward the front .
There are two main types of cells: neurons and glial cells. Neurons are fully developed by approximately 2 years of age; there are 100 billion to 150 billion: there are hundreds of different types; they serve as units of communication: they lose their ability to divide but can always branch out & make more connections, which underlies brain plasticity; they connect to 1,000 to 10,000 other units; they have axons and dendrites; there are 3 different classifications for neurons: shape, size and function; the three types of neurons that are classified by shape are: unipolar, bipolar, and multipolar (which is about 90-95% of neurons); the small neurons are called spindle, stellate, and granule; the three types of neurons classified by function are motor, sensory and interneurons.
Glial cells, on the other hand, have their own distinct functions. They provide housekeeping and produce myelin (which speeds up signal transmission and requires less energy in signal transmission) for neurons: they can keep dividing unlike neurons: they're 10x's more numerous than neurons: there are 3 main types: microglia, astroglia, and oligodendrocyte (myelin-CNS); they do not have axons or dendrites. but they do have feet/extensions (used for myelination); they help develop the cell type of neurons: they produce neurotransmitters; they send Ca++ signals: couple (electrical gap junction = faster) and uncouple neurons; they direct neurons to target destination during development.
Communication between cells is made possible by synapses. Axons form synapses upon the cell body or dendrites of a neuron. They are composed of the synaptic bouton, the postsynaptic membrane and the synaptic cleft. The axon hillock is where the axon originates from and is the place where depolarization first reaches a critical threshold for the neuron to transmit a nerve impulse. Afferent axons carry information into the brain region and efferent axons carry information away from the brain region. There are two types of transportation systems: fast and slow .
Receptors are molecules embedded in the cell membrane that have specialized recognition sites for neurotransmitters. There are two main types: ligand-gated and voltage-gated channels. Ligand-gated channels need ligand (another molecule) to attach to it in order to open, whereas, voltage-gated channels are only sensitive to voltage change in the cell. Some channels can be both.
Ramón y Cajal came up with the concept of dynamic polarization, which wasn't completely correct but which still provided good information to use. Dynamic Polarization says that dendrites are what receive input, the cell body is what integrates and fires the action potential (AP) and the axon is involved in the output of information. Cajal also found that the AP starts at the axon hillock, which is the lowest threshold.
There are two types of electrical signals: action potentials and graded potentials. Action potentials are actually graded potentials that have reached a certain threshold (~65mV). Action potentials do not change in amplitude, have a regenerated signal and are used in long distance communication. Graded potentials can be either excitatory postsynaptic potentials (EPSP) or inhibitory postsynaptic potentials (IPSP). They lose amplitude over time and are not regenerated. They are used for short distance communication.
At the neuronal level information can be coded by five different mechanisms. First is through the frequency of firing. Second by the intensity, which is the changes in the graded potential (amplitude). Third, temporal patterns have a pattern across time: doublets, triplets, or bursts. This is thought to be the main way the brain codes information. The fourth way is spatial patterns: when two different areas firing at different frequencies are combined, a message is sent. Fifth, information can be transmitted via population codes, which are the total summation of the vector. For example: cells in motor cortex fire in different directions depending on the desired movement. Cell firing is probabilistic.
There are four types of input/output pathways: sensory, motor, interneurons and modulatory. Sensory and motor pathways have long axons, are myelinated and are fast. They are organized in an hierarchical structure, show one-to-one mapping and are global. One-to-one mapping means that the connections between one cell to the next remains constant throughout the stages of processing. Global refers to going from low to high level processing. Interneurons have short axons, are myelinated and are fast. They show one-to-one mapping as well but are locally organized and thus, are restricted to a certain area. The modulatory systems use long axons, are unmyelinated and are therefore, slow. They are single source divergent, meaning that they map out in a one-to-many fashion. Each pathway uses functionally distinct systems. The sensory/motor systems use the amino acid neurotransmitters. The interneurons use peptides and the modulatory use biogenic amines.