Topic 4: The brain and neuropsychology Revision Notes

Understanding Neuronal Structure and Function

Neurons are the basic units of the nervous system, responsible for transmitting information. Each neuron typically consists of a cell body (soma), which contains the nucleus and essential cell machinery; dendrites, which are branch-like extensions that receive chemical signals from other neurons; and an axon, a long projection that carries electrical impulses away from the cell body to other neurons. Some axons are covered in a myelin sheath, a fatty layer that insulates the axon and significantly increases the speed of electrical signal transmission. Neurons operate on an 'all-or-nothing' principle, meaning that once a threshold of electrical stimulation is reached, the neuron fires an action potential at its full strength; otherwise, it does not fire at all. There is no partial firing or varying strength of action potentials.

Synaptic Transmission, Neurotransmitters, and the CNS

Synaptic transmission is the process by which neurons communicate at a synapse, the junction between two neurons. When an electrical impulse (action potential) arrives at the axon terminal of the presynaptic neuron, it stimulates the release of neurotransmitters from synaptic vesicles into the synaptic cleft, the microscopic gap between neurons. These neurotransmitters then diffuse across the cleft and bind to specific receptor sites on the postsynaptic neuron's dendrite or cell body. This binding causes a change in the electrical potential of the postsynaptic neuron, either making it more likely to fire its own action potential (excitation) or less likely (inhibition). Examples of neurotransmitters include serotonin (involved in mood, sleep) and dopamine (involved in reward, motivation, motor control). These continuous electrochemical signals across vast neural networks are essential for the functioning of the Central Nervous System (CNS), which includes the brain and spinal cord. The CNS processes sensory input, integrates information, and coordinates responses, enabling all complex behaviours and psychological processes such as learning, memory, and emotion.

Early Psychology & Introspection

Psychology's roots are in philosophy. Wilhelm Wundt established the first psychology laboratory in 1879, pioneering experimental psychology. His method, introspection, involved trained individuals systematically observing and reporting their own conscious mental processes (sensations, feelings, images). While a revolutionary step towards scientific inquiry, introspection was criticised for its subjective nature, lack of empirical agreement across observers, and inability to study unconscious processes or animal behaviour effectively. This limited its scientific reliability and generalisability.

Early Neuropsychological Insights

Even before Wundt, 19th-century clinical observations provided early evidence linking brain to behaviour. Paul Broca (1861) and Carl Wernicke (1874) studied patients with specific language deficits resulting from brain damage. Broca's area was linked to speech production, and Wernicke's area to speech comprehension. These 'lesion studies' demonstrated localisation of function, showing that specific brain regions are responsible for particular mental processes. This early neuropsychological evidence was crucial, establishing a biological basis for psychological functions and paving the way for later brain-based research.

Mid-20th Century Shifts: Behaviourism and Cognitive Revolution

In the early 20th century, behaviourism (e.g., Watson, Skinner) emerged, advocating for psychology to study only observable behaviour, rejecting introspection and unobservable mental states. Concurrently, psychodynamic theories (e.g., Freud) focused on unconscious drives. However, the 1950s-60s saw the 'cognitive revolution,' a significant shift that brought internal mental processes (e.g., memory, perception, problem-solving) back into focus, often using information-processing models. This marked a return to studying 'the mind' but with greater scientific rigour than introspection.

Modern Brain-Based Research & Neuropsychology

The cognitive revolution laid the groundwork for cognitive neuroscience. From the 1970s onwards, technological advancements led to advanced neuroimaging techniques. Positron Emission Tomography (PET) scans (1970s-80s) and functional Magnetic Resonance Imaging (fMRI) scans (1990s) allow researchers to observe changes in blood flow (and thus indirectly, brain activity) during cognitive tasks. Electroencephalography (EEG) measures electrical activity, providing excellent temporal resolution. These methods provide objective, quantitative data, allowing for empirical testing of theories about brain-behaviour relationships. Neuropsychology integrates these tools with studies of cognitive deficits following brain injury, offering specific insights into how brain structures support mental functions. This has bolstered psychology's empirical foundation and scientific credibility.

Aim of Sperry's (1968) Study

Sperry's study aimed to investigate the extent of functional specialization in the two cerebral hemispheres and whether they could operate independently following the surgical severing of the corpus callosum (commissurotomy). The research focused on understanding hemispheric lateralisation and the unique contributions of each hemisphere to perception and cognition in patients treated for severe epilepsy.

Key Procedure and Findings

Participants were patients who had undergone commissurotomy. They sat in front of a screen with their gaze fixed on a central point. Visual stimuli (e.g., images, words) were flashed for a mere 0.1 seconds to either the left or right visual field using a tachistoscope, ensuring information reached only one hemisphere initially. Tactile stimuli were also placed in either hand under a screen. Key findings included: information to the left visual field (right hemisphere) could not be verbally named but could be identified by the left hand. Information to the right visual field (left hemisphere) could be named verbally. This indicated left hemisphere dominance for language and right for visuospatial tasks, showing independent awareness.

Strengths and Weaknesses of the Study

A strength is the highly controlled laboratory setting, which minimised extraneous variables and allowed for precise observation of hemispheric functions. This yielded robust data on lateralisation. A key weakness is the limited generalisability; the sample was small (11 patients initially) and unrepresentative. Participants were epileptics who had undergone brain surgery, meaning their brains were not neurologically typical. This makes it difficult to extend findings directly to the healthy population.