Presentation on theme: "Cognitive Neuroscience An introduction Emanuele De Luca | Department of Psychology."— Presentation transcript:
Cognitive Neuroscience An introduction Emanuele De Luca | Department of Psychology
Overview Part 1 A. Background and introduction B. Neuroimaging techniques Part 2 Mirror neurons
Part 1(A) Background and introduction
Mind and Brain: An Empirical Example Wilder Penfield (btw 1928 and 1947) Direct electrical stimulation of cortex Produces "mental" sensations of thinking, perceiving, etc. rather than a sense of the brain being stimulated "a star came down and towards my nose", "those fingers and my thumb gave a jump", "I heard the music again; it is like the radio"
Mind and Brain: Cognitive Neuroscience Approach Cognitive neuroscience aims to provide a brain-based account of cognitive processes (thinking, perceiving, remembering etc.) Made possible by technological advances in studying the brain that are safer and less crude than, say, Penfield’s method
Historical Foundations Do mental experiences arise in the heart (e.g. Aristotle ca. 384 BC) or brain (e.g. Hippocrates ca. 460 BC)? How can a physical substance (brain/body) give rise to mental experiences? = MIND–BODY PROBLEM - Dualism – mind and body are separate substances (e.g. Descartes) - Dual-aspect theory – mind and body are two levels of explanation of the same thing (e.g. like wave-particle duality) - Reductionism – mind eventually explained solely in terms of physical/biological theory These issues still relevant to modern cognitive neuroscience
Historical Foundations Early anatomists believed ventricles important Cortex was often schematically drawn (top) or misrepresented like intestines (bottom left) until 18th century Gall and Spurzheim (1810, bottom right) provide an accurate depiction
Phrenology (Franz Joseph Gall,1796) Different parts of cortex serve different functions Differences in personality traits manifest in differences in cortical size and bumps on skull Crude division of psychological traits (e.g. "love of animals") and not grounded in science Mark Sykes/Science Photo Library
Functional Specialization without Phrenology Although phrenology is discredited, the notion that different regions of the brain serve different functions has stood the test of time Termed FUNCTIONAL SPECIALIZATION Modern cognitive neuroscience uses empirical methods to ascertain different functions It does not assume that each region has one function or each function has a discrete location (unlike phrenology), but does assume some degree of specialization of neurons in particular regions
Functional Specialization: Broca’s Observations ( ) Patient with left frontal lesion (Tan) who could not speak but had otherwise good cognitive abilities Suggested a specialized language faculty in the brain Copyright 2002, with permission from Elsevier
Functional Specialization: After Broca Wernicke ( ) later observed a patient with poor speech comprehension, but good production Suggests at least two language faculties in the brain (comprehension vs production) that can be independently affected by brain damage Note that the faculties were inferred from empirical observation (unlike phrenological faculties) This inference can be made without necessarily knowing where in the brain they are located This approach later became known as COGNITIVE NEUROPSYCHOLOGY
The New Phrenology? Uttal has argued that functional imaging is the new phrenology Stories in the popular press do little to allay these concerns Important to consider computational processes rather than simple localization and also to consider how brain systems interact. This may avoid a new phrenology
The Return of the Brain: Cognitive Neuroscience 1970s: structural imaging methods (CT, MRI) enable precise images of the brain (and brain lesions) 1980s: PET adapted to models of cognition developed by psychologists 1985: TMS is first used (a non-invasive, safer equivalent of Penfield’s earlier studies) 1990: Level of oxygen in blood used as a measure of cognitive function (the principle behind fMRI)
The Methods of Cognitive Neuroscience Temporal resolution Spatial resolution Invasiveness
Part 1(B) Neuroimaging techniques
Brain Reading? To what extent can we tell what someone is thinking by monitoring momentary changes in their brain? What do techniques such as fMRI actually measure? What are the limitations and usefulness of the various methods?
The Active Brain: overview Cognitive activity is associated with increased activity of neurons Neurons performing similar functions tend to cluster together (functional specialization) Neural activity generates electrical signals – measured by electrophysiological techniques Neural activity leads to oxygen consumption and this leads to localized changes in blood flow (a haemodynamic response) – measured by functional imaging (PET, fMRI)
Revision of Neural Electrical Activity Axons propagate action potentials (sudden depolarization of membrane) The electrical input from lots of different neurons is summed together. If it exceeds a threshold then the receiving neuron will also generate an action potential.
Action Potentials summed
Two Main Electrophysiological Techniques (1) Single-cell recordings Electrode/s placed in or near a neuron (invasive) Measure number of action potentials per second (2) Event-related potentials (ERP) Electrode/s placed on the skull Measures summed electrical potentials from millions of neurons (sensitive to dendritic currents, pyramidal neurons)
Single-Cell Recordings Enable researchers to understand how individual neurons code information How specific is the response of neurons? For example, could one find a neuron that responded to just one face (a "grandmother cell")? Neurons may respond to faces more than objects, and to some faces more than others Have you a granma neuron?
Single-Cell Recordings Neurons may respond to conceptual properties of a stimulus too For example, this neuron responds when gaze is oriented downwards (even though the physical pose is very different)
Event-Related Potentials (ERPs) Based on EEG (electroencephalography) recordings EEG signal is averaged over many events (to reduce effects of random neural firing) and synchronized to some aspect of the event (e.g. onset of stimulus, pressing a button) Electrodes record a series of positive and negative peaks Timing and amplitude of the peaks is related to different aspects of the stimulus and task (e.g. consider face recognition)
Event-Related Potentials (ERPs)
Pyramidal neurons of the cortex
N170 component for faces
Advantages and Disadvantages of ERP ERP signal is directly related to neural activity and this electrical activity is conducted instantaneously to the scalp Therefore, ERP has an excellent temporal resolution The ERP signal is derived from different sources in the brain and it is not possible to infer exactly where these sources are from the scalp Therefore, ERP has a poor spatial resolution
Functional Imaging Neural activity consumes oxygen as well as generating electrical signals In order to compensate for increased oxygen consumption, more blood is pumped into the active region PET measures the blood flow in a region, whereas fMRI measures the blood oxygenation The time taken for this response is slow (several seconds) and so functional imaging has a poor temporal resolution, but a good spatial resolution This is the complementary profile to EEG
Positron Emission Tomography (PET) Measures local blood flow Radioactive tracer injected into blood stream Tracer takes up to 30 seconds to peak Catherine Pouedras/Science Photo Library
Functional Magnetic Resonance Imaging (fMRI) Does not use radioactivity, but directly measures the concentration of deoxyhaemoglobin in the blood This is called the BOLD response (Blood Oxygen Level Dependent contrast) The change in BOLD response over time is called the haemodynamic response function and it has a number of distinct phases (not to be confused with the ERP waveform, which is completely unrelated) The Haemodynamic Response Function peaks in 6–8 seconds and so this is the temporal resolution of fMRI
Functional Magnetic Resonance Imaging (fMRI) Haemodynamic response function (change in BOLD signal over time)
Cortex: laminated gray matter covering of brain MRI (T1weighted): Gray matter = cells White matter = axons
Face processing areas in the human brain
What does it mean to say a brain region is "active"? The brain has a constant supply of blood and oxygen; if it didn’t, it would die This means we cannot literally stick someone in a scanner and read their thoughts (because the whole brain would look active) In order to infer functional specialization, one needs to compare RELATIVE differences in brain activity between two or more conditions A region is "active" if it shows a greater response in one condition relative to another If the experimenter chooses inappropriate conditions the regions of activity will be meaningless (junk in, junk out) – functional imaging isn’t straightforward
Traditional polygraph measures bodily response (e.g. sweating, heart rate), but what if someone doesn’t feel guilty? Brain is the organ that creates the lie Anterior cingulate cortex is active when asked to generate false answers to questions relative to truthful ones (e.g. “where was your last vacation?”) This region believed to be involved in monitoring conflicts between responses BUT, not necessarily active if lie is memorized in advance Lie Detection
Is Brain Reading Possible? Most experiments manipulate cognitive processing and measure changes in brain response Can the reverse be done, i.e. can the brain response be used to infer what people are thinking? Brain response can be used to predict which object is seen or imagined from a limited set (e.g. face vs house vs cats vs shoes) with ~90% accuracy However, can only be done if experimenter pre- tests the participants on these items (to know where to look in that particular person)
Limits on Brain Reading Each brain is subtly different anatomically May be able to measure general contents of thought (e.g. a face) rather than specific contents (e.g. Tony Blair’s face) Not easy to distinguish between conscious and unconscious cognition just from looking at the brain
TMS: Reverse Engineering Infer the function of a region (or cognitive mechanism) by removing it and measuring the effect on the rest of the system For example, if damage to a region disrupts reading, but not speaking or seeing, then one might conclude that the region is specialized for some aspect of reading Disruption of brain function comes about through natural damage (strokes, etc.), elicited damage (e.g. animal models), or harmless temporary changes induced electro-magnetically (TMS)
Transcranial Magnetic Stimulation (TMS) Coil contains a wire carrying an electric current A rapid change in the current creates a magnetic field The magnetic field induces a current in the nearby neurons (causing them to "fire", i.e. generate action potentials) This disrupts the cognitive function that they may be doing at that point in time ( virtual lesion) Simon Fraser/Science Photo Library
Part 2 Mirror neurons
Definition: Mirror neurons are a class of neurons in the parietal lobe. Discovered monkeys in 1992 (Rizzolatti Lab) and studied ever since then. They fire both when the animal performs an action (grasps an object) and when sees another individual make a similar action (monkey or human).
Research Paradigm Microscopic electrodes inserted into single neurons. Monkey hooked to machine which records Discovery of the function of these neurons happened by accidental (anecdote of the nut).
Finding mirror neurons Pre-motor and posterior parietal cortex neuron responses in the monkey a) Observed action b) Executed action Only when movement has a purpose (is an action) neurons fire Specific to action type (pick up or move peanut)
Mirror Neuron facts For these neurons to fire the monkey has to see an object-directed action (object alone or hand movement alone is not enough). The mere observation of an object not acted upon does not evoke a response either. Reward not relevant to firing. Mirror neurons can be driven not only by action execution and observation, but also by the sound produced by the same action
Mechanism of action understanding Each time monkey sees action by another individual, neurons that are activated when the same action is executed by himself are firing. Thus the monkey has knowledge of the other’s action from his own activity.
Human Mirror Neurons The mirror neuron system for actions in humans cannot be directly studied. fMRI paradigms Rizzolatti (in 2005) found that when people listened to sentences describing actions, the same mirror neurons fired as would have fired had they performed the actions or witnessed them being performed.
Human Mirror Neuron System
Empathy - emotions Mirror neuron network in man involves and activates Amygdala (emotions) We do not accomplish understanding of other’s feelings by analogy or thinking processes. Rather, the other’s emotion is experienced by ourselves and therefore directly understood. It is a shared body state.
Evolution of language Once you have two abilities in place; 1) the ability to read someone's intentions (through your own) 2) the ability to mime their vocalizations then you have set in motion the evolution of language. (You need no longer speak of a unique language organ and the evolution of language doesn't seem quite so mysterious any more).
Social Skills Anytime you watch someone else doing something (or even starting to do something), the corresponding mirror neuron fires in your brain, thereby allowing you to "read" and understand another's intentions, and thus to develop a sophisticated "theory of other minds."
Mirror neurons in Autism Mirror neurons that fired in nonautistic people watching someone else make meaningless finger movements didn’t fire in autistic children. Both autistic and nonautistic adolescents watched pictures of people with distinctive facial expressions. Both subjects could imitate the expressions and say what emotions they expressed. But while the nonautistic teens showed robust activity in mirror neurons corresponding to the emotions expressed, the autistic teens showed no such activity. They understood the expressions cognitively but felt no empathy.
Conclusions: Ramachandran "I predict that mirror neurons will do for psychology what DNA did for biology," "They will provide a unifying framework and help explain a host of mental abilities that have hitherto remained mysterious and inaccessible." Mirror neurons primed the human brain for the great cultural leap forward. Once we had the ability to imitate, and learn through imitation, transmission of culture is possible.