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Lecture 2

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PSYC 2650
Anneke Olthof

LECTURE TWO – JANUARY 15/2013 Introduction Video: because he is having no physical reaction when he sees them, his brain tells him that the people are not people he knows. Principal Structures of the Brain - Different parts of the brain do different things; and when they get damaged they can do weird things. - The simplest fact illustrated by Capgras syndrome is that different parts of the brain perform different jobs. - Researchers began to realize this in the nineteenth century by studying the cognition and behaviour of patients with lesions to the brain. Phineas Gage was one such famous patient. In 1848, an explosion during the construction of a railway sent a tamping iron through his frontal lobes, resulting in a variety of cognitive and emotional changes; after the accident his personality had completely changed; he seemed to have lost the part that told him what was inappropriate (acted on animal instincts and could not make decisions). - The study of people with brain lesions help us learn about the function of these brain regions in healthy people; the approach is referred to as the localization of function. - Even for simple cognitive tasks, multiple regions are involved; one area isn’t responsible for one certain thing. HINDBRAIN - One of the first areas to develop, one of the oldest parts - Sits directly atop the spinal cored - Control rhythms of the heart and breathing - Regulates levels of alertness - Includes cerebellum which coordinate movement and balance, in addition to more recently discovered sensory and cognitive roles (ex. finding short cuts). MIDBRAIN - Sits above the hindbrain - Coordinates movement, especially eye movement - Includes parts of auditory pathways (from ears to parts of forebrain) - Regulates experience of pain FOREBRAIN - The rest of the brain - Most of what you see when you look at the brain - Includes the cortex (outer layer of thin convoluted sheet of tissue, 3mm thick) - A variety of subcortical structure Cortex: divided into left and right cerebral hemispheres by the longitudinal fissure; commissures are thick bundles of nerve fibers that connect the two hemispheres; the largest one is corpus callosum; the cortex is divided into anterior and posterior regions by the central fissure. Frontal lobes: whole part of the brain Parietal lobes: Temporal lobes: processing auditory information Occipital lobes: visual information Subcortical Parts Thalamus: every sense except smell; sensory information goes through here for processing Hypothalamus: motivated behaviours (eating, sleeping, sex) Limbic system: Amygdala (emotional); hippocampus (new information; H.M. frozen in time had no hippocampus) Recall that neuroimaging allows researchers to take high quality, 3D images of the living brain; takes x-ray slices and assimilates all the slices into an image. – Computerized axial tomography (CT): good for getting brain structure – Positron emission tomography (PET): better for brain function; has a way of measuring blood flow – Magnetic resonance imaging (MRI): good for structure; uses magnets and measures magnetic properties of cells in your brain; can plot of variations. – Functional magnetic resonance imaging (fMRI): good for brain function; measures where the blood is flowing when you are doing various tasks; also uses magnets.  Ex. fMRI study:  When viewing images of faces, the fusiform face area (FFA) is active.  When viewing images of houses, the parahippoccampal place area (PPA) is active  Binocular rivalry: showed that the activation in these two regions reflects what the person is conscious of; not just the presented stimuli.  *note that the regions indicated by fMRI studies are not always necessary for the task in question; instead they may only be correlated with the task, in the way a speedometer is correlated with (but not needed for) the movement of the car; ex. you can still have those areas active, but may not be the part of the brain that is processing Transcranial magnetic stimulation (TMS): used to ask whether an area of the brain is necessary; knock out a part of the brain. Primary projection areas: primary arrival and departure points for information entering (sensory areas) and leaving (motor areas) the cortex; located in the posterior frontal lobes; more cortical space is devoted to the regions of the body we move with the greatest precision; right side is controlling left and left is controlling right. • The primary somatosensory projection area is located in the anterior parietal lobes. • The primary auditory projection area is located in the superior temporal lobes. • The primary visual projection area is located in the occipital lobes. The cortical maps represent sensory or motor information in an orderly manor; organization is by region of the body, region in space or auditory frequency. Cortical space is assigned disportionately; greater sensory acuity or motor precision is associated with larger cortical representation. Cortical organization is contralateral; the left side of the body or perceptual world has more representation on the right side of the brain, and vice versa. The rest of the cortex has been considered association cortex. Neurological syndromes that reflect damage to regions of the association cortex include: – Apraxia – problems with the initiation or organization of movement – Agnosia – problems identifying familiar objects – Aphasia – problems with language – Neglect syndrome – problems in which half of the visual world is ignored – Prefrontal damage – problems with planning and implementing strategies, inhibiting behaviours THE VISUAL SYSTEM Vision is the modality through which much of our knowledge is acquired. Vision provides an excellent illustration of how the close study of the brain can proceed, and what it can teach us. The structure of the eye is designed to project a sharp image onto the retina, the light-sensitive tissue that lines the back of the eye. Two types of photoreceptors are cells respond to light and are found on the retina: 1) Rods - Higher sensitivity - Lower acuity - Colour-blind - Found in the periphery of the retina 2) Cones - Lower sensitivity - Higher acuity - Colour-sensitive - Found in the fovea A series of neurons communicates information from the retina to the cortex. - In the eye: o Photoreceptors o Bipolar cells o Ganglion cells and the optic nerve - In the thalamus: o Lateral geniculate nucleus (LGN) - In the cortex: o V1, the primary visual projection area, or primary visual cortex, located in the occipital lobe; back of the brain. Much of what we know about the visual system comes from a technique known as single-cell recording; to understand this technique, we need to learn a few things about neurons. The basic parts of a neuron are: - Dendrites, which detect incoming signals - The cell body, which contains the nucleus and cellular machinery - The axon, which transmits signals to other neurons Communication between neurons is done via chemical signals. Neurotransmitters are chemicals released by one neuron to communicate with another neuron. The space between the two is called a synapse; thus, the first neuron is called the presynaptic neuron and the second neuron the postsynaptic neuron. Communication within neurons is done via electrical signals. – Neurotransmitters affect the post-synaptic neuron by changing ion distributions and resulting electrical potentials. – If the post-synaptic cell reaches threshold, an action potential is fired and propagates down the axon, releasing neurotransmitter that affects the next neuron. • In single-cell recording, a neuron’s firing rate, or frequency of action potentials, is recorded as various kinds of visual stimuli are presented to the subject. • Using these methods, researchers map out the receptive field – the kinds of stimuli to which the neuron best responds – for various cells of the visual system. • The receptive fields of the bipolar cells, ganglion cells, and cells in the lateral geniculate nucleus have a center-surround organization. • The receptive fields of the primary visual cortex (V1) are lines of particular orientations; these cells are sometimes called edge detectors. • Because of their different receptive fields, the neurons in area V1 are each specialized for a particular kind of analysis. • This is an example of parallel processing, a system in which many different steps or kinds of analysis occur at the same time. • The opposite of this is serial processing, in which steps are carried out one at a time. Another example of parallel processing is found earlier in the visual pathway in the ganglion cells, optic nerve, and LGN. – Parvocellular cells have smaller receptive fields and tend to continue firing as long as the stimulus is present. – Magnocellular cells have larger receptive fields and respond more strongly to changes in stimulation. • Parallel processing is also demonstrated by the higher visual pathways. • From area V1, information is sent too many secondary cortical visual areas for further parallel processing. These secondary visual areas lead to two major processing streams, the what system and the where system. The what system: - Is concerned with the identification of objects - Involves an occipital-temporal pathway - Damage to this system can result in visual agnosia The where system: - Is concerned with determining the locations of objects and guiding our actions in response - Involves an occipital-parietal pathway - Damage to this system can result in problems with reaching for seen objects With the great extent of parallel processing in the visual system, different aspects of a single object (e.g., shape, colour, movement) are analyzed in different parts of the visual system. How the brain reunites these different features into a coherent, integrated perception of the objects in the visual scene is referred to as the binding problem. • Elements that help solve the binding problem: – Spatial position – the visual areas processing features like shape, colour, and motion each know the spatial position of the object – Neural synchrony – the visual areas processing features of the same object fire in a synchronous rhythm
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