Mind, Brain & Behavior Wednesday February 5, 2003

Announcement  The syllabus mistakenly says that Feb 24 is Veteran’s Day – a holiday.  There will be class on Feb 24 – we will use the day to catch up and to do a Q&A review for the next midterm.

Sample Extra Credit Answers  When the brain is developing after birth, connections are being formed between neurons. “Cells that fire together, wire together” explains that if two neurons fire at the same time then the connection or pathway between them is strengthened.  Cells that have action potentials together strengthen and make it more likely that the receiving neuron will have an action potential.  What Hebb meant was that, in early development, the brain is wired with many superfluous connections. Neurons that ‘fire together’ form circuits and these pathways are strengthened whereas those connections that do not fire are lost.

Mental Representation  Mental representation is at the heart of cognition.  Mental representation – how the brain constructs images and abstracts information from sensory inputs and represents it in specific pathways and brain regions. The brain does not receive pictures of the world Perception as direct, precise images of the world is an illusion.

Cognitive Neural Science  Cells are important, but the connections between cells contain the information. The study of cognition emphasizes populations of neurons. Computational models can test inferences about how populations work together. Imaging can show activity of groups of neurons.

Cognitive Psychology  Expanding on the science of the Behaviorists, early cognitive psychologists suggested that: Perception is constructed Perception shapes behavior  Mental processes intervening between stimulus and behavior can be studied.

Five Main Approaches  Focus on single-cell activity in primates.  Shift to studying information processing, not how a stimulus evokes a response.  Study of patients with lesions suggests which processes are independent modules.  Imaging permits changes in populations of neurons to be related to processes.  Computers permit modeling of activity in large populations of neurons and provides an analogy.

Representation of Personal Space  Touch – the sense of the texture of objects and their movement across the skin.  Proprioception – the sense of the static position and of movement of our fingers and limbs.  Somatic sensory system – includes touch and proprioception plus nociception (tissue damage) and temperature sensation.

Mapping the Sensory Cortex  Single cell measurements show specific responses when part of the body is touched. Penfield mapped sensory cortex. Different sizes of representation correspond to amount of innervation in that body region.  Different species of animals rely on different parts of the body for information and thus have different sensory maps.

Maps Can Be Modified  Maps depend on experience (use).  Inputs to the sensory cortex are formed based on Hebbian correlated firing: Cells that fire together, wire together.  Syndactyly (webbed fingers) – fingers are not represented independently. When surgery separates the fingers, they become independently represented in cortex.

Phantom Limb Syndrome  Patients with amputated limbs continue to sense the missing limb.  Originally thought to be caused by signals coming from the spinal cord from scar tissue.  Now thought to originate from representation areas as they are remapped (other functions expand into the area for the lost limb).

Receptive Fields  Mountcastle – identified receptive fields of sensory neurons in skin. Receptive fields overlap, so each area of skin is monitored by multiple neurons.  Size of the receptive field varies in different parts of the body.

Organized in Columns  Sensory cortex contains separate columns for each modality (touch, pressure, temperature, pain). Within each column, all neurons respond to the same type of sensory receptor. Different types of sensory receptors are dominant in different areas of the sensory cortex.

Integration of Modalities  Integration is accomplished through layered processing: The submodalities converge on common cells. Response properties of neurons at higher levels become more complex. The size of the receptive field increases at each level of processing.  Complex stimulus properties emerge from elementary properties.

Parietal Association Areas  Areas 1 & 2 merge inputs from areas 3a, 3b.  Posterior parietal cortex (areas 5 & 7) integrate sensory information with visual and auditory information. Lesions affect spatial perception, visuomotor integration, directed attention. Astereognosia – inability to recognize objects by touch. Neglect syndrome – ignore half of body.

Attention  Both posterior parietal cortex and frontal cortex are active when attention is shifted from one object to another. Posterior when sensory input shifts attention. Frontal when a motor response is made.  Extinction – inability to shift attention to an object on opposite side from lesion.