Mind, Brain & Behavior Friday February 7, 2003

From Nerve Cells to Cognition (Cont.) Chapter 18

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.

Cognition and the Cortex Chapter 19

Association Areas  Once thought to be “silent” areas.  Modern evidence suggests that association areas are higher order processing centers for sensory or motor information.  Two sources of information: Those with brain damage due to accident or disease. Those with surgery – well-defined lesions.

Three Association Areas  See Figure 19-2  Prefrontal (1) – weigh consequences of future actions and plan motor responses.  Limbic (2) – allows emotions to affect motor planning.  Parietal-temporal-occipital (3) – processes sensory information for perception and language.

Hemispheric Specialization  The left and right hemispheres have different cognitive capabilities.  The corpus callosum and anterior commissure permit hemispheres to coordinate activity.  Epileptic patients may have a commisurotomy in which pathways between hemispheres are severed. Called “split-brain” patients.

Split-Brain Functioning  Normally, split-brain patients show little impairment. Hemispheres function independently.  Information must be presented to just the left or right hemisphere, to see differences.  Left hemisphere – aware and verbal.  Right hemisphere – automatic.

Normal Brains  Both hemispheres work together in the normal brain – it makes little sense to talk about “right brain” or “left brain.” Interaction of hemispheres needed to identify objects by touch.  Wada test – used to demonstrate which hemisphere speech is lateralized to. Patient unable to continue counting with sodium amytal.

Lateralization of Language  Nearly all right-handed people (96%) have language in the left hemisphere.  Most left-handed people also have language in left hemisphere, 15% have language in the right hemisphere, a small percentage have it in both.  The hemispheres are anatomically asymmetrical in the areas associated with language.