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Consciousness, Memory, Language

Consciousness. Selective attention, abstract thinking, the ability to verbalize experiences, the capacity to plan activities based on experience, self-awareness and the concept of values are some of the many characteristics of consciousness. Consciousness enables us to deal with difficult environmental conditions (adaptation). Little is known about the brain activity associated with consciousness and controlled attention (LCCS, see below), but we do know that subcortical activation systems such as the reticular formation (^ p. 322) and corticostriatal systems that inhibit the afferent signals to the cortex in the thalamus (^ p. 326) play an important role.

Attention. Sensory stimuli arriving in the sensory memory are evaluated and compared to the contents of the long-term memory within fractions of a second (^ A). In routine situations such as driving in traffic, these stimuli are unconsciously processed (automated attention) and do not interfere with other reaction sequences such as conversation with a passenger. Our conscious, selective (controlled) attention is stimulated by novel or ambiguous stimuli, the reaction to which (e.g., the setting of priorities) is controlled by vast parts of the brain called the limited capacity control system (LCCS). Since our capacity for selective attention is limited, it normally is utilized only in stress situations.

The implicit memory (procedural memory) stores skill-related information and information necessary for associative learning (conditioning of conditional reflexes; ^ p. 236) and non-associative learning (habituation and sensitization of reflex pathways). This type of unconscious memory involves the basal ganglia, cerebellum, motor cortex, amygdaloid body (emotional reactions) and other structures of the brain.

The explicit memory (declarative/knowledge memory) stores facts (semantic knowledge) and experiences (episodic knowledge, especially when experienced by selective attention) and consciously renders the data. Storage of information processed in the uni-and polymodal association fields is the responsibility of the temporal lobe system (hip pocampus, perirhinal, entorhinal and parahip-pocampal cortex, etc.). It establishes the temporal and spatial context surrounding an experience and recurrently stores the information back into the spines of cortical dendrites in the association areas (^p.322). The recurrence of a portion of the experience then suffices to recall the contents of the memory.

Explicit learning (^ A) starts in the sensory memory, which holds the sensory impression automatically for less than 1 s. A small fraction of the information reaches the primary memory (short-term memory), which can retain about 7 units of information (e.g., groups of numbers) for a few seconds. In most cases, the information is also verbalized. Long-term storage of information in the secondary memory (long-term memory) is achieved by repetition (consolidation). The tertiary memory is the place where frequently repeated impressions are stored (e.g., reading, writing, one's own name); these things are never forgotten, and can be quickly recalled throughout one's lifetime. Impulses circulating in neuronal tracts are presumed to be the physiological correlative for short-term (primary) memory, whereas biochemical mechanisms are mainly responsible for long-term memory. Learning leads to long-term genomic changes. In addition, frequently repeated stimulation can lead to long-term potentiation (LTP) of synaptic connections that lasts for several hours to several days. The spines of dendrites in the cortex play an important role in LTP.

Mechanisms for LTP. Once receptors for AMPA (a-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid) are activated by the presynaptic release of glutamate (^ p. 55 F), influxing Na+ depolarizes the postsynaptic membrane. Receptors for NMDA (N-methyl-D-aspartic acid) are also activated, but the Ca2+ channels of the NMDA receptors are blocked by Mg2+, thereby inhibiting the influx of Ca2+ until the Mg2+ block is relieved by depolarization. The cyto-solic Ca2+ concentration [Ca2+]i then rises. If this is repeated often enough, calmodulin mediates the auto-phosphorylation of CaM kinase II (^ p. 36), which persists even after the [Ca2+]i falls back to normal. CaM kinase II phosphorylates AMPA receptors (increases their conductivity) and promotes their insertion into the postsynaptic membrane, thereby enhancing synaptic transmission over longer periods of time (LTP).

I— A. Storage of information in the brain (explicit memory)

109bits/s

109bits/s

Forgotten due to fading

Sensory memory

Storage time < 1 s

Tertiary memory Very large capacity Lifetime storage

Sensory memory

Storage time < 1 s

Primary memory 7 bits

Storage time: seconds to minutes

Secondary memory Very large capacity Minutes to years

Primary memory 7 bits

Storage time: seconds to minutes

Tertiary memory Very large capacity Lifetime storage

Secondary memory Very large capacity Minutes to years

Disremembered due to disturbance (interference) by previous or later knowledge

Recall time (access)

Fast

Slow

Fast

Amnesia (memory loss). Retrograde amnesia (loss of memories of past events) is characterized by the loss of primary memory and (temporary) difficulty in recalling information from the secondary memory due to various causes (concussion, electroshock, etc.). Anterograde amnesia (inability to form new memories) is characterized by the inability to transfer new information from the primary memory to the secondary memory (Korsakoff's syndrome).

Language is a mode of communication used (1) to receive information through visual and aural channels (and through tactile channels in the blind) and (2) to transmit information in written and spoken form (see also p. 370). Language is also needed to form and verbalize concepts and strategies based on consciously processed sensory input. Memories can therefore be stored efficiently. The centers for formation and processing of concepts and language are unevenly distributed in the cerebral hemispheres. The left hemisphere is usually the main center of speech in right-handed individuals ("dominant" hemisphere, large planum temporale), whereas the right hemisphere is dominant in 30-40% of all left-handers. The non-dominant hemisphere is important for word recognition, sentence melody, and numerous nonverbal capacities (e.g., music, spatial thinking, face recognition).

This can be illustrated using the example of patients in whom the two hemispheres are surgically disconnected due to conditions such as otherwise un-treatable, severe epilepsy. If such a split-brain patient touches an object with the right hand (reported to the left hemisphere), he can name the object. If, however, he touches the object with the left hand (right hemisphere), he cannot name the object but can point to a picture of it. Since complete separation of the two hemispheres also causes many other severe disturbances, this type of surgery is used only in patients with otherwise unmanageable, extremely severe seizures.

Glia

Sense of Taste c to T3 C ra

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