Understanding Consciousness

• Cognitive abilities like language, problem-solving, and social interactions rely on consciousness, which encompasses self-awareness, surroundings, and the ability to focus.
• Consciousness arises from collaboration between the brainstem, diencephalon, and cerebral hemispheres
• Cognitive abilities are linked to specific cortical areas and their connections, with one hemisphere often more dominant.

Aspects of Consciousness

• Content reflects cortical activity at a given moment.
• Level reflects activity in brainstem and diencephalic nuclei, projecting to the thalamus and cortex with excitatory neurotransmitters.
• Rostral parts of the locus ceruleus and raphe nuclei use norepinephrine and serotonin, respectively.
• Cholinergic neurons from the pons and midbrain project to the thalamus
• A larger collection in the basal nucleus of Meynert projects to the cerebral cortex.
• Hypothalamic neurons, including the tuberomammillary nucleus (using histamine) and the lateral tuberal hypothalamus (using orexin/hypocretin), project to the thalamus and cortex.
• Brainstem components are referred to as the ascending reticular activating system, which activates the forebrain and maintains consciousness.

Variations in Consciousness

• Level is dynamic, ranging from alertness to drowsiness, influenced by modulatory nuclei.
• Content is affected by damage to specific cortical areas.
• Bilateral damage to modulatory nuclei, their axons, or large cortical/subcortical areas impairs or eliminates consciousness.
• Consciousness requires interaction among brain structures rather than activity in a specific area.
• Small brainstem lesions affecting ascending fibers can cause coma, a prolonged unconscious state.
• Coma typically resolves within weeks, leading to emergence, death, or a vegetative state with some brainstem function but absent cortical functions.

Sleep and Wakefulness

• The level of activity vary rhythmically among all animals
• Sleep is a reversible state of decreased consciousness
• Sleep is actively induced with changes in the modulatory network.

Sleep Stages

• Mammalian sleep has stages defined by electroencephalography (EEG).
• EEG signals reflect synaptic currents in cortical pyramidal cells.
• Attentive wakefulness shows low-voltage, desynchronized EEG.
• Sleep onset progresses through four stages with larger, slower, synchronized EEG waves, indicating deeper sleep; stage 4 is known as slow-wave sleep.
• Every 90 minutes, EEG patterns retrace, entering a desynchronized state resembling wakefulness, though awakening is more difficult.
• This stage is called REM sleep (rapid eye movement sleep), where eyes dart under closed lids, contrasting with non-REM sleep (stages 1-4).
• Sleep cycles through these stages multiple times per night but non-REM episodes shorten and REM episodes get longer as the night progresses
• Most slow-wave sleep occurs early with REM dominating near dawn.
• Non-REM sleep features decreased heart rate, blood pressure, respiratory rate, and hypothalamic thermostat set point, alongside increased gut motility and decreased brain blood flow.
• Muscle tone decreases, allowing movement such as rolling in bed
• Slow-wave sleep is associated with sleepwalking.
• People awakened from non-REM sleep rarely report dreams with detailed imagery.
• REM sleep has tonic and phasic characteristics
• Heart rate blood pressure and respiratory rate increase and fluctuate
• Muscle tone disappears due to motor neuron inhibition with temperature regulation becoming reptilian
• Bursts of rapid eye movements and muscle twitches occur
• Cerebral blood flow increases nearly to waking levels except in multimodal cortical association areas
• Those awakened are likely to recall detailed, emotional dreams that can be explained by changes in blood flow
• Selective REM sleep deprivation leads to increased REM sleep afterward
• REM sleep occupies 50% newborns sleep time possibly rising to 80% for premature infants with adults only experiencing REM sleep 20-25% of time
• Percentage decreases as humans age

Neurobiology of Sleep

• Non-REM sleep occurs when the modulatory activating network is inhibited.
• REM sleep is triggered by pontine reticular formation neurons.

Non-REM Sleep Specifics

• Modulatory nuclei in the hypothalamus and brainstem excite each other during wakefulness
• Orexin neurons project to all other nuclei with degeneration resulting in narcolepsy
• Patterned inhibition from the preoptic hypothalamus and medulla initiates non-REM sleep
• Cholinergic neurons become virtually silent
• Other nuclei also slow down resulting in diminished consciousness

REM Sleep Specifics

• The reticular formation facilitates REM
• Experimental animals endure constant wakefulness with precise damage
• Cholinergic neurons in the pontine reticular formation become active during non-REM sleep

Higher Cortical Function

• Humans have seemingly limitless behavioral response options
• Association cortex provides a substrate to modify behavioral responses to fit different situations
• Primary visual cortex initially sorts stimuli before dorsal association areas (analyzing location/movement) and ventral areas ("what" pathway for shape, color)
• Unimodal areas project to multimodal areas where visual input is integrated with other properties
• Certain multimodal areas direct attention and assess significance in relation to needs and prior experiences
• Posterior multimodal areas are important for identifying and directing attention
• Anterior areas are important for devising responses

Hemispheric Asymmetry

• Primary visual cortices have asymmetric properties and continue into cortices.
• Unimodal visual association cortex has more cells in the left hemisphere encode shapes of letters, while more cells in right hemisphere encode faces.
• One hemisphere is more important for language and is commonly referred to as the dominant hemisphere
• The "nondominant" hemisphere is important for other functions
• Corpus callosum and anterior commissure coordinate and unify the activities of the two hemispheres mitigating asymmetries in activities.
• Lesions in unimodal areas cause a loss in the ability to utilize a certain type of sensory informaiton Lesions in multimodal areas cause particular deficits in cognitive abilities
• Lesions in limbic areas cause abnormal responses in emotional or drive-related situations, as well as memory deficits.

Unimodal Areas

• Primary sensory and motor areas connect outside and rest of cerebral cortex
• Damage to one these areas causes deficit (I.E. Loss of vision in contralateral field)
• Primary sensory area projects to unimodal association cortex, and includes sub areas for different aspects of analysis.
• Visual association cortex is highly understood

Sensory-Specific Agnosias

• Partial damage can cause selective losses and cause agnosias, where fundamental sensation is maintained, but recognition of objects by of that sense is hindered.
• Impairment is restricted to the specific sense
• Damage to prosopagnosia impedes recognizing face
• Tactile agnosia impairs palpitation recognition
• Auditory agnosia impairs recognizing sounds
• Motor Association Cortex (pre motor cortex) plans different movement aspects

Limbic Cortex

• Limbic cortex is thought of as a group of unimodal association areas connecting multiple modal areas and the hypothalamus.
• Used to coordinate autonomy and behavior
• Crucial to memory of events
• Language is intrinsically multimodal, words have symbolic meaning
• Aphasia needs damage to multimodal language

Aphasia

There are more in depth sections of the brain that control language than just one area

• Posterior language areas are more involved in compression and anterior are more involved in expression Therefore posterior lesions result in fluent aphasias where speaking is plentiful but inaccurate Left handed people more likely to have language parts split among the 2 hemispheres

Perisylvian Language Zone

• Borders the left lateral suicus and entails Wernicke’s area.
• Wernicke’s area connects the objects to concepts.
• Connections are not always direct or complete
• There are different types of Aphasia based on damaged areas

Attention and Spatial Orientation

• It is important to focus on the body.
• Neglect occurs through damage in the left side (stimulus to the right is perceived even in sounds)
• Neglect varies Frontal Lobe
• Resourcefulness in primates and humans
• Collectively known as “executive function”
• Patterns can lead to actions we construe as someone’s character or personality

Prefrontal Cortex

• Like other association areas there are a collection of subareas
• Later surface - lateral prefrontal cortex
• Medial surface - orbital gyri
• Prefrontal functions are not as strong as other multimodal areas
• There can be some relatively limited effects depending the damage is unilateral

Doral Prefrontal Cortex

• Uses many inputs as the basis of the “working memory”
• Used to find the proper way of thinking
• Used in keeping thoughts at a good amount of time

Ventromedial Damage

• Likely someone will factor in Motivational Issues.
• Phineas Gage had traumatic wounds, and change could happen but wouldn’t affect functions

Disconnection Syndrome

Cognitive functions can also be distorted by damage