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Download Topic 14 - Center for Complex Systems and Brain Sciences
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Cognitive Neuroscience Consciousness Consciousness is central to all of cognitive neuroscience. Yet, it is the most difficult phenomenon to define, and also to study. As we saw at the beginning of the course, the conscious mind is at the core of the "hard problem" because of the subjectivity it entails. However, we can study the phenomenon of consciousness objectively as another cognitive function. We have seen consciousness enter the descriptions of cognitive function at many places in the study of cognitive neuroscience, often cloaked as volition. Some cognitive functions, such as working memory, may be conscious or unconscious. Others, such as perceptual attention, seem to always be conscious. But we have not yet discussed consciousness itself as a cognitive function. Here, we will examine the neuroscientific evidence for the neural basis of consciousness. We will not consider what is the essential difference between consciousness and unconsciousness. Different types of study used in the investigation of consciousness in the brain 1. Anesthetized and sleep states 2. Alternating perception a. Binocular rivalry b. Multistable perception 3. Blindsight and hemispatial neglect 4. Subliminal perception 5. Study of vegetative state patients 6. Computer simulations Guiding premises about consciousness 1. It is generated by the brain 2. There is no single "center" for consciousness in the brain - it is widely distributed - halothane anesthesia studies show that multiple brain areas are active when a person is awake, and they all uniformly go silent under anesthesia 3. Consciousness varies quantitatively - the level of consciousness in animals or the human fetus, for example, may differ 4. It varies qualitatively - auditory and visual consciousness may differ, for example 2 types of consciousness Damasio distinguishes core consciousness (basic awareness) from extended consciousness (having higher organizational complexity). Anatomical orientation: consciousness primarily depends on three brain structures 1. brainstem 2. thalamus 3. cerebral cortex 1. Brainstem The brainstem “arouses” the cortex, and thereby modulate wakefulness. The reticular activating system (RAS) lies in the core of the brainstem and sends projections to the cortex. Two main pathways project from the RAS to the cortex: a) dorsal pathway thru the thalamic intralaminar nucleus (ILN) b) ventral pathway thru the hypothalamus & basal forebrain The RAS is involved in: a) arousal b) sleep-wake cycle regulation c) attention The permanent lack of core consciousness that results from damage to the RAS is called coma. The locus coeruleus (LC) is located in the pons. In it are the soma of neurons that project to the cortex & other brain structures, where the axonal terminals release the neurotransmitter norepinephrine (NE). The projections follow a similar pathway to the dorsal RAS pathway. Activation of, and subsequent NE release in, the cortex by the LC prevents sleep. The brainstem receives input from a large number of sources that inform it about the state of the internal and external environments of the organism. The RAS and LC can change the state of arousal of the organism accordingly. 2. Thalamus Bilateral lesioning of the ILN, or blocking brainstem inputs to ILN, produces permanent loss of core consciousness. 3. Cerebral cortex The cortex maintains wakefulness and contributes to selective attention. Damage to cortex results in loss of specific abilities à cortex contributes to extended consciousness. Conscious versus unconscious processing and access to information Most cognitive processing is unconscious. We are only conscious of the content of the mind, not what generates that content. The question of whether consciousness is required for cognitive processing has been investigated in patients with blindsight. Blindsight is the phenomenon in which a person is blind in part of their visual field due to a lesion of the primary visual cortex, and yet the person demonstrates the use of visual information. Blindsight suggests that visual perception can occur in the absence of consciousness. Evidence suggests that the damaged primary pathway (retinaàLGNàV1) is involved in blindsight. Thus, there does not seem to be any reason to invoke alternative pathways, such as thru the superior colliculus to extrastriate cortex. Patients with hemispatial neglect also demonstrate above-chance performance based on visual input but without visual awareness. For example, neglect patients with right hemisphere damage are unable to name stimuli presented in the left visual field, but they can make judgments about whether such stimuli are the same as or different from stimuli in the right visual field. In fact, they deny having seen anything in the neglected left visual field. Conclusion: neglect patients (with damaged parietal cortex and spared visual cortex) can make perceptual judgments outside of conscious awareness. The extent of subconscious processing Right-sided neglect patients can make perceptual judgments by matching stimuli presented in the two visual hemifields even though they deny seeing anything in the left hemifield. They respond faster when the stimuli are related. In the intact, healthy brain, unconscious processing is shown by word association (word priming) tasks. The extent and type of information processed unconsciously is studied with the technique of subliminal perception - judgment may be biased by unconscious elements of a stimulus presented very quickly. For example, in a cross-form (picture-word) priming paradigm, a picture can unconsciously affect word stem completion, even though the picture is presented very briefly and followed by a mask. Gaining access to consciousness It has been proposed that optimal information processing requires that only currently relevant information be allowed access to consciousness. Treisman’s attentional binding hypothesis posits that the visual attentional system binds elementary sensory features together into percepts at attended locations. Each point in the visual field is processed for these elementary features, such as color, shape, and motion, outside of conscious awareness. Thus, attention is needed to bind the results of unconscious processes into conscious percepts. The scaffolding-to-storage hypothesis of Raichle & Petersen proposes a conscious-to-unconscious transition in learning. First, conscious processing is required for practice while learning complex skills or memories (erecting the scaffolding). Then, after learning, the same skills are performed, or memories retrieved, unconsciously (removing the scaffolding in favor of more permanent “storage” structures). Neurons, neuronal groups, and conscious experience Newsome found that a monkey’s psychophysical performance capacity could be predicted by the neuronal response pattern of single neurons in area MT (motion-processing area of visual cortex). He also discovered that careful alteration of the response rate of those same neurons by microstimulation could change the animals performance on a perceptual task toward making the correct decision. The implication is that conscious processing can be altered by changing neuronal activity. Libet studied neural timing factors in conscious and unconscious. His most famous result is that the cortex begins to change its action-related activity as many as 300 ms before the conscious intention to act. These experiments are the basis for his backward referral hypothesis, which claims that conscious awareness of an event is delayed by around half a second, and that this awareness is referred back in time to the beginning of the event.