Neural Basis of Cognition Lecture 8 Attention Attention • What is attention? – There is a limit to how much information the human brain can process – The process of choosing what information to further process is “attention.” Attention • How can we characterize attention? – Alertness and arousal: general awareness of the world (decreases during sleep) – Vigilance (“sustained attention”): the ability to maintain alertness continuously over time, important when a task must be performed in a nonstop matter – Selective attention: the selection of information essential to a task, such as ignoring background noise during a conversation in a crowded restaurant Attention • Are these “attention systems” distinct? Do they use the same resources? – Evidence to the contrary: the brain’s processing capacity is larger when tasks draw from different resource pools (e.g. spatial, auditory) than from the same one Attention • What brain structures are involved in attention? – Reticular activating system – Superior colliculus – Thalamus – Parietal lobe – Anterior cingulate cortex – Frontal lobe Reticular Activating System (RAS) • The RAS is involved in alertness and arousal and is responsible for controlling sleep-wake cycles • Cell bodies in the RAS have diffuse connections to most regions of the cortex, allowing them to modulate the arousal and alertness of the entire brain • Damage to the RAS or its functioning results in coma Superior colliculus • Responsible for saccades (to quickly bring peripheral visual stimuli into foveal vision) – Express saccades: fast (~120ms), triggered by novel visual stimulus in the periphery, disappear upon destruction of the superior colliculus – Regular saccades: voluntary, take about 200300ms Thalamus • Some nuclei (medial dorsal, intralaminar, reticular nuclei) in the thalamus modulate the level of arousal of the cortex • Sensory information is relayed to the brain through the thalamus, so it is in a logical position to have a role in selective attention – The pulvinar seems to play an important role in selective attention • PET studies indicate that the thalamus is more engaged when filtering of sensory information is required, such as in a task where an item must be detected among eight other items as opposed to when it is shown alone • ERP and MEG studies indicate that such filtering occurs very early after the receipt of the stimulus Parietal Lobe • The parietal lobe, important for visual and spatial aspects of attention, is thought to be involved in more fine-grained selection of sensory information • The parietal lobe is also taught to be responsible for the overall allocation of attentional resources to a particular stimulus or task Parietal Lobe • Evidence: – Single-cell recordings in monkeys show that the firing rate of some parietal neurons is enhanced any time attention is directed to a visual object (independently of motor actions). In particular, the lateral intraparietal region is important for the representation of attended or salient spatial locations, responding regardless of the modality of information about a location or whether there is a motor response – Neuroimaging in humans has provided converging evidence: increased activation in parietal regions across a variety of tasks that involve increased visual attention but not by highly demanding tasks in general Parietal Lobe • The parietal lobe is thought to play a role in binging together visual attributes with their position in space • Feature integration theory: basic visual features such as color are detected relatively automatically, but we cannot know which features go together unless we direct our attention to a particular location – Attention is the “glue” that binds this information • Classic example: – Finding a single red X in a table of green Xs happens at the same speed regardless of the number of green Xs (preattentive processing) – Finding a single red X in a table of green Xs and red Os takes longer with larger tables – This increase in time occurs because attention can be directed to only one point at a time; directing attention to a point in space precedes the identification of information, which means that directing attention to a particular spatial location allows the features at that location to be bound together so that an item can be identified Parietal Lobe • Bilateral damage to the parietal regions disrupts the ability to bind together features • Individuals with such deficits cannot detect conjunction of features, whereas their ability to detect a single attribute remains intact Anterior Cingulate Cortex • Once the brain has filtered sensory information, it must choose a response; the region responsible for that is the cingulate cortex, which can be thought of as an interface between subcortical and cortical regions • Recall from lecture 3 that the anterior cingulate cortex is involved in choosing a novel response • Activity in the anterior cingulate cortex during conflicting responses but not during noncongruent responses in the Stroop task supports that the cingulate is critically involved in response selection • Cingulate activity is observed when there is a need to select between directly conflicting responses, and when selecting the correct response is demanding or complicated (such as wel there are multiple possible responses) • Greater cingulate activity is also found when the determination of a response is complicated because it relies on multiple attributes of a stimulus (e.g. color, form speed) rather than a single one (e.g. color) • Also, cingulate activity is thought to be correlated with task difficulty Frontal Lobe • The frontal lobe is involved in selection of information for more abstract characteristics, such as selecting words that have particular meanings or selecting information that must be held in working memory • Discussion of the frontal lobe’s role in attention falls more in the realm of discussions of executive function Attentional selection • Early selection (early stage of processing, before items are identified) or late selection (after sensory processing is complete and items have been identified)? • Classic example of an experiment designed to answer this question: – Individuals are instructed to listen and count the number of target tones, such as long tones, interspersed within more frequent non-targets, such as short tones – They are told to attend only to information in one ear – Responses are compared for targets when they are attended for as compared to when they are unattended – The point in time as which the amplitude of the ERP to the attended stimulus begins to diverge from that of the unattended stimulus is noted – This happens approximately 80ms after stimulus presentation, suggesting that attention can happen early • However, the P300 component (mentioned in lecture 2) occurs only in response to stimuli being attended to • So, attentional selection can happen early or late Selection by features • What aspect of the sensory world is used when selecting information? – Space-based viewpoint of attention – Object-based viewpoint of attention Space-based viewpoint of attention • Neuroimaging studies have provided evidence that regions of both visual and parietal cortex mediate space-based attentional deficits – The mapping of the visual world in early visual processing areas (V1-V4) is retinotopic – Attending to information in one visual field increases activation over V2-V4 regions of the opposite hemisphere – ERP and MRI studies show that this space-based attentional modulation occurs early in processing, ~100ms after stimulus presentation, in the secondary visual cortex Object-based viewpoint of attention • In a task where attention is directed to a spacially constant attribute associated with faces, increased activation was observed in the fusiform face area (which recognizes faces); in contrast, when attention was directed to an attribute associated with houses, increased activation was observed in the parahippocampal place area (which recognizes scenes) • This modulation of attention appeared to occur relatively early on in processing, when visual features are first recognized as forming a particular object Attention • The conclusion of all the evidence mentioned in this lecture is that attention manifests as increased activation of the areas of the brain involved in processing the type of information being attended to • There is also some evidence that taskirrelevant information undergoes decreased processing Hemineglect • Hemineglect (sometimes referred to as hemi-inattention) is a syndrome in which an individual ifnotes, or does not pat attention to, the side of space contralteral to a lesion • The side of space is usually defined with reference to the body midline but may occur with regard to other spatial reference ftames as well • This inattention is seen regardless of the modality in which information is presented • Depending on the severity, an individual might fail to eat food on the left side of a plate, draw the left side of objects, read the left side of words, or use the left side of the body • In severe cases, an individual may even deny that the left side of this body belongs to him Hemineglect • One severr case: – A patient with hemineglect complained to a nurse that a staff member had played a cruel practical joke on him by placing a severed leg in his bed. – The patient then attempted to throw the leg out of his bed, hurling himself onto the floor. • Another: – A woman had a stroke and fell in her bathroom. – While being examined, she insisted that her left arm was not hers but the examiner’s. When the examiner brought the patient’s left arm into view and asked whose it was, she answered, “It’s not mine. I found it in the bathroom, when I fell. It’s not mine because it’s too heavy; it should be yours. It can move and do everything; when I feel it too heavy, I put it on my stomach. It doesn’t hurt me, it’s kind.” – When she was asked where her own arm was, she replied, “behind the door.” • Denying ownership of a limb and claiming it belongs to someone else without any other deficit in reasoning is called somatoparaphrenia. Hemineglect • Neglect is usually observed after vascular damage to the parietal regions extending into subcortical regions. • Neglect is observed more commonly and is more severe after right then left hemisphere lesions, and so neglect is observed more often for the left side of space. • Neglect can also occur after damage to fronta regions, the basal ganglia, and the thalamus • Usually, neglect is severe at first (all items on one side of space are ignored), but, with weeks to months, some information on the neglected side is processed. However, it rarely, if ever, disappears completely. Hemineglect • Hemineglect does not occur from low-level sensory processing deficits; patients with hemineglect can still perform motor acts such as showering, dressing, and eating with both sides of the body (though one side may be preferred). • Lack of information from one half of the visual field cannot account for neglect because re-orienting the head would bring that side of space into view, but patients with hemineglect ignore it. • Sensory processing deficits cannot account for hemineglect because information from the contralateral side of space is generally ignored regardless of modality, but ipsilateral and contralateral projections in the auditory system, for example, make that unlikely. Hemineglect • Drawing and line bisection tasks: Hemineglect • Bringing attention to the left side of the line in the line bisection task can improve performance • If information on the neglected side is critical for understanding or compregension, it tends to receive attention – “antiballistic” might be read as “ballistic” though only the letters “llistic” are past the midline of the word • Performance on a task can improve if a greater reward is promised • Hemineglect seems to be associated with attention • “The patient with hemineglect treats one side of space the way you normally treat the space behind your head.” Hemineglect • Perhaps hemineglect is the result of the lack of an internal mental representation of the neglected side of space • Bisiach and Luzzatti, 1978: – Two patients with hemineglect for the left side of space were asked to imagine standing at one end of the Piazza del Duomo of Milan and to describe what they saw. They described landmarks on the right. – The patients were then asked to imagine standing at the opposite end of the plaza, facing the opposite direction, and to describe what they saw. The described landmarks on the remaining half of the plaza • This rules out the theory of lack of a full mental map Hemineglect • One theory says that hemineglect is due to preference for stimuli on one side of space, which can be interpreted as an uneven competition between hemispheres for controlling the direction of attention • Oliveri et al. 1999: – Using TMS to disrupt the processing of the intact hemisphere briefly eliminated hemineglect Hemineglect • Actively or passively moving the limb on the neglected side of space to make the neglected side of space more salient • Caloric stimulation: – Water at least 7 degrees Celsius cooler than body temperatere is poured into the ear canal, inducing motion in the semicircular canals of the vestibular system – Can result in vertigo • Neck-proprioceptic stimulation (vibration in the left neck muscles) • No permanent treatment Attentional Bias • Each hemisphere appears to have an attentional bias for the contralateral side of space. • Damage to the right hemisphere appears to slow response times to simple stimuli more than damage to the left hemisphere, suggesting that the right hemisphere had a larger role in alerting and arousal. • PET studies indication that the right hemisphere is important in sustaining overall attention, such as in vigilant tasks. Attentional Bias • Which face looks happier? Attentional Bias • Most right-handed individuals perceive the face on the left as happier. • It is suggested that the left half-face is perceived as more expressive because the right hemisphere is more adept at processing emotional and facial information.