Psych 216:Psych 216:MovementMovement

Attention

What is attention?What is attention?

There is too much information available in the world to process it all. Demonstration: change-detection performance

Thus, attention mechanisms exist to select the most relevant subset of the available information and focus processing resources on that subset.

Overt and covert Overt and covert mechanismsmechanisms

• One way to influence what is processed is via overt movements. For example, moving your head or eyes toward stimuli leads to faster and more accurate processing of those stimuli due to having more receptors in your fovea.

• However, selective processing of stimuli can occur in the absence of any movements, this is known as covert attentional selection.

What is selected?What is selected?

• Our goals (‘top-down’ factors) and the properties of the stimuli themselves (‘bottom-up’ factors) co-determine what is selected.

• This interaction of factors has been extensively studied using visual search tasks.– Visual search tasks are those in which subjects are

instructed to look for a certain stimulus presented within an array of irrelevant stimuli.

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Attention and eye Attention and eye movementsmovements

• One of the primary functions of visual attention is to select stimuli that will be the targets of future eye movements.

• Movements take a long time to program and execute (by the cognitive clock). For example, human eye movements take approximately 200-250 ms to produce where as the effects of attentional selection of stimuli can be observed in as little as 20-100 ms.

Behavioral evidence that Behavioral evidence that covert attention is fastcovert attention is fast

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Electrophysiological evidence Electrophysiological evidence that covert selection is fastthat covert selection is fast

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Rapid covert selection during Rapid covert selection during visual searchvisual search

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C75 & C25in Different Hemifields-200200400600800-3µV+3µVTime Poststimulus (ms)

more negativecontralateralto C75more negativecontralateralto C25

Neuropsychological studies Neuropsychological studies of attentionof attention

• Different types of patients manifest different types of attention deficits (or even advantages).

1) Split-brain patients

2) Visual Neglect

3) Balint’s Syndrome

Split-brain patientsSplit-brain patients

• In order to alleviate severe epilepsy some patients have had their corpus callosum cut.

• This creates a situation in which the two hemispheres of the brain are essentially independent. For example, only the left hemisphere can read.

From World Book © 2001 World Book, Inc., 233 N. Michigan Avenue, Suite 2000, Chicago, IL 60601. All rights reserved.Image from ÅThe Dynamic Human: the 3D visual guide to anatomy and physiology,Ä © Wm. C. Brown Publisher, 1996.

Two brains are better than Two brains are better than oneone

• Because covert visual attention mechanisms are not lateralized split-brain patients can perform visual search at almost twice the rate of normal subjects. This suggests that as ‘normals’ our two hemispheres are linked such that we only have one focus of attention during visual search.

Neglect & ExtinctionNeglect & Extinction

• Damage to the right parietal lobe (usually from a stroke) leads to “unilateral neglect”– Patients do not attend to the contralateral side of space

(usually the left side following a right hemisphere lesion)– Although they have intact sensory inputs, they fail to

respond

• Patients typically recover over a period of weeks or months, but may be left with long-lasting “extinction”– They will respond to a single event on the contralateral

side, but they fail when a stimulus is presented simultaneously on the “good” side

Line Cancellation TestLine Cancellation Test

CopyingCopying

A Sensory Deficit?A Sensory Deficit?

• Some neurologists believed that neglect is a result of diffuse damage to sensory systems

• Several sources of evidence now indicate that neglect is not a result of sensory deficits– Neglect in imagery– Object-based effects

Mental ImageryMental Imagery

The Piazza del Duomo in Milan, Italy

Object-Based NeglectObject-Based Neglect

Features & ConjunctionsFeatures & Conjunctions• In a visual search

experiment, patients with neglect showed normal search speeds when looking for a simple-feature target

• When they searched for a color-form conjunction target, search times were abnormally slowed if the target was in the contralateral field

Balint’s SyndromeBalint’s Syndrome• Patient R.M. has bilateral

parietal damage, leading to a complete lack of focused attention (Balint’s Syndrome)

– He can detect simple features, but he cannot localize them– He can perform simple feature search tasks accurately, but

he is at chance in conjunction search tasks– His performance is improved by an artificial “attentional

prosthesis”

Imaging studies of attentionImaging studies of attention

• Functional imaging studies have been conducted in which blood flow or oxygen consumption in the brain was measured while visual attention tasks were performed by to determine what parts of the brain control covert attentional selection.

• These studies show parietal lobe activation during visual search and cueing tasks. This activity is consistent with several existing hypotheses.– 1) That parietal cortex controls the voluntary orienting

of attention to a location of interest.– 2) That parietal cortex controls the reorienting of

attention to new locations once one object or location has been attended.

Corbetta et al. (2000)Corbetta et al. (2000)

Measured blood flow while subjects performed a cueing task.