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ATTENTIONAL SYSTEMSAs we have seen last week, different types of visual information are processed in different part of the visual system. In particular, we studied that information about location and spatial characteristics and information about non-spatial features (shape, color, motion, etc.) are processed in two different visual pathways. * The ventral pathway, or "WHAT" pathway (from the occipital visual cortex to the temporal lobe) processes features important for object recognition
* The dorsal or "WHERE" pathway (from the visual cortex to the parietal lobe) process features important for localization and motor action.
Attention seems to follow a similar distinction: orienting of attention in space is related to activity in the parietal lobe. Attention to objects or to object features is related to activity in the ventral visual pathway and in the frontal lobe.
Michael Posner proposed to distinguish between two attentional systems: the POSTERIOR attentional system that controls orienting of attention in space and involves parietal brain regions and the ANTERIOR attentional system, that controls the resolution of conflicts and selective attention and involves regions in the frontal lobe.
Spatial and nonspatial attention have different properties and are studied with different experimental paradigms and we will study them separately.
SPATIAL ATTENTION
We will examine two different paradigms that investigated spatial attention: the spatial cueing task and the flanker task. We will examine how each experimental paradigm works and which kinds of information we can get using these paradigms
What is an experimental paradigm?
Spatial cueing is a DETECTION task. The participant is asked to press a key as soon as possible any time he or she sees a target stimulus. In this particular case the target is an asterisk and it can appear in the left or in the right box. The participant has to fixate the plus sign in the center and not move his/her eyes. Why is it important that the person does not move his eyes?
After a certain interval the plus sign changes in a "greater than" or "smaller than" sign. This sign is called CUE and gives information to the observer on where the target will appear. In one version of the spatial cueing paradigm, 80% of the time the target will appear in the box indicated by the cue (i.e., a > will indicate the right box and a < will indicate the left box), and only 20% of the time the target will appear in the other box. Another cue "<>" is used for "neutral" trials, that is, trials in which the participant does not have any information about the probable spatial location of the target.
VALID TRIAL is a trial in which the target appears where is expected, that is, where the cue was indicating. Eighty percent of the trials are valid.
INVALID TRIAL is a trial in which the target appears in the box that was not cued (20 % of the times)
NEUTRAL TRIAL is a trial in which the cue does not give any indication on where the target will appear.
Which strategy do you think the participant will use? What do you predict the results will be?
RESULTS:
As you can see from the figure, people are faster when they know were the target will appear (VALID CONDITION) as compared to the condition in which they do not have information on the target location (NEUTRAL CONDITION). Mike Posner proposed that this happens because people ORIENT attention where they expect the target. Because people are not allowed to move their eyes in this task, we talk of "COVERT ORIENTING" of attention (we move attention but there is no behavioral correlate such as an eye movement). Covert orienting of attention produces an advantage in target processing in the attended location.
Also, people are slower when the target appears in the non cued location (INVALID CONDITION) as compared to the NEUTRAL CONDITION. This is the COST associated to orienting of attention.
Remember from last week that according to signal detection theory there are two parameters that determine the response to a signal: sensitivity and response bias. Can the advantage in valid trials and the cost in invalid trial be an effect of response bias? Some researchers explored this issue and computed ß and d' for valid and invalid trials and found that the difference was in SENSITIVITY and not in RESPONSE BIAS. This means that orienting of attention change the efficiency of information processing in the cued location and not our response criterion.
This is a very simple paradigm, but it is very useful because it can be used with animals (for example with monkeys) and with patients that can be too impaired to perform complicated tasks. Posner proposed that attentional orienting is the combination of three mental operations: disengagement, movement and engagement. Where in the brain do you think these operations are computed? Neuroimaging and lesion studies have showed that the parietal lobes are very important areas for spatial orienting of attention.
One deficit that is related to attentional orienting is VISUAL NEGLECT. Patients with parietal lesions have a hard time orienting attention on locations contralateral to the lesion.
SUMMARY
I want briefly to examine another experimental paradigm that is used to study spatial attention.
SELECTION BASED ON SPACE: THE FLANKER TASK
In this task, the participant sees groups of three letters as in the figure below:
The target in this paradigm is the central letter and the person has to press a key if the middle letter is an H and another key if the middle letter is an S. So, target selection is based on the position of the letters. Eriksen showed that people are faster to respond in the congruent condition (when target and flanker letters are the same) than in the incongruent condition (when target and flanker letters are different). This experiment shows that even if we can improve processing in the attended location, stimuli presented in the unattended location influence our behavior, in this case slowing down our response time. However, if the flanker letters are far enough from the target, the difference between congruent and incongruent conditions starts to disappear. When the distance is more than 1 degree of visual angle, the flanker letters do not influence reaction time any more.
These data suggest that the selectivity of our spatial attention is limited. If we think of spatial attention as a spotlight we have to conclude that the minimum diameter of the spotlight is 1 degree of visual angle and everything within that region will be "lighted up" by attention.
SUMMARY
* Attention can move through space even if our eyes are not moving
* Allocating attention to a certain location improve processing
* Moving attention produce a cost
* Spatial orienting is controlled by the parietal lobe and other subcortical structures (superior colliculus and thalamus)
* The focus of attention has a certain size; it cannot be smaller than 1 degree of visual angle. Stimuli that fall within the focus of attention are processed and influence behavior.
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