3. Orienting - Visual Locations

Orienting to visual locations refers to moving our eyes and possibly head such that some target (stimulus) falls on our fovea.  It allows us to attend to that location of the target.  It is also possible to attend to a location without actually moving our eyes or head (this is known as covert attention whereas moving the eyes/head is referred to as overt attention).  Within the first 150ms of attending to some location, we are able to respond faster to events occurring there and detect events occurring at that location with lower thresholds. Actually, improvements begin before any eye or head movements (shift of attention) Furthermore monkey brain studies have shown increased neuronal responses in the following areas, indicating their importance to attention.
  1. Posterior parietal lobe
  2. Lateral pulvinar nucleus
  3. Superior colliculus
  4. Parietal cortex
Further evidence of the role in attention of the above areas comes from the study of primate and human brain lesion subjects who have suffered damage to these areas.  Lesions in any of these areas will reduce the ability to shift attention to some other location.  For example, damage to the superior colliculus affects the ability to shift attention to some other location regardless of whether we previously are or not attending to some other target.

3.1 Hypothesis for the shifting of visual attention

Examining humans and animals with brain lesions, has confirmed that there are definitely physical areas in the brain responsible for attention and it has also provided an idea of the process required for the shifting of visual attention.  Based on these studies, the authors suggests the following hypothesis for shifting of visual attention:
  1. Parietal lobe disengages attention from the current focus.
  2. Mid-brain area moves the "index of attention" to the target area.
  3. Pulvinar collects data from the target location and processing begins.
Brain lesion studies have also revealed (although still controversial) that that the left and right brain hemispheres have their own attentional specialty.  Both carry out the operations required to shift attention, however, they differ with respect to the level of detail.  "Large and small" letter tests (e.g. forming large letters out of small letters - see [6]) indicate that the right hemisphere is oriented towards global processing whereas the left hemisphere is mainly responsible for local processing.  For example, the image of figure 3 illustrates the combination of many small letter "L"s to form one large letter "L".   Global processing allows one to notice the single large "L" while local processing will focus on the constituent small letter "L"s.
 
 
Figure 3.  Large letter "L" constructed from many small letter "L"s.

3.2 Pattern recognition - Visual Search

Attention is used in pattern recognition tasks from detecting features to storing new visual patterns.  An example of a pattern recognition task is shown in figure x, where the task may be to locate the single red triangle in the presence of other different color shapes.
 
 
Figure 4.  Pattern recognition task involving a single target shape with one distinct feature. Reprinted from [10] 

In these pattern recognition tasks, increasing the number of non-targets and their features does not influence the search for a one feature (e.g. color, or shape as in figure 4 where the red target is separated by all other non-target shapes by color only).  However, increasing the number of target features to more than one, does decrease the speed of the task.  For example, consider the example shown in figure 5.  Finding the red triangle (target) now takes more time than finding the same target triangle from the example of figure 4..
 
 

Figure 5.  Pattern recognition task involving a multiple feature target. Reprinted from [10].

Further evidence that attention is involved in pattern recognition comes from studies involving single cell recordings.  Such studies indicate the attention system interacts with the pattern recognition system through the thalamus, requiring approximately 90ms to do so.