6.4 Neural Correlates of Attentional Selection
6.4.2 Salience Processing Unit
The job of the salience processing unit is to calculate the relative saliency of the objects represented by the subject. Since I have argued that the salience unit plays a crucial role in the capturing of attention, the area that calculates saliency should also be active prior to the deployment unit so that can be the causal source of the information that results in the redeployment of attention to the salient object. What area seems like it can play the role needed by the theory?
The lateral intraparietal area (LIP) is the most likely candidate for having the function of tracking the salience of represented objects. Part of the reason for this is that neurons in LIP do not have a fixed preferences. That is, unlike the neurons in visual cortex, they do not have a preferred stimulus that remains the same over time. Instead these neuron appear to respond more strongly when the stimulus at that locationis moresalient, regardless of
the particular features that make it salient (Bisley and Goldberg 2003).4 There are two parts of this claim. First, it seems that LIP represents items by their location, which means that we can think of the salience signal as a ‘map’ of locations of varying salience. Itti and Koch (2001) describe it as “a scalar, two-dimensional map whose activity topographically represents visual saliency, irrespective of the feature dimension that makes the location salient” (p.198). Second, the map in LIP represents the general salience of an object, not any particular feature. This helps explain why, for example, Bisley and Goldberg (2006) found that neurons in LIP respond strongly to briefly flashed stimuli regardless of their other properties.
If there are salience maps in LIP, in order for them to play the role of the salience processing unit posited by my theory, these maps must be able to influence the deployment of attention. Happily there is some causal evidence that induced changes in the map in LIP can cause changes the bias signal sent to the deployment unit. Mirpour et al. (2010) found that stimulating LIP biases visual search toward the corresponding part of the visual field, presumably because such stimulation directly increases the represented salience at that location.
The salience maps in LIP are able to quickly ( 80 ms) represent the location of a ‘pop- out’ stimulus (Ipata et al. 2006). Interestingly, it seems that such signals can be suppressed after learning that those stimuli are not task relevant. That is, ‘top-down’ signals regarding the relevancy of the pop-out item seem to be able to reduce the activity of the LIP neurons representing the salience of that item. This means that the salience map is not determined solely by the visual input itself. In subsequent research multiple ‘top-down’ influences have been found to affect the salience map in LIP. For example, it can be influenced learned rules about the task-relevance of the object at that location (Ipata et al. 2009) and by the reward value of the object at that location (Anderson et al. 2011). Interestingly, the salience of a
4For example a neuron in LIP would respond equally strongly to the sudden onset of a red letter amongst blue letters as a blue letter amongst red letters. The features do not matter in and of themselves, only the relative salience.
stimulus can be suppressed after being closely examined and determined to be irrelevant at the moment (Mirpour et al. 2009). These results indicate that the salience maps in LIP are formed on the basis of both the actual salience of the object at that location and information from the attentional template about what is relevant. This means that subjects have a degree of control over the ability of salient stimuli to capture attention.
It seems, then, that LIP contains maps that represent the salience of objects in each part of the visual field. However, in order for LIP to play the role proposed for the salience unit, it must occupy the right place in the ‘flow chart of the mind’. In particular, it must be activepriorto the deployment module (FEF) in cases of attentional capture and in cases of the voluntary selection of an item for attention (if LIP is active at all) it must be activeafter the deployment module has initiated attentional modulation. Buschman and Miller (2007) have done work to address exactly this issue. They found that when a pop-out stimulus is the target in a task, the signal representing the location of the target appeared first in LIP and then in FEF (and dlPFC). This suggests that in cases of capture the signals flow from visual cortex to LIP, and then on to the deployment unit in FEF. In the case where there was no pop-out target and the subject simply had to search for the target on the basis of the task instructions, they found that the signal representing the location of the target turned up first in FEF (and dlPFC) and then in LIP. This suggests that the two areas are connected in the proper fashion to work as required by the model.5