As discussed above, executive processes send control parameters to the input selection system that determine what types of inputs should be selected. These parameters will cause attention to be focused onto a given input, which in turn causes a facilitation of processing for the attended input and an inhibition of processing for the unattended inputs. One set of processes is used to identify the input that should be selected, and another set of processes is used to produce differential processing of the selected and unselected inputs. We call the first set of processes the control of selection and the second set of processes the implementation of selection. In the context of the spotlight metaphor of attention, the control of selection is analogous to pointing the beam in the correct direction, and the implementation of selection is analogous to the strength of the beam. The control of selection typically involves prefrontal and parietal cortices, and the implementation of selection typically occurs within the areas that process the inputs (e.g., within visual cortex).
Consider, for example, the visual search task shown in , in which the observer must indicate whether the array contains a green vertical bar. In healthy individuals, attentional control parameters can be set to favor the color green, allowing these individuals to limit search to a subset of the items (26
). indicates this with circles around the green items, which indicate that attentional control mechanisms will flag these items for preferential treatment. However, flagging these items is only half of the problem; selective processing of the flagged items must somehow be implemented. That is, an additional mechanism must be present that allows the features of the flagged items to be identified without interference from surrounding items. This example of the implementation of selection is illustrated by a ring of suppression around the target item, which allows the features of the target item to be apprehended more clearly.
This distinction can also be seen in the example shown in . Mechanisms of control determine which of the two letters is flagged, whereas mechanisms of implementation allow the flagged item to suppress the unflagged items and gain control over processing.
Unfortunately, it can be extremely difficult to separately measure the control and implementation of selection by means of behavioral measures. If responses are slow or inaccurate for a given target item, this could be due to a failure to direct attention to this item or a failure to effectively process this item and filter out distractors once attention has been directed to the item. However, control is easier to isolate than implementation because there are several techniques that can be used to determine whether attention has been misdirected. For example, if overt shifts of gaze are measured rather than shifts of covert attention, it is trivial to determine whether attention was directed to the right item or the wrong item (27
). Similarly, one can determine whether a salient distractor can attract attention to itself, thus slowing the allocation of attention to the target (29
). Many of the best studies of the implementation of selection involve electrophysiological recordings, which can provide independent measures of the processing of attended and ignored stimuli (7