The human visual cortex is divided into specialized modules that code a variety of different visual features, like motion in area MT/MST [1
], faces in the fusiform face area (FFA; [3
]) and houses in the parahippocampal place area (PPA; [4
]). This division of labor entails a well-known problem: When confronted with multiple objects, how does the visual system 'know' which features belong together in one object?
This so-called 'binding problem' [5
] calls for the integration of information into object representations or 'object files' [6
]. The immediate consequences of such integration have been demonstrated in an elegant study by O'Craven et al. [7
]. Their subjects saw overlapping pictures of a house and a face, with either the house or the face moving. When subjects were asked to respond to the direction of the motion, attention spread from the motion to the object, regardless of which object was moving: Functional magnetic resonance imaging (fMRI) results showed that the PPA was activated more strongly when the house moved, and the FFA was activated more strongly when the face moved. This suggests that attending to an event creates some sort of functional link between the representations of its features, whether they are relevant (like the direction of the motion in this example) or irrelevant (like the faces or houses). Further support for this notion comes from a recent fMRI study by Yi et al. [8
] who found that face-selective regions in the FFA and lateral occipital cortex exhibited significantly less activation when (task-relevant) faces were repeated in (task-irrelevant) continuous versus discontinuous trajectories. Again, this suggests that attending to a moving object creates an object file in which object identity and spatiotemporal parameters are closely integrated.
To ensure stable percepts of objects (e.g., tolerating small changes in viewpoint or lighting), the functional links or object files need to be persistent over time. Indeed, behavioral research suggests that object files outlive the events they represent by several seconds and that they affect subsequent behavior in a systematic fashion [8
]. For example, if subjects respond to one feature (e.g., shape) of a two-dimensional stimulus (e.g., varying in shape and location), they respond faster and more accurately if the two stimulus features both repeat or both alternate, than if one feature repeats while the other alternates [9
]. Consistent with the notion of object files, this finding suggests that processing an object binds its features such that if one or all of these features are encountered again, the whole object file is retrieved. If this involves reactivation of a feature that mismatches with features of the present object (which happens when one feature repeats and another alternates), performance is impaired because of the conflict between retrieved and perceptually available features and/or because the old associations need to be deconstructed [14
]). Note that the task described here did not require participants to integrate features; therefore the obtained effects provide a relatively pure measure of automatic, implicit integration processes, free of particular task-dependent strategies [15
Automatic retrieval of object files has the theoretically interesting property of mimicking several effects that are often attributed to executive control processes. For example, there is evidence that at least substantial portions of the flanker-compatibility effect [16
], the Simon effect [17
], inhibition of return [18
], and negative priming [19
] are actually produced by the impact of object files formed in the previous trial. However, the neural mechanisms underlying the hypothesized object-file retrieval are unknown and direct demonstrations that feature repetition actually induces the retrieval of corresponding object files are lacking. Accordingly, the present fMRI study was designed to test whether reviewing a particular stimulus feature reactivates the features of the object it previously accompanied. The features/objects that we used to address this question were motion, faces, and houses, which, as noted above, activate distinguishable regions of the occipitotemporal cortex [7
]. These stimuli have been shown to integrate in a similar way as more basic features such as location and color [20
]. As in previous studies [9
], participants were presented with two stimuli: A task-irrelevant prime (S1) and a probe (S2). Both stimuli consisted of blended pictures of a face and a house. On each trial, either the face or the house moved in one of two possible directions and participants were instructed to respond as quickly as possible to the direction of the moving object in S2. Thus, each stimulus consisted of two features (motion direction and moving object) that were orthogonally repeated or alternated between S1 and S2 (Figure ).
Figure 1 An example trial. On each trial two face-house compound stimuli (S1 and S2) were presented. Either the face or the house moved, in a left-up right-down or right-up left-down oscillatory fashion. Participants were instructed to watch S1 and to give a two-choice (more ...)
We expected to obtain the standard behavioral result: Repeating the motion direction and the moving object, or alternating both, should yield better performance than repeating one feature and alternating the other. The fMRI measures were used to test whether this pattern actually reflects object-file retrieval. In particular, our approach was to use activity in the FFA and PPA as an effective index of the degree to which the task-relevant stimulus feature (motion direction in S2) reactivated the task-irrelevant feature (moving object) it accompanied in S1. Thus, we examined whether repeating the motion direction reactivated the object (face or house) that moved in this direction in S1. Diagnostic for this reactivation effect are conditions in which the moving object changes (e.g., if a house moved in S1 but a face moved in S2): Repeating the motion direction in S2 should tend to reactivate the representation of the house that moved in this direction in S1, which should lead to a greater activation of the PPA than if motion direction alternates.