Facial recognition is an essential component of human social interaction. Because of this importance, and unlike common objects that are typically identified at the category level (e.g., “cat”), faces are identified at the level of the individual (e.g., “Mary”) [1].

Humans can differentiate the identity of individuals rapidly and accurately from relatively small differences in facial structure [2]. To explain the effectiveness of human face recognition, a range of evidence suggests that rather than processing facial features independently, information is integrated (holistic processing ^[3][4][3,4]).

Thus, a key characteristic of facial processing is its holistic nature ^[5][6][7][5,6,7], with minimal engagement in part-based analysis. Instead, there exists a robust integration among facial components. Most faces possess identical features (eyes, nose, and mouth) and consistent gross configurational information (eyes positioned above the nose, nose above the mouth), and different individuals exhibit comparable facial characteristics (e.g., eye color). Nevertheless, most adults can distinguish and recognize thousands of individual faces [8]. This suggests that details regarding specific facial features are not reliable for identifying an individual face. Holistic processing is deemed essential for distinguishing visually similar objects, such as faces, by using subtle variations in the configuration or relationships among different visual features.

Three experimental paradigms are widely acknowledged as standard metrics for holistic processing in the literature on face recognition ^[9][10][11][9,10,11]. These include the inversion effect, the composite task, and the part–whole task. Inversion effects, where upside-down stimuli are harder to recognize (i.e., recognized with lower accuracy) than upright stimuli [12], are considered an indirect measure of holistic face processing because they demonstrate that faces are processed differently from other objects. While inversion impacts both faces and objects, it has a disproportionately larger impact on faces, suggesting that holistic processing plays a more significant role in face recognition. However, this effect does not directly quantify the nature of holistic processing, as it does not differentiate between differences in face and object processing. There is evidence suggesting that people are less change-blind to faces than to objects. Research employing the “flicker paradigm,” which involves alternating images with minor changes, demonstrates that alterations in faces are identified more rapidly and accurately than alterations in other things, particularly when the faces are upright and displayed alongside various objects [13]. This advantage for faces appears to be associated with their biological and social importance, and it diminishes when faces are reversed or displayed in isolation.

Other tasks are more direct in quantifying holistic processing. In the part–whole task, the perception of a feature (e.g., eyes) is enhanced when it is presented within a whole object (a face) rather than in isolation, demonstrating the facilitative effect of the whole on part processing [3]. This effect has been interpreted to indicate that face parts are encoded not in isolation but rather as integral elements of a larger visual unit which is a (upright) face, improving performance for parts when integrated in the whole face.

A definitive illustration of holistic face processing is represented by the composite effect, which denotes that the perception of a significant portion of a face (e.g., the upper half) is affected by an irrelevant portion (e.g., the lower half). The composite effect is typically implemented using a same–different paradigm, where two faces are presented in quick succession, and the participant must determine whether the target half (e.g., the upper half) is identical in the sequentially displayed faces. Individuals often encounter difficulties when focusing their attention on a single facial feature, which apparently reflects (automatic) attention to all parts.

Do these three tasks measure the same process (or the same aspects of a single process)? Rezlescu and collaborators [10] investigated the interrelations among composite, inversion, and part–whole effects. The three holistic processing measures exhibited minimal overlap, with a significant and moderate correlation identified solely between the inversion and part–whole effects. Consequently, the three tasks represent distinct holistic mechanisms. Ventura and collaborators [14] found that variations in the same composite task (sequential vs. simultaneous presentation) may capture different aspects of holistic processing, necessitating a thorough investigation of these differences. Furthermore, various composite tasks may engage different aspects or subprocesses of holistic processing, including early and experience-dependent holistic processes.

An additional consideration thus pertains to the distinction between early holistic processes and experience-dependent holistic processes. Indeed, the significance of experience in the establishment of holistic processing has been challenged. Zhao and collaborators [15] discovered holistic processing resembling facial recognition for non-facial, novel stimuli without the influence of perceptual expertise. In the absence of training, line patterns exhibiting prominent Gestalt features (such as connectedness, closure, and continuity among components) were processed holistically, similar to faces.