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Composite face illusion is an emerging finding in the field of cognitive science. Faces are one of the important source to access social information and how face processing is done by the people is major area of research. Two different halves of a face when taken from separate individuals forms a composite stimuli. When both the upper and bottom halves of the face are aligned together then it provides an innovative configuration of the face. Both the identical halves of the same face are recognized as different when the other halves are of different faces. This accounts towards the understanding of holistic processing that says one tends to recognize a whole face more easily as compared to individual feature. The composite face procedures have enormously contributed towards the awareness of concepts such as aberrant face perception, detrimental effects of inversion but it is still deceptively hard to interpret and measure and have become the subject of intense scrutiny.

Composite paradigms have advanced our knowledge and understanding of face perception and the underlying mechanisms involved in dynamic face processing. However, there are gaps in the knowledge which needs to be evaluated since the functional significance still remains uncertain for the composite face illusion.

The composite face effect
The matching paradigm of the composite face has been used in a large number of studies during the last decade. During a study in 1994, participants were presented with faces side by side at the same time in which top halves of the presented faces were kept identical and bottom halves were different. The observers tend to make mistakes or take more time in recognizing the identical top halves because the bottom halves belong to different faces. However, the presence of non-identical bottom halves were not the only reason that caused error in recognition and slow response to a stimuli because it was found that the composite illusion disappears if the top halves are spatially misaligned from the bottom halves. There was no difficulty in perceiving the misaligned halves as different when participants were provided with the same stimuli which was harder to recognize as different when aligned.

Role of decisional response bias in composite face effect
Due to the existing evidence based on the interdependent processing of both the halves of composite faces, many researchers has proposed that functional locus of composite face effect (CFE) is perceptual. However, the perceptual nature of the CFE has been studied and challenged by other researchers stating the role of decisional factor as well in facial recognition. The study based on signal detection theory which is also called as the general recognition theory (GRT) with the aid of multidimensional generalization was used to analyze the data involving behavioral discrimination tasks with faces. When GRT was used, the researchers led to the conclusion that both decisional and perceptual factors are responsible for driving the composite effect. However, the signal detection theory could not answer the question that how decisional biases are gained during the composite face task and later it was put forward that substantial experience with the faces might lead towards the development of highly ingrained assumptions in people regarding the parts of the faces during a task.

Evidence of perceptual locus in composite face effect
It is generally assumed that the decisional response biases might emerge as a result of perception of the composite face illusion because when the participants in a study were provided with identical top halves and non-identical bottom halves, they tend to take extra time and make more errors while matching the halves. The available electrophysiological and neuroimaging evidence from studies such as fMRI have shown this adaptation. The brain areas that were found to be involved in composite face effect are right middle fusiform gyrus, right inferior occipital gyrus and areas of visual cortex that primarily respond towards the faces.

When faces were inverted and spatially misaligned then neural composite face effect was not found which shows that information of aligned face halves is integrated by the neurons of these visual areas and is recognized as a single whole face. The same evidence was also represented by the studies that involved event related potentials of the scalp on N170 face sensitive component. The face sensitive N170 component accounts for the early stage during which facial recognition is accessed at the recording sites of occipito-temporal lobe and is independent of the decision making, which suggests that at an initial stage of facial recognition in human brain the perceptual representation of face is inherently holistic.

Holistic face processing
Holistic encoding is a very significant phenomenon in the process of facial recognition. It is defined as a simultaneous integration of different components and features of the face into one perceptual representation. Many of the behavioral experiments explain that upright faces elicit holistic effect in which the face is perceived as a whole rather than recognized through individual features, which forms the basis of composite effect during the process of face recognition and representation. Till now most of the studies and research have focused on the fact that how holistic encoding plays an important role in face recognition, but it is also observed that it can be linked to facial dimension and identity which shows its significant role in identification of facial expression. There are some key points highlighted by many studies in holistic face processing- firstly, inversion of the picture plane disrupts the face recognition. Secondly, the memory of an individual part of face is more accurate when the face is presented as whole rather than presented alone. Lastly, the performance of individuals in recognizing the faces decreases when two half faces of different individuals are aligned together which is better when independent stimuli of half faces is presented.

Research directions
Research on visual perception face challenges to merge neurophysiological approach with phenomenological approach and understanding the concept that how human brain is responsible for building a unified perception of face is still a dispute in visual neuroscience research field. The question does not only outline the concerns related to facial recognition but also involves general problem in understanding the human vision. Cognitive neuroscientists and experimental psychologists have used composite face illusion as an adaptation to methodological paradigm that aims at measuring various stimulus transformation.

Future research in the area can address the relationship between static and dynamic composites which remains unclear in terms of sensitivity to orientation and alignment. Despite many studies on the composite face effect it still remains a fascinating and captivating visual illusion which still holds a potential that can be further researched to refine the paradigm of the concept.