Impaired holistic processing in congenital prosopagnosia

Abstract

It has long been argued that face processing requires disproportionate reliance on holistic or configural processing, relative to that required for non-face object recognition, and that a disruption of such holistic processing may be causally implicated in prosopagnosia. Previously, we demonstrated that individuals with congenital prosopagnosia (CP) did not show the normal face inversion effect (better performance for upright compared to inverted faces) and evinced a local (rather than the normal global) bias in a compound letter global/local (GL) task, supporting the claim of disrupted holistic processing in prosopagnosia. Here, we investigate further the nature of holistic processing impairments in CP, first by confirming, in a large sample of CP individuals, the absence of the normal face inversion effect and the presence of the local bias on the GL task, and, second, by employing the composite face paradigm, often regarded as the gold standard for measuring holistic face processing. In this last task, we show that, in contrast with controls, the CP group perform equivalently with aligned and misaligned faces and was impervious to (the normal) interference from the task-irrelevant bottom part of faces. Interestingly, the extent of the local bias evident in the composite task is correlated with the abnormality of performance on diagnostic face processing tasks. Furthermore, there is a significant correlation between the magnitude of the local bias in the GL and performance on the composite task. These results provide further evidence for impaired holistic processing in CP and, moreover, corroborate the critical role of this type of processing for intact face recognition.

Figure 1

Composite experiment: Examples of the stimuli used in the experiment showing the aligned (top row) and misaligned (bottom row) conditions.

Figure 2

Supporting evidence for impaired holistic/configural processing in CP. a. Stimuli used in the compound letter global/local task b. Results obtained for the entire group of control participants, age-matched controls and CP participants on the global/local task showing that CPs do not exhibit the expected global advantage and, instead, evince a local advantage and local-to-global interference. Asterisks denote significance level * p<0.05; **p<0.005 ***p<0.0005. Error bars indicate ± standard error of the mean across participants.

Figure 3

Composite experiment: Mean accuracy (a) and reaction time (b) on the composite face experiment for the entire group of control participants, age-matched controls and CP participants on the ‘same top’ trials. Note the lack of interference effect in the CP group in accuracy and in RT, indicating that CPs are not affected by the experimental manipulation. Asterisks denote significance level * p<0.05; ***p<0.0005. # indicates p value that is marginally significant (p=0.06 in top graph and p=0.07 in the lower graph). Error bars indicate ± standard error of the mean across participants.