Abstract
Humans can accurately tell the age, gender, current emotional state, and direction of gaze of person by a mere glimpse at his face. In normal development, this amazing capacity develops early, within the first year or two. Yet, what happens when early visual experience is lacking? We examined a unique population of Ethiopian children (N=24) who had congenital bilateral cataracts that were surgically removed only in late childhood (Mean age at surgery 10.8 years). These children had extremely poor vision prior to cataract removal (M=0.99 cpd). After the operation, their visual acuity substantially improved (M=5.59 cpd), but the improvement varied considerably. As early as five months after cataract removal, many patients (but not all) were able to distinguish the age (M=85.8% correct, p<0.001 compared to chance level performance) and gender (M=64.0% correct, p=0.005), differentiate between emotional expressions (M=68.9% correct, p<0.001) and select a face with straight-ahead gaze based on head (M=64.6% correct, p<0.001) or eye orientation (M=61.9% correct, P=0.003). Postoperative visual acuity was a major factor explaining the variance in the patients’ performance across these face understanding tasks (r=0.59-0.69 across tasks, all p<0.003), while preoperative was significantly related only to selection of faces with straight-ahead gaze based on head orientation (r=0.59, p=0.005). Crucially, control participants who did the same tasks under severe image blur, matching the conditions experienced by the patients with the poorest visual acuity (i.e., visual acuity = 1 cpd), could perform all the above tasks extremely well (M=87.4%-95.8% correct across tasks, all p<0.001). Thus, sufficient visual acuity is probably required for learning these tasks, but once acquired, useful facial information can be extracted even in conditions when the face is dramatically blurred. We conclude that there is evidence for a critical level of acuity, but not a critical period for the acquisition of social information from faces.