Explanations and Experiments: An Example

The Phenomenon and the Explanations

• Prosopagnosia: the acquired inability to recognize human faces, normally with little or no difficulty with other sorts of visual recognition
• Explanation 1: Faces are recognized in a separate part (module) of the brain from other things. When this part is damaged, we see effects on face recognition but not on other visual recognition.
• Explanation 2: Face recognition is handled by general-purpose visual recognition regions of the brain, but it is a very difficult task. When the region is damaged, we may see the effects only for the most difficult tasks.

Experiments to Verify Explanation 1

1. • Prediction: If we compare performance of normals with prosopagnosics on very difficult vision tasks not involving faces, we should not see a great difference.
   • Experiment: Subjects presented with cards showing objects, later given a second set and asked whether they had seen each before. Objects included faces and easily confusable other objects.
   • Result: While normals performed about equally well on faces and other objects. prosopagnosics did significantly more poorly on faces.
2. • Prediction: If face recognition is a module (and operates in parallel with the other module(s)), there should also be people with damage to the other recognition areas who show difficulty recognizing some objects but not faces.
   • Experiment: Such a person exists, given the first experiment.
   • Result: He tends to do better on faces than on other objects.
3. • Prediction: Upside-down faces are much more difficult to recognize than right-side-up ones; they are apparently not processed by the face recognition module. The difference in performance on right-side-up and updside-down faces show be much less, or nonexistent, for prosopagnosics.
   • Experiment: Normals and prosopagnosics compared on recognition of right-side-up and upside-down faces.
   • Result: Prosopagnosic performed better on upside-down faces.

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Last updated: 24 October 1995
URL: http://www.indiana.edu/~gasser/exp_ex.html
Comments: gasser@salsa.indiana.edu
Copyright 1995, The Trustees of Indiana University