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
Experiments to Verify Explanation 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
- 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
- Result: He tends to do better on faces than on other objects.
- 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
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