A particular difficulty in discriminating between mirror images.

We investigated the selective impairment of mirror image discrimination in a patient with bilateral parieto-occipital lesions (FIM). We report a difficulty with the discrimination between mirror images more selective than has been previously reported (Turnbull OH, McCarthy RA. Failure to discriminate between mirror-image objects: a case of viewpoint-independent object recognition? Neurocase 1996;2:63). FIM was asked to judge, in five same/different experiments, whether pairs of simultaneously presented line drawings of objects were identical. FIM demonstrated only a minor impairment in discriminating between orientations in the picture plane but was at chance in making discrimination between mirror images. An experiment with normal observers established that our results were not due to differences in task difficulty. Two further experiments investigated the effects of rotation on the discrimination of letters and geometric shapes. FIM's impairment extended to geometric shapes but not to letters. These results would be consistent with the preservation of an abstract representation for object recognition that did not code the difference between mirror image views.