Another set of disorders does not increase or decrease our imagery but rather, distorts it. Neurologic disorders that affect association cortices impair the way we process perceptual information. Simultanagnosia, for example, results from bilateral parieto-occipital damage, and patients have trouble mapping space. Visual acuity is preserved so that when a target is located, it is seen, but finding the target is the problem.
Images courtesy Dr. Richard J. Caselli
Patients with simultanagnosia attempting to "draw a clock, put in all the numbers, and make it say three o’clock."
Patients with posterior cortical atrophy often have severe constructional apraxia. Some are better able to copy a complex figure than draw a well-known item such as a clock (see figure).
Prosopagnosia, a form of visual agnosia that impairs our ability to recognize familiar faces results from damage to the temporal lobes, particularly in the right hemisphere. A prosopagnosic can discern a face is that of an attractive young woman with ruby red lipstick but may fail to identify Marilyn Monroe.
by damage to visual association cortex in which patients lose the ability to perceive color. Artists who have developed achromatopsia, not surprisingly, have lost their ability to use color in their paintings (Neuropsychologia 2004;42:1568-83).
Mental imagery is the idea we wish to fulfill, but it remains a daydream until it is acted upon. Before we act, we need a plan, and next month we shall consider disorders of strategic formulation.