It is characterized by a distinctive, "elfish" facial appearance, along with a low nasal bridge; an unusually cheerful demeanor and ease with strangers, coupled with unpredictably occurring negative outbursts; mental retardation coupled with an unusual facility with language; a love for music; and cardiovascular problems, such as supravalvular aortic stenosis and transient hypercalcaemia. Williams syndrome shares some features with autism (such as difficulty understanding the state of mind of conversational partners), although persons with Williams generally possess very good social skills, such that this condition is sometimes called "cocktail-party syndrome". Temple Grandin, author of Thinking in Pictures, has claimed that the brain abnormalities of Williams syndrome are the opposite of those of autism. There also appears to be a higher prevalence of left-handedness and left-eye dominance in those with Williams, and cases of absolute pitch appear to be significantly higher amongst those with the condition.
Another symptom of Williams syndrome is lack of depth perception and an inability to visualize how different parts assemble into larger objects (in assembling jigsaw puzzles, for example). This problem is caused by a defect in the brain that creates a sparsity of tissue in the visual systems of the brain. A team of researchers at the National Institute of Mental Health used functional magnetic-resonance imaging (fMRI) to watch the blood flow of the brains of test subjects while they were performing two tasks involving spatial relations. People with Williams Syndrome showed weaker activity in the section of the brain associated with spatial relations. Scans of brain anatomy of test subjects with Williams indicated a deficit of brain tissue in an area of the same section of the brain mentioned above. This deficit partly blocks transmission of visual information to the spatial-relations region of the brain. In the test, all participants of the study measured in the average intelligence range, to remove the possibility that the retardation aspect of Williams syndrome would have an effect on the visual systems of the tested individuals.
An interesting experiment documented the fact that when shown a picture, Williams Syndrome people drew the small details while people with Downs Syndrome drew the big picture. Thus it can be generalized that people with Williams Syndrome "miss the forest for the trees".