Agnosia ('failure to know') refers to an inability to perceive or identify a stimulus, even though its details can be detected and a person with this disorder can retain a normal intellectual capacity. Apperceptive visual agnosias are failures in high-level perception, where they cannot recognize objects visually by their shape. In most cases, the visual field of a patient with an apperceptive visual agnosia is in full use; they can see everything, but they cannot verbally report what something is (Clark, 1994). A common symptom of apperceptive visual agnosia is prosopagnosia, the inability to recognize faces.

    There have also been questions as to which areas of the brain control which functions, an ongoing debate that is aimed at discovering the most profound underpinnings of the brain. For example, the theory of modularity has been examined and argued for and against. Martha Farah, a researcher in the area of neuroscience, argues against this notion by using examples such as visual agnosias (Farah, 1994). She argues that human brain processing is graded, interactive, and distributed, that processes occur across the brain and not in a linear fashion which was the long held belief. But evidence from disorders such as visual apperceptive agnosias furthers the notion that the brain is actually much more complex that once believed. When referring to most neurological problems, they are usually termed as a loss or deficit, delineating an impairment or incapacity of neurological function. Many case studies have shown that the complexity of the internal structure and mental performances of the brain are quite sophisticated in nature and are more profound that we once theorized.

    Take the example of Dr. P, also known as "the man who mistook his wife for a hat," who was a patient of Dr. Oliver Sacks. Dr. P was a musician teaching at the university level. He was a man of great talent and great intellect. One day, for seemingly no reason at all, his students noticed strange things about him. When they would approach him with a question, Dr. P would sometimes not recognize who they were right away, but would recognize them as soon as they spoke (Sacks, 1985). This strange behavior carried over into other areas of his life; when walking down the street, Dr. P would sometimes see faces where they were not, on water hydrants or parking meters. Because he did not feel ill, these symptoms were overlooked and ignored by not only Dr. P but also his students and family. On a routine visit to the eye doctor some three years later, his doctor told him that there was nothing wrong with his eyes; there was something wrong with the visual part of his brain.

    Dr. Sacks became his neurologist at that point and it was only then that the bizarre and quirky behaviors that Dr. P were exhibiting were only part of a deep rooted neurological dysfunction. Dr. Sacks noted upon their first meeting that the musician tended to face him with his ears; that he was not really looking at him, but instead was listening to him. Dr. P said there was nothing wrong with him that he was aware of and that he had never felt better. However, upon closer examination, Dr. Sacks recognized some of the typical symptoms of someone with a visual agnosia. For example, when looking at a photograph on the cover of National Geographic Magazine his eyes darted back and forth, his attention caught by small details or bright colors. When asked what he saw, Dr. P responded, "I see a river...and a little guest-house with its terrace on the water." He was unable to see the whole picture, only focusing on small bits and pieces of the entire image (Sacks, 1985). It was after this examination that Dr. P got his name; he stood up from the table, reached over to his wife, and grabbed her head and tried to lift it, clearly thinking that her head was a hat!

    Upon further examination of the patient, Dr. Sacks noticed many other visual deficits. For example, when he presented Dr. P with a glove and asked him what it was he responded in a way that was at the same time childish but also deeply wrought in intellectual. Though he could not say 'it's a glove,' he gave a detailed description of what the function and shape of the object was. He said, "A continuous surface...it appears to have five outpouchings...a container of some sort." Dr. P was unable to see past abstractions; he was incapable of looking at something in its' entirety and functioned much more like a machine than like a human, detecting subtle features and markers of an object rather than the full picture.

Furthermore, when asked to visualize a local square and walk through it in his mind, Dr. P noted all of the buildings on the right side but ignored all on the left (Sacks, 1985). However, when told to turn around and come back the way he came, he listed all of the buildings previously forgotten. Therefore, not only was he suffering from a visual agnosia involving prosopagnosia, he was also suffering from a neglect syndrome that interrupted his imagination and visual imagery.

It was unclear at the time what to make of his condition. As he could still play and sing music as well as before his symptoms started, it was hard to determine the exact deficits in their totality and even more difficult to diagnose him. While visiting the professor at his home, Dr. Sacks learned that he was also a painter of formidable skill. He glanced through a hallway full of paintings done by the doctor, which were hanging in chronological order. He noticed that all of his earlier work was very realistic and detailed. As time went on, however, he noted that the paintings became more abstract and even seemed to be nonsense (Sacks, 1985). What it signified to Sacks was a parallel in his pathology and his artwork; as his paintings became less and less concrete so did his mental processes, signifying a long, drawn out loss that occurred over the span of years.

Dr. Sacks did not diagnose Dr. P that afternoon at his house. Knowing that his life's work was based in music and song, the neurologist simply told him to make it the whole of his life, the center of his universe. 'The man who mistook his wife for a hat' did everything to music, and if he lost touch with it, even for a moment, he would lose himself in his surroundings and become completely disconnected. For a man who had lost the ability to recognize objects in their entirety, to see things past the abstract, to make judgments based on visual information, he retained a skill that is essential, one that science sometimes leaves behind, that is the ability to relate to something on a personal level, in his case, music. Dr. Sacks never performed a follow-up with Dr. P, but often wondered what became of him.