PITTSBURGH - When visual scenes contain multiple objects and people, we take it for granted that we can recognize them all with ease and accuracy. But just how the brain gathers and makes sense of raw visual material remains something of a mystery.
Fueled by the development of impressive new behavioral and scientific techniques, scientists now are uncovering more clues about how the human brain "sees." And in the process, scientists like Carnegie Mellon University Associate Professor of Psychology Marlene Behrmann are creating a better understanding of our mind's "eye" and changing the landscape for possible treatment of brain disorders.
Behrmann will discuss "The mind's eye and the brain's matter: The neural basis of visual imagery" at a session on Saturday, Feb. 16, during the American Advancement of Science meeting in Boston, Mass. The presentation, which is part of the Cognitive Neuroscience of Mental Imagery session, will address questions about the mechanisms recruited for visual mental imagery and the relationship between these mechanisms and those used for perception.
Research in Behrmann's lab focuses on how the brain interprets signals from the eyes; specifically her goal is to shed light on the psychological and neural processes that underlie our ability to interpret visual information. These processes allow us to recognize objects, faces and words, and they enable us to know where these objects appear so that we can reach out to pick them up or move our eyes to inspect them further.
Behrmann skillfully uses patients with discrete, or distinct, damage to the visual areas of the brain in her research. For example, she examines the behavior of individuals suffering from visual agnosia, a deficit in the ability to recognize even common objects shown visually. Patients who fail to recognize the objects still recognize the objects when they are presented in another way -- for example, through touch.
She also conducts research with on hemispatial neglect, a condition that typically follows a stroke to the superior regions of the brain. Individuals suffering from hemispatial neglect ignore information appearing on one side of space. Behrmann and Carnegie Mellon Associate Professor of Psychology David Plaut have created a computational model of the brain that mimics the symptoms of hemispatial neglect she observes in patients.
Through detailed examination of the behavior of these individuals, as well as using functional magnetic resonance imaging with non-impaired subjects, Behrmann attempts to address three major questions:
- How are form and identity of objects represented in our brain?
- How is location, or spatial information, "coded" by the brain?
- How are form and identity integrated with location to present the unitary visual experience that most of us enjoy?
Behrmann said the generation of an internal visual representation - what is often referred to as "seeing with the mind's eye" -- in the absence of retinal stimulation is known as visual mental imagery, and is thought to be a critical component of many cognitive tasks including learning, reasoning, problem solving and language.
"Perhaps the most hotly debated current issue is whether mental imagery exploits the same underlying mechanisms as visual perception. Indeed, such a brain organization might be thought of as an instance of two-for the price of one: two processes piggybacking on the same basic mechanism," Behrmann said.
She added that research findings obtained from functional imaging studies and studies using psychophysical and behavioral methods to characterize perception and imagery largely support the two-for-one proposal.
However, there are some challenges. One provocative source of data comes from detailed studies of individuals who have sustained brain damage in adulthood. These individuals are, for example, unable to draw or describe objects from memory, to dream or to verify propositions based on memory ("does the letter 'W' have three strokes?") despite their ability to recognize objects or pictures shown to them.
Behrmann said the converse situation also exists. There are individuals with profoundly impaired perceptual function but remarkably intact imagery. These individuals might fail to recognize even everyday objects such as a hairbrush or a tennis racquet yet, when asked, can draw and can derive detailed visual images of these very same objects. This independence between perception and imagery is assumed to indicate two completely separable and independent processes and, as such, presents a challenge for views that argue for a common underlying mechanism.
Behrmann expects that the discrepancy between the views will be reconciled through a detailed analysis of the cognitive processes required for imagery and perception. Although the same processes may be involved in both functions, some of the processes likely play a greater role in imagery than in perception and vice versa, she added.
"While the two-for-one principle operates in general, it is possible to see some separation between processes under brain damage that selectively affects either imagery or perception. The conclusion is that, while the same brain-behavior mechanisms might well support imagery and perception, they do so to a differential extent such that divergences may be observed within a system whose psychological and neural substrate is shared. In this way, the system gets the best of both worlds," she said.
Marlene Behrmann behrmann@cnbc.cmu.edu, 412-268-2790