Dr. Nancy Kanwisher, Professor of Brain and Cognitive
Sciences at MIT, described some of her recent studies
of the neural and cognitive mechanisms underlying human
visual perception and cognition. In her previous work,
she has investigated object recognition, visual attention,
and perceptual awareness, as well as response selection,
social cognition and the human understanding of numbers.
Kanwisher is best known for her pioneering work that
has identified several regions of the human brain that
seem to play specialized roles in the perception of
specific categories of visual stimuli such as faces,
places, and bodies.
Some of Kanwisher’s evidence that face perception is
mediated by special cognitive and neural mechanisms
comes from her functional magnetic resonance imaging
(fMRI) studies of the human brain’s fusiform face area
(FFA) and from behavioral studies of the “face inversion
effect.” Kanwisher combined these two methods to ask
whether face perception mechanisms are stimulus specific,
process specific, or both. In those experiments subjects
discriminated pairs of upright or inverted faces or
house stimuli that differed in either the spatial distance
among parts (configuration) or the shape of the parts.
The FFA showed a much higher response to faces than
to houses, but no preference for the configuration task
over the part task. Similarly, the behavioral, face
inversion effect was as large in the part task as the
configuration task for faces, but absent in both part
and configuration tasks for houses. According to Kanwisher,
these finding indicate that face perception mechanisms
are not process specific for parts or configuration
but are domain specific: that is, they are selective
face stimuli per se.
Kanwisher acknowledged that function of the fusiform
face area (FFA), a face-selective region in human extra
striate cortex, remains a matter of active debate. To
bring clarity to the issue, she measured the trial-by-trial
correlation between FFA activity measured by functional
magnetic resonance imaging (fMRI) and behavioral outcomes
in perceptual tasks. Her results show that FFA activation
is correlated on a trial-by-trial basis with both detecting
the presence of faces and identifying specific faces.
However, for most non-face objects (including cars seen
by car experts), within-category identification performance
was correlated with activation in other regions of the
ventral occipitotemporal cortex, not the FFA. These
results indicate that the FFA is involved in both detection
and identification of faces, but that it has little
involvement in within-category identification of non-face
objects (including objects of expertise).
Together with her post-doctoral student, Chris Baker,
and Eli Pelli of Harvard’s Department of Ophthalmology,
Kanwisher has explored neural plasticity and reorganization.
In these studies she exploited disease-related changes
in retinal function that are associated with macular
degeneration (MD), the leading cause of visual impairment
in the developed world. MD damages the central retina,
obliterating fovea vision and severely disrupting everyday
tasks such as reading, driving, and face recognition.
In such cases, the macular damage eliminates the normal
retinal input to a large region of visual cortex, comprising
tens of square centimeters of surface area in each hemisphere,
which is normally responsive only to fovea stimuli.
Using functional magnetic resonance imaging, Kanwisher
and colleagues asked whether this deprived cortex simply
becomes inactive in subjects with MD, or whether it
takes on new functional properties. In two adult MD
subjects with extensive bilateral central retinal lesions,
Kanwisher and colleagues found that parts of visual
cortex (including primary visual cortex) that normally
respond only to central visual stimuli are strongly
activated by peripheral stimuli.
Such activation was not observed (1) with visual stimuli
presented to the position of the former fovea, or (2)
in control subjects with visual stimuli presented to
corresponding parts of peripheral retina. These remarkable
results demonstrate large-scale reorganization of the
brain and of visual processing in MD, and will likely
prove important in any effort to develop new strategies
for rehabilitation of MD subjects.
