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June 2003 Vol 4 No 6 HIGHLIGHTS
Nature Reviews Neuroscience 4, 430 (2003); doi:10.1038/nrn1151
[2150K]
VISUAL PROCESSING
Face it
Rachel Jones
We humans are specialists at face processing. We can identify individual
faces despite their similarities, and we can tell a lot about how a person is
feeling from their facial expression. Vuilleumier et al., writing in Nature
Neuroscience, have found evidence that these two functions are carried out
by different visual pathways in the brain, and that these pathways use
different elements of the visual information contained by the image of a face.
Examples of the face
pictures used in the study.
The broad-spatial-
frequency images (left
panel of each set of three
images) were filtered to
produce images that
contain only high (middle
panel of each set of three
images) or low (right
panel of each set of three
images) spatial
frequencies. Reproduced
with permission  (2003)
Macmillan Magazines Ltd.
Functional imaging techniques have added to our
understanding of how the brain processes faces.
We know, for example, that part of the fusiform
cortex -- the 'fusiform face area' (FFA) -- is
selectively activated by images of faces, and that
images of fearful faces also activate the amygdala,
which is important for processing emotion.
Intriguingly, the amygdala, unlike the FFA, is
activated by fearful faces even if those faces are
not consciously perceived or attended.
There is indirect evidence that the amygdala
receives visual information through a subcortical
pathway, which might allow it to carry out 'quick
and dirty' processing -- for example, rapidly
checking for danger signals (such as fearful
expressions) without waiting for the cortex to
carry out more detailed analysis. The same
pathway, through the thalamus, is thought to
mediate 'blindsight' -- the residual visual abilities
of cortically blind patients who can, for example, point accurately to a
stimulus they cannot see. Vuilleumier et al. tested the idea that this
subcortical pathway relies on low-frequency information extracted from the
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visual scene by the magnocellular channels, whereas the cortical processing
of facial identity relies on high-frequency information extracted by
parvocellular channels.
Subjects were asked to look at pictures of faces that contained either broad-
spatial-frequency information (normal images), or just the high- or low-
spatial-frequency elements. Their task was to identify the gender of the faces
-- a judgement that relies equally on high- and low-frequency data. Using
event-related functional magnetic resonance imaging, the authors studied
the activity of the brain in response to the different types of image.
As expected, normal pictures of faces reliably activated the FFA -- and, if the
faces were fearful, they also elicited the activation of the amygdala. Pictures
that contained only high spatial frequencies were equally able to activate the
FFA -- but they did not activate the amygdala even if their expressions were
fearful. By contrast, low-spatial-frequency pictures did not activate the FFA
as strongly, but the amygdala responded well to fearful expressions in these
pictures.
To see whether the lack of amygdala activation was because subjects could
not discern the expressions of the high-spatial-frequency faces, the
researchers asked another group to rate the pictures for fearfulness. The
results showed that, at least when they were attending to expression, people
could identify fearful expressions just as well in either type of picture. So it is
likely that although the amygdala carries out fast processing of expression
through subcortical inputs, a cortical visual pathway can also be used to
identify fearful expressions.
The findings of this study are consistent with previous psychophysical results
showing that low-spatial-frequency components of visual scenes convey
global, configurational information that carries emotional cues, whereas high
spatial frequencies are needed for detailed analysis of fine-grained
information for tasks such as face recognition. A fast
magnocellular­pulvinar­amygdala pathway might allow us to recognise
danger or emotional cues rapidly and regardless of the focus of attention,
and might also mediate the early visual abilities of babies, who can use low
spatial frequencies to detect emotional cues before the cortical visual system
matures.
References and links
ORIGINAL RESEARCH PAPER
Vuilleumier, P. et al. Distinct spatial frequency sensitivities for processing
faces and emotional expressions. Nature Neurosci. 11 May 2003
(10.1038/nn1057) | PubMed |
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