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Normal and impaired face perception:
implications for the optometrist
Andrew J. Logan BSc (Hons), MCOptom, Dr Gael E. Gordon PhD, Dip. Optom and Dr Gunter
Loffler PhD
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Course code: C-34648 | Deadline: February 28, 2014
Learning objectives
To understand the difficulties experienced by patients with an impairment of
face processing (Group 2.2.1)
To understand the neurological implications which relate to impairments of
face perception (Group 6.1.14)
Learning objectives
To understand the difficulties experienced by patients with an impairment of
face processing (Group 2.2.1)
About the author
Andrew Logan is an optometrist studying for a PhD in vision science at Glasgow Caledonian University (GCU) under the supervision of the coauthors, Dr Gael E. Gordon, optometrist and lecturer in vision science at GCU and Dr Gunter Loffler, optometrist and reader in vision science at GCU.
The group have published widely on the subject of face perception and presented their work at both national and international conferences.
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Being able to recognise a familiar face is a skill often taken for granted. This article gives an account of the
underlying mechanisms for normal face perception and the implications of impairment to this important visual
function due to ocular and neurological disease.
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Introduction
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Faces are among the most important objects
that humans visualise and they can contain a
wealth of information. A brief glimpse of a face is
sufficient to allow an accurate judgement about
someone’s age, gender or ethnic background
and whether that face is familiar or not. Humans
can easily discriminate between hundreds or
even thousands of different faces. Faces also
play an important role in communication;
humans broadcast cues about their feelings and
emotions through facial expressions and, for
example, we can infer the focus of someone’s
attention just from the direction of their eye
gaze. That we are so good at discriminating
between faces is in many ways remarkable.
After all, faces are based upon the same basic
configuration; two eyes, above a nose, above
a mouth and are, hence, broadly similar.
Nevertheless, the visual system is very sensitive
to subtle differences in the position and shape of
facial features. As a result, humans are extremely
accurate at recognising faces that are familiar
to them. From an evolutionary perspective, this
ability is critical for any species to interact with
others of that species and form social bonds.
Our ability to recognise faces allows for changes
in lighting, viewing angle, facial expression and
cosmetic details (for example make-up and
facial hair) which all significantly transform the
retinal image.1 Consider the case of a woman
returning home from the hairdresser with an
expensive new hairstyle which her partner fails
to notice. It could be argued, perhaps, that
this omission is not (entirely) due to a lack of
attention on his part but rather the result of a
very sophisticated visual system that has set its
priority at recognising a face as familiar rather
than noticing other changes.
Humans are so good at recognising faces that,
despite years of research, computer scientists
have been unable to produce an algorithm to
match the ability of the human visual system.
Figure 1 The Thatcher illusion with former president of the College of Optometrists, Frank Munro.
Adapted from Thompson, 198013
However, this ability is limited to familiar faces
– when we look at unfamiliar faces, our ability
to differentiate between them is considerably
more prone to error.2 The consequences of
this can be serious; it has been estimated
that mistaken eye-witness identification is
responsible for approximately 75% of wrongful
convictions in the USA.3
Early experiences
Faces are important right from the start; newborn infants have an innate preference to fixate
on a face rather than other visual patterns.4
There is evidence that new-born babies learn
to recognise their mother’s face soon after
birth5 and show an early preference for
attractive compared to unattractive faces.6
Despite this innate face processing ability, it
takes years to become expert at recognising
faces. Recent estimates suggest that the face
processing system may continue to mature over
the first 30 years of life.7 Normal face-viewing
experience appears to be a requirement for
the normal development of face perception.
Adults deprived of visual stimulation in early
life by congenital cataracts demonstrate
marked face recognition deficits.8 On the other
hand, similar levels of visual deprivation do
not impair recognition of non-face objects
such as houses or even animal faces.9 These
findings suggest that human face recognition
is particularly reliant on visual experience.
As visual experience is so important
for recognising faces, the processing
system becomes tailored by experience
with particular face categories. This is
best demonstrated by the own-race bias
(sometimes referred to as the cross-race
effect). People are substantially worse at
discriminating between faces of an unfamiliar
race than faces that belong to their own race.10
This own-race bias is not fixed and can be
recalibrated by a change in experience. For
example, Korean children adopted into French
families when they were less than 10 years old
were shown to perform more accurately on a
face recognition test with Caucasian faces than
with Korean faces.11
Original faces
Top half (both)
Bottom half (left)
Figure 2 The composite face effect
Illusions and effects
Given our remarkable sensitivity to faces,
it has long been speculated that the visual
system may process faces differently to other
objects. Studies with upside-down faces
provide support for this assumption. Although
recognition accuracy for most objects
decreases slightly if they are viewed inverted,
face recognition accuracy is disproportionately
reduced.12 The classic explanation for this
effect is that inversion impairs the extraction
of the fine differences in feature spacing and
position which give a person’s face its unique
identity. This is perhaps best illustrated by
the Thatcher illusion, first described by Peter
Thompson in 1980 (see Figure 1).13 The two
upside-down faces in Figure 1 appear similar.
However, if you rotate the page so that you
view them in the typical, upright orientation,
it is clear that they are markedly different. The
face on the right appears grotesque because
the mouth and eyes have been inverted. This
was first (and famously) demonstrated with
a picture of Margaret Thatcher. The Thatcher
illusion demonstrates that the visual system
is curiously insensitive to differences in facial
features when faces are viewed upside-down.
Verbal descriptions of faces are often based
on prominent individual facial features, for
example, large ears for Prince Charles, or full
feature (for example the nose) taken from a
face with which they had been familiarised
(see Figure 4, page 52). When the target nose is
shown alongside noses from other individuals,
the task is difficult (Figure 4 – top). When
the same noses are put inside an otherwise
identical head, the task is much easier (Figure 4
– bottom). Simplified faces, akin to police e-fits,
have been used here to increase the clarity of
the part-whole effect.
The part-whole effect is the opposite of
what one would expect. Typically, isolated
targets are easier to identify than when extra
information is added to a scene (see Figure 5).
This does not apply to faces; adding extra face
parts makes the task easier. The part-whole
effect provides strong evidence that facial
features are not recognised individually but
are combined into a unified representation.
Neither the composite nor the part-whole
effect is found with non-face objects, for
example, cars, dogs or houses.15 These results
suggest that the visual system treats faces as a
special class of object and processes them in a
unique, holistic way.
Impairments of face
perception
Given that recognising familiar faces is an
important social skill, it is unsurprising that
patients who are unable to recognise faces
report anxiety in social situations, with feelings
of isolation and depression.16 Generally, face
recognition ability is diminished by reduced
visual acuity and contrast sensitivity.17 For
example, patients with age-related macular
degeneration (AMD) perform face recognition
tasks less accurately than people of a similar
age with healthy eyes,18 with one study
reporting that patients with AMD needed to
be eight times closer to a face image than
age-matched controls to correctly recognise
the face.19 Recently, it has been demonstrated
that similar impairments of face perception
are also found in patients with advanced
glaucoma.20 These results suggest that face
recognition impairment may be an important,
but perhaps overlooked, difficulty experienced
by patients with ocular disease. Patients with
AMD regularly highlight problems with face
recognition as a determinant of their quality
of life.21
Impairments of face perception are also
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Composite faces
lips for Angelina Jolie.
This is paradoxical
because it is well
established that
face recognition
relies heavily upon
the pooling of facial
features across the
whole face and
combining these
into a single, holistic
face, rather than the
recognition of isolated
features. This can
be illustrated by the
composite face effect
as shown in Figure 2.
The two composite
faces in the figure
look rather different
Bottom half (right)
from each other. The
top halves of both
composite faces,
however, are identical (check this by covering
the bottom face halves) but combining
identical top halves with different bottom
halves creates the impression that the faces
are completely different.14 The visual system
appears to automatically merge the top
and bottom halves together, creating the
perception of a new face identity. Note that
the two composite faces (top row) bear very
little resemblance to any of the three original
faces (bottom row) which were used to create
the composites. As a result, the presence of
the identical upper part in these images goes
unnoticed and requires careful inspection to
verify.
The mental processes responsible for
combining features into a single face identity
can be disrupted by introducing a horizontal
offset between the top and bottom parts, as
shown in Figure 3. As a result, the visual system
no longer treats the two halves as a single unit
and the similarity of the top halves becomes
apparent while the difference between the
bottom halves remains.
A compelling demonstration of the
involuntary and automatic nature of the
processing that combines facial features into
contextual units is provided by the part-whole
effect.15 In this classic experiment, observers
are asked to recognise a particular facial
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Figure 3 The composite face effect removed. Introduction of a horizontal disparity between
the top and bottom face halves mitigates the illusion
an established characteristic of autistic
spectrum disorders (ASD). Compared to
typically-developing children, those with
ASD spend significantly less time looking at
faces when they are embedded in a complex
visual scene.22 When they do look at faces,
patients with ASD often avoid looking at the
eye region of a face and focus on the mouth.23
This suggests that the face-viewing experience
of patients with ASD during development
is markedly different to that of typicallydeveloping individuals. While patients with
ASD have difficulty recognising familiar faces,
differentiating between unfamiliar faces and
interpreting facial expressions,24 their ability to
recognise non-face objects (for example cars,
houses) is unimpaired.25 This suggests that face
perception represents a particular problem
for patients with ASD. Impairments of face
perception have also been associated with a
range of other prevalent conditions including
Alzheimer’s disease, Parkinson’s disease and
schizophrenia.26,27,28
The best-studied and most dramatic
impairment of face perception is the rare
Target face
Figure 4 The part-whole effect
neurological condition prosopagnosia
(sometimes referred to as face blindness).
The term prosopagnosia was first coined
by Bodamer in 1947 to describe a specific
impairment of familiar face recognition in
three patients.29 The loss of face recognition
ability in prosopagnosia can be profound;
some patients become unable to recognise
their spouse or even their own reflection in a
mirror.30 Prosopagnosia is debilitating. Patients
often report that they are unable to follow film
plots because they cannot recognise central
characters. In extreme cases, the condition
can necessitate a change in the patient’s
career and can lead to the development
of social anxiety disorders or depression.
Patients with prosopagnosia frequently have
normal language, memory and intellectual
functioning. Moreover, the patient’s low-level
vision (visual acuity and contrast sensitivity)
remains largely intact. Thus, despite being
able to view faces clearly, patients with
the condition often complain that all faces
seem indistinguishable. Indeed, one patient
described faces as appearing:
“Strangely flat, white, with
emphatic dark eyes, as if made from
a flat surface, like white, oval plates,
Which of these is
all alike.”31
the target nose?
The aetiology of prosopagnosia
involves damage to the specific brain
region linked to facial processing,
Which of these
contains the
the fusiform gyrus in the temporal
target nose?
lobes. One of Bodamer’s original
patients was a young male who
developed acquired prosopagnosia
after sustaining a bullet wound to the head. The
patient had no history of difficulty with face
recognition prior to suffering brain damage.
The most common cause of acquired
prosopagnosia, however, is stroke. Less frequent
causes include carbon monoxide poisoning,
head trauma and surgical treatment for
intractable epilepsy.32
Depending on the type and extent of
brain damage, the resulting impairment can
be unique to faces. Patients with acquired
prosopagnosia may perform very poorly on
face recognition tests yet be able to recognise
other complex objects including fruit, birds,
furniture, cars and even animal faces, as
accurately as neurologically-normal people.33
For example, one patient with acquired
prosopagnosia was reported to be able to
identify individual members of his flock of
sheep, despite being unable to recognise
familiar faces.34 Furthermore, although patients
with the condition are unable to recognise faces,
some can make accurate judgments about the
gender, attractiveness and age of individual
faces. Interestingly, patients with prosopagnosia
do not exhibit any of the hallmarks of face
perception, demonstrated by the three effects
described earlier (the face inversion, composite
or part-whole effects).33 This is in line with the
view that the condition represents a specific
impairment of the face recognition system.
More recently, a different form of
prosopagnosia has been described.
With manifestations similar to the acquired
form, developmental prosopagnosia is a
lifelong impairment of face recognition, where
patients have no structural brain abnormalities
(as confirmed by MRI and CT scans) and no
impairment of intelligence or memory. Low-level
visual function (visual acuity, colour vision and
visual fields), as in the acquired form, is typically
normal. The precise aetiology of developmental
prosopagnosia has not been determined;
although patients with the condition are
often related to other people who experience
difficulty with face perception suggesting a
genetic basis. One report identified significant
impairments of face perception within 10
members of the same family.35 Although cases
of acquired prosopagnosia are relatively rare, it
does seem that the developmental form of the
condition may be considerably more common.
In children, the symptoms of developmental
an individual
changes their
appearance
(for example a
new hairstyle,
switching from
spectacles to
contact lenses).
When meeting
new people,
such as visiting
an optometry
Figure 5 Try to find the letter ‘d’ in both squares. The task is considerably
practice,
more difficult when extra visual information is added
patients with
prosopagnosia
may not, however, be immediately obvious
find name badges particularly helpful.
during a typical optometric consultation that
Patients who experience difficulties with
problems with face perception exist. In the
face recognition also prefer to interact with
case of ocular disease, the optometrist needs
only small numbers of new people. It may
to be aware of perceptual problems with
be helpful, then, to ensure that, on visiting
faces as a consequence of the impaired retinal
an optometry practice, patients with known
and/or optic nerve functioning. In the case
prosopagnosia are consistently assisted by the
of neurological disorders, neither the ocular
same members of staff. health nor the visual acuity may point towards
face perception deficits. Where face deficits
Conclusion
Faces engage mechanisms which are
distinct from those used to recognise other
objects. The normal visual system is adept
at recognising minute differences between
individual faces, which are important for
normal social functioning. Impairments of face
perception can result from both ocular disease
and neurological disorders. Optometrists are,
therefore, likely to encounter patients who
present with difficulties in face recognition. It
are identified, the optometrist should keep
in mind the considerable impact these can
have on a patient’s quality of life and provide
appropriate advice and support.
Acknowledgements
The authors would like to thank Frank Munro
for generously allowing us to use his face
photograph. The authors would also like
to thank Lesley Miller and Eilidh Martin for
comments on an earlier version of this article.
MORE INFORMATION
References Visit www.optometry.co.uk/clinical, click on the article title and then on ‘references’ to download.
Exam questions Under the new enhanced CET rules of the GOC, MCQs for this exam appear online at www.optometry.co.uk/cet/exams.
Please complete online by midnight on February 28, 2014. You will be unable to submit exams after this date. Answers will be published on
www.optometry.co.uk/cet/exam-archive and CET points will be uploaded to the GOC every two weeks. You will then need to log into your CET
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Reflective learning Having completed this CET exam, consider whether you feel more confident in your clinical skills – how will you change the way you
practice? How will you use this information to improve your work for patient benefit?
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prosopagnosia can lead to an erroneous
diagnosis of ASD or the child simply being
labelled as antisocial and unfriendly. Adults with
prosopagnosia often report feeling guilty and
embarrassed about their inability to recognise
people from previous social interactions.
For many patients, the diagnosis and an
explanation about the nature of the condition
provides reassurance. When asked about their
experiences of living with developmental
prosopagnosia before and after diagnosis,
one patient commented: “I didn’t want to let
anybody know how weak I was – whereas now I
don’t put it down to weakness, I put it down
to just the way I’m wired, and it’s not a problem
in terms of telling people.”16 There is, as yet, no formal treatment for the
condition. Although preliminary evidence
suggests that intensive training may improve
the face recognition ability of patients with
developmental prosopagnosia.37 Others have
suggested that improvements can result from
inhalation of oxytocin, a hormone that has a
key role in social bonding.38
It has been estimated that the prevalence
of the developmental form is 2–3%.36 The
condition may be under-diagnosed because
patients may be unaware that their faceperception ability was abnormal, with deficits
camouflaged by coping strategies, such as
relying on distinctive voices, gait, clothing,
spectacles or hairstyles to recognise individuals.
These strategies, however, are considerably
less robust than face recognition; patients
with prosopagnosia experience difficulty if

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