Agnosia - Sydney Cognitive Development Centre

Agnosia may be defined as an impairment in the
higher visual processes necessary for object
recognition, with relative preservation of elementary
visual functions. Studies in agnosia help provide
both ideas and constraints to theories of the
process of visual recognition. However, there is
much debate on theories of visual recognition.
There is also much controversy on how agnosia
should be classified. These classifications often
reflect different understandings of the process of
visual recognition. Two recent classifications will be
discussed in terms of their own hypotheses of visual
recognition.

Farah (1995) distinguishes between apperceptive
and associative agnosia. Apperceptive agnosia is
thought to reflect any failure in object recognition in
which perceptual impairments are clearly at fault,
despite relatively preserved elementary visual
functions. Farah proposes four subdivisions within
apperceptive agnosia. The first category is
apperceptive agnosia (narrow sense). These
patients show adequate elementary visual functions,
yet are impaired on the simplest form of shape
discrimination. They cannot recognise, match, copy,
or discriminate both simple stimuli and complex
objects. The next category is labelled dorsal
simultanagnosia. These patients have full (or near
full) visual fields but can usually see only one object
at a time. They have difficulties in localising an
object in space, however, as long as they can see
an object, they can recognise it. Ventral
simultanagnosia is similar to dorsal simultanagnosia
as patients are able to recognise only single
objects. However, even though they cannot
recognise multiple objects they can see them. The
last category described by Farah is a perceptual
categorisation deficit. These patients have great
difficulties in matching three-dimensional objects
across shifts in perspective and therefore have
problems in naming objects when viewed from an
unusual perspectives.

In comparison, associative agnosia is thought to be
due to problems in accessing semantic knowledge
about an object even though the perceptual
representation of the object is intact. Farah
describes several subtypes of associative agnosias.
Firstly, associative visual object agnosia (narrow
sense) describes patients who have difficulty in
recognising visually presented objects as seen
though inabilities to name visually presented objects
both verbally and non-verbally, and problems
grouping objects according to their semantic
category. These patients do, however, show normal
recognition of objects though modalities other than
vision and also retain the ability to copy objects or
drawing and match similar pairs of stimuli. The next
category of associative agnosia is prosopagnosia
which reflects an inability to recognise faces.
Another category, pure alexia, includes patients that
cannot read normally despite apparently normal
visual capabilities and preservation of writing and
other language abilities. Farah also distinguishes
other categories of agnosia such as having a
selective deficit for living or nonliving things, or
other category specific impairments.

Farah interprets both apperceptive and associative
agnosia in terms of several theories of normal visual
recognition. However, she concludes by advancing
her own hypotheses of visual recognition. She
postulates that higher vision begins with the
grouping of local elements of the visual field into
larger-scale contours, regions, and/or surfaces.
Patients with apperceptive agnosia (narrow sense)
are examples of a disruption in these grouping
processes. They have perception of local contour,
colour, and brightness yet have severe impairments
of overall form. This stage of grouping separately
registered local elements into higher order
geometric representations is a prerequisite for
virtually all higher visual processing and is a purely
stimulus-driven process. The array of grouped
stimulus elements interacts with two higher-level
systems: the spatial attention system and the object
recognition system. A disruption in the spatial
system results in dorsal simultanagnosia, in which
the contents of the grouped array that can be
attended to in a given amount of time are
abnormally limited. In contrast, a disruption in the
object recognition system results in ventral
simultanagnosia, in which the contents of the
grouped array that can be recognised in a given
amount of time are abnormally limited. The spatial
attention system selects portions of the grouped
array, and thus stimuli in this portion of the array
are likely to be detected and recognised by the
object recognition system.

The object recognition system redescribes portions
of the grouped array in a more abstract format and
captures the invariant 3-D geometric properties
more fully than the grouped array. All objects are
composed of parts which have certain spatial
relations, and this system decomposes most objects
into their constitute parts. These parts are then
matched against object representations which
results in the recognition of the object. This has led
Farah to suggest that there are two underlying
types of recognition ability being damaged in
associative agnosia. Firstly, the ability to represent
parts themselves, including complex parts for
objects that undergo little decomposition. Secondly,
the ability to rapidly encode multiple parts especially
for objects that undergo much decomposition.
Impairments in the ability to represent parts reflects
the range of associative agnosias. That is, the more
the representation of parts is impaired, the greater
the recognition deficits. This can explain differences
between prosopagnosia, and other object agnosias,
as with severe deficits only the simplest objects will
be recognised. In contrast, if the ability to represent
objects is intact but the ability to rapidly encode
multiple parts is impaired, then most objects will be
recognised. Only objects which undergo
decomposition into many parts will be affected. This
type of deficit will result in alexia.

In contrast, Humphreys and Riddoch (1987)
propose a different classification system which they
believe reflects the complexity of the component
processes involved in visual object recognition, and
the manner in which such processes may selectively
break down. They propose that there are six types
of deficits that may be applied to patients with visual
agnosia. These include impairments in shape
processing, transformation processes, integration
processes, access to form knowledge, access to
semantics, loss of stereoscopic vision, and impaired
semantic knowledge. The impaired shape
processing classification refers to patients whose
recognition problems are attributable to deficits in
processing the primitive characteristics of shape
such as edges at different orientations and spatial
scales. Impaired transformation processes involve a
classification of patients that can perform simple
shape matching tasks, identify objects from a
prototypical view, but are unable to identify objects
from an unusual view. In contrast, patients with loss
of access to form knowledge typically have difficulty
in accessing or using stored information about
visual form. Patients may show difficulties in drawing
from memory yet will be good at copying.

Impaired access to semantics reflects the next
category of visual agnosia. These patients have
access to the object form system as well as intact
semantic knowledge, however, they are impaired in
accessing knowledge visually. However, as their
semantic knowledge is intact, they can access this
knowledge via another modality. Loss of
stereoscopic vision constitutes another
classification. Patients with this problem are able to
derive well integrated 2-D representations of objects
but fail to assign depth to these representations.
This deficit impairs object recognition when
recognition is dependent upon the derivation of
accurate surface descriptions. The last classification
involves impaired semantic knowledge. Patients
show no deficits on pre-semantic visual object
processing yet are unable to recognise these
objects. Unlike patients with semantic access
impairment, semantic agnostic patients have
damage to the semantic system and thus cannot
recognise objects via other modalities.

This type of classification is directly reflected by an
information processing framework proposed by
Humphreys and Riddoch. This model begins with a
visually presented object. The first two processes of
visual recognition are local geometric feature
processing and global shape processing. They
propose that if a lesion occurs at the global shape
processing site, shape processing will be impaired.
Information from these first two processes help to
form a viewpoint-dependent object description. A
lesion at this site results in impaired 2-D shape
integration or the coding of integrated shape
descriptions. Information from all three processes so
far form an abstract episodic object description.
Damage to the abstract episodic object description
will impair transformation processes. Information
from both the abstract episodic object description
and the viewpoint dependent object description then
access object form knowledge. Damage to the
object form knowledge site will result in impairments
in accessing knowledge of visual form. The next
stage is semantic knowledge of the object, and
damage will impair semantic knowledge of all objects
in all modalities. Object form knowledge and
semantic knowledge send information to one
another and lesions in the flow of knowledge to one
another will affect access to semantic knowledge,
however, semantic knowledge is spared thus it may
be accessed via other modalities. It is evident that
the classification system of Humphreys and Riddoch
is derived directly from their model of visual
recognition and the deficits that can occur at each
stage of the model.

Both classifications described are based on vastly
different models. Farah’s model of recognition is
much better in explaining dissociations between
different types of associative agnosias such as
prosopagnosia, pure alexia, and other selective
object agnosias. In contrast, Humphreys and
Riddoch’s information processing model of visual
recognition is much more open to further expansion
in a way in which Farah’s apperceptive-associative
distinction is not. For example, it is becoming
increasing clear that perception is not entirely
normal in the vast majority of associative agnosia,
thus, not all types of agnosia can be easily
categorised into Farah’s broad classifications of
apperceptive and associative agnosia.

In conclusion, there is currently much debate over
the processes underlying visual recognition. Both
classifications discussed are based on different
models, and it is evident that each of the disorders
in both classification systems illuminate disruptions
of different stages of normal visual recognition.
Further research of visual agnosia will increase
knowledge of normal recognition. Subsequently, a
greater understanding of normal visual recognition
will help researchers devise new and better ways of
classifying agnostic patients.