PSY 2C 08: BRAIN, BODY AND
UNIT–I: BRAIN AND BEHAVIOUR
Agnosia is a perceptual disorder in which sensation is preserved but the ability to recognize a
stimulus or knowing it’s meaning is lost. Agnosia means “with out knowledge”. patients with
agnosia cannot understand and recognize what they see, hear, or feel. Agnosia result from
lesions that disconnect and isolate visual, auditory and somatosensory input from higher level
processing. Perceptual skills have a hierarchical and parallel organization. The nature and
severity of perceptual impairment depends upon modality affected and the level at which
sensory processing has been interrupted. It is rare in its in it’s pure form. Less than one
percent of all neurological patients have agnosia. When assessing agnosia, it is important to
establish that sensation is preserved; the patients is alert, intelligence is intact (or near intact)
with no language or memory disorder. Examination involves assessing what the patient sees,
hears or feels when presented with objects, pictures or sounds using a combination of clinical
procedures and neuropsychological tests.
The history of agnosia contains several striking examples of the interplay between cognitive
theory and clinical practice, and represent a good example of how scientific advancement is
not always linear or cumulative. There has been a recent revolution in the field of agnosia as
we have more primarily from an almost exclusive emphasizes on disconnection concept to a
more cognitive neuropsychological perspective. Because models of normal perception have
always driven conceptualization of agnosia, brief review of four broad models will be
provided before discussing the major agnostic syndrome.
The earliest neuropsychological idea of the process of object recognition were embodied in
“stage models” which held that the cortex first built up a percept from elementary sensory
impressions. Lissauser (1890) argued that recognition proceeds in two stages : apperception
(he meant that the conscious perception of sensory impression; the construction of a separate
visual attribute in to whole) and association ( the imparting of meaning to content of
perception by matching and linking in to a previous experience). A central idea that object or
face recognition depends not just an integrity of early perceptual process but also on later,
culminating “gnostic” stage in which the visual impressions are combained in such a way as
to assess a internal representation.
In 1965 Geschwind’s view agnosia resulted from a disconnection between visual and verbal
processes. He cited anatomic evidence from the syndrome visual object agnosia, which in his
view was most often seen in the context of left mesial occipital lobe damage. According to
Geschwind, this lesion not only induced a right homonymous hemianopia but also prevented
information perceived by the right hemisphere from reaching the naming area because of
impingement of crossing fibers. In advancing this hypothesis Geschwind described several
examples of patient who after failing to identify on formal testing, later used or interacted
normally with the object. In bringing attention to these phenomena, he provided clear
evidence that recognition is not a unitary phenomena.
The model proposed by Lissauure and Geschwind attempt to explain agnostic symptoms in
terms that were consistent with available theoretical constructs. This approach begins by
specifying the tasks that sensory perceptual systems must perform to achieve the kind of
powerful and flexible recognition abilities we as humans possess. We are able to recognize
everyday objects and faces with remarkable ease across wide ranges in viewing distance,
orientation and illumination. We are able to infer depth, volume, and structure from relatively
impoverished two-dimensional stimuli such as photographs and line drawings. Thus from
perceptual analysis we can derive enormous amount of structural and semantic information
about the world around us.
COGNITIVE NEUROPSYCHOLOGICAL MODEL
A fourth class of model has recently emerged in the tradition of the cognitive modular, or
“box model” approach. These models attempts to out line, in cognitive terms the functional
components involves in object recognition. According to Ellis and Young the process of
recognition begins by comparing viewer- centered and object - centered representations to
stored structural descriptions of known objects (so-called object recognition units) ORUs. The
ORU acts as a interfrence between visual representation and semantic information. When
information in viewer and object centered representations adequately matches structural
information in some ORU, the ORU becomes activated. This in turn, gives rise to a sense of
familiarity and unlocks semantic information about the object. Since the ORU receives
independent input from viewer and object centered representations, it can be activated by
either independently if a sufficient matches is obtained. Name retrieval occurs in the final
stage of the model. There is no direct link between ORU and speech output lexicon; all
retrieval of object names occur via the semantic representation.
THREE CRITERIA OF RECOGNITION:
a) The patient’s ability to overtly identify the stimulus can be assessed.
b) Responses that indicate semantic knowledge about the object.
c) The presence or absence of discriminative responses adequate to the stimulus.