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Ans. According to Campbell, DeJong and Haerer, "agnosia refers to the loss or impairment to know or recognize the meaning or import of a sensory stimulus, even though it has been perceived (Campbell et al., 2005, p.91). Numerous types of agnosia has been reported till now, like finger agnosia, visuospatial agnosia, optic agnosia , apperceptive and associative agnosia to name a few. The following essay discusses the last two forms.
Apperceptive and associative agnosia was originally distinguished by Lissauer in Andrewes in 2001. Apperceptive agnosia is identified as failure in perception of vision despite intact visual sensation, put forward by Lissauer It is reported that these patients are unable to identify because their perception of objects in impaired in this form of agnosia (Andrewes, 2001). However, in associative agnosia perception remains unaltered but the patient is fails to recognize what the object is (Andrewes, 2001). Associative agnosia can be rightly defined as 'normal percept stripped of meaning' (Teuber, 1968 as cited in Andrewes, 2001, p.50)
The two terms, apperceptive and associative agnosia is used in two different ways (Farah, 1990 as cited in Andrewes, 2001) The first way is to do with rare neuropsychological syndromes and closely relating to Lissauer's description of agnosia (Andrewes, 2001). The second way of using the terms is much broader and includes numerous neuropsychological signs (Andrewes, 2001). Hence, it can be said, "a patient may be described as showing some signs of apperceptive agnosia without actually having all the features of the clinical syndrome" (Farah 1990 as cited in Andrewes, 2001, p.50). This can be better understood from the following example.
Signs of apperceptive agnosia may coexist with problems in recognizing pictures from atypical views or when it is surrounded by shadows (Warrington & Taylor as cited in Andrewes, 2001) in a single patient. Some of the patients may also find it difficult to correctly recognize figures which is in midst of confusing and distracting shapes (Andrewes, 2001). When signs of apperceptive agnosia exist alone in an individual then he is able to recognize the object and corresponds it with its use. For instance, if a patient sees a bucket not only he will recognize it but when it is kept in its usual /normal orientation tell its use as well (Andrewes, 2001).It is often seen that patients showing signs of apperceptive agnosia have an unaffected conceptual knowledge but knowledge of common objects in unusual orientation deters (Andrewes, 2001). Hence, it can be said that "top down information about the structural features of the objects are impoverished" (Andrewes, 2001, p. 50). Such patients having signs of apperceptive agnosia are commonly referred to as "apperceptive perceptive disorders".
Apperceptive agnosics have better acuity, colour and brightness differentiation skills from the other visual capabilities although their shape perception is markedly impaired (Farah & Feinberg, 1997). In fact, they have very good local perception of local visual properties, it is only when they are asked to extract a structure from an image they fail (Farah & Feinberg, 1997). Associative agnosics have far better visual perception than apperceptive agnosics (Farah & Feinberg, 1997). For example, they are able to recognize an object from its feel or spoken definition, thus implying that the general knowledge of the object is still at place (Farah & Feinberg, 1997). It should be noted that associative agnosics fail to recognize an object (by sight) when kept alone (Farah & Feinberg, 1997) thus indicating towards that this is not just a naming deficit but failure to recognize an object by nonverbal means (Farah & Feinberg, 1997). Associative agnosia varies from person to person. For instance, some associative agnosics may suffer from face recognition, object recognition and printed word recognition problems, face recognition is being the most common and printed word recognition being rare (Farah & Feinberg, 1997).
The scans (MRI and CAT) of brains of apperceptive and associative agnosics have helped in understanding the localisation of lesion in this disorder (Kemp et al., 2004). Jankowiak & Albert (1994) have put forward that in apperceptive agnosia lesions are found to be localized in the posterior cerebral hemisphere including occipital, parietal and posterior temporal regions bilaterally. Unilateral lesion is also found in this agnosia however, the possibility of it is very scarce (Kemp et al., 2004). Furthermore, poisoning by carbon monoxide is a very common cause of apperceptive agnosia (Adler, 1950; Benson & Greenberg, 1969; Champion & Latto, 1985; Mendez, 1988; Sparr et al., 1991 as cited in Kemp et al., 2004), it is reported that carbon monoxide poisoning results in spread of large number of small lesions known 'salt and pepper' lesions resulting in scotomas all across the visual field (Champion & Latto, 1985 as cited in Kemp et al., 2004). It is believed that since apperceptive agnosia results from bilateral lesions, its occurrence is much rare. However, it is opined by Jankowiak & Albert (1994) that due to the paucity of accurate PET scans and imaging studies confirming the localisation of lesions no concrete conclusion can be drawn at this stage (Kemp et al., 2004). Associative agnosia is an outcome of bilateral posterior lesions (Jankowiak & Albert, 1994 as cited in Kemp et al., 2004). The lesions occur in the region of posterior cerebral artery whose function is to supply of blood to visual cortex and temporal lobe (Jankowiak & Albert, 1994 as cited in Kemp et al., 2004). It is suggested that lesion size is a decisive factor here, as large lesions will lead to perceptual deficits; additionally, considering the symptoms it is also said that the lesions in associative agnosia might cause damage to the perceptual pathway which links visual information with stored visual memory in posterior hemisphere of either side (Jankowiak & Albert, 1994 as cited in Kemp et al., 2004). Moreover, occurrence of associative agnosia is more than apperceptive agnosia (Kemp et al., 2004).
A very useful and a practical method through which we can distinguish apperceptive and associative agnosia can be testing them on the basis of their ability to copy drawings (Kemp et al, 2004). Rey figure copying test can come very handy here. Apperceptive agnosics are unable to copy a drawing due to their impaired perception of a picture, associative agnosics on the other hand can successfully copy a drawing although they are unable to recognize what the object is (Kemp et al., 2004).
However, Lissauer gave his distinction between apperceptive and associative agnosia hundred years before but it is found to closely relate to the David Marr's distinction of the two forms of agnosia given in 1982 (Kemp et al., 2004). According to Marr, apperceptive agnosia is failure to form a three dimensional picture or a representation of a object whereas in associative agnosia an individual achieves a three dimensional picture but fails to connect it to the stored knowledge of the object perceived (Kemp et al.,2004).