Progressive impairment of constructional abilities: a visuospatial sketchpad deficit?

Abstract

The case of a patient, PC, with an impairment of constructional abilities due to a progressive degenerative disease is described. Ideomotor apraxia was also present. PC showed difficulties in all tasks requiring a “manipulation” of a visual model or requiring a visual model to address a specific movement or a choice among alternatives. The spatial component of mental imagery was also severely impaired. The conclusion was that the patient presented a deficit of the rehearsal component of the visuospatial sketchpad (VSSP). This produced an impairment of spatial learning, but spatial retrograde memory was preserved, as well as object recognition. The most prominent sites of atrophy were located in the parietal lobe bilaterally. A 3-year follow-up is reported, showing a progressive impairment of verbal abilities.

Introduction

In recent years, there have been a large number of case reports of patients showing a progressive breakdown in language abilities, with sparing of other cognitive functions [40]. There are also descriptions of progressive dementia in which the prominent features are visuo-perceptual and spatial impairments [12], [18], [31], as well as two reports of patients with progressive degeneration of the right temporal lobe and progressive prosopoagnosia [15], [36]; finally, two cases of progressive visual agnosia have been described [9].

Visuo-perceptual impairment is reported in patients with Lewy bodies dementia [26]: these patients are particularly impaired in size and form discrimination, overlapping figure identification and visual counting.

Disorders of constructional abilities are very common in the course of degenerative dementia [17] and they may also be the first symptom [12], or the most severe [21]. However, no attempt has been made in these cases to understand the “functional” locus of the impairment. In contrast, cases with selective cognitive deficits may provide an interesting contribution to the development of cognitive models of visuospatial processing. In one of such cases, it has been shown that mental representation of horizontal and vertical spatial relations in an egocentric co-ordinate system are functionally dissociated [21].

There are two main problems regarding constructional apraxia (CA). The first concerns the fact that CA can, indeed, be caused by a variety of cognitive deficits: visuo-perceptual, mental representation, motor implementation or executive disorders. The second problem is related to methodological issues, because CA has been assessed in different ways: copying designs; drawing from memory; arranging sticks in a pattern; building three-dimensional models; block design. Even if all of these tasks require some basic common abilities, they nevertheless differ in other important aspects, such as the load of general intelligence or the involvement of grapho-motor abilities. Recently, the study of CA has become the study of drawing, more precisely of copying. Copying requires different stages to be carried out. Several models have been proposed [20], but essentially, they all include three main stages. During the preliminary stage, a search for an interpretative hypothesis of the model is performed: on the one hand, the individual tries to identify in the stimulus objects that have already been drawn in the past, and at the same time he/she analyses the spatial relationships between elements of the picture and those between the picture and the paper on which it is drawn. During this stage a visual representation of the model is stored in a visual short-term memory system. This stage requires visuo-perceptual abilities and short-term memory. In the second stage, the elements identified by this analysis are processed in order to formulate a drawing plan. It has also been proposed that a subject can retrieve separate parts of the model, which are known configurations (such as squares, circles, etc.), stored in a sort of “constructional lexicon” [1]. This stage requires mental imagery and retrieval from long-term memory. Then, in the third, executive stage, the plan is preserved in a short-term memory system for as long as is necessary to complete its translation onto paper, due to activation of motor programmes [20].

Visuospatial short-term memory in constructional tasks has not received much attention, but from the description above, it is evident that a short-term memory system plays a role. I will refer to the visuospatial sketchpad (VSSP) component of working memory (WM) [2]. The VSSP is assumed to maintain and manipulate visual information and to be involved in visual imagery. Logie [23] suggests that the VSSP can be divided in two sub-components (in analogy with the phonological loop): the visual cache and the inner scribe, working in partnership. The visual cache is thought to deal with information that is visual in nature, such as form and colour, and to be closely linked to the activities of the visual perceptual system. The inner scribe, in contrast, is proposed to handle information about movement sequences and, in a manner somewhat analogous to the sub-vocal rehearsal process of the phonological loop, to refresh the contents of the visual cache. It is obvious, therefore, that in case of a damage of the visual cache or the inner scribe, a constructional task would not be performed. Baddeley and Andrade [4] suggest that the maintenance of information by active rehearsal is necessary for imagery. They assume that the regeneration process is part of the function of the slave systems.

A number of studies, both psychological and neuropsychological, now exist to suggest separable subsystems of visuospatial memory for dealing with spatial information (such as the location of an object) and visual information (such as appearance) [13], [16], [32]. Recent research has hypothesised that WM is not a form of transit lounge that acts to hold perceptual input on its way to long-term memory, but as a workspace [24].

Here, the case of a woman who showed a severe CA, due to a progressive degenerative disease, is described. A mild degree of ideomotor apraxia was also present, while oral apraxia was absent. The functional locus of impairment is discussed in the frame of the WM model [2].

With regard to neural correlates, all data from the literature seem to confirm that CA is generally due to lesions encroaching upon the posterior (parieto-temporo-occipital) regions [17]. In the past, the qualitative differences observed between the constructional performance of right and left brain-damaged subjects led to advance the hypothesis that an executive, planning defect subsumed constructional disturbances in left brain-damaged patients, while visuospatial deficits could be responsible for the disorder in right brain-damaged patients [17]. This distinction, however, has proved to be inconsistent [19].

Recently, four patients have been described with a progressive degenerative pathology, showing early visuospatial problems, agraphia of a predominantly peripheral type and difficulty with bimanual tasks [34]: neuroimaging disclosed bilateral parietal lobe atrophy and hypoperfusion, which was out of proportion to that seen elsewhere in the brain.

Given that CA can result from a variety of deficits (visuo-perceptual, visuospatial short-term memory, mental imagery, motor translation and execution), it seems obvious that lesions in different sites—causing a functional damage in different loci along the processing system can be associated with CA.

A 3-year follow-up is reported, showing in the third year, an initial involvement of verbal abilities. A brief discussion of the clinical aspects of the disease is also given, together with an attempt to clarify the neural substrates of it.