Comprobación de la Operacionalización de Hipótesis 152

As aready mentioned in chapter one, Heilman and Valenstein (1979) proposed some years ago that the rightward bisection errors found in patients with hemispatial neglect might be due to constant errors in directing an action in egocentric space, rather than to defects in sensation (Denny-Brown et a l,

1952), attention (Kinsbourne, 1970) or internal space representation (Bisiach et ah, 1981). They attributed these errors in turn to an under-activation of right- hemisphere premotor systems which (in normal subjects) would initiate action in a leftward direction. In neglect patients however, intended acts are biased by a predominance of rightward vectors. In two later studies (1983; 1985b) Heilman and his colleagues demonstrated this phenomenon of 'directional hypokinesia' directly. In the earlier study (Heilman et a l, 1983) control subjects and patients with left- sided hemispatial neglect were asked to close their eyes, point their right index finger to their sternum, and then point to an imaginary point in space which was midline with respect to their chest. The patients with neglect pointed approximately 9cm to the right of the midline, whereas the controls pointed slightly to the left of the midline. It was argued, that because this task did not require visual or somesthetic input from left hemispace, the defective performance could not be attributed to hemispatial inattention or to a defect in hemispatial visual or somesthetic memory. Similarly, because the patient did not need to explore left hemispace, the deviation could not be due to an exploratory or gaze defect. Consequently the findings seemed most compatible with the directional hypokinesia hypothesis. In the later study, Heilman et a l

(1985b) tested the ability of patients with left-sided hemispatial neglect to move a lever toward or away from the side of their lesion in response to a central

visual stimulus. They demonstrated that the neglect patients needed more time to initiate movement toward the neglected left hemispace than the right hemispace, an asymmetry which was not found in brain damaged controls without neglect. The results of this study were again interpreted in favour of a directional hypokinesia operating in these patients.

Rather recently, a different model has been proposed that could also explain the nature of rightward bisection errors in neglect patients, although the underlying assumptions are rather different: Halligan and Marshall (1991) examined visuospatial localization in a patient with severe visual neglect. In the critical task condition, the subject was required to judge the spatial location of an arrow on a TV screen, the arrow being located either at the bottom or top of the screen, pointing inwards. The subject made her location judgements by indicating verbally which of an array of 15 numbers, on the opposite edge of the screen, the arrow was aligned with. It was found that the patient (PP) erred rightwards in these judgements, with an average error of about 1 cm. However the errors of judged location tended to be greater the nearer the arrow was presented to the left edge of the screen (up to an average of 1.75 cm), and less when it was near the right edge of the screen (ultimately declining to zero). The authors liken their results to the compression of a spring from left to right: it is as if the coordinates of PP's subjective space were pushed over to the right but uniformly 'shrunk' at the same time, thus maintaining Euclidean properties of equal spacing.

Generalization of this model could possibly account for the line bisection behaviour of neglect patients. If the horizontal dimension of space is visualized as a compression spring, a horizontal line might be equivalent to (say) 4 coils of the spring. If this subjective space is uniformly compressed in a neglect patient, a 20 cm horizontal line should appear to be (say) 17 cm long, irrespective of where it appears in objective space. However, if the compression is toward a

fixed point on the right, as hypothesized by Halligan and Marshall (1991), then the horizontal line should appear to be shifted as a whole rightwards in space. This would predict a corresponding rightward error of the hand in moving to bisect the line, just as is assumed to occur in directional hypokinesia. Interestingly, this space-compression model would also predict a variation in line bisection error as a function of spatial location. Although all lines should appear subjectively shifted rightwards in location, a line placed on the patient's right should be shifted rather less, but one to the left shifted more, than a central line. Thus the arm should be misdirected more in respect of lines placed in left hemispace and less for lines in right hemispace. This has indeed been reported several times (Schenkenberg et al., 1980; Nichelli et al., 1989).

An alternative kind of spatial misperception that might be present in neglect patients was proposed by Milner (1987). It was suggested that there might be a distortion of the subjective space of a neglect patient that was non-

Euclidean. That is, subjective space might be compressed, but progressively more so in more leftward parts of space. According to this view, the space occupied by the left half of a line would literally appear shorter to a neglect patient than the right half, therefore bisection would tend to be performed in a rightward direction. This idea would predict rightward bisection errors, but can only account for the increased errors in left space and the lesser ones in right space if it is assumed that the gradient of distortion is steeper in the leftward parts of ego-space than in the rightward parts.

All experiments presented in this chapter were designed to test this hypothesis, but the data of Halligan and Marshall (1991) may also be regarded as a direct test of this hypothesis in one patient: for each of 15 locations, 1 cm apart, their patient was asked to indicate the arrow's subjective position. According to Milner (1987), the subjective spacing between adjacent pairs of these 15 points should increase as one passes from left to right. The actual

mean spacing of PP's judgements, however, show no trend for such an increase. Therefore the distortion of PP's subjective space appears to be more closely modelled by a uniform compression that maintains Euclidean geometry, rather than a distortion of space that is MOM-Euclidean.

On the other hand, there could be a problem with the task used by Halligan and Marshall in that it might underestimate abnormalities in subjective spatial relationships in neglect. Changes in the perception of the arrow’s location might be matched by similar changes in the perceived locations of the numbers used to code the response. In addition, there is a necessary ceiling effect on judgements near the right edge of the screen, since large rightward errors would have hit or exceeded the edge. In any case, Milner’s hypothesis might apply to the perceived sizes of objects rather than to the distances between

them. Gainotti and Tiacci (1971) showed several years ago that in many right hemisphere damaged patients, visual shapes tended to be underestimated in size when compared with similar shapes shown on the right. They attributed this to the presence of patients with neglect in their group, and presented evidence to support this contention. Their data thus lend support at least to the more restricted version of Milner's hypothesis.

Interestingly neither the directional limb hypokinesia theory (Heilman and Valenstein, 1979) nor the uniform compression of space theory (Halligan and Marshall, 1991) would predict that a given line would appear of different length when located in different parts of visual space, nor that its true midpoint would appear to be shifted from centre. Instead, rightward bisection errors are explained on these theories as resulting from a spatial misdirection of the act of bisecting. One simple test of such accounts is therefore to present a /?re-bisected line (Marshall and Halligan, 1989; Reuter-Lorenz et a l, 1990) in particular to use the 'landmark task' (see also chapter two). When the pre-transection is at the objective midpoint of a line, then on either the uniform-compression or the

hypokinesia theory, a neglect patient should see the mark veridically as being at the midpoint. If, however, the patient literally sees the left half of the line as shorter than its right half, then a central transection should be judged as nearer to the left end of the line. On the additional assumption that the distortion of size scaling might change more steeply in left hemispace, the errors in landmark judgements should be more pronounced there than in central or right space.

In the following experiments six patients with clinically manifest neglect syndromes are described. They were first tested on the traditional line bisection test, where they demonstrated typical rightward bisection errors which varied in the usual way when the lines were presented in different parts of visual space. Subsequently the performance of these patients on the landmark task was assessed, containing an examination of the effects of spatial location, as Milner's hypothesis would predict that the gradient of distortion experienced by the neglect patients is steeper in the leftward parts of ego-space than in the rightward parts. Finally, cueing procedures were included. Although visual cues have been reported to reduce the amount of rightward displacement shown in the traditional bisection task (Riddoch and Humphreys, 1983; Halligan et a l,

1991) it is hardly ever abolished completely. It remains to be seen whether providing cues compensates for the (presumed) distortion of the patient's subjective space. Although it has been shown that normal control subjects overestimate whichever part of a line it explicitly cued (chapter two; Milner et al. , 1992) this might not necessarily be replicated for neglect patients.

EXPERIMENT 5A: LINE BISECTION PERFORMANCE OF RIGHT AND