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It is puzzling that differences between the performance of elderly and younger subjects were found both in the traditional line bisection task and the 'landmark' test. Only cueing procedures provided consistently clear and robust results throughout the whole set of experiments. For both subject groups and both tests (bisection and 'landmark') use of a single letter biased judgements in a manner consistent with a relative overscaling of the cued end of the line. So for both subject populations it may be inferred that the influence of cueing upon active line bisection (found in Experiment 1 and 3, see also Nichelli et al.,

(overestimation of the cued half-line) and little if at all at the level of any putative orienting response bias. It seems likely that this perceptual bias can be attributed to a differential attentional salience of the two ends of a unilaterally cued line, perhaps due to visual areas within the contralateral hemisphere being especially activated.

Repeating the findings of Bradshaw et al. (1985; 1987a) significant leftward displacements of bisection responses in the central presentation condition were found for both subject groups. Both groups also showed an overall leftward bisection error but this proved significant for the elderly subjects only. It is commonly assumed that this bisection error is caused by a right hemisphere advantage for spatial processing. Tasks which involve the appreciation of visual space are assumed to selectively activate the right hemisphere (Bowers and Heilman, 1976; Kinsbourne and McMurray, 1975; Tartaglione et al., 1983). However only a task such as the 'landmark' test could illuminate whether this activation is reflected in a relatively magnified percept of the leftward part of the line (Milner, 1987) or by a response bias in the form of a predominantly leftward orienting tendency (Heilman et al.,

1985a). The landmark data of the elderly subjects indicate that activation is reflected in leftward orienting responses, as judgements remain toward the left (Experiment 4). However no consistent bias on the landmark task was found in the younger subjects. There are two possible explanations for this: one is that both factors (perceptual and motor) were operating together in the younger subjects and thus cancelled each other out. This explanation would be consistent with a dual role of the right hemisphere in both enhancing the perceptual salience of spatial stimuli in the left hemispace, and also in activating leftward orienting response tendencies. On the other hand it could be argued that the overall effect (and the effect in central space) was simply not strong enough to be replicated in the landmark task (Experiment 2). It was certainly statistically

weaker in the bisection experiment (Experiment 1) than the overall effect found in the elderly subjects (Experiment 3).

Regarding spatial location, the data of the younger subjects (Experiment 1) confirmed most previous reports (Bradshaw et al., 1987a; Dudgeon, 1988; Reuter-Lorenz et at. , 1990) that the leftward error in bisection tends if anything to increase in the left half of egocentric space, but to reverse direction in the

right half. However, the leftward errors found in central and left space in the younger subjects were significantly larger than the rightward errors in right

space (Experiment 1). Right hemisphere involvement appeared even larger in the elderly subjects, as leftward bisection errors were found in all three spatial positions (Experiment 3). As use of lateral presentation conditions is assumed to activate the contralateral hemisphere it thus seems that, for these particular tasks, the contribution of the right hemisphere was greater than that of the left hemisphere.

In the landmark task younger subjects (Experiment 2) showed no consistent bias regarding spatial location. Again this could be due to a failure to replicate a weak effect. On the other hand Pagliarini (1988) demonstrated a perceptually- determined response bias toward the midline of laterally places

lines. His subjects indicated the midline (the half of the line closer to the centre) of lines presented laterally in left and right hemispace, as being shorter

than the lateral part. These data indiacte that the lateral part of laterally displayed lines appears relatively enlarged. However in Pagliarini's experiment, subjects made a verbal not a directional motor response. It is therefore possible that in Experiment 2, where a motor response was made, a perceptual bias toward the midline (indicating that part of the line as being shorter) may have been counteracted by a motor- orienting bias in terms of a lateral overshoot, independent of the actual perception.

This would be in direct opposition to the location effect reported in the elderly subjects (Experiment 4) who indicated the lateral part of laterally presented lines as being shorter. (This was only significant for left space which was probably due to the existence of a general leftward motor bias added on to the spatial biases; see also Discussion Experiment 4). So if a perceptual bias is at work in the elderly subjects it seems to enlarge the medial rather than the lateral part of laterally presented lines.

So regarding the effects of space in young and elderly subjects, it is not at all clear why elderly subjects should overscale the medial extent of a line while younger subjects if anything overscale the lateral extent. Consequently the results on spatial location presented here should, if possible, be replicated and the interpretations offered regarded as preliminary.

SUMMARY

Normal young and elderly subjects were tested in a series of 4 experiments to examine the influences of spatial location and cueing upon line bisection judgements. Judgements in all 4 experiments were strongly influenced by cueing with a letter at one or other end of the line. The spatial location of the line (in left, central or right body space) also had a minor effect in Experiments 1, 3 and 4, where also evidence was found for a small constant error when lines were presented centrally. It is argued from the results of Experiments 2 and 4, where no explicit bisection response was required, that perceptual/attentional factors, rather than a motor bias, play the major role in mediating the cueing effect. It is concluded that there is a substantial attentional effect upon judgements of extent, whereby paying greater attention increases perceived relative line length. However the constant error and the effect of spatial location, seem to be both perceptual and motor in nature and it seems

that the perceptual effect operates differently in younger than elderly subjects: whereas younger subjects seem to overestimate the lateral half-line of laterally presented lines, elderly subjects showed the opposite pattern, i.e. an overestimation of the medial part.

CHAPTER THREE

LINE BISECTION PERFORMANCE OF PATIENTS WITH