To attempt to resolve this question, in the next experiment the landmark task (see Experiments 2 and 4, chapter two) was presented to the RCVA, LCVA and control patients. Two influences were apparent in the bisection behaviour described in Experiment 5A: (a) a bias toward the cued side, and (b) an overall leftward bias for the LCVA patients and the control group when using their left hand. This constant error might be due to a leftward motor bias, with or without a perceptual overestimation of the left part of the line. If the predominant bias was leftward-motor, then in the landmark task, subjects would be expected to point generally to the lefi when faced with a centrally transected line. The contrary effect would occur with a predominantly perceptual bias: if subjects perceived the left half of a line as being longer, n<g/?fward pointing would be predicted. If the two effects are both present, then they would presumably subtract from one another.
The same principle applies for the cueing effect: if a motor bias is operating (through an activation of the contralateral hemisphere), pointing judgements should be towards the cued end of the line. If however a perceptual bias affects performance than judgements should be made towards the end of the line opposite the cue.
No hand effects were found for the control group in Experiment 3 (chapter two). Furthermore as this task required pointing only, no difficulties were experienced by the LCVA group when asked to do this with their left hand. It also seems unlikely that differences between hands would become relevant in this psychophysical judgement task when they were not apparent in the bisection task, so hand was not included as a factor and the data of all three groups (RCVA, LCVA and controls) analysed together. An objection could be raised against direct comparison of right and left CVA patients as the LCVA group proved younger than the RCVA group. On the other hand both groups had little difficulty in performing the bisection task and produced virtually the same results as the controls, suggesting that age was not a relevant factor. However an additional analysis excluding the three youngest patients of the LCVA group, was performed in order to ensure that age differences did not affect the results.
METHOD
Subjects. Subjects were the same as in Experiment 5A with 12 patients with unilateral right hemisphere infarct, 12 patients with unilateral left hemisphere infarct and 12 normal control subjects.
Materials. Materials were the same as in Experiment 2 except that only half of the centrally prebisected stimuli and only 10 asymmetrically pretransected lines were used. This set of 22 lines was presented once in each spatial location (left, right or central with respect to the subject's body midline), the order of spatial presentations being counterbalanced between subjects.
Procedure. Seating and instructions for the subjects were the same as in Experiment 2. However, in order to ensure that all subjects understood the instructions correctly, randomly cued lines which were transected noticeably to
the right or left of the true centre (2 to 4 cm from the middle) were first presented to the subjects. Each subject had to give five continously correct responses before the landmark experiment was started.
Statistical Analyses. For the lines pretransected in the centre, the
number o f rightward responses (maximum three per cell) were subjected to two separate three-way analysis of variance: in the first analysis all subjects were included and group (LCVA patients, RCVA patients and controls) was a between-subjects factor, and space and cueing were within-subjects factors. The second analysis was the same apart from excluding the three youngest LCVA patients. The significance of main effects and interactions involving repeated measures was assessed using a Geisser-Greenhouse adjustment to the degrees of freedom where appropriate. Finally, significant main effects and interactions were examined in detail through the Newman-Keuls testing procedure, using the 5% level of significance throughout. In addition, one-sample t-tests were used for testing against chance performance in particular test conditions.
RESULTS
Confirming the expectations, no significant differences between LCVA and RCVA group or interactions with group were found for the second analysis, which excluded the three youngest subjects of the LCVA group. Consequently only the results of the first analysis shall be reported in detail.
As in Experiment 5A, no main effects of group or interactions with group were found. There was however a highly significant effect of cueing for all three groups [F(2.3,76.6) = 8.88, p < 0.001], with unilateral left and right cue conditions being significantly different from each other. However only the left cue condition differed significantly from the no-cue and bilateral cue conditions. Judgements made under the right cue conditions, although
significantly different from those made under the no-cue condition, did not differ from the bilateral cue condition. Nonetheless no-cue and bilateral cue condition did not differ from each other. With a unilateral left cue subjects perceived the right end of the line as being closer to the transection mark, whereas a unilateral right cue had the opposite effect: subjects judged the left end of the line as being closer (Figure 3.4). However, only the unilateral right cue condition differed reliably from chance performance (t(35) = 4.60, p < 0.01). Responses under the no-cue, bilateral-cue and left cue conditions showed approximately equal numbers of rightward and leftward responses, and did not differ significantly from chance performance.
Similarly all subjects in the three groups divided their responses evenly between left and right at each of the three spatial locations, and when summed over all conditions.
All subjects performed perfectly for the stimuli pretransected 5mm from the true midpoint. Subjectively however, they found it as hard to judge transections that deviated from either side of the midpoint as lines that were actually centrally bisected. As already mentioned in Experiment 4, errors of the control subjects on the 1 to 4 mm deviating lines were unequally distributed towards leftward responses, in that all subjects more frequently judged the marks deviating to the right as being closer to the left end of the line than judging the left marks as closer to the right. This bias was highly significant (t=(ll) = 3.51, p < 0.01). The same bias was found for LCVA patients (t(ll) = -3.08, p < 0.01). The RCVA group, on the other hand, showed no significant difference between the numbers of errors made with rightwardly versus leftwardly transected lines (t(ll) = 1.48).
RCVA Patients LCVA Patients Controls 2.5- 2.0- cs % 0.0
No Letter 2 Letters Letter left Letter right
50%
Figure 3.4: Manual landmark judgements of the RCVA and LCVA patients and the control group, as a function of cueing (no letter, 2 letters, letter left, letter right): the mean number of forced-choice manual indications (out of 3) that an objectively central transection is placed right of centre. Errors bars indicate intersubject variability.
EXPERIMENT 6B: LANDMARK PERFORMANCE OF NEGLECT