shows the results. Main effect of Group (F [2, 42] = 3.34, p = .05) and Level were significant (F [1, 42] = 4.5, p = .04). The interaction of Level x Group was significant (F [2, 42] = 6.17, p =.004).
Figure 2 (a) Median RTs (ms) for the LPD, RPD, and NC groups in the no-bias condition. (b) Median RTs (ms) for the LPD, RPD, and NC groups in the biased-attention conditions. The left half of the graph represents median RTs to targets occurring at the global or (more ...)
Simple effects tests examined further this Level x Group interaction. Between global and local levels, RTs were comparable for the NC group (F [1,16] = 0.26, p =.62), replicating prior findings of level equivalence with this task (Lamb et al., 1988). While the RPD group showed the predicted global precedence numerically, this was not significant (F [1, 12] = 0.39, p =.55). By contrast, the LPD patients demonstrated an abnormal, local precedence because RTs were significantly slower at the global than local level (F [1,14] = 19.64, p =.001), indicating relatively impaired global level processing.
Between groups, for local level targets, the LPD group had significantly slower RTs than the NC group (F [1, 30] = 12.5, p =.001). The RPD group was numerically but not significantly slower than the NC group (F [1, 28] = 2.58, p =.12). However, RPD and LPD groups did not differ (F [1, 26] = 0.03, p =.87), suggesting that the RPD group like the LPD group was also impaired at the local level. For global level targets, again, the LPD group was slower than the NC group (F [1, 30] = 46.46, p <.001), and the RPD group tended to be slower than the NC group (F [1, 28] = 2.9, p =.098). RTs did not differ significantly between LPD and RPD groups (F [1, 26] = 1.12, p =.30), again, suggesting that the RPD group like the LPD group was also impaired at the global level. In sum, the LPD group (with right hemisphere damage) was impaired at both levels but more at the global than local levels, whereas the RPD group (with left hemisphere damage) showed some evidence of an impairment at both levels like that of the LPD group, but numerically (but not significantly) more at the local than global level.
To examine further if the groups had different RT advantages to either the global or local level, a difference score (global RT minus local RT) was calculated for each participant. The difference scores showed that NC participants responded with comparable speed (difference score =19 ms) to targets occurring at either the local or global level. LPD patients showed local primacy, responding on average 181 ms faster for detecting targets at the local than global level, whereas RPD patients showed global primacy, responding on average 35 ms faster to global than local targets. A one-way ANOVA on difference scores with a between-subjects factor of Group demonstrated a main effect of Group (F [2, 42] = 6.17, p < .005). Three univariate ANOVAs with the between subjects factors of order and version included were conducted to explore this group effect. The scores of the LPD and RPD groups differed significantly (F [1,26] = 9.73, p = .004), and scores of the LPD and NC groups differed significantly (F [1,30] = 8.5, p = .007), but the scores between the RPD and NC groups did not differ (p = .42). This pattern of results demonstrated that the LPD group showed an abnormal global level precedence that the RPD and NC groups did not show.
shows the results. Main effect of Group was not significant (F [2,27] = 0.11, p = .90). Level was marginal (F [1,27] = 2.94, p = .098). The main effect of Bias was significant (F [1,27] = 6.41, p < .02]. The interaction of Group x Level was significant (F [2,27] = 8.22, p =.002). This reflected the observation that overall across both bias conditions, NC and RPD responded faster to global than local targets, whereas LPD showed the opposite, responding faster to local than global targets. This was the pattern observed between RPD and LPD groups under no bias conditions, providing further evidence for this finding. The interaction of Group x Bias was significant (F [2,27] = 6.36, p=.005), as NC and RPD were faster overall under local than global bias, whereas LPD showed the opposite, being faster under global than local bias, regardless of hierarchical level. The LPD effect reflects the observation that this group had comparable RTs in three conditions but much slower RTs to global targets under the condition where attention was biased away toward the opposite, local level (i.e., global targets under local bias), resulting in overall slower RTs under local bias than global bias for only the LPD group. The Bias x Level interaction was significant (F [1,27] = 77.35, p < .0001), confirming the efficacy of our bias manipulation on hierarchical processing. The critical interaction of Bias x Level x Group was not significant (F [2,27] = 0.20, p = .82), suggesting that LPD, RPD, and NC participants were able to benefit from probability information. These results support the significant results and trends found under no bias conditions that LPD patients have problems with global more than local processing, while RPD patients have problems with local more than global processing. The bias manipulation served to exaggerate these processing problems so that the weaker problem in RPD patients could be detected more clearly.
To examine further differences in processing local and global targets in each Bias condition and for each group, we conducted a repeated measures ANOVA on each group and bias condition, separately. Analyses included Global Version and Order as between subject factors. Under global bias conditions, the Level effect was significant for NC (F [1,16] = 8.2, p = 0.01) and tended towards significance in the RPD (F [1,13] = 3.8, p = .07) but not LPD groups (F [1,14] = 0.38, p = .55), as NC and RPD were faster on the biased global target level than the local level, whereas LPD were about as fast at both levels, and even showed the opposite, being slightly slower at the global than local level, suggesting abnormal global processing in LPD.
Under local bias conditions, the Level effect was significant for NC (F [1,17] = 15.7, p = .001) and LPD (F [1,14] = 7.3, p < .02) but not RPD groups (F [1,13] = 0.05, p = .83), as NC and LPD were faster on the biased local target level than the global target level, whereas RPD were about as fast at both levels, and even showed the opposite, being slower at the local than global level, suggesting abnormal local processing in RPD. When attention was biased to the local level, RPD patients were not able to benefit entirely normally from the attentional bias; apparently impaired local level processing in RPD results in RTs that are slower for local than global targets, despite attending to the local level. As a general note, consistent with normal attentional biasing, LPD patients responded faster to global targets, despite impaired processing of them, when presented under global bias relative to under local bias conditions, whereas RPD patients respond faster to local targets, despite impaired processing of them, when presented under local bias than global bias conditions.
To investigate the impact of global or local processing deficits on attentional resources as a function of side of motor symptom onset, the difference score for each participant was compared for each bias condition (global, local). An omnibus, two-way mixed ANOVA on difference scores with a within-subjects factor of Bias and between-subjects factor of Group demonstrated a significant main effect of Bias (F [1,27] = 77.35. p < .0001) and Group (F [2,27] = 8.23, p = .002) but not the interaction of Bias x Group (F [2, 27] = .20, p =.82).
The differences scores for the local and global biased attention conditions were also compared to each other using repeated measures ANOVA for each group separately with Local/Global Version and Order as between subjects. To determine if differences scores for the separate bias conditions significantly differed from zero, repeated measures within group were run and Order and Version were included as between group factors. To assess group effects, we used a Univariate ANOVA with Group, Version, and Order entered as between subject factors for each pair of groups. For NC, under local bias, a positive direction indicating local precedence was predicted, whereas under global bias, a negative direction indicating global precedence was predicted. If only processing is affected, then RPD should be impaired under local bias (i.e., less positive than NC), whereas LPD should be impaired under global bias (i.e., less negative than NC). If attention or both processing and attention are affected, then PD patients should be impaired in all conditions, showing less biasing in the appropriate direction than NC.
Results for the NC group (who showed no primacy effects in the no-bias condition) revealed that, under local-biased attention, RTs were faster to targets at the local than global level, and this RT change (Mdn difference score = 107 ms) differed significantly from zero (F [1, 17] =15.7, p =.001). Under global-biased attention, RTs were faster to targets at the global than local level, and this change (Mdn difference score = -178 ms) differed significantly from zero (F [1, 16] =8.2, p =.01). In sum, the NC group showed RT gains under both local- and global-biased attention conditions. There was a difference between the scores under local versus global bias attention (F [1, 11] =181.5, p <.001). These results demonstrate that the NC group benefited significantly from probability information in both biasing conditions.
Results for the LPD group (who demonstrated impaired global level processing in the no-bias condition) revealed that, under local-biased attention, RTs were faster when a local than global target appeared, and this change (Mdn difference score = 293 ms) differed significantly from zero (F [1, 14] =7.4, p =.02). This RT change in the LPD group tended to differ from that for the NC group (but not RPD [see below]) in this bias condition (F [1, 31] =3.7, p =.06), suggesting abnormally large local precedence for the LPD group under locally biased attention. By contrast, under global-biased attention, the LPD group lacked the normal RT advantage for targets occurring at the global relative to local level, and this difference score (Mdn = 14 ms) did not differ reliably from zero (F [1, 14] =0.38, p =.55). However, the difference between the LPD and NC groups under global bias was not significant (F [1, 30] =0.44, p =.51) nor was the difference between the LPD and RPD group (F [1, 27] =1.05, p =.31). In sum, the LPD group showed an RT gain under local but not global attention bias conditions, unlike the NC group who showed a gain under both. Like the NC group, the LPD group showed that difference scores under global versus local bias attention differed significantly (F [1,8] = 14.68, p = .005), but, for the LPD group, it was because they showed an RT advantage for probability information provided at the local level, but when attention was instead biased to the global level, the RT benefit for global level targets did not occur. Taken together with the finding of abnormally worse global than local processing in LPD under no-bias conditions, this pattern of bias attention findings provide further support for an LPD impairment at global more than local level processing.
The opposite pattern of results was obtained for the RPD group, who had appeared to show impaired local level processing in the no-bias condition. Under local-biased attention, unlike both NC and LPD groups, the RPD group lacked the normal RT advantage for targets occurring at the local relative to the global level, and this difference score (Mdn = -39 ms) did not differ reliably from 0 (F [1, 13] =0.05, p =.83). While the comparison of difference scores between RPD and NC groups did not reach significance (F [1,30] = 2.83, p = .10), the RPD pattern did differ significantly from the LPD group, who did show a normal RT advantage in this condition (F [1,27] = 6.74, p = 0.02), indicating the RPD group did not show a normal pattern under these conditions. This result demonstrates a single dissociation for local level processing in the RPD group. Under global-biased attention, the RPD group demonstrated slower RTs to local than global targets, and there was a trend for this RT change to be significant (Mdn difference score = -250 ms) (F [1, 13] =3.8, p =.07), and this global bias pattern did not differ reliably from that of the NC group (F [1,29] = .01, p = .92), suggesting relatively normal global level processing in RPD, and providing evidence for a double dissociation, though the RPD and LPD patterns did not differ (F [1,27] = 1.05, p = .31). The RPD group seemed to show an RT gain under global but not local attention bias, unlike the NC group who showed a gain under both, and, indeed, in the RPD group, difference scores under global versus local bias differed significantly (F [1,8] = 19.48, p = .002). In sum, the RPD group showed the opposite RT pattern relative to the LPD group who benefited from local but not global attention bias. The RPD results demonstrate clearly that these patients benefited when attention was biased to the global level but to a lesser extent when attention was biased to the local level.
The difference scores provided evidence that the NC group but neither PD group showed comparable bias effects under local and global attention. In particular, the LPD group exhibited a clear benefit of local bias but not global bias of attention on local processing, consistent with the global processing impairment demonstrated under no-bias conditions. By contrast, the RPD group showed a greater benefit of global bias than local bias of attention on global level processing, consistent with the local processing problems suggested under no bias conditions.