Perceptual and response-related components of unilateral neglect may evolve independently of one another: Evidence from five single-case studies

Abstract

The Milner Landmark Task allows the disentanglement of perceptual and response-related components of unilateral neglect. If these two components reflect separate functional systems, then cases should be observed in which the two components evolve differently across time. To test this hypothesis we surveyed a continuous series of 21 right hemisphere stroke patients. Five patients from the sample were affected by unilateral neglect at the outset and could be submitted to repeated administrations of the Landmark task in the first weeks post stroke. Two versions of the task were used, Landmark-Manual and Landmark-Verbal, differing in the type of response required. Two patients showed independent changes in the perceptual and the response-related component of neglect, hence confirming the view of separate functional systems underlying them. Dissociations between the task versions were found, witnessing a role of the type of response. Unexpectedly, one patient showed an initial leftward deviation of the subjective midpoint of the stimulus line, which later reversed to a classical rightward deviation. We interpreted such a pattern in terms of co-existing “productive” and “negative” components of perceptual neglect.

• Unilateral neglect
• Neglect evolution
• Landmark test
• Input-related neglect
• Output-related neglect
• Paradoxical neglect

This project was partially supported by a Milan University FIRST Grant and by a PRIN Grant to EC. No conflict of interest affects this manuscript. We thank one of the reviewers for his/her thoughtful comments.

Notes

1. ¹ We chose to use the terms “IRN” and “ORN” for the two neglect varieties, because the expressions “perceptual bias” and “response bias”, used by Bisiach et al. (1998) are also the names of the scores used to measure the two deficits.

2. ² Toraldo et al. (2004) observed one patient with left homonymous hemianopia and without neglect (DB) who showed the same effect on the Landmark-V task, i.e., a PSE of –10 mm.

3. ³ One further complication is that, according to the famous Premotor Theory of Attention (PTA, Rizzolatti et al., 1987), movements of the visuo-spatial attention focus are nothing else than premotor programs of (not necessarily performed) eye movements. So according to the PTA, attentional and premotor biases in our Figure 3 are partly the same thing. The separation between attentional and premotor processes depicted in Figure 3 is more consistent with other models of visuo-spatial attention, e.g., Treisman and Gelade’s (1980).

4. ⁴ Case 2 is not an exception: by looking at Figure 1, in only one patient (Case 4) and in only one task (L-V) PSE and M seemed to follow a similar trajectory across time (Figures 1G, 1H, gray plots), with all other cases showing largely independent trajectories in the two tasks.

5. ⁵ Many other combinations of the factors depicted in Figure 3 can explain the empirical profile of Case 2 (e.g., anisometry on sessions 1–3 plus delusional elongation on sessions 1–2 plus premotor bias on sessions 1–3, etc.). However, none of those combinations is composed of a single factor—the discrepant PSE and M trajectories make a single-factor explanation impossible, and all of those combinations include the delusional leftward elongation of the line, as this is the only factor (in Figure 3) which can explain the leftward migration of PSE on sessions 1–2.

6. ⁶ Note that single-case analyses are fruitful also when large patient samples are available (e.g., Toraldo et al., 2006).