Neuropsyrhologia . Vol.30,No.12 .pp .1101-I loft 1992 Paned m Genet Britons.
0328-2932, 52 55 .00+0,00 1992 Pcrgamon Pros Ltd
NOTE A STIMULUS-RESPONSE RELATIONSHIP IN UNILATERAL NEGLECT : THE POWER FUNCTION ANJAN CHATTERIES,* MARK MENNEMEIER and KENNETH M . HEILMANt Center for Neuropsychological Studies and the Department of Neurology, University of Florida College of Medicine and the Neurology Service, Department of Veterans Affairs Medical Center . Gainesville, Florida, U .S .A . (Received 6 January 1992 : accepted 5 August 1992) Abstract We have previously suggested that patients with unilateral neglect may be limited in their ability to sequentially attend or act upon stimuli . To assess the nature of this capacity limitation, we examined the relationship between number of stimuli presented on cancellation arrays and howmany targets a patient with neglect cancelled . This relationship was systematic and described by a power function : targets cancelled= K (targets prcscntcd)°, in which the constant and exponent were derived empirically . Density of targets and time taken to cancel targets did not account for the relationship . Improvement on subsequent testing was reflected in an increase in the constant . However. the exponent of the power function did not change . suggesting that same critical aspect of her dysfunction remained the same . These data also imply that she had implicit knowledge of quantity of stimuli presented, and that this knowledge systematically influenced her explicit behavior .
INTRODUCTION PATIENTS WITH UN 11 AmRAI . NECI F.c1 fail to orient towards, attend to, or act on stimuli in space contra lateral to their lesion  . On cancellation tasks these patients cancel ipsilesional targets and tend to ignore contralateral ones  . However, if explicitly instructed to act in contralateral space, some patients are capable of doing so . We recently reported the performance of a patient with left-sided neglect on a number of cancellation tasks  . On traditional cancellations, her performance was typical of most patients with neglect . She cancelled targets on the right side of the array and ignored those on the left . She was then instructed to cancel targets such that after cancelling a target on the right she had to cancel a target on the left . She continued in a right- and-left alternating manner until she indicated she had cancelled all the targets . Under these conditions, rather than exhibiting a change in the extent of her neglect (as measured by number of cancelled targets), she demonstrated a change in the spatial distribution of neglect . In some trials she cancelled targets on the extreme right and left sides of the array, while neglecting targets in the middle . despite passing over these target between every cancellation . These results were interpreted as demonstrating that she had a limited capacity to attend to or act upon targets sequentially, and this capacity limitation was not confined to a specific spatial location . In this report, we further investigate the nature of this capacity limitation in the same patient . A limited capacity to sequentially attend to or act upon stimuli might be expressed in a number of ways . If this patient neglected some fixed percentage of space, such as 50% . then she would always fail to cancel targets encompassed within this space regardless of how many targets were present . This failure could he related to a defective internal representation of space  . When presented with 16 evenly distributed targets she would cancel eight, and when presented with 32 she would cancel 16 . Alternatively, her defect could be related to the means by which stimuli recruit processing resources . If she has a reduced attentional capacity that is fixed such that there is a limited number of targets of which she can be aware, then only that many targets would he cancelled, irrespective of how many were presented in the array . For example, if she cancelled eight targets on an array of 16, she would also *Now at The University of Alabama at Birmingham, 454 Sparks Center, 1720 Seventh Avenue South, Birmingham, AL 35294, U .S .A . tAddress for correspondence : Kenneth M, Heilman, M .D ., Box J-100236, JIIMHC. University of Florida, Gainesville, FL 32610-0236, U .S .A . Hot
canal eight on an array of 32 targets . There could he other more complex, albeit lawful, relationships between number of targets presented and targets cancelled . The elucidation of such relationships may provide Curt her insight into the mechanisms underlying neglect. The following experiments attempt to sort out these possible relationships between stimuli and her response .
PA IIENT HISTORY M .F. was a 73-year-old . right-handed woman who was tested between 8 and 14 weeks after a right-hemisphere stroke. She was alert, oriented and aware of her stroke . I Icr memory, language comprehension and repetition were normal . I ler language production was fluent . grammatie, well articulated, but aprosodic . She did not suffer from an agnosia or apraxia . She demonstrated extinction of left-sided visual, auditory and tactile stint ti She did not haven persistent deviation other eyes or head and did not have a visual field defect . She had a dense left hemiplegia . A computer tomography ICI )scan of her head done I weak after her stroke showed a Large infarct in the right middle cerebral artery distribution, involving the right parietal and frontal lobes (Fig . I ) .
METHODS Proc . ,dure
The patient was tested in her room . which was quiet and secluded, in an assisted living home. Stimulus arrays were placed in front of M .F, centered al tier mid-saggital plane . There. were no distractors, and the targets were arrayed with an equal number of targets randomly distributed In each quadrant. Each cancellation task was considered completed when M .F . indicated she was done . If she stopped cancelling the targets, but made no indication, after 15 see she was asked if the task was completed . The next task was initiated only when she said she was done . Conditions were given randomly for each trial . Short breaks were taken every 20 min . and testing sessions were about 2 hr long- Testing sessions were stopped at the patient's request or if she appeared tired . txperinent I
M-F . was asked to cancel "0 's distributed war 466 .1 sq . can on a white sheet of paper, 21 .6 x 279 cm . There were three conditions, which consisted of either 16, 31 or 64 identical targets . She performed 20 trials . . with 32 targets she cancelled 21 .2 Re .,ults . On arrays with 16 targets M F cancelled 14 .6 (+2 .1I or 91 _570 I ±3 .8) or 66 .25 ., and wilt 64 targets she cancelled 27 .611 5 .21 or 43 .13".. of [he targets . In seeking a relationship between the nunther of targets presented and how many she cancelled, we realized that the variance in her performance increased on stitmdus arrays with more targets . A logarithmictranslormation could he u,sel Lo stabilise this increasing variance  . The number of targets on the stimulus arrays also increased exponentially 12' . ^_`, 1") . Thus, both the dependent variable Inumber of targets cancelled) and the independent variable Inumber of tar gets on the array) were logarithmically transformed . Regression resulted in the linear equation : log(targets cancelled)-0 46 logltargets presented)+0 .61 . This relationship is mathcmaIteally equivalent Lo [lie power function targets cancelled-4 .07 (targets pi esentedI" .-1, 5R : F=120 .14, P