For more than a century, it has been known that damage to the right hemisphere of the brain can cause patients to be unaware of the contralesional side of space. This condition, known as unilateral neglect , represents a collection of clinically related spatial disorders characterized by the failure in free vision to respond, explore, or orient to stimuli predominantly located on the side of space opposite the damaged hemisphere. Recent studies using the simple task of line bisection , a conventional diagnostic test, have proven surprisingly revealing with respect to the spatial and attentional impairments involved in neglect. In line bisection, the patient is asked to mark the midpoint of a thin horizontal lie on a sheet of paper. Neglect patients generally transect far to the right of the center. Extensive studies of line bisection have been conducted, manipulating-among other factors-line length, orientation, and position. We have simulated the pattern of results using an existing computational model of visual perception and selective attention called MORSEL (Mozer, 1991). MORSEL has already been used to model data in a related disorder, neglect dyslexia (Mozer & Behrmann, 1990). In this earlier work, MORSEL was “lesioned” in accordance with the damage we suppose to have occurred in the brains of neglect patients. The same model and lesion can simulate the detailed pattern of performance on line bisection, including the following observations: (1) no consistent across-subject bias is found in normals; (2) transection displacements are proportional to line length in neglect patients; (3) variability of displacements is proportional to line length, in both normals and patients; (4) position of the lines with respect to the body or the page on which they are drawn has little effect; and (5) for lines drawn at different orientations, displacements are proportional to the cosine of the orientation angle. MORSEL fails to account for one observation: across patients, the variability of displacements for a particular line length is roughly proportional to mean displacement. Nonetheless, the overall fit of the model is sufficiently good that we believe MORSEL can be used as a diagnostic tool to characterize the specific nature of a patient's deficit, and thereby has potential down the line in therapy.

Neglect dyslexia, a reading impairment acquired as a consequence of brain injury, is traditionally interpreted as a disturbance of selective attention. Patients with neglect dyslexia may ignore the left side of an open book, the beginning words of a line of text, or the beginning letters of a single word. These patients provide a rich but sometimes contradictory source of data regarding the locus of attentional selectivity. We have reconsidered the patient data within the framework of an existing connectionist model of word recognition and spatial attention. We show that the effects of damage to the model resemble the reading impairments observed in neglect dyslexia. In simulation experiments, we account for a broad spectrum of behaviors including the following: (1) when two noncontiguous stimuli are presented simultaneously, the contralesional stimulus is neglected (extinction); (2) explicit instructions to the patient can reduce the severity of neglect; (3) stimulus position in the visual field affects reading performance; (4) words are read much better than pronounceable nonwords; (5) the nature of error responses depends on the morphemic composition of the stimulus; and (6) extinction interacts with lexical knowledge (if two words are presented that form a compound, e.g., COW and BOY, the patient is more likely to report both than in a control condition, e.g., SUN and FLY). The convergence of findings from the neuropsychological research and the computational modeling sheds light on the role of attention in normal visuospatial processing, supporting a hybrid view of attentional selection that has properties of both early and late selection.