Object substitution masking and its relationship with visual crowding.

Object substitution masking (OSM) occurs when the perceptibility of a brief target is reduced by a trailing surround mask typically composed of four dots. Camp et al. (Journal of Experimental Psychology: Human Perception and Performance, 41, 940-957, 2015) found that crowding a target by adding adjacent flankers, in addition to OSM, had a more deleterious effect on performance than expected based on the combined individual effects of crowding and masking alone. The current experiments test why OSM and crowding interact in this way. In three experiments, target-flanker distance is manipulated whilst also varying mask duration in a digit identification task. The OSM effect-as indexed by the performance difference between unmasked and masked conditions-had a quadratic function with respect to target-flanker distance. Results suggest it is OSM affecting crowding rather than the converse: Masking seems to amplify crowding at intermediate target-distractor distances at the edge of the crowding interference zone. These results indicate that OSM and crowding share common mechanisms. The effect of OSM is possibly a consequence of changes to the types of feature detectors which are pooled together for target identification when that target must compete for processing with a trailing mask in addition to competition from adjacent flankers.

The role of distractors in object substitution masking.

In object substitution masking (OSM) a surrounding mask (typically comprising of 4 dots) onsets with a target but lingers after offset; under such conditions, the ability to perceive the target can be significantly reduced. OSM was originally claimed to occur only when a target was not the focus of attention, for instance, when embedded in an array of distractors (Di Lollo, Enns, & Rensink, 2000). It was argued that the distractors influenced the time taken for focal attention to reach the target. Some recent work, however, failed to find any such distractor influence; the effect of mask duration was found to be independent of set size when steps were taken to avoid ceiling effects in the smallest set size condition (Argyropoulos, Gellatly, Pilling, & Carter, 2013; Filmer, Mattingley, & Dux, 2014). In 3 experiments, we repeatedly found that set size manipulations can interact with mask duration (in which neither ceiling nor floor effects are evident), with the effect of the mask on target perceptibility being amplified according to the number of distractor items. However, a further experiment (Experiment 4) showed that crowding by nearby distractors was actually responsible for this "set size" effect. When decoupled from crowding, set size alone did not interact with masking, though it did influence overall accuracy. Thus, the presence of distractors does influence OSM, but not in the way originally assumed by Di Lollo and colleagues in their model. The Crowding × OSM interaction suggests that the 2 phenomena involve partly overlapping mechanisms.

Patients with hemispatial neglect are more prone to limb spasticity, but this does not prolong their hospital stay.

OBJECTIVE: To determine whether stroke patients who suffer from hemispatial neglect tend to stay in hospitals longer because they are prone to limb spasticity. DESIGN: Retrospective analysis of inpatient medical notes. SETTING: Inpatient neurorehabilitation unit of a regional UK teaching hospital. PARTICIPANTS: All patients (N=106) admitted to the neurorehabilitation unit between 2008 and 2010 who had suffered a stroke, as confirmed by computed tomography or magnetic resonance imaging. INTERVENTIONS: Not applicable. MAIN OUTCOME MEASURES: Statistical coincidence of hemispatial neglect and spasticity; length of hospital stay. RESULTS: Chi-square analyses indicated that individuals with left neglect were nearly one third more likely to develop spasticity than those without neglect (87% vs 57%), while nearly one half of those with left-sided spasticity showed neglect (44% vs 13%). Individuals with neglect stayed in the hospital 45 days longer than those without neglect, but the presence or absence of spasticity did not affect length of stay. CONCLUSIONS: The results provide the first statistical evidence, to the best of our knowledge, that neglect and limb spasticity tend to co-occur poststroke, though it is only the former that significantly prolongs stay. Diagnostic value aside, these results are important because they tell us that the treatment of neglect should not be overshadowed by efforts to reduce comorbid spasticity. Despite its poor prognosis, hemispatial neglect continues to receive little targeted therapy in some units.