Sustained Splits of Attention within versus across Visual Hemifields Produce Distinct Spatial Gain Profiles

Visual attention can be focused concurrently on two stimuli at noncontiguous locations while intermediate stimuli remain ignored. Nevertheless, behavioral performance in multifocal attention tasks falters when attended stimuli fall within one visual hemifield as opposed to when they are distributed across left and right hemifields. This “different-hemifield advantage” has been ascribed to largely independent processing capacities of each cerebral hemisphere in early visual cortices. Here, we investigated how this advantage influences the sustained division of spatial attention. We presented six isoeccentric light-emitting diodes (LEDs) in the lower visual field, each flickering at a different frequency. Participants attended to two LEDs that were spatially separated by an intermediate LED and responded to synchronous events at to-be-attended LEDs. Task-relevant pairs of LEDs were either located in the same hemifield (“within-hemifield” conditions) or separated by the vertical meridian (“across-hemifield” conditions). Flicker-driven brain oscillations, steady-state visual evoked potentials (SSVEPs), indexed the allocation of attention to individual LEDs. Both behavioral performance and SSVEPs indicated enhanced processing of attended LED pairs during “across-hemifield” relative to “within-hemifield” conditions. Moreover, SSVEPs demonstrated effective filtering of intermediate stimuli in “across-hemifield” condition only. Thus, despite identical physical distances between LEDs of attended pairs, the spatial profiles of gain effects differed profoundly between “across-hemifield” and “within-hemifield” conditions. These findings corroborate that early cortical visual processing stages rely on hemisphere-specific processing capacities and highlight their limiting role in the concurrent allocation of visual attention to multiple locations

Role of Visual Hemifields in Processing and Storage of Motion Information

PURPOSE: Studies on motion processing using multiple-object tracking indicates that the bottleneck of information processing occurs at the visual-short term memory (VSTM) stage. In contrast, recent studies reported the bottleneck to occur prior to VSTM at the stimulus encoding stage. Performance in motion processing is greatly influenced by attention to the task. On the other hand, studies on attention, e.g. Alvarez and Cavanagh (2005), suggest that there are partial or independent resources for attention across each visual hemifield. The purpose of this study was to investigate, using a cross-cuing approach, whether attentional resources are partial or independent in motion processing across different memory stages. METHODS: In the first (N=9) and second experiments (N=8) observers reported the direction of motion of a target through a partial-report technique where the target’s terminal position was cued. The targets were distributed equally and either confined to one hemifield (unilateral) or both visual hemifields (bilateral). The cue was presented either immediately after the stimulus presentation, or with a delay that ranged from 50 to 3000 ms to investigate the processing of motion across stimulus encoding, sensory, and VSTM stages. A third experiment (N=8) was designed to replicate the findings of Alvarez and Cavanagh. RESULTS: At the stimulus encoding stage, performance gradually declined (30%) as a function of set size but there was no statistical significance between the unilateral and bilateral conditions. At the sensory memory and the VSTM stages, the main effect on performance was due to set size. Again, there was no effect on performance due to the visual-field location of the targets. In the third experiment, performance in the bilateral and unilateral conditions was similar and with no statistically significant difference. CONCLUSIONS: I did not find any evidence for independent attentional resources for each hemifield in processing the direction of stimulus motion. The results of this study support findings of previous studies that the bottleneck of motion processing occurs prior to VSTM, at the stimulus-encoding stage.Optometry, College o