Visual masking with presentations in same and opposite visual fields: evidence for contralateral masking

Brief intermittent light stimulation disrupts saccadic oculomotor control

PURPOSE

This study sought to determine the effect of very brief, single and multiple pulses of light on spatial and temporal aspects of saccadic eye movements.

METHODS

Twelve visually normal, young adult subjects participated in the experiments. Horizontal eye position was monitored as subjects attempted to track target step displacements in the presence of either single or multiple brief flashes of light in the visual field.

RESULTS

Three primary findings were observed: (1) increased saccadic latency, (2) increased time for target acquisition and (3) increased initial saccadic error.

CONCLUSION

The present findings suggest the influence of attentional processes and/or visual masking effects on saccadic eye movement control.

Contralateral visual masking may be an artifact

There are serious methodological problems in studies which report contralateral visual masking. Contralateral masking occurs when detection of a hemifield target stimulus is impaired by a pattern-masking stimulus presented to the opposite hemifield. We demonstrate that, in studies which used positive stimuli (i.e., black letters on a white field), contralateral masking may be an artifact. Although we observed contralateral masking when positive stimuli were presented, there was no evidence of masking with negative stimuli (i.e., white letters on a black field). A special masking stimulus with a positive mask contralateral to the target and a black hemifield ipsilateral to the target also failed to produce masking. Contralateral masking in this experiment was due to the flash of light in the field ipsilateral to the target; it was this ipsilateral stimulation, rather than contralateral interference, which impeded target recognition.

Hemispheric patterns in visual search

A visual search paradigm was employed to examine hemispheric serial and parallel processing. Stimulus arrays containing 4, 9, or 16 elements were tachistoscopically presented to the right visual field-left hemisphere (RVF-LH) or left visual field-right hemisphere (LVF-RH). Subjects judged whether all of the elements within an array were physically the same (all X's) or whether one (O) was different from the rest. Left hemisphere presentations were processed more quickly and accurately than LVF-RH presentations for all stimulus conditions. As the number of array elements increased, more errors and longer response times were obtained for different stimulus items whereas fewer errors and somewhat shorter response times were obtained for same stimulus items. These and previous results suggest that the left hemisphere obtains an advantage for visual search because of that hemisphere's superiority for fine-grained feature analysis rather than because of a fundamental hemispheric serial/parallel processing dichotomy.

Backward contralateral masking by light and by random pattern

Letter-stimuli as targets were presented to the right or left visual fields and followed either by a flash of light or by a flash of light plus a patterned mask. The patterned mask always appeared in the opposite visual field of the letter targets. Analysis showed that masking occurred for both types of masks but that subjects produced more errors at each of five intervals between onset of the target and onset of the mask for the flash of light plus a patterned mask in the opposite visual field than for the flash of light alone. A pattern mask, when presented to the opposite visual field of a target stimulus, interferes with target processing at short target-mask intervals. These findings suggest that central backward masking may involve target-mask interactions beyond the visual cortex (Area 17).