Academic journal article Canadian Journal of Experimental Psychology

Fundamental Properties of the N2pc as an Index of Spatial Attention: Effects of Masking

Academic journal article Canadian Journal of Experimental Psychology

Fundamental Properties of the N2pc as an Index of Spatial Attention: Effects of Masking

Article excerpt

Abstract

Masking is an important tool in many paradigms used to study the cognitive architecture. The N2pc is an electrophysiological event-related potential (ERP) that has been used as a tool to study the deployment of visual spatial attention. The aim of this paper was to study the effects of masking on the N2pc. Two stimuli were presented on the screen, one to left and one to right of fixation, and subjects reported the identity of one of them. The targets could be discriminated both by their category (letters vs. digit) and by their colour (pink vs. green). Backward masking was produced by presenting a second pair of bilateral stimuli after the offset of the first pair. The second pair of stimuli consisted of characters of the same colour and category as in the first pair. Forward masking was produced by using the very same stimuli as in the backward masking condition, but by instructing subjects to report the second stimulus. The forward mask trials had longer response times compared to no-mask trials, and backward mask trials had even longer response times, and also a higher error rate. Although the different masking procedures lead to clear behavioural effects, the N2pc was not affected, suggesting that the deployment of visual spatial attention, per se, was not affected by pattern masking. A sustained posterior contralateral negativity (SPCN) following the N2pc was also found (300 ms post-target, and beyond), and the amplitude of the SPCN was strongly modulated by the number of presented stimuli and the duration of the SPCN was positively correlated with RT in the behavioural task. We hypothesize that the SPCN reflects neural activity associated with the passage of information through visual short-term memory.

The N2pc is a lateralized event-related potential (ERP) component that reflects the locus of attention in the visual field (Luck & Hillyard, 1994a). It usually occurs in the temporal window of the N2 component and is measured at posterior electrode sites, over the hemisphere contralateral to the target. Usually the N2pc is measured by taking the difference in the ERP observed at electrode sites contralateral to the visual field of a target and the ERP observed at a corresponding electrode on the ipsilateral side, for visual displays that are physically equivalent across the left and right visual fields. By physically equivalent, we mean displays that should produce equivalent bottom-up sensory activation in each hemisphere, as a function of purely sensory factors. The observation of a difference in electrophysiological response across cerebral hemispheres for such displays, therefore, must reflect differential processing of the initially equivalent display. Luck and Hillyard found that the amplitude of the N2pc was higher for more attention-demanding tasks, and they argued that the N2pc reflects neuronal activity associated with the filtering out of distractors in visual search tasks. Luck, Girelli, McDermott, and Ford (1997) found that adding nearby distractors increased the amplitude of the N2pc, which provided converging evidence for the hypothesis that the N2pc reflects a process of distractor suppression. More generally, these results suggest that increasing the perceptual difficulty of visual search might increase the amplitude of the N2pc.

Backward masking renders visual tasks more difficult by superimposing new contours over the location previously occupied by visual targets. This type of masking often increases response time and increases error rates. If increasing task difficulty increases the amplitude of N2pc and if masking increases task difficulty, we hypothesized that backward masking might also increase the amplitude of the N2pc. This hypothesis was tested in the present experiment.

Woodman and Luck (2003a) showed that a particular form of masking, four-dot masking, did not modulate the N2pc, although this masking lowered the accuracy of the report of the targets. …

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