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Visual N1
The visual N1 is a visual evoked potential, a type of event-related electrical potential (ERP), that is produced in the brain and recorded on the scalp. The N1 is so named to reflect the polarity and typical timing of the component. The "N" indicates that the polarity of the component is negative with respect to an average mastoid reference. The "1" originally indicated that it was the first negative-going component, but it now better indexes the typical peak of this component, which is around 150 to 200 milliseconds post-stimulus. The N1 deflection may be detected at most recording sites, including the occipital, parietal, central, and frontal electrode sites.〔Mangun, G.R., & Hillyard, S.A, (1991). Modulations of sensory-evoked brain potentials indicate changes in perceptual processing during visual-spatial priming. Journal of Experimental Psychology: Human perception and performance, 17(4): 1057-1074.〕 Although, the visual N1 is widely distributed over the entire scalp, it peaks earlier over frontal than posterior regions of the scalp,〔〔Ciesielski, K.T., & French, C.N. (1989). Event-related potentials before and after training: Chronometry and lateralization of visual N1 and N2. Biological Psychology, 28: 227-238.〕 suggestive of distinct neural and/or cognitive correlates.〔Makeig, S., Westerfield, M., Townsend, J., Jung, T., Courchesne, E., & Sejnowski, T.J. (1999). Functionally independent components of early event-related potentials in a visual spatial attention task. Royal Society, 354: 1135-1144.〕 The N1 is elicited by visual stimuli, and is part of the visual evoked potential – a series of voltage deflections observed in response to visual onsets, offsets, and changes. Both the right and left hemispheres generate an N1, but the laterality of the N1 depends on whether a stimulus is presented centrally, laterally, or bilaterally. When a stimulus is presented centrally, the N1 is bilateral. When presented laterally, the N1 is larger, earlier, and contralateral to the visual field of the stimulus. When two visual stimuli are presented, one in each visual field, the N1 is bilateral. In the latter case, the N1’s asymmetrical skewedness is modulated by attention.〔Wascher, E., Hoffman, S., Sanger, J., Grosjean, M. (2009). Visuo-spatial processing and the N1 component of the ERP. Psychophysiology, 46: 1270–1277.〕 Additionally, its amplitude is influenced by selective attention, and thus it has been used to study a variety of attentional processes.〔Luck, S. J., Woodman, G. E., and Vogel, E. K. (2000). Event-related potential studies of attention. Trends in Cognitive Sciences, 4, 432-440.〕〔Rugg, M.D., Milner, A.D., Lines, C.R., & Phalp, R. (1987). Modulations of visual event-related potentials by spatial and non-spatial visual selective attention. Neuropsychologia, 25, 85-96.〕
==History==
Although the N1 is an early visual component that is part of the normal response to visual stimulation, it has been studied most extensively with respect to its sensitivity to selective attention. Initial studies focusing on the modulation of the N1 amplitude with respect to attention found limited evidence for N1 attention effects. However, uncertainty about the relationship between N1 amplitude and attention was resolved by Haider, Spong, and Lindsley’s (1964) groundbreaking study in which levels of attention were found to systematically relate to variation in the amplitude of the N1. Specifically, Haider et al. (1964) employed a vigilance task requiring visual discrimination and response to ensure that participants attended to the stimuli, instead of passively observing the visual images. Participants observed an array of light flashes and were told to respond with a button press to dim flashes. These dim flashes were interspersed with brighter flashes that did not require a response. The experiment lasted for approximately 100 minutes, and, typical of this type of vigilance task, accurate responding to the dim flashes decreased over time, which is indicative of the decline in attention across the experiment. Importantly, the amplitude of the N1 systematically varied with the response to the dim flashes. As accuracy and attention decreased, the amplitude of the N1 decreased, suggesting that the amplitude of the N1 is intimately tied to levels of attention.〔Haider, M., Spong, P., & Lindsley, D.B. (1964). Attention, vigilance, and cortical evoked-potentials in humans, Science, 145, 180-182.〕
Subsequent studies employing different attention manipulations found similar results, providing further support for the link between the N1 and attention. In one study, subjects directed attention to different types of visual stimuli, and the amplitude of the N1 to the visual stimuli varied according to whether they were attended. More specifically, the N1 was greater for stimuli that were attended to versus those that were ignored.〔Eason, R.G., Harter, M.R., & White, T.C. (1969). Effects of attention and arousal on visually evoked cortical potentials and reaction time in man. Physiology and Behavior, 4(3): 283-289.〕 A later study by Van Voorhis & Hillyard (1977)〔Van Voorhis, & Hillyard, S.A. (1977). Visual evoked potentials and selective attention to points in space. Perception and Psychophysics, 22(1): 54-62.〕 examined amplitude changes in the N1 during a task in which light flashes were concurrently delivered to the left or right visual field in independently random sequences. Subjects were instructed to attend left, attend right, or attend to both fields. Enhancement of the N1 at the occipital site was found when attention was directed to the field in which light flashes were delivered. In comparison, the N1 were smaller for flashes that occurred in the field opposite of attentional focus. When attention was divided between the left and right fields, the N1 amplitude was intermediate. Thus, visual information at attended locations appeared to be amplified. The attention-related modulation of the N1 produced evidence of selective visual attention similar to the attention effect discovered in the auditory modality, in which the auditory N100 varies according to selective attention within the auditory field.
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