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. Although, the visual N1 is widely distributed over the entire scalp, it peaks earlier over frontal than posterior regions of the scalp, suggestive of distinct neural and/or cognitive correlates. 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. Additionally, its amplitude is influenced by selective attention, and thus it has been used to study a variety of attentional processes.
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. 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. A later study by Van Voorhis & Hillyard (1977) 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.
Main paradigms Filtering Paradigm After the amplitude of the N1 was found to vary according to levels of attention, researchers became interested in how identical stimuli were perceived when they were attended versus unattended. An experimental paradigm, sometimes referred to as the Filtering Paradigm, was developed to assess how attention influences perception of stimuli. In the Filtering Paradigm, participants are instructed to focus their attention on either the right or left visual field of a computer screen. The visual field is typically counterbalanced within subjects across trials or experimental blocks. Thus, for the first set of trials, participants may pay attention to the right visual field, but subsequently they may pay attention to the left visual field. Within each trial and across visual fields, participants are presented with the same stimuli, for example flashes of lights varying in duration. Participants are told that when a particular stimulus, such as a short duration flash of light, referred to as a target, appears in the visual field they are attending, they should respond with a button press. The number of targets within each visual field is less than that number of non-targets, and participants are also told to ignore the other visual field and to not respond to the targets presented in that visual field. When targets in the attended visual field are compared to targets in the unattended visual field, the unattended targets are found to elicit a smaller N1 than the attended targets, suggesting that attention acts as a sensory gain mechanism that enhances perception of attended (vs. unattended) stimuli.
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