Last updated An EEG waveform showing a typical N100 peak
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.[1] Although, the visual N1 is widely distributed over the entire scalp, it peaks earlier over frontal than posterior regions of the scalp,[1][2] suggestive of distinct neural and/or cognitive correlates.[3] 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.[4] Additionally, its amplitude is influenced by selective attention, and thus it has been used to study a variety of attentional processes.[5][6]
An evoked potential or evoked response is an electrical potential recorded from the nervous system of a human or other animal following presentation of a stimulus, as distinct from spontaneous potentials as detected by electroencephalography (EEG), electromyography (EMG), or other electrophysiologic recording method. Such potentials are useful for electrodiagnosis and monitoring.
An event-related potential (ERP) is the measured brain response that is the direct result of a specific sensory, cognitive, or motor event. More formally, it is any stereotyped electrophysiological response to a stimulus. The study of the brain in this way provides a noninvasive means of evaluating brain functioning.
The brain is an organ that serves as the center of the nervous system in all vertebrate and most invertebrate animals. The brain is located in the head, usually close to the sensory organs for senses such as vision. The brain is the most complex organ in a vertebrate's body. In a human, the cerebral cortex contains approximately 14–16 billion neurons, and the estimated number of neurons in the cerebellum is 55–70 billion. Each neuron is connected by synapses to several thousand other neurons. These neurons communicate with one another by means of long protoplasmic fibers called axons, which carry trains of signal pulses called action potentials to distant parts of the brain or body targeting specific recipient cells.
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.[7]
Attention is the behavioral and cognitive process of selectively concentrating on a discrete aspect of information, whether deemed subjective or objective, while ignoring other perceivable information. It is a state of arousal. It is the taking possession by the mind in clear and vivid form of one out of what seem several simultaneous objects or trains of thought. Focalization, the concentration of consciousness, is of its essence. Attention has also been described as the allocation of limited cognitive processing resources.
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.[8] A later study by Van Voorhis & Hillyard (1977)[9] 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.
In neuroscience, the N100 or N1 is a large, negative-going evoked potential measured by electroencephalography ; it peaks in adults between 80 and 120 milliseconds after the onset of a stimulus, and distributed mostly over the fronto-central region of the scalp. It is elicited by any unpredictable stimulus in the absence of task demands. It is often referred to with the following P200 evoked potential as the "N100-P200" or "N1-P2" complex. While most research focuses on auditory stimuli, the N100 also occurs for visual, olfactory, heat, pain, balance, respiration blocking, and somatosensory stimuli.
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.[5][6][9][10]
Visuospatial Cuing Paradigm
In Visuospatial Cuing Paradigms, attention is cued to one area of the computer screen, but the validity of the cue with respect to the presentation of the target stimulus varies. Thus, this paradigm provides insight into how putting attention in the correct versus incorrect location influences the amplitude of the N1. For example, participants are presented with a visual array in which there are four boxes at the upper and lower right- and left-hand corners of the computer screen. In the first frame of the visual display, they are told to fixate on a small dotted line in the center of the computer screen. To prepare participants to locate the cue, a warning frame follows in which the dotted line is replaced with a cross. The warning frame is followed by the cued frame, in which an arrow points in the direction of one or all four of the squares. In some cases, the cue is accurate and points to the square in which the target will be presented. In other cases, the cue is inaccurate and points to the square in which the target will not be presented. In the remaining cases, a neutral cue that points in the direction of all the squares is presented. Next, a target frame is displayed in which a small dot appears in one of the four squares. In the last frame, an arrow points to one of the four squares and participants respond with a button press to whether the cue appeared in the square. The amplitude of the N1 varies with respect to accurately cued, inaccurately cued, and neutrally cued trials. In trials in which attention was directed toward the square in which the target was presented (accurately cued trials), the amplitude of the N1 is larger than in both a) trials in which attention was directed to all squares (neutrally cued trials) and b) trials in which attention was directed to the wrong square (inaccurately cued trials), suggesting that the amplitude of the N1 represents a benefit for placing attention in the correct location.[11]
Factors that influence amplitude and latency
The amplitude, or the size, of the N1 is measured by taking the average voltage within the window that typically encompasses the N1 (about 150 to 200 ms post-stimulus). Because the N1 is a negative-going component, "larger" amplitudes correspond to being more negative, whereas "smaller" amplitudes correspond to being less negative.
The amplitude of a periodic variable is a measure of its change over a single period. There are various definitions of amplitude, which are all functions of the magnitude of the difference between the variable's extreme values. In older texts the phase is sometimes called the amplitude.
Research has suggested that the amplitude of N1 is affected by certain visual parameters, including stimulus angularity and luminance, both of which are directly related to the size of N1.[12][13] The amplitude of N1 is also greater in response to stimuli in attended vs. unattended locations. Conversely, amplitude is decreased when the interstimulus interval (i.e., the amount of time between successive presentations of stimuli) is increased for stimuli at attended locations.[14] Amplitude effects on the N1 are absent during simple Reaction Time tasks, which only require subjects to make a rapid response to stimuli.[1] This finding suggests that N1 is linked to visual discrimination processes.
Researchers interested in understanding selection effects of attention have been especially interested in amplitude variation of the N1 because amplitude differences are believed to represent a gain control mechanism (see Filtering Paradigm above). For example, because the amplitude of the N1 for targets in unattended visual fields is smaller than for targets in attended visual fields, it is believed that attention serves to amplify the processing of sensory inputs from attended locations and suppress sensory inputs from unattended locations.[5][6] Thus, amplitude differences in the N1 are useful in providing evidence for whether attention serves to select certain types of sensory stimuli for further processing.
One of the factors that influences the latency of N1 is processing effort: N1 latency increases as effort at processing is also increased.[15] Specifically, latency seems to increase during tasks that are significantly complex or difficult and, thus, require greater active attention or effort. For example, the onset, peak, and offset latencies of the N1 occur significantly earlier in response to moving stimuli in a simple detection task vs. an identification task.[16] N1 is also sensitive to the manipulation of a visual stimulus' intensity. N1's peak latency is shortened as the brightness of stimulus flashes is increased.[17] Therefore, it appears that N1 latency is affected by perceptual factors, such as flash intensity, as well as the level of attentional demand or processing effort.
Color and motion
Amplitude differences in the N1 have provided evidence that attention allows for more extensive analysis of visual information, such as color and motion. For example, in a Filtering Paradigm (see description above), participants were instructed to identify targets based on either color or motion. In some cases, participants were told to attend to one side of the visual field, while in other cases participants' attention was not focused on one side of the visual field. It was found that the amplitude of the N1 was greater for targets of the correct color and motion when participants were instructed to attend to one side of the visual field versus when they were not instructed to do so. These findings suggest that attention to a particular location serves to facilitate further processing of visual information and suppress further visual processing in unattended locations.[18]
Objects and location
Although spatial attention has been shown to be unique in selection for perceptual information that will be further processed, objects have also been shown to be important in filtering information for further processing. For example, in a Filtering Paradigm (see above), rectangles were presented on either side of the visual field. Participants were directed to attend to one side of the visual field and to the top 50% of the object within that visual field. The target was a shaded region of the top right-hand side corner; however, similar targets were presented in the unattended bottom half of the object in the attended visual field and in the top and bottom halves of the object in the unattended visual field. As expected, when comparing targets in the attended visual field to targets in the unattended visual field, it was found that the amplitude of the N1 was greater for attended (vs. unattended) objects. Additionally, although the amplitude of the N1 was greatest for targets in the attended visual field and the attended part of object, the amplitude of the N1 for targets in the unattended portion of the attended object was larger than the amplitude of the N1 for targets at an equivalent distance from the locus of attention but on an unattended object. These results provide evidence that while spatial attention does serve as a selection mechanism for further processing, spatial attention can spread across objects and influences further perceptual processing.[19]
Emotional stimuli
More recently, research on the N1 has expanded into the processing of socially relevant stimuli. Attention is especially relevant to the processing of emotional stimuli because emotional stimuli (vs. unemotional stimuli) are believed to receive preferential attention and perceptual processing. ERP research has been useful in understanding how emotion relates to attention because the N1 provides a means of examining the significance of emotion in capturing attentional resources. Several studies, using a variety of paradigms, have found that emotional stimuli are influential in capturing attention. For example, in one study, both stimuli that were positively valenced (e.g., nude person of the opposite sex) and negatively valenced (e.g., snarling wolf) were shown to elicit greater N1 amplitudes than neutrally valenced (e.g., wrist watch) stimuli.[20] Similarly, the valence of interpersonal stimuli has been found to influence the amplitude of the N1. Positive stimuli (e.g., smiling faces) and negative stimuli (e.g., sad faces) have been found to elicit a greater N1 than neutral stimuli (e.g., neutral faces).[21] These findings support the claim that emotional stimuli are more effective in capturing attentional resources than non-emotional stimuli.
What the N1 has revealed about attentional processes
The large corpus of studies focused on factors that modulate the amplitude of the visual N1 have provided a wealth of evidence suggesting that, while the visual N1 is a sensory component evoked by any visual stimulus, it also reflects a benefit of correctly allocating attentional resources and that it is a manifestation of an important sensory gating mechanism of attention. When attention is focused on areas of the visual field in which relevant information is presented (vs. evenly distributed across the visual field or focused on an area in which relevant information is not presented), the amplitude of the N1 is largest and indicates a benefit of correctly allocating attentional resources.[22] Additionally, the amplitude of the N1 is believed to represent a sensory gain control mechanism because focusing attention on one area of the visual field serves to increase the amplitude of the N1 to relevant perceptual information presented in that field (vs. the other visual field), and thus facilitates further perceptual processing of stimuli.[5][6] This finding supports the Early Selection Model of Attention, which contends that attention acts (i.e., filters information) on a stimulus set early in the information processing stream.
Additionally, research on the visual N1 suggests that spatial and object attention serve as an early selection mechanism that influences the selection of other perceptual features (e.g., color, motion) for further processing. The amplitude of the N1 is largest for perceptual features in attended (vs. unattended) locations and on attended (vs. unattended) objects, providing evidence that perceptual features are only selected for further perceptual processing if they are in attended locations or on attended objects.[18][19]
Lastly, the visual N1 has also been interpreted to reflect a discrimination process that takes place within the locus of attention. As compared with conditions that simply require a response, the N1 component is enhanced in conditions that require a differentiation between classes of stimuli. This effect is similar for color- and form-based discriminations, regardless of the level of difficulty of the discrimination. The N1 may, therefore, reflect a discrimination mechanism that is applied to an attended spatial area.[23]
Neural sources
Identifying the neurological sources of ERP components based on the topographical distribution of the N1 on the scalp is especially difficult because the number of potential sources (referred to as dipoles), orientations, and magnitudes that can produce the topographical distribution of the N1, like any other ERP component, is theoretically infinite. This problem of working from the topographical distribution of ERP components to identifying neural sources, is referred to as the Inverse problem.[24] Although the neural generators of the N1 are not definitively known,[10] evidence suggests that the N1 does not originate in the primary visual cortex, but instead from multiple generators in the occipito-parietal, occipito-temporal, and (possibly) frontal cortex.[25]
Wishful thinking describes decision-making and the formation of beliefs based on what might be pleasing to imagine, rather than on evidence, rationality, or reality. It is a product of resolving conflicts between belief and desire.
Also known as perceptual blindness, inattentive blindness results from a lack of attention that is not associated with vision defects or deficits, as an individual fails to perceive an unexpected stimulus in plain sight. When it becomes impossible to attend to all the stimuli in a given situation, a temporary “blindness” effect can occur, as individuals fail to see unexpected but often salient objects or stimuli. The term was coined by Arien Mack and Irvin Rock in 1992 and was used as the title of their book of the same name, published by MIT press in 1998, in which they describe the discovery of the phenomenon and include a collection of procedures used in describing it. A famous study that demonstrated inattentional blindness asked participants whether or not they noticed a gorilla walking through the scene of a visual task they had been given.
The N400 is a component of time-locked EEG signals known as event-related potentials (ERP). It is a negative-going deflection that peaks around 400 milliseconds post-stimulus onset, although it can extend from 250-500 ms, and is typically maximal over centro-parietal electrode sites. The N400 is part of the normal brain response to words and other meaningful stimuli, including visual and auditory words, sign language signs, pictures, faces, environmental sounds, and smells.
Inhibition of return (IOR) refers to an orientation mechanism that briefly enhances the speed and accuracy with which an object is detected after the object is attended, but then impairs detection speed and accuracy. IOR is usually measured with a cue-response paradigm, in which a person presses a button when they detect a target stimulus following the presentation of a cue that indicates the location in which the target will appear. The cue can be exogenous, or endogenous. Inhibition of return results from oculomotor activation, regardless of whether it was produced by exogenous signals or endogenously. Although IOR occurs for both visual and auditory stimuli, IOR is greater for visual stimuli, and is studied more often than auditory stimuli.
The P300 (P3) wave is an event related potential (ERP) component elicited in the process of decision making. It is considered to be an endogenous potential, as its occurrence links not to the physical attributes of a stimulus, but to a person's reaction to it. More specifically, the P300 is thought to reflect processes involved in stimulus evaluation or categorization. It is usually elicited using the oddball paradigm, in which low-probability target items are mixed with high-probability non-target items.
The mismatch negativity (MMN) or mismatch field (MMF) is a component of the event-related potential (ERP) to an odd stimulus in a sequence of stimuli. It arises from electrical activity in the brain and is studied within the field of cognitive neuroscience and psychology. It can occur in any sensory system, but has most frequently been studied for hearing and for vision. In the case of auditory stimuli, the MMN occurs after an infrequent change in a repetitive sequence of sounds For example, a rare deviant (d) sound can be interspersed among a series of frequent standard (s) sounds. The deviant sound can differ from the standards in one or more perceptual features such as pitch, duration, or loudness. The MMN is usually evoked by either a change in frequency, intensity, duration or real or apparent spatial locus of origin. The MMN can be elicited regardless of whether the subject is paying attention to the sequence. During auditory sequences, a person can be reading or watching a silent subtitled movie, yet still show a clear MMN. In the case of visual stimuli, the MMN occurs after an infrequent change in a repetitive sequence of images.
The contingent negative variation (CNV) was one of the first event-related potential (ERP) components to be described. The CNV component was first described by W. Grey Walter and colleagues in an article published in Nature in 1964. The importance of this finding was that it was one of the first studies which showed that consistent patterns of the amplitude of electric responses could be obtained from the large background noise which occurs in electroencephalography (EEG) recordings and that this activity could be related to a cognitive process such as expectancy.
Auditory spatial attention is a specific form of attention, involving the focusing of auditory perception to a location in space.
The P3a, or novelty P3, is a component of time-locked (EEG) signals known as event-related potentials (ERP). The P3a is a positive-going scalp-recorded brain potential that has a maximum amplitude over frontal/central electrode sites with a peak latency falling in the range of 250–280 ms. The P3a has been associated with brain activity related to the engagement of attention and the processing of novelty.
In neuroscience, the visual P200 or P2 is a waveform component or feature of the event-related potential (ERP) measured at the human scalp. Like other potential changes measurable from the scalp, this effect is believed to reflect the post-synaptic activity of a specific neural process. The P2 component, also known as the P200, is so named because it is a positive going electrical potential that peaks at about 200 milliseconds after the onset of some external stimulus. This component is often distributed around the centro-frontal and the parieto-occipital areas of the scalp. It is generally found to be maximal around the vertex of the scalp, however there have been some topographical differences noted in ERP studies of the P2 in different experimental conditions.
The N200, or N2, is an event-related potential (ERP) component. An ERP can be monitored using a non-invasive electroencephalography (EEG) cap that is fitted over the scalp on human subjects. An EEG cap allows researchers and clinicians to monitor the minute electrical activity that reaches the surface of the scalp from post-synaptic potentials in neurons, which fluctuate in relation to cognitive processing. EEG provides millisecond-level temporal resolution and is therefore known as one of the most direct measures of covert mental operations in the brain. The N200 in particular is a negative-going wave that peaks 200-350ms post-stimulus and is found primarily over anterior scalp sites. Past research focused on the N200 as a mismatch detector, but it has also been found to reflect executive cognitive control functions, and has recently been used in the study of language.
The C1 and P1 are two human scalp-recorded event-related brain potential components, collected by means of a technique called electroencephalography (EEG). The C1 is named so because it was the first component in a series of components found to respond to visual stimuli when it was first discovered. It can be a negative-going component or a positive going component with its peak normally observed in the 65–90 ms range post-stimulus onset. The P1 is called the P1 because it is the first positive-going component and its peak is normally observed in around 100 ms. Both components are related to processing of visual stimuli and are under the category of potentials called visually evoked potentials (VEPs). Both components are theorized to be evoked within the visual cortices of the brain with C1 being linked to the primary visual cortex of the human brain and the P1 being linked to other visual areas. One of the primary distinctions between these two components is that, whereas the P1 can be modulated by attention, the C1 has been typically found to be invariable to different levels of attention.
The P3b is a subcomponent of the P300, an event-related potential (ERP) component that can be observed in human scalp recordings of brain electrical activity. The P3b is a positive-going amplitude peaking at around 300 ms, though the peak will vary in latency from 250–500 ms or later depending upon the task. Amplitudes are typically highest on the scalp over parietal brain areas.
The N170 is a component of the event-related potential (ERP) that reflects the neural processing of faces.
N2pc refers to an ERP component linked to selective attention. The N2pc appears over visual cortex contralateral to the location in space to which subjects are attending; if subjects pay attention to the left side of the visual field, the N2pc appears in the right hemisphere of the brain, and vice versa. This characteristic makes it a useful tool for directly measuring the general direction of a person's attention with fine-grained temporal resolution.
The oddball paradigm is an experimental design used within psychology research. Presentations of sequences of repetitive stimuli are infrequently interrupted by a deviant stimulus. The reaction of the participant to this "oddball" stimulus is recorded.
The sensory enhancement theory assumes that attentional resources will spread until they reach the boundaries of a cued object, including regions that may be obstructed or are overlapping other objects. It has been suggested that sensory enhancement is an essential mechanism that underlies object-based attention. The sensory enhancement theory of object-based attention proposes that when attention is directed to a cued object, the quality of the object’s physical representations improve because the spread of attention facilitates the efficiency of processing the features of the object as a whole. The qualities of the cued object, such as spatial resolution and contrast sensitivity, are therefore more strongly represented in one's memory than the qualities of other objects or locations that received little or no attentional resource. Information processing of these objects also tends to be significantly faster and more accurate as the representations have become more salient.
The Posner Cueing Task, also known as the Posner paradigm, is a neuropsychological test often used to assess attention. Formulated by Michael Posner, the task assesses an individual’s ability to perform an attentional shift. It has been used and modified to assess disorders, focal brain injury, and the effects of both on spatial attention.
References
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