C1 and P1

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The C1 and P1 (also called the P100) are two human scalp-recorded event-related brain potential (event-related potential (ERP)) 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 (when using a mastoid reference point) 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 (when also using a mastoid reference point) 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 (striate cortex) of the human brain and the P1 being linked to other visual areas (Extrastriate cortex). 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.

Contents

History

The different components within the category of VEPs were first described by Spehlmann in 1965, who compared human ERPs when viewing patterned and diffuse stimuli that were quickly flashed on the screen while a person was viewing the general area where the flash was to appear. [1] However, it was not until Jeffreys and Axford that the earliest individual components of those VEPs were delineated, including the C1 component. They had human participants view stimulus patterns of squares for a very short time (25ms), aperiodically, in different parts of the participant's visual fields while being recorded using electrodes placed towards the back of the head. Specifically, they recorded from three electrode sites placed on the longitudinal midline of the head: one 3 cm anterior to the inion (the bony projection at the posteroinferior part of the skull), and two 3 cm to either side of the midline. After averaging between like trials (trials where the stimuli were presented in the same part of the visual field) and looking at the ERPs, Jeffreys and Axford postulated that there are two distinct components in the first 150 milliseconds, the C1 and the C2. But of the two components, the C1 tended to show polarity shifts across the scalp for trials where a stimulus was shown on one side of the visual field was compared to trials where stimuli were shown on the opposite side of the visual field. The C1's polarity is also inverted whenever trials where the stimuli were presented in the top half of the visual field versus when stimuli were presented in the lower half of the visual field. Based on this evidence, Jeffreys and Axford proposed that the C1 reflected activity in the striate cortex as the activity tends to reflect a retinotopic map very similar to the one in the striate cortex. [2]

Since its initial discovery, the common theory about the C1 continues to state that it is an early component when viewing stimuli and that it represents activity in the primary visual cortex.

One of the initial descriptions of the P1 can be credited to Spehlmann with his categorization of components within the VEPs. [1] Whereas previous papers had looked at human ERPs to visual stimuli, and, undoubtedly, recorded P1 components as can be seen by visually inspecting the waveforms in the early articles, [3] Spehlmann was one of the first to describe a "surface positive component at 80-120ms." In his experiment, Spehlmann showed participants patterns of black and white squares, arranged in a checkerboard manner. These patterns were flashed to the participant by using a strobe light that had a frequency of 1-2 flashes per second. Averaging across trials, Spehlmann noted two different positivities, the first of which would later go on to be known as the P1.

In the last quarter of the 20th century, the P1 started to be studied looking at what is called the P1 "effect" in the selective attention domain. Van Voorhis and Hillyard found modulations in the P1 due to attention using the famous paradigm used by Eason, Harter, and White. [4] [5] For their experiment, Van Voorhis and Hillyard had participants view circular flashes of light to the left and to the right of a central fixation with the right and left flashes occurring independently with each side having flashes 2 to 8 seconds apart (a replication of Eason et al.), the flashes occurring randomly with 1 to 4 seconds between each flash (left or right), or the flashes occurring randomly with 300 to 600ms between each flash. Participants were instructed to either attend to the left visual field, the right visual hemisphere, or both visual hemispheres for a double flash (two flashes within 70ms of each other). Participants were also instructed to either look for the target passively or press a button whenever the double flash occurs. To record the ERPs, they had two electrodes down the midline (Cz and Oz) all referenced to the right mastoid. Van Voorhis and Hillyard found that the P1 had a greater positive amplitude when the target was presented in the attended field than when it was presented outside the attended field across all conditions. This was one of the first papers to show that attention could modulate a visually evoked potential as early on as the P1. Ever since this experiment, the difference between the P1 amplitude when the participant is attending in the correct and incorrect visual field (or the P1 effect) has been extensively studied as part of selective attention. [4]

Component characteristics

The C1 component typically peaks anywhere from 50–100ms and its polarity and scalp distribution are dependent on where the stimulus is presented. [2] [6] Roughly speaking, the C1 has a negative polarity if the stimuli is presented in the upper half of the visual field (when using a mastoid reference) but it has a positive polarity if the stimuli is presented in the lower half of the visual field. The C1 scalp distribution is fairly broad with greatest polarity typically along the occipito-parietal sites, [6] although the scalp can be lateralized with greater polarity along the occipito-parietal sites contralateral to the stimulus. [2]

The P1 component is a positive going component that typically begins around 70–90ms with a peak around 80-130ms. [7] Its amplitude maximum is over the lateral occipital scalp, approximately right over the ventrolateral prestriate cortex, contralateral to the visual field in which the stimuli is presented. [6]

Main paradigms

C1s are evoked whenever a visual stimulus is presented. As such, virtually any paradigm that presents visual stimuli can be used to look at the C1 component. However, one of the main paradigms used to look at the differential effects of viewing stimuli in different visual fields, and the one used to originally identify the C1 component, involves presenting visual stimuli in all different visual hemifields, one at a time. [8] The participant is typically warned that a series of stimuli are to be presented and then exactly the same type of stimuli are presented all while participants fixate at a cross at the center of the screen.

Early P1 research centered on looking at what components are present when visual stimuli was viewed. This is reflected by the main paradigm used to elicit a P1. In this paradigm, geometric objects, patterns of geometric shapes, [1] colors, [9] or even just flashes of white light, [3] for a very short time. ERPs are then recorded from sites above occipital regions and those waveforms are averaged across trials.

Later research on the P1 started to look at the P1 effect with regards to selective attention. These paradigms vary with type of stimuli used and time in between stimuli but the base paradigm mainly involved the participant attending to a specific part of the visual field while looking for a target in his or her entire visual field. Blocks of stimuli are presented one at a time and the participant must indicate (or at the very least look for) the target stimuli's presence. Before each block specific instructions are given as to what part of the visual field to attend to as well as any experiment specific instructions. [4] The important comparison is between the P1 for targets that are presented in the space where a participant was attending versus targets that appear in parts of the visual field where the participants were not attending.

A variant of this paradigm is the filter paradigm. In this paradigm participants are asked to attend to a certain part of the visual field and to not pay attention to or "filter out" the rest of the visual field. Blocks of stimuli are presented one at a time in both attended and unattended space. Participants are to look for a target that differs from the rest of the stimuli on some number of dimensions such as size, length, luminance, etc. within only the attended space as indicated before every block. However, targets are also presented in the unattended space. [9] The important comparison in this paradigm is between the P1 for targets presented within the attended visual field versus targets that were presented out of the attended visual field.

Another variant of the basic paradigm of selective attention is the visuospatial cueing paradigm. In this paradigm stimuli are presented one at a time in a fixed number of locations in the visual field. Participants are to look for and indicate if a particular stimulus is the target stimulus. The main aspect of this paradigm is that prior to every presentation of a stimulus there is a cue, indicating where the stimulus is going to be present. The cues, though, are not entirely accurate with some percentage indicating the wrong spatial location. [10] In some experiments there could even be cues that do not indicate any specific location whatsoever or a neutral cue. [11] The critical comparison in this paradigm is the comparison between the P1 on trials where the stimulus was presented in the location indicated by the cue versus trials when the stimulus was presented in a location not indicated by the cue. For those experiments where a neutral cue is given, another important comparison is between the P1 of the two trials where a directional cue is given (either correct or incorrect) versus the P1 on those trials where the cue gives no indication of a direction.

Functional sensitivity

Spatial location in the visual field

The C1 component is sensitive to where a stimulus is presented in a visual field. [8] The C1 has been shown to be negative when items are presented in the top half of the visual field and positive when the visual stimuli are presented in the bottom half of the visual field. The scalp distribution of the C1 component can also lateralized based on the lateralization of the stimuli. [8] Stimuli presented in the left half of the visual field will elicit more negativity over the rightward occipital and parietal channels. Stimuli presented in the right half of the visual field will elicit a negativity over the leftward occipital and parietal channels.

While the polarity is consistent across presentations of visual stimuli in different visual fields, the P1s scalp topographic maps do change in that the positivity is elicited contralaterally to the visual field in which a stimulus is presented although not to the extent shown in the C1 component. [6]

Attention

One of the main differences between the C1 and the P1 is the effects of attention on each component. Although multiple studies have shown that there is no effect of increased attention on the C1, [12] [13] more recent studies suggest that C1 may be more sensitive to internal states than previously thought. [14] [15] However studies using different variants of spatial cueing paradigms have shown that the P1 shows greater amplitude when a stimulus is shown in an area where the participant was attending. In an experiment by Mangun and Hillyard, they had had participants do a size discrimination task between two bars, one on the left and one on the right with the target stimuli either being the smaller or taller bars, depending on the block of trials. A cue was given before each pair of blocks was given. This cue was only correct 75% of the time. When comparing the P1 when the participant was attending to the correct side to the P1 when the participant was not attending to the correct side, the former had a greater amplitude than the latter. [10]

However, the P1 effect is not necessarily modulated by having participants attend to a certain area. By adding a neutral cue, they showed that there was no difference between the amplitude of the P1 when the correct area was attended and when a neutral cue was given, not giving any indication as to where the target stimulus was to show up. [11]

Theory/source

P1 reflects the "cost of attention"

Luck, Hillyard, Mouloua, Woldorff, Clark and Hawkins proposed that the P1 effect is a reflection of a "cost of attention." [11] As has been shown previously, whenever a participant is paying attention to a particular area and the target stimulus was presented outside wherever the participant was attending, there is a decrement in the P1 amplitude. [4] [10] Luck et al. claim that this decrement is actually a cost of attending someplace and being incorrect. This decrement or suppression of the P1 represents the cost of having to stop attending to one area and shift the attention to the place where the target stimulus is located. This is as compared to another component called the N1. The N1 shows an increment in amplitude when a participant is attending to a certain area and the stimulus is shown in that area. Luck et al. call this the "benefit" of attention. [11]

Early vs. late selection attention models

One of the critical debates that the C1 and P1 have helped to contribute to is that of early versus late selection models. Early selection models such as Broadbent's early filter theory [16] claim that attention filter out unattended information while in the middle of processing that information. However late selection models claim that information is processed to a much later stage and attention serves to choose between that processed information. Attentional effects on the P1 show that attention can affect visual processing as early as 65ms with stimuli appearing in unattended regions of space, having a lower P1 amplitude. [4] However, the lack of modulation of the C1 component due to attention or lack thereof shows that not all information is being filtered out immediately. Instead, early aspects of visual processing (as reflected in the C1) seem to unfold in a manner that is unaffected by the allocation of attention over space. [12]

C1 and the striate cortex

When it was first discovered by Jeffreys and Axford in 1972, they suggested that the source of the C1 component was somewhere in the V1 (or the striate cortex or Brodmann's Area 17) as the polarity reversals and the reversals from side to side mirrored the retinotopic map of V1. More specifically, they suggested that the C1 is generated in Brodmann's Area 17 or the V1. [2] In the early years the findings of some studies helped to support this hypothesis. [6] [17] Conversely, others found that the C1 might be located in areas such as Brodmann's Area 18, [18] or Brodmann's Area 19. [19]

However more recent evidence using source localization techniques such as brain electrical source analysis (BESA) in conjunction with fMRI points to the C1 being generated in the primary visual cortex of Brodmann's area 17. Clark, Fan, and Hillyard using a paradigm whereby circular checkerboards were presented in different visual fields, localized the C1 to the striate cortex using a 2-dipole BESA approach. [20] Di Russo, Martinez, and Hillyard used sinusoidally modulated black and white checkerboard circles in the four different hemifields (upper-right, upper-left, lower-right, and lower-left) to look at the location of the C1. They found also using a BESA method, using 7 pairs of dipoles, that the C1 originated in the striate cortex. Their BESA results also matched up with the concurrent fMRI results for the same participants. [12]

The extrastriate cortex

The source of P1 component, as opposed to the C1 component, is not entirely known. Work presenting bars in different sections of the visual field, some of which were presented in attended parts of the visual field and some were not, points to the neurological source of the P1 somewhere over the ventrolateral prestriate cortex or Brodmann's Area 18. To make this judgment, they used both current source density maps and structural MRI of the participants to localize the source of the P1. [6] Other papers using a combination of fMRI and BESA dipole modeling have also pointed to the P1 coming from the ventrolateral prestriate cortex. [12] [21]

Further evidence that the P1 is located along the ventral pathway comes from a studies using both ERPs and positron emission tomography. These studies have shown that the P1 is associated with activation in the dorsal occipital areas, [22] and the posterior fusiform gyrus. [23]

See also

Related Research Articles

An evoked potential or evoked response is an electrical potential in a specific pattern recorded from a specific part of the nervous system, especially the brain, of a human or other animals following presentation of a stimulus such as a light flash or a pure tone. Different types of potentials result from stimuli of different modalities and types. Evoked potential is distinct from spontaneous potentials as detected by electroencephalography (EEG), electromyography (EMG), or other electrophysiologic recording method. Such potentials are useful for electrodiagnosis and monitoring that include detections of disease and drug-related sensory dysfunction and intraoperative monitoring of sensory pathway integrity.

<span class="mw-page-title-main">Event-related potential</span> Brain response that is the direct result of a specific sensory, cognitive, or motor event

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 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.

<span class="mw-page-title-main">P300 (neuroscience)</span> Event-related potential

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.

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 which case it is abbreviated to vMMN. The (v)MMN occurs after an infrequent change in a repetitive sequence of stimuli For example, a rare deviant (d) stimulus can be interspersed among a series of frequent standard (s) stimuli. In hearing, a deviant sound can differ from the standards in one or more perceptual features such as pitch, duration, loudness, or location. The MMN can be elicited regardless of whether someone 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) is a negative slow surface potential, as measured by electroencephalography (EEG), that occurs during the period between a warning stimulus or signal and an imperative ("go") stimulus. The 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 EEG recordings and that this activity could be related to a cognitive process such as expectancy.

In neuroscience, the lateralized readiness potential (LRP) is an event-related brain potential, or increase in electrical activity at the surface of the brain, that is thought to reflect the preparation of motor activity on a certain side of the body; in other words, it is a spike in the electrical activity of the brain that happens when a person gets ready to move one arm, leg, or foot. It is a special form of bereitschaftspotential. LRPs are recorded using electroencephalography (EEG) and have numerous applications in cognitive neuroscience.

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 is 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.

Auditory spatial attention is a specific form of attention, involving the focusing of auditory perception to a location in space.

Difference due to memory (Dm) indexes differences in neural activity during the study phase of an experiment for items that subsequently are remembered compared to items that are later forgotten. It is mainly discussed as an event-related potential (ERP) effect that appears in studies employing a subsequent memory paradigm, in which ERPs are recorded when a participant is studying a list of materials and trials are sorted as a function of whether they go on to be remembered or not in the test phase. For meaningful study material, such as words or line drawings, items that are subsequently remembered typically elicit a more positive waveform during the study phase. This difference typically occurs in the range of 400–800 milliseconds (ms) and is generally greatest over centro-parietal recording sites, although these characteristics are modulated by many factors.

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.

<span class="mw-page-title-main">Visual N1</span>

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.

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.

Somatosensory evoked potential is the electrical activity of the brain that results from the stimulation of touch. SEP tests measure that activity and are a useful, noninvasive means of assessing somatosensory system functioning. By combining SEP recordings at different levels of the somatosensory pathways, it is possible to assess the transmission of the afferent volley from the periphery up to the cortex. SEP components include a series of positive and negative deflections that can be elicited by virtually any sensory stimuli. For example, SEPs can be obtained in response to a brief mechanical impact on the fingertip or to air puffs. However, SEPs are most commonly elicited by bipolar transcutaneous electrical stimulation applied on the skin over the trajectory of peripheral nerves of the upper limb or lower limb, and then recorded from the scalp. In general, somatosensory stimuli evoke early cortical components, generated in the contralateral primary somatosensory cortex (S1), related to the processing of the physical stimulus attributes. About 100 ms after stimulus application, additional cortical regions are activated, such as the secondary somatosensory cortex (S2), and the posterior parietal and frontal cortices, marked by a parietal P100 and bilateral frontal N140. SEPs are routinely used in neurology today to confirm and localize sensory abnormalities, to identify silent lesions and to monitor changes during surgical procedures.

<span class="mw-page-title-main">P3b</span>

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 to 500 ms or later depending upon the task and on the individual subject response. 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, familiar objects or words. Furthermore, the N170 is modulated by prediction error processes.

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.

<span class="mw-page-title-main">Oddball paradigm</span> Psychology research paradigm

The oddball paradigm is an experimental design used within psychology research. The oddball paradigm relies on the brain's sensitivity to rare deviant stimuli presented pseudo-randomly in a series of repeated standard stimuli. The oddball paradigm has a wide selection of stimulus types, including stimuli such as sound duration, frequency, intensity, phonetic features, complex music, or speech sequences. The reaction of the participant to this "oddball" stimulus is recorded.

The Posner cueing task, also known as the Posner paradigm, is a neuropsychological test often used to assess attention. Formulated by Michael Posner, it assesses a person'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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  5. ↑ Eason, R. Harter, M., & White, C. (1969). Effects of attention and arousal on visually evoked cortical potentials and reaction time in man. Physiology and Behavior, 4, 283-289.
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  8. 1 2 3 Jeffreys and Axford, 1965.[ full citation needed ]
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  15. ↑ Rauss, Pourtois, Vuilleumier, Shwartz. (2009). Attentional load modifies early activity in human primary visual cortex. Human Brain Mapping. vol. 30 (5) pp. 1723-1733
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  17. ↑ Parker, D. M., Salzen, E. A., & Lishman, J. R. (1982). The early wave of the viusla evoked potential to sinusoidal gratings: Responses to quadrant stimulation as a function of spatial frequency. Electorencephalogy and Clinical Neuropsychology, 53, 427-435.
  18. ↑ Lesevre, N. (1982). Chronotopographical analysis of the human evoked potential in relation to the visual field (data from normal individuals and hemianopic patients). Annals of the New York Academy of Sciences, 388, 156-182.
  19. ↑ Maier, J., Dagnelie, G., Spekrijse, H., & van Dijk, B. W. (1987). Principal components analysis for source localization of VEPs in man. Vision Research, 27, 165-177.
  20. ↑ Clark, V. P., Fan, S., & Hillyard, S. A. (1995). Identification of early visual evoked potential generators by retinotopic and topographic analyses. Human Brain Mapping, 2, 170-187.
  21. ↑ Martinez, A., Anllo-Vento, L., Sereno, M. I., Frank, L. R., Buxton, R. B., Dubowitz, D. J., Wong, E. C., Hinrichs, H., Heinze, H. J., & Hillyard, S. A. (1999). Involvement of striate and extrastriate visual cortical areas in spatial attention. Nature Neuroscience, 2(4), 364-369.
  22. ↑ Woldorff, M. G., Fox, P. T., Matzke, M., Lancaster, J. L., Veeraswarmy, S., Zamarripa, F., Seabolt, M., Glass, T., Gao, J. H., Martin, C. C., and Jerabek, P. (1998). Retinotopic organization of early visual spatial attention effects as revealed by PET and ERPs. Human Brain Mapping, 5(4), 280-286.
  23. ↑ Mangun, G. R., Hopfinger, J. B., Kussmaul, C. L., Fletcher, E. M., & Heinze, H. J. (1997). Covariations in ERP and PET measures of spatial selective attention in human extrastriate visual cortex. Human Brain Mapping, 5, 273-279.
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