Visual N1
[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.
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.
In the Filtering Paradigm, participants are instructed to focus their attention on either the right or left visual field of a computer screen.
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.
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.
[11] 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).
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.
[14] Amplitude effects on the N1 are absent during simple Reaction Time tasks, which only require subjects to make a rapid response to stimuli.
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.
[15] Specifically, latency seems to increase during tasks that are significantly complex or difficult and, thus, require greater active attention or effort.
[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.
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.
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.
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.
[23] 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.
