![]() When a single stimulus attribute is modulated periodically as a function of time, the evoked response generated by that stimulus has a periodic time course. More importantly for the purpose of this article, the technique has been extended to stimuli of increasing complexity, from luminance flicker to pictures of faces, and therefore this method has become more broadly applicable in visual science research. The signal processing methods used at the time were rudimentary (Regan, 1989) and have been steadily improved over time (Nelson, Seiple, Kupersmith, & Carr, 1984 Norcia, Clarke, & Tyler, 1985 Tang & Norcia, 1995 Tyler, Apkarian, Levi, & Nakayama, 1979). Interest crescendoed in the 1960s among researchers who studied the processing of luminance information (Kamp, Sem Jacobsen, Storm Van Leeuwen, & van der Tweel, 1960 Regan, 1964, 1966 Spekreijse, 1967 van der Tweel & Lunel, 1965 van der Tweel & Spekreijse, 1969). The result is a series of potential waves having the same frequency as that of the flicker” (p. As was typical of the time, some details are left to the imagination, but the following quote from the article leaves little doubt: “At a signal the eyes are opened and the shutter lifted to turn on the flickering light. Steady-state evoked potentials in response to visual stimuli were first reported by Adrian and Matthews (1934a) in a remarkable article that also demonstrated suppression of the alpha rhythm by attention. Because the responses to such periodic stimuli can be very stable in amplitude and phase over time, those responses have been referred to as the steady-state visually evoked potential (Regan, 1966). In contrast to these transient ERPs, exogenous ERPs can also be generated in response to a train of stimuli presented at a fixed rate. In order to achieve this isolation, stimuli in an ERP experiment are typically separated from each other by a long and/or variable interstimulus interval, allowing for the estimation of a stimulus-independent baseline reference. In their most common form, ERPs are recorded in response to an isolated, discrete stimulus event. These two types of responses comprise event-related potentials (ERPs), which are “the general class of potentials that display stable time relationships to a definable reference event” (Vaughan, 1969, p. Evoked potentials can be generated not only as a result of physical stimulation by a sensory stimulus (exogenously generated evoked potentials) but also by internal cognitive or motor processes (endogenously generated evoked potentials). ![]()
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