In the study of choice reaction time (CRT), it has been a long standing research objective to estimate the time taken by a particular component of information processing, – or at least by a subset of processing components – uncontaminated by the time taken by the remaining components. An example is the measurement of the time taken by perceptual processing up to and including stimulus identification uncontaminated by the time needed for response selection. The time taken by motor processes is probably always included in a reaction time but this may not be problematic when it can be considered to represent a constant. A particular advantage of separate measures of the time taken by perceptual processing and by response selection is that it would aid in assessing the loci of effects of the various variables affecting CRT.

In the classical tradition, the above objective was at the basis of subtraction method of Donders (1969). Briefly, a simple (a)-reaction was thought to reflect merely the time taken by peripheral sensory and motor processes, a two-choice (b)-reaction would add the time needed for stimulus identification and for response selection, whereas a selective (c)-reaction was supposed to add the time for identification but not for response selection. Hence, (b)–(c) would reflect the time taken by response selection and (c)–(a) the time taken by stimulus identification.

In the (c)-reaction subjects should respond at the presentation of one out of two alternative stimuli whereas they should refrain from responding when the other one is presented. The presence of only one overt response led Donders to believe that response selection does not contribute to CRT; it was soon realised, however, that responding and witholding a response reflects a choice as well. To meet this problem, Wundt (1883) proposed the (d)-reaction in which a response is always carried out, irrespective of which one of two alternative stimuli is presented. In that case response selection should be really excluded so that (d)–(a) would be an appropriate measure of the time taken by perceptual processing. Unfortunately – besides in the case of Wundt and his research assistant ! – the duration of the (d)-reaction and the (a)-reaction proved indistinguishable. Berger (1886) noted the apparent flaw that, in the case of a single response to any stimulus, subjects might simply fail to identify the stimulus before responding so that the processes involved in the (d)-reaction and the (a)-reaction are virtually the same.

The notion of the (d)-reaction has revived in the paradigm of the functional visual field (e.g. Sanders, 1963; Sanders, 1993 and Sanders, 1998). In the standard version of this paradigm two stimuli (SL, SR), subtending a sizeable – say 45° binocular – horizontal visual angle are presented at eye level and at an equal distance to the left (SL) and to the right (SR) of the visual meridian. At the start of a trial subjects are instructed to fixate the spot at which SL will be presented. Following inspection of SL, the eyes have to be shifted to SR, whereupon a joint response to SL and SR – a same–different response is most common – is required. Standard recording of horizontal eye movements allows measurement of (a) a time TL, defined as the interval elapsing between presentation of SL and the start of the saccade, (b) a time TM, defined as the duration of the saccade, and (c) a time TR, defined as the interval between arrival of the eyes at SR until the completion of the response. The hypothesis is that the saccade is triggered upon completion of perceptual identification of SL, so that TL represents a (d)-reaction. The flaw of identification failure is avoided, since completion of the ultimate same-different response requires an identified SL. The decision about which response is required – same or different – should not affect TL. It is evident that, in addition to perceptual processing of SL, TL includes motor processes needed to initiate the saccade but, indeed, these might be considered as a constant.

Yet, this reasoning entails some assumptions which, if invalid, would fully undermine the notion of a more satisfactory version of the (d)-reaction. One assumption is that no perceptual processing of SL takes place during the saccade to SR. The well established finding that intake of information is virtually impossible during a saccade does not mean that central perceptual processing information is equally hampered. It could well be that the main function of a fixation is to achieve stimulus intake, whereas further perceptual processing might occur equally efficient during the fixation and during the subsequent saccade. A saccade is quite time consuming – some 100 ms in the case of a 45° saccade – which renders the issue of central processing during a saccade particularly relevant.