Training conditional relationships using a matching-to-sample (MTS) procedure typically consists of providing one sample, A1, and then rewarding only subsequent choices of B1 (and not, for example, B2). In this case, B1 is the positive stimulus or S+ and B2 is the negative stimulus or S-. When A2 is given, the opposite occurs: B1 is not associated with any consequences, but now B2 is followed by reinforcement (i.e., B1 is the S- and B2 is the S+, when A2 is the sample).
According to Carrigan and Sidman (1992), two main factors can modulate the learning of conditional relationships during MTS procedures. The first regards the pairing of stimuli during training, more specifically, whether the sample is related to S+ or S-. The second factor concerns the behavioral component that defines relationship between the sample and the comparison stimuli. A select control occurs when the sample relates to S+ and the established behavioral component is to choose (e.g., if A1, then choose B1). A reject control, in contrast, occurs when the sample relates to S- and the established behavioral component is “do not choose” or “reject” (e.g., if A1, then reject B2).
Perez and Tomanari (2008) have identified three main experimental variations used to study the occurrence of selection and rejection controlling relationships: (i) procedures that add new stimuli to replace one of the comparison stimuli (e.g., Cumming and Berryman 1965; Dixon and Dixon 1978; Goulart et al. 2005; McIlvane et al. 1987; Stromer and Osborne 1982); (ii) procedures that add masks to replace one of the comparison stimuli (e.g., McIlvane et al. 1987; Wilkinson and McIlvane 1997); and (iii) procedures that use different amounts of S+ and S- to establish the two types of stimulus control (e.g., Johnson and Sidman 1993; Magnusson 2002).
The experiment designed by Stromer and Osborne (1982) is an example of the use of new stimuli. Four adolescents with atypical development were taught to conditionally relate visual stimuli of two distinct sets (AB relations). Given A1, the participants should choose the comparison B1 and not choose the comparison B2. Given A2, they should choose the comparison B2 and not the comparison B1. After achieving a learning criterion, participants were exposed to symmetrical trials (BA relations) with the introduction of new stimuli to evaluate the establishment of control by selection and rejection. For example, given B1, the comparisons could be A1 and a new stimulus. Participants choosing A1 would demonstrate that they had established the relationship between the sample and S+ (i.e., select control). On the other hand, given the same sample B1, another comparison could be A2 and a new stimulus. In this second case, participants choosing the new stimulus would demonstrate that they had established the relationship between B1 and S- (i.e., reject control). In the study by Stromer and Osborne, participants exhibited a high percentages of correct responses in both types of trials, demonstrating that the initial procedure had established both selection and rejection controlling relationships.
The study carried out by McIlvane et al. (1987) is an example of the use of masks. Eight adults with typical development were trained on conditional relationships between nonsense words that were composed of three letters. Given the samples “CUG,” “VEK,” and “DAX,” the correct responses were “ZID,” “PAF,” and “BEH,” respectively. Each trial presented a word as the sample (e.g., “CUG”) in the center of a computer screen and another two words as comparison stimuli on the bottom of the screen (e.g., “ZID” and “PAF”). After participants learned the conditional relationships between the words, probe trials with masks were used to test selection and rejection relationships. Each probe trial presented a word as the sample (e.g., “CUG”) and, as comparison stimuli, a word and a mask or “empty” stimulus (e.g., “ZID” and “===” or “===” and “PAF”). In trials in which the S- was replaced by a mask, participants chose the words more frequently than the masks, thus demonstrating the establishment of the relationship between the sample and S+ (i.e., select control). In trials in which the S+ was replaced by a mask, participants more frequently chose the mask instead of the words, thus demonstrating a learned relationship between the sample and S- (i.e., reject control).
Finally, the study by Johnson and Sidman (1993) is an example of the third experimental variation: each sample was related to a single S- stimulus and four S+ stimuli. Thus, given A1, some trials had B1 and B2 as comparison stimuli, other trials had X1 and B2, and so on. Four conditional relationships were therefore led to selection control (i.e., if A1, then choose B1, X1, X2, or X3), and only one conditional relationship led to rejection control (i.e., if A1, do not choose B2). Overall, this procedure more strongly fostered the reject control, since learning to reject B2 was sufficient for obtaining correct responses on all trial types.
Observing the stimuli selected and, more specifically, the duration of eye fixation on each alternative, can be used to assess the establishment of control by selection or rejection (Kato et al. 2008; Wilkinson and McIlvane 1997). Kato et al. (2008) suggested that participants who spend more time observing S+ stimuli have an increased likelihood of establishing selection control, whereas participants who spend more time observing S- stimuli have an increased likelihood of establishing rejection control. Importantly, all of the procedures used to study selection and rejection relationships seem to require some specific patterns of visual contact with the stimuli in each trial and this fact could be related to the controlling relations established. In the procedure by Johnson and Sidman (1993), for example, considering the trials in which A1 was presented, B2 (S-) was presented four times more often than the each S+ (B1, X1, X2 and X3). McIlvane et al. (1987) and Stromer and Osborne (1982) did not manipulate the frequency of each stimuli, but they did manipulate the possibility of visual contact by replacing one of the comparison stimuli with masks or new stimuli.
The link between learning conditional relationships and eye movements was directly assessed in an experiment by Dube et al. (2006). Each trial began with the presentation of a sample composed of four elements in a 2 × 2 matrix. When the participant touched any point in the sample, it disappeared. After 1 s, three comparison stimuli were presented each in one of the four corners of the monitor screen. One comparison stimulus was identical to one of the elements of the sample previously presented, and touching it was defined as the correct response. Eye movements were recorded during the procedure and participants who spent more time observing the samples achieved a higher percentage of correct responses in this task, whereas participants who spent less time observing the samples achieved a lower percentage of correct responses. The participants who demonstrated lower percentages of correct responses were given additional training sessions using the same procedure. Their rate of performance improvement was associated with the extent to which they increased time spent observing the components of the sample stimulus. Dube et al. (2010) replicated this study and showed that for participants who performed poorly, increasing the duration of observation of sample stimuli increased accuracy. Similar results have been described in other experiments that have related one’s time spent observing stimuli and pattern of choosing the stimuli in simple discrimination tasks (Pessôa et al. 2009; Schroeder 1970).
Magnusson (2002) described another experiment that related selection and rejection controls and eye movements. The aim of the study was to compare eye movements during the training of selection and rejection control conditional relationships based on the procedure used by Johnson and Sidman (1993). Eye movements were defined by the frequency with which participants observed one of the comparison stimuli. Duration of observation was not considered. Participants observed S- more frequently than S+ when establishing a rejection control relationship, but observed S+ more frequently than S- when establishing a selection control relationship. Thus, this experiment demonstrated that particular relationship to be learned can modulate the frequency of stimuli observations. The relationship between the duration of eye fixation and the establishment of controls, however, has not yet been systematically investigated.
One important issue about eye movements is that the duration of stimulus observation in MTS tasks can be modulated by different response topographies, as suggested by Kato et al. (2008). According to these authors, the use of a computer mouse to select a comparison stimulus can provide the participant with an occasion to spend more time observing the stimuli, because responding requires looking at the S+ and positioning the cursor on it. The use of a keyboard does not require this added looking time. If this is true, an increase in the time spent observing the S+ stimuli would contribute to establishing selection control relationships among those participants who use a mouse. Another issue to consider regarding the measurement of eye movements is that in general looking frequency and duration will be correlated. However, the method of responding may bias measurements. By using a keyboard, often participants look back and forth between the stimuli and the keyboard many times within the same trial. This results in a greater frequency of short-duration observations of stimuli. Such patterns do not occur when a mouse is used.
Considering that (i) visual contact and time spent observing stimuli in each trial are important aspects involved in the establishment of conditional relationships, and that (ii) the topographies of responses can bias eye movements, the aim of the present study was to investigate the eye movements of participants learning conditional relationships via MTS and to compare those indicating responses with a mouse to those using a keyboard. We evaluated whether eye movements, analyzed in terms of gaze fixation duration, are related to establishing selection and rejection control relationships and whether the use of different response topographies influences the time spent observing the stimuli presented in each trial.