52 Methodological Preliminaries
scope for promising visual comparison experiments. As we will see later, only a selection of those factors can indeed be reviewed: Even when such a limitation is imposed, the possible observations become very numerous and yield rather complex interactions that have to be analysed. But, which tasks and which strategy determining the course of consecutive experiments would be most sensible now? The following section shows the available options and aims to motivate the chosen procedure(s).
3.3 Procedure 53
Figure 3.6: Which of the two line segments is longer?
line segments’ length (comparison stimulus) in order to match the length of the corresponding segment (target stimulus), the following processing steps must be performed for task completion:
(a) Visual exploration and memorisation of the target stimulus.
(b) Shift of attention to the comparison stimulus.
(c) Visual exploration of comparison and matching with memorised length infor-mation of target.
(d) Adjustment of comparison according to memorised length – if necessary.
(e) Shift of attention to target.
(f) Validation of adjustment.
(g) Re-iteration of the previous steps in case validation fails or is unsatisfactory.
These items represent the various steps that determine line segment length compar-ison. They present an “extension” to the cognitive structure and also describe the assessment–memorisation–comparison steps in greater detail – given such a task.
However, it is already clear that such a complex setting and the possibly interacting processes could turn out to be too difficult to understand all at once. Thus, it may be a good approach to first choose a simpler setting in order to observe isolated phenomena which may then be easier to explain. Using the method of constant stimuli it should be possible to eliminate at least one factor – namely the influence oflength adjustment– to achieve this goal. Length adjustment must be considered a dynamic process as the stimulus changes its length (its end points “move”) during the adjustment step(s).
Method of constant stimuli: Rather than dynamically adjusting the line segment length to match the target and comparison stimuli, the method of constant stim-uli that we favour for this investigation requires subjects to make a simple binary decision – for example, which of the two stimuli is the longer one. The sequence of processing steps for task completion is similar to the previous one, but does not include the step of adjustment of line segment length.
54 Methodological Preliminaries
When dynamic stimuli are used, it is usually difficult to attribute particular shifts of attention uniquely to either these adaptation processes – as dynamic processes, i.e. movement or stimulus changes, are well known to be prime attractors for visual attention – or to an influence of specific stimulus attributes. The elimination of the dynamic process of line segment length adjustment should facilitate the monitoring and understanding of comparison processes and the influence of line segments at-tributes thereupon. This static procedure should be particularly beneficial for the interpretation of eye-movement patterns and associated attention processes.
If considered in detail, the proposed discrimination task implicitly suggests a vari-ation of discrimination difficulty. Here, it appears to be particularly sensible to distinguish between an “easy” and a “difficult” discrimination task condition, as these might lead to rather interesting and very different processing strategies. As outlined in Section 2.4, an easy discrimination task could be solved “holistically”
without much focused information acquisition whereas the difficult condition might require an“analytic” processing mode.
But, what could be appropriate definitions for the easy and difficult conditions?
In fact, the experiment described above, which applies the method of adjustment, should help to solve this problem. The results obtained here yield information on how accurately subjects can match the length of two line segments. We can then use this data to infer which differences between line segment lengths are difficult to distinguish – obviously those that lie within this accuracy – and which are easy to distinguish – those that lie considerably outside the accuracy. This distinction thus determines the easy and difficult conditions for an experiment using the method of constant stimuli.
An aspect not explicitly accounted for in the sequence of processing steps so far, but one that was hypothesised to greatly affect the visual exploration, inter-stimulus compari-son and attention processes, is the contribution ofperipheral vision, viewed in the context of stimulus decomposition and fusion. As formulated in the hypotheses (see Section 2.4), it should initially be considered in isolation and, in a next step, must be integrated in the final explanatory model. The next paragraphs address procedural considerations as-sociated with experiments investigating peripheral vision in general and propose specific experimental procedures for the line segment assessment and comparison paradigm.
The investigation of peripheral vision always presents a challenge to experimenters and requires a particularly sophisticated experimental design: It must be ensured that subjects do not foveally look at the stimulus relevant to the investigation, but that the stimulus is visible for the subject in a specificeccentricity region instead. Maintaining such a “seeing without looking” condition is usually not too difficult for short presentation times, for example when tachistoscopic displays are used . Here, cues are presented and foveally viewed by subjects prior to the stimulus in question which subsequently appears in the designated periphery of the visual field for a very short time only. However, with prolonged stimulus presentation times, it is found to be increasingly difficult to prevent subjects from foveally looking at the stimulus. Even with sophisticated pre-cuing and distractor tasks, it
3.3 Procedure 55
cannot always be ensured that subjects look where they are supposed to for the duration of the experiment. Furthermore, it is difficult to reliably state if subjects obey to the rules of “not looking”. Finally, it is at least controversial if the distractor tasks do not influence the performance of peripheral perception and consequently bias the experimental findings.
Rather than indirectly generating peripheral viewing conditions accompanied by the above-mentioned uncertainties regarding validity, an alternative approach seems feasible:
The monitoring of eye movements to ensure that peripherally presented stimuli are indeed viewed peripherally. Specifically, two options are available with such a method:
(a) Offline: Eye movements are monitored “in the background”, i.e. they are recorded during the experiment and analysed offline after task completion to select “valid”
(see below) trials.
(b) Online: The EyeLink eye tracker makes online tracking of eye movements feasible.
As already described in Section 1.4 and, in more detail, in Section 3.1, this feature allows for the almost instantaneous analysis of the eye-gaze data just measured during the experiment.
In both cases, trials are rendered valid if, for the entire display time, subjects view the stimulus peripherally only and make no fixations outside a small, pre-defined region around a designated fixation point. The “active” monitoring of eye movements online, however, has clear advantages over the “passive” offline method: Online eye-movement monitoring may provide feedback as to whether a trial was valid or notduring the experi-ment. In contrast, feedback is only available after the experiment when eye movements are monitored offline. Online feedback must certainly be preferred as it usually encour-ages subjects to produce more valid trials. This generates more valid data per subject for analysis. Furthermore, the procedure can be interrupted as soon as fixations are made outside a pre-defined region. In addition, invalid trials can be repeated to obtain an equal number of valid trials from all subjects in order to minimise bias induced by individual subjects.
Taking these preliminary considerations into account, it is evident that the choice of an active, online eye-movement monitoring method is most favourable for validity tests in experiments that investigate peripheral vision. For the research projected here it appears to be most promising to investigate the assessment oflengths and orientations of line segments presented in different eccentricity regions. We expect eccentricity to have a considerable influence on the processes involved in the perception and assessment of a line segment itself as well as on the processes that guide comparison in the above-mentioned line segment adjustment and discrimination tasks.
For the investigation of eccentricity effects, we shall use a variant of the method of adjustment. Rather than simultaneous comparison, we will use sequential comparison so that (roughly) the following sequence of procedural steps emerges:
56 Methodological Preliminaries
Figure 3.7: Experimental setting for the investigation of eccentricity effects on position and line segment perception. The central dot serves as a fixation marker the subjects have to observe while trying to assess either length or orientation of the peripherally presented line segment or the position of the peripherally presented cross. The possible eccentricity regions I–IV are marked and shaded in the figure for clarity reasons only.
(a) Presentation of a line segment (target stimulus) at an eccentric position relative to a central fixation marker (see Figure 3.7) – restricted, gaze contingent viewing condition.
(b) Blanking of the display.
(c) Presentation of a line segment (comparison stimulus).
(d) Adjustment of comparison length or orientation, respectively, to match correspond-ing dimension of previously viewed target – unrestricted free gaze condition.
With respect to the effects of eccentricity on line segment perception relevant here, sequential comparison is, in fact, the best option for obtaining the desired data. By pre-senting only one stimulus at a time, subjects have a single task to accomplish with every procedural step. This is in particular important during the gaze contingent viewing of the target where interference from concurrent stimuli or parallel tasks is not intended (see above). Rather than trying to assess the designated dimension of a peripherally visible target and simultaneously adjust the comparison line, subjects “only” have to accomplish the assessment part – which alone is difficult enough. No task interference from the si-multaneous adjustment will bias or even dominate the perception processes. On the other hand, it can be argued that the time between the end of the presentation of the target
3.3 Procedure 57
and beginning of the presentation of the comparison has a negative effect on reproduction accuracy (“memory effect”). However, we argue that this possible objection is more than compensated for by the procedural advantages in the previously accounted respects.
As line segment length assessment can be thought of as a process that involves stim-ulus decomposition and the calculation of the distance between the line segment’s two end points (data fusion) these end points evidently play an important role. Particularly important for accurate length assessments would thus be the accurate assessment of end point positions. In analogy, the same is true for line segment orientation assessment when this is viewed as a process that is guided by the calculation of the relative positions of the two end points to each other. As a consequence, it appears sensible to conduct a further experiment in order to explore how accurately position (or location) can be assessed.
For such an experiment, the design of the previous setting would have to be altered only in one minor point: A position marker rather than a line segment is displayed within a certain eccentricity region of the fixation point (see Figure 3.7). Furthermore, the ad-justment procedure (see item (d) above in the sequence of procedural steps) now requires the positional reproduction of the target marker.
As the performance in position estimation, i.e. the levels of accuracy achieved, might vary with the position of the target relative to the fixation point (“meridial position”), either horizontal or vertical, an investigation of this factor is also advisable (cf. Gre-gory, 1970; Schneider, 1978; Prinzmetal & Gettleman, 1993; Armstrong & Marks, 1997).
The thin diagonal lines in Figure 3.7 indicate these distinct areas. As results from posi-tion assessment might serve as the basis to understand more about the processes involved in line segment length and orientation assessment – not only in peripheral vision – this (position assessment) experiment should be conducted first.
Thus, in order to obtain valuable data for analysis that yield useful results regard-ing the visual perception of line segments and the assessment of selected attributes, the following sequence of experiments is proposed:
(1) Sequential comparison –
Eccentricity effects in position and line segment perception
• Experiment E0: The basis – Position assessment
• Experiment E1: Length assessment
• Experiment E2: Orientation assessment (2) Simultaneous comparison –
Similarity effects in line segment perception
• Experiment S1: The basis – Dynamic adjustment in length matching
• Experiment S2: Holistic vs. analytic processing – Binary judgements in length discrimination
So far, we have established the methodological preliminaries for the upcoming experi-ments with respect to technological aspects, stimuli and procedural strategy. In the course
58 Methodological Preliminaries
of this, we incidentally introduced the relevant factors the investigation will focus on and their effects on specific variables as well. The next section will now more formally define and specify these parameters, i.e. independent and dependent variables.