The types of studies that will be reported in the following chapters involve two main technical requirements in relation to timing accuracy. Firstly, visual stimuli need to be presented at specific times for precise durations. Secondly, motor responses need to be recorded with high temporal accuracy. To meet these requirements in a practical and cost-effective fashion, a combination of a standard Pentium grade PC running MS Windows operating system, a cathode ray tube monitor, a mouse response device, and the Inquisit psychological testing software package (Draine, 2003) were selected.
A difficulty in choosing these components is that psychological experiments have conflicting requirements. It is normally suitable to use a multi-tasking operating system to simplify the development of experimental tasks and stimuli. However, multi- tasking operating systems tend to share their processing power between a number of computing tasks that are not all related to the experimental parameters being measured. This has the unwanted consequence of limiting the timing accuracy of the system.
The use of off-the-shelf cathode ray tube monitors is also suitable since these monitors are widely available and cheap. Unfortunately, the display on these monitors is
typically refreshed only every 10 to 16 ms. Thus, if presentation of the stimulus is not synchronised with the refresh rate of the monitor, a stimulus that should be presented for 50 ms might be presented for a period varying between 34 and 66 ms. It is therefore particularly important to choose a software package that links the presentation of stimuli on screen to the start of the monitor refresh cycle. It is also important for the testing software to use every possible avenue to limit the sharing of processing resources with unnecessary processes during the timing of experimental trials.
Finally, there are also advantages in using standard input devices (mouse, keyboard). This facilitates the transfer from one computer to another or the ability to test in different and/or multiple locations. However, the way in which information is transferred between these input devices and the computer is not well known by most users and can dramatically influence the accuracy of the measures. Data transmitted by certain input devices may only be “read” by the computer relatively infrequently, and the input device itself may buffer inputs for significant periods of time. These technical variables may induce measurement errors amounting to tens of milliseconds. Furthermore, these errors may be random or follow a pattern and therefore they may not only overestimate measurements but also add significant noise to them which may hide subtle experimental effects which could otherwise have been detected.
To address these issues, a systematic audit of the selected hardware and software has been conducted and is presented in detail in Appendix A. The main conlusions were that the hardware and software selected were near millisecond accurate but that response times tended to be systematically overestimated and that the mouse hardware added some variability to the response time measure. This was not deemed an important factor as all experimental conditions would be affected in a similar fashion.
2.5 Summary
This chapter was concerned with methodological issues and specifically interested in determining whether the Poffenberger task was an adequate paradigm to assess interhemispheric transfer time; whether a four-stimulus name letter-matching task was a suitable task to assess hemispheric interaction; and to determine whether the timing accuracy of the software and hardware planned for the present research were sufficiently accurate.
The Poffenberger task, although not perfect, was found to be suitable and to have a number of advantages over other techniques. Of particular importance was the fact that the Poffenberger paradigm assesses interhemispheric transfer time in the context of real behaviour and therefore is more likely to reflect subtle individual differences over a wide range of neural processes; processes that may be differentially affected by variables which are of particular interest to this research such as sex, age, handedness, and functional lateralisation.
The letter-matching task was also found to be highly suited to assessing hemispheric interaction and to reveal differences between experimental groups of interest. It was particularly encouraging to find that two methodological experiments which were not specifically designed to study variations in individual variables, uncovered such differences. Furthermore, this task was found to produce important variations in the level of hemispheric interactions between individuals which makes its data ideal for use in regression analyses involving independent variables whose influence might be very subtle.`
Finally, the audit of the experimental apparatus revealed that it was extremely accurate and well suited to the type of tasks and experiments that will be conducted.
Based on the above findings, the following chapter will investigate whether a clear link can be established between the efficiency of hemispheric interaction and interhemispheric transfer time and if present, how this relationship is modulated by other variables such as sex, age, and handedness.