Summary And Conclusions

This chapter presents an extensive, although nonexhaustive, survey of the effects of auditory and visual signal modality on interval timing. It is clear from the literature described here that the modality of a timing signal can have a significant influence on the discriminability of a timing signal, its subjective duration, or both. It is also clear that visual-auditory modality effects on timing take at least two forms.

In some cases, the magnitude of the modality effect is an absolute constant amount in that the duration of the timing signal does not influence the size of the modality effect. Results of this form are consistent with modality-dependent latency differences in initiating and terminating timing. Within the scalar timing theory, this type of effect is localized to differences in switch closure and opening efficacy. Importantly, such effects can manifest themselves either as differences in the subjective duration of equivalent duration auditory and visual signals or as differences in duration discriminability (cf. Grondin, 2001).

In other cases, the magnitude of the modality effect is a constant proportion of the duration of the signal being timed. Results of this form are consistent with differences in clock speed. Interestingly, work in a number of labs (e.g., Droit-Volet, this volume; Penney et al., 2000; Wearden et al., 1998) has shown that although switch and clock speed modality effects can manifest themselves in the same experiment, they are separable effects. As described above, there are at least two plausible accounts of the source of clock speed differences within the scalar timing theory framework. One possibility is that the pacemaker runs at a faster rate for auditory than for visual signals (e.g., Wearden et al., 1998). The alternative, and the position promoted here, is that auditory and visual signals are differentially efficient at maintaining the switch in a closed state. The probability that the mode switch will oscillate between a closed and an open state is greater for visual than for auditory signals, with the result that more pacemaker counts are lost when the signal is visual than when it is auditory. As is the case for timing latency effects, clock speed effects can manifest themselves either as differences in the subjective duration of equivalent duration auditory and visual signals or as differences in duration discriminability.

In some cases, whether a modality effect is obtained also depends on the specific nature of the timing task used. For example, if the task requires participants to compare auditory and visual signals to a common memory representation, then auditory signals will seem subjectively longer than equivalent duration visual signals. In the case of tasks that measure duration discrimination acuity, then either timing onset-offset latency or clock speed effects can result in better discrimination for auditory than for visual signals, regardless of whether the auditory and visual signals are directly or indirectly compared to one another (cf. Grondin, 2001; Raamsayer and Lima, 1991).

As noted in the introduction, the influence of signal characteristics on timing is intimately related to the cognitive and physiological nature of the timing process itself. A strong claim has been made here that auditory and visual signals are not equally efficient in either initiating switch closure or maintaining the switch in a closed state. The obvious next question is whether a single pacemaker-switch-accumulator system is activated by both auditory and visual signals or whether there are modality-specific clocks. Following Rousseau and Rousseau (1996), we propose that distinct switch-accumulator modules process auditory and visual timing signals, although these modules receive input from a common amodal pacemaker.

Of course, if modality-specific switch-accumulator modules are assumed, then it should be possible to isolate these timing modules in the brain. The evidence from the study with persons at high risk for developing schizophrenia is valuable in this regard because it indicates that it is possible for modality differences to be exacerbated in some populations. Significantly, the exaggerated modality effect appeared to be mediated by the visual modality because differences in auditory response functions between the HrSz and NC groups failed to obtain. This outcome provides additional support for the idea that there are multiple modality-specific switch-accumulator modules and that these modules can be independently modulated.

In conclusion, modality effects raise a number of fundamental questions about the nature of interval timing in their own right and also provide a window for examining basic aspects of the interval timing system. Although modality effects on timing have been investigated since the beginning days of experimental psychology, much remains to be learned.

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