Consensual Issues And Major Advances

First of all, the book masterfully demonstrates how the scalar timing model, or scalar expectancy theory (SET) (Gibbon et al., 1984), fuels many distinct fields of interval timing research. Initially grounded on animal studies using conditioning schedules for timing (see Church, this volume) and counting (see Brannon and Roitman, this volume), SET has inspired ecological foraging theories and the analysis of other natural behaviors (see Bateson, this volume; Hills, this volume; MacDonald and Meck, this volume), has invaded the ontogenetic and aging fields (see Droit-Volet, this volume; Lustig, this volume), and provides a foundation to the attentional models developed in the frame of human timing (see Fortin, this volume; Pang and McAuley, this volume). As any vivid and comprehensive model, it also elicits alternative and contradictory views (see Crystal, this volume; Hopson, this volume; Malapani and Rakitin, this volume; Matell et al., this volume; Meck, this volume), reminding us of the fragility of consensus, which could someday dissolve. Will appealing new models freed from the pacemaker-accumulator postulate become prominent in the next few years? Future research will decide. Major advances in the field of neural bases will certainly be crucial in selecting the most plausible formal model(s).

Another issue that has received increasing consensus since the 1980s concerns the role of attention in timing performance. It is extensively documented in the present volume (see Buhusi, this volume; Fortin, this volume; Lewis and Miall, this volume; Lustig, this volume; Pang and McAuley, this volume; Penney, this volume) and inspires interpretations even in unexpected experimental contexts (e.g., genetic manipulations — see Cevik, this volume). An impressive set of data has been obtained in dual-task paradigms, showing that compared to full attention conditions, divided attention produces a systematic bias to shorten time estimation, as if the quantity of pulses stored by the accumulator (within the SET framework) were reduced (for neuronal correlates of divided attention in the frontal motor cortex, see Pang and MacAuley, this volume). This bias appears in both humans (for a review, see Brown, 1997) and animals (Lejeune et al., 1999), and cross-species similarities are also evident when gaps or breaks occur during the target interval (see Buhusi, this volume; Fortin, this volume). Dual-task and break studies bear clear analogy: during the target interval, an attention shift toward a concurrent task seems to produce an interruption in the pulse accumulation process (Macar et al., 1994; Rousseau et al., 1984). Similar effects are observed whether an interfering event takes place or is simply expected (Casini and Macar, 1997; see Fortin, this volume). Such data have important implications for modeling the functioning of the accumulator. They indicate that the concept of a flickering switch (Lejeune, 1998; see Penney, this volume; cf. switch modes — see Buhusi, this volume) placed under attentional control during the entire target interval has much greater plausibility than the idea of a switch that merely delimits this interval (Zakay and Block, 1995). Note that interpreting the attentional bias within the framework of accumulator models does not imply that accumulator-free concepts may not account for it. However, in most accumulator-free models, attentional bias has not been considered explicitly, as it should be in view of its high consistency and theoretical importance.

Remarkable advances have also been made concerning the neural bases of timing, due to research on brain lesions and pharmacological manipulations and, obviously, to the development of brain imaging methods (see Hinton, this volume). Two cerebral networks are now identified as having plausible timing functions: the striato-thalamo-frontal pathways involved in dopaminergic systems (see Matell et al., this volume; Malapani and Rakitin, this volume; Meck, 1996, this volume), and the cerebello-thalamo-frontal pathways (see Diedrichsen et al., this volume; Meck, this volume). Strong debates exist between tenants of each system. In this line, the current literature contains incompatible assumptions, such as the possibility that the two networks subserve different components of the timer (Gibbon et al., 1997), or that all components involve tight interactions within single striatal neurons (see Matell et al., this volume).

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