2. General intro and overview
The study of the neural basis of temporal processing is in its infancy
However, it is arguably in the auditory domain that timing is most prominent, owing to its importance in vocalization and speech recognition.
Our brains measure time continuously. We are aware of how long we have been doing a particular thing, how long it has been since we last slept, and how long it will be until lunch or dinner. We are ready, at any moment, to make complex movements requiring muscle coordination with microsecond accuracy, or to decode temporally complex auditory signals in the form of speech or music. Our timing abilities are impressive, diverse and worthy of investigation. But they are not very well understood.
Many models of time perception have been put forward (for example, see [1–3]), collectively postulating a wide variety of different mechanisms. Regardless of their diversity, the models all agree that temporal information is processed in many ways: it is remembered, compared to othertemporal information, combined with sensory information, and used in the production of motor outputs.
The holy grail of timing research is to understand the ‘time-dependent process’: a mechanism equivalent to a piezoelectric crystal in a man- made clock or the movement of a shadow on a sundial. This has proven an elusive goal, to the extent that ideas about how this mechanism might work remain near the level of conjecture. Researchers have had great difficulty in pinning timing- related activity in the brain to any specific type of function. This is largely because mosttime measurement tasks draw upon more than one process, making it difficult to tease the various components apart.
In comparison with spatial stimuli, there is a significant gap in our understanding of how the brain discriminates simple temporal stimuli, such as estimating the duration of time for which a light or tone is presented. Recent studies have begun to examine the neural (Kilgard and Merzenich, 2002; Hahnloser et al., 2002; Leon and Shadlen, 2003) and anatomical (Rao et al., 2001; Lewis and Miall, 2003; Coull et al., 2004) correlates of temporal processing. However, the neural mechanisms that allow neural circuits to tell time and encode temporal information are not clear. Indeed, it has not yet been determined if timing across different time scales and modalities relies on centralized or locally independent timing circuits and mechanisms (Ivry and Spencer, 2004).
Timing is critical in both the discrimination of sensory stimuli (Shannon et al., 1995; Buonomano and Karmarkar, 2002; Ivry and Spencer, 2004; Buhusi and Meck, 2005) and the generation of coordinated motor responses (Mauk and Ruiz, 1992; Ivry, 1996; Meegan et al., 2000; Medina et al., 2005). The nervous system processes temporal information over a wide range, from microsecondsto circadian rhythms (Carr, 1993; Mauk and Buonomano, 2004; Buhusi and Meck, 2005).
Time and space are the fundamental dimensions of our existence. Although space is gradually losing its value in a world of computer networks, cellular phones and virtual libraries, time is becoming the essence of our times, as is reflected by ever increasing speed, rate of return and productivity — concepts that are intrinsically related to time. Time is also crucial for everyday activities, from our sleep–wake cycle to walking, speaking, playing and appreciating music, and playing sports. We can engage in these activities because, like most animals, we process and use temporal information across a wide range of intervals (FIG. 1) — in contrast to, for example, the limited range of the light spectrum that we can see.
Being able to tell the time is also advantageous for gathering spatial information. Just as a position in space can be triangulated by using distance to landmarks, the GLOBAL POSITIONING SYSTEM (GPS) provides current position by triangulating temporal information (the difference or coincidence in phase of signals) from satellites. COINCIDENCE DETECTION is also used by bats, owls and frogs to form an accurate, topographic representation of space from INTERAURAL TIME DIFFERENCES1. For these species, telling space is telling time. Timing and time perception are fundamental to survival and goal reaching in humans and other animals..
Many actions manifest precise timing. The musicians in an orchestra time their movements to the gestures of the conductor. The drag racer uses the countdown lights to anticipate the start of a race. A pitcher must temporally coordinate muscular activity across different joints to ensure that the ball is delivered to a targeted region of the strike zone. One basic question in motor control concerns how the timing of those different types of actions is controlled and whether a common process is invoked across different task domains so that such temporal precision can be achieved (see Keele & Ivry, 1987).
Time perception is an ability that is taken for granted, yet relatively little understood. Without it, other cognitive functions, especially motor actions and visual awareness, would be severely impaired. Basic tasks such as crossing the road would be near impossible.