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Neuronal oscillations of different frequencies are hypothesized to be basic for temporal perception; this theoretical concept provides the frame to discuss two temporal mechanisms that are thought to be essential for cognitive processing. One such mechanism operates with periods of oscillations in the range of some tens of milliseconds, and is used for complexity reduction of temporally and spatially distributed neuronal activities. Experimental evidence comes from studies on temporal-order threshold, choice reaction time, single-cell activities, evoked responses in neuronal populations or latency distributions of oculomotor responses. The other mechanism refers to pre-semantic integration in the temporal range of approximately 2-3 s. Experimental evidence comes from studies on temporal reproduction, sensorimotor synchronization, intentional movements, speech segmentation, the shift rate of ambiguous stimuli in the visual or auditory modality or the temporal modulation of the mismatch negativity. These different observations indicate the existence of a universal process of temporal integration underlying the mental machinery. This process is believed to be basic for maintenance and change of perceptual identity. Owing to the omnipresence of this kind of temporal segmentation, it is suggested to use this process for a pragmatic definition of the states of being conscious or the 'subjective presence'.

Memory is an essential element to adaptive behavior since it allows consolidation of past experience guiding the subject to consider them in future experiences. Among the endogenous molecules that participate in the consolidation of memory, including the drug-seeking reward, considered as a form of learning, is dopamine. This neurotransmitter modulates the activity of specific brain nucleus such as nuclei accumbens, putamen, ventral tegmental area (VTA), among others and synchronizes the activity of these nuclei to establish the neurobiological mechanism to set the hedonic element of learning. We review the experimental evidence that highlights the activity of different brain nuclei modulating the mechanisms whereby dopamine biases memory towards events that are of motivational significance.

The concept of a temporal integration process in the timing mechanisms in the brain, postulated on the basis of experimental observations from various paradigms (for a review see P$oUppel, 1978), has been explored in a sensorimotor synchronization task. Subjects synchronized their finger taps to sequences of auditory stimuli with interstimulus-onset intervals (ISIs) between 300 and 4800 msec in different trials. Each tonal sequence consisted of 110 stimuli; the tones had a frequency of 500 Hz and a duration of 100 msec. As observed previously, response onsets preceded onsets of the stimuli by some tens of milliseconcls for ISIs in the range from about 600 to 1800 msec. For ISIs longer than or equal to 2400 msec, the ability to time the response sequence in such a way that the response 5 were placed right ahead of the stimuli started to break clown, i.e., the task was fulfilled by reactions to the stimuli rather than by advanced responses. This observation can he understood within the general framework of a temporal integration puce 55 that is supposed to have a maximal capacity (integration interval) of approximately 3 sec. Only if successive stimuli fall within one integration period, can motor programs be initiated properly by a prior stimulus and thus lead to an appropriate synchronization between the stimulus sequence and corresponding motor acts.