Learning about time: plastic changes and interindividual brain differences.

Learning the timing of rapidly changing sensory events is crucial to construct a reliable representation of the environment and to efficiently control behavior. The neurophysiological mechanisms underlying the learning of time are unknown. We used functional and structural magnetic resonance imaging to investigate neurophysiological changes and individual brain differences underlying the learning of time in the millisecond range. We found that the representation of a trained visual temporal interval was associated with functional and structural changes in a sensory-motor network including occipital, parietal, and insular cortices, plus the cerebellum. We show that both types of neurophysiological changes correlated with changes of performance accuracy and that activity and gray-matter volume of sensorimotor cortices predicted individual learning abilities. These findings represent neurophysiological evidence of functional and structural plasticity associated with the learning of time in humans and highlight the role of sensory-motor circuits in the perceptual representation of time in the millisecond range.

No inherent left and right side in human 'mental number line': evidence from right brain damage.

Spatial reasoning has a relevant role in mathematics and helps daily computational activities. It is widely assumed that in cultures with left-to-right reading, numbers are organized along the mental equivalent of a ruler, the mental number line, with small magnitudes located to the left of larger ones. Patients with right brain damage can disregard smaller numbers while mentally setting the midpoint of number intervals. This has been interpreted as a sign of spatial neglect for numbers on the left side of the mental number line and taken as a strong argument for the intrinsic left-to-right organization of the mental number line. Here, we put forward the understanding of this cognitive disability by discovering that patients with right brain damage disregard smaller numbers both when these are mapped on the left side of the mental number line and on the right side of an imagined clock face. This shows that the right hemisphere supports the representation of small numerical magnitudes independently from their mapping on the left or the right side of a spatial-mental layout. In addition, the study of the anatomical correlates through voxel-based lesion-symptom mapping and the mapping of lesion peaks on the diffusion tensor imaging-based reconstruction of white matter pathways showed that the rightward bias in the imagined clock-face was correlated with lesions of high-level middle temporal visual areas that code stimuli in object-centred spatial coordinates, i.e. stimuli that, like a clock face, have an inherent left and right side. In contrast, bias towards higher numbers on the mental number line was linked to white matter damage in the frontal component of the parietal-frontal number network. These anatomical findings show that the human brain does not represent the mental number line as an object with an inherent left and right side. We conclude that the bias towards higher numbers in the mental bisection of number intervals does not depend on left side spatial, imagery or object-centred neglect and that it rather depends on disruption of an abstract non-spatial representation of small numerical magnitudes.

Physiological correlates of subjective time: evidence for the temporal accumulator hypothesis.

Clock-counter models, the most influential cognitive models of temporal computation, have been successful in explaining a large set of behavioral data. However, it remains unclear whether the component operations postulated in these models correspond to any specific biological mechanism. Using stimuli in different sensory modalities and manipulating physical properties known to bias the 'subjective' perception of time (speed for vision and pitch for audition), the present study aimed to highlight brain areas where activity correlates with the 'subjective' perception of time: a time accumulator according to clock-counter models. Using functional MRI we found that during the encoding of a temporal interval in the millisecond range (600 and 1000 ms), the hemodynamic response of a few brain regions correlated with the interval reproduction performance. For the visual modality, the activity of the putamen, the mid-insula and the mid-temporal cortex reflected the subjective interval duration, which was biased according to the different speeds of the visual stimuli. This effect was found only when subjects encoded the stimulus duration and was specific for the visual modality, where a significant overestimation of time with increasing speed was observed. These results demonstrate a definite relation between 'subjective time' and brain activity, supporting the hypothesis of a physiological correlate of time 'accumulation'.

Auditory temporal expectations modulate activity in visual cortex.

Temporal expectation is the ability to make predictions and to use temporal information to anticipate the occurrence of future events. This capacity is associated with highly efficient perceptual and motor behaviors. However, how cognitive systems use temporal information to optimize behavior and what brain structures are engaged during these processes remains largely unknown. Neurophysiological and recent neuroimaging data have suggested that temporal expectations modulate activity not only in parietal and motor-related frontal regions, but also in occipital visual cortex, when the expected stimulus is a simple visual object. Here we investigate crossmodal properties and category selectivity of temporal expectations examining activity in visual cortex during expectation of auditory stimuli (the sound of hand-clapping or of a hammer-hammering). We found that activity in occipital cortex changed over time, reflecting the subject's temporal expectations about the upcoming auditory event. This modulatory effect included extrastriate visual areas known to process body-parts and tools, despite these were never presented visually during the experiment. However activity in these areas was not specific for the expected sound category, but it was rather related to the overall probability of the auditory target to occur. We conclude that crossmodal associations can influence activity in sensory-specific visual areas in an anticipatory manner, consistent with temporal expectations affecting activity in a distributed system of motor-related and sensory-related brain regions.

Sensory and association cortex in time perception.

The recent upsurge of interest in brain mechanisms of time perception is beginning to converge on some new starting points for investigating this long under studied aspect of our experience. In four experiments, we asked whether disruption of normal activity in human MT/V5 would interfere with temporal discrimination. Although clearly associated with both spatial and motion processing, MT/V5 has not yet been implicated in temporal processes. Following predictions from brain imaging studies that have shown the parietal cortex to be important in human time perception, we also asked whether disruption of either the left or right parietal cortex would interfere with time perception preferentially in the auditory or visual domain. The results show that the right posterior parietal cortex is important for timing of auditory and visual stimuli and that MT/V5 is necessary for timing only of visual events.