Barbara Magnani

Prismatic Lenses Shift Time Perception

Previous studies have demonstrated the involvement of spatial codes in the representation of time and numbers. We took advantage of a well-known spatial modulation (prismatic adaptation) to test the hypothesis that the representation of time is spatially oriented from left to right, with smaller time intervals being represented to the left of larger time intervals. Healthy subjects performed a time-reproduction task and a time-bisection task, before and after leftward and rightward prismatic adaptation. Results showed that prismatic adaptation inducing a rightward orientation of spatial attention produced an overestimation of time intervals, whereas prismatic adaptation inducing a leftward shift of spatial attention produced an underestimation of time intervals. These findings not only confirm that temporal intervals are represented as horizontally arranged in space, but also reveal that spatial modulation of time processing most likely occurs via cuing of spatial attention, and that spatial attention can influence the spatial coding of quantity in different dimensions.

Time and spatial attention: Effects of prism adaptation on temporal deficits in brain damaged patients

Growing evidence indicates that the representations of space and time interact in the brain but the exact neural correlates of such interaction remain unknown. Neuroimaging and neuropsychological studies show that processing of temporal information engages a distributed network in the right hemisphere and suggest a link between deficits in spatial attention and deficits in time perception. In the present study we used the procedure of prismatic adaptation (PA) to directionally manipulate spatial attention in order to explore the effect of attentional deviation on time perception in patients with right (RBD) vs. left (LBD) brain damage. In a first experiment, two groups of RBD and LBD patients and two groups of age-matched healthy subjects were submitted to a time reproduction task before and after rightward or leftward PA (between-group design). In a second experiment RBD and LBD patients were submitted to the same task, before and after rightward and leftward PA (within-group design). RBD but not LBD patients presented a time deficit with a clear tendency to underestimate the real time. PA inducing leftward attentional deviation biased time perception toward an underestimation in RBD patients and controls, while it was ineffective in LBD patients. PA inducing a rightward attentional deviation failed to affect time perception in either group. These results underline the effects of PA on temporal deficits in brain damaged patients. The novel finding is that, while a right hemispheric network is critical for explicit timing, a left hemispheric network is necessary for mediating the effects of prismatic adaptation on spatial and temporal perception.

Prismatic adaptation effects on spatial representation of time in neglect patients.

Processing of temporal information may require the use of spatial attention to represent time along a mental time line. We used prismatic adaptation (PA) to explore the contribution of spatial attention to the spatial representation of time in right brain damaged patients with and without neglect of left space and in age-matched healthy controls. Right brain damaged patients presented time underestimation deficits, that were significantly greater in patients with neglect than in patients without neglect. PA inducing leftward attentional deviation reduced time underestimation deficit in patients with neglect. The results support the hypothesis that a right hemispheric network has a role, per se, in time perception. Moreover, they suggest that right hemisphere is important in time perception for its control of spatial attention, engaged in spatial representation of time. Procedures that ameliorate left spatial deficits could also be useful for modulating temporal deficits in right brain damaged patients with neglect.

The role of posterior parietal cortex in spatial representation of time: A TMS study

The existence of a spatial representation of time, where temporal intervals are represented on a mental temporal line (MTL), oriented in ascending order from left to right, was demonstrated manipulating spatial attention by means of Prismatic Adaptation (PA). In young healthy subjects, prisms adaptation inducing a rightward shift of spatial-attention produced an overestimation of time intervals, whereas prisms adaptation inducing a leftward shift of spatial-attention produced an underestimation of time intervals [4]. The aim of the present study was to investigate the neural basis mediating the effects of PA on spatial time representation. Posterior-Parietal-Cortex (PPC) is the best candidate to discharge this function. Indeed, neuropsychological and neurophysiological studies designate right-PPC as the site of space-time interaction [1,3,7]. Concerning the neural bases of PA procedures, left and right-PPC are involved in different phases of PA procedure [2,5, 6]. Here we investigated, by using TMS, the role of the Posterior-Parietal-Cortex (PPC) in spatial representation of time and in cerebral plasticity phenomena mediating prismatic adaptation effects on time processing. To this aim, healthy subjects were submitted to a tem-

Prismatic Adaptation as a Novel Tool to Directionally Modulate Motor Cortex Excitability: Evidence From Paired-pulse TMS

BACKGROUND The prismatic adaptation (PA) is a visuo-motor procedure that has captured the attention of neuroscientists in the last decades, hence it seems to affect high-order cognition. However, the basic neural processes related to PA and its effects on cortical plasticity are not clear yet. OBJECTIVE/HYPOTHESIS The aim of the present study is to explore whether PA induces a direct effect on the motor cortices (M1) excitability. METHODS Fourteen healthy participants were submitted to paired-pulse TMS to measure short-intracortical-inhibition (SICI) and intracortical-facilitation (ICF) on both the left and the right M1, before and after PA, that could induce a leftward or rightward after-effect. RESULTS An increase of intracortical-facilitation was found in the M1 contralateral to the after-effect direction. Moreover the extent of facilitation and of the after-effect were correlated to each others. CONCLUSION This finding reveals that PA influences M1 cortices directly, raising their excitability. The present investigation represents an innovative step for the understanding of neurophysiological processes by which PA affects brain functions.

The role of posterior parietal cortices on prismatic adaptation effects on the representation of time intervals

Previous studies provided evidence of an ascending left-to-right spatial representation of time durations by using a technique affecting high levels of spatial cognition, i.e. prismatic adaptation (PA). Indeed, PA that induced a leftward aftereffect distorted time representation toward an underestimation, while PA that induced a rightward aftereffect distorted time representation toward an overestimation. The present study advances previous findings on the effects of PA on time by investigating the neural basis subtending these effects. We focused on the posterior parietal cortex (PPC) since it is involved in the PA procedure and also in the formulation of the spatial representation of time. We conducted two experiments where right-handed healthy adults were submitted to a time task, before and after PA, that could induce a leftward or rightward aftereffect. Repetitive TMS (rTMS) was used to inhibit the left or right PPC before PA administration. In a first experiment the time task consisted of reproducing an half duration (time bisection task) by pressing a key and the participants responded and adapted to prisms with their right hand. In a second experiment the time task consisted of reproducing a whole duration (time reproduction task) by pressing a key and the participants responded and adapted to prisms with their left hand. We found an abolition of the effects of PA on time when rTMS was delivered on the left and not on the right PPC, regardless of the task and moreover, when the participants responded and adapted with the right hand and also with the left hand. This result suggests a direct involvement of the left PPC in the interactive process, between spatial modulations induced by PA and the spatial representation of time, that does not depend on motor processes. This study provides useful results for future investigations on the neural mechanisms subtending the effects of PA on spatial representations.

Left insular cortex and left SFG underlie prismatic adaptation effects on time perception: Evidence from fMRI

Prismatic adaptation (PA) has been shown to affect left-to-right spatial representations of temporal durations. A leftward aftereffect usually distorts time representation toward an underestimation, while rightward aftereffect usually results in an overestimation of temporal durations. Here, we used functional magnetic resonance imaging (fMRI) to study the neural mechanisms that underlie PA effects on time perception. Additionally, we investigated whether the effect of PA on time is transient or stable and, in the case of stability, which cortical areas are responsible of its maintenance. Functional brain images were acquired while participants (n=17) performed a time reproduction task and a control-task before, immediately after and 30 min after PA inducing a leftward aftereffect, administered outside the scanner. The leftward aftereffect induced an underestimation of time intervals that lasted for at least 30 min. The left anterior insula and the left superior frontal gyrus showed increased functional activation immediately after versus before PA in the time versus the control-task, suggesting these brain areas to be involved in the executive spatial manipulation of the representation of time. The left middle frontal gyrus showed an increase of activation after 30 min with respect to before PA. This suggests that this brain region may play a key role in the maintenance of the PA effect over time.

Exploring the reciprocal modulation of time and space in dancers and non-dancers

We explored whether time and space representations modulate each other in subjects that are trained to integrate time and space dimensions, i.e., professional dancers. A group of dancers, and one of non-dancers, underwent two different tasks employing identical stimuli. A first static central line could last one of three possible durations and could have one of three possible lengths. A second growing line appeared from the left or right of the screen and grew up toward the opposite direction at constant velocity. In the Spatial task, subjects encoded the length of the static line and stopped the growing line when it had reached half the length of the static one, regardless of time travel. In the Temporal task, subjects encoded the duration of the static line and stopped the growing line when it had lasted half the duration of the static one, regardless of space traveled. Dancers, differently from non-dancers, anticipated time in the Temporal task. However, both dancers and non-dancers were biased by the stimulus length when performing the Temporal task, while they were not biased by the stimulus duration when performing the Spatial task. Concluding, this study underlines the plasticity of time dimension that can be influenced by spatial information and by sensorimotor training for the synchronization in space and time.