دانلود رایگان مقاله فعال سازی هیپوکامپ مربوط به پردازش "زمان واقعی"

Abstract

The delay associated with cerebral processing time implies a lack of real-time representation of changes in the observed environment. To bridge this gap for motor actions in a dynamical environment, the brain uses predictions of the most plausible future reality based on previously provided information. To optimise these predictions, adjustments to actual experiences are necessary. This requires a perceptual memory buffer. In our study we gained more insight how the brain treats (real-time) information by comparing cerebral activations related to judging past-, present- and future locations of a moving ball, respectively. Eighteen healthy subjects made these estimations while fMRI data was obtained. All three conditions evoked bilateral dorsal-parietal and premotor activations, while judgment of the location of the ball at the moment of judgment showed increased bilateral posterior hippocampus activation relative to making both future and past judgments at the one-second time-sale. Since the condition of such ‘real-time’ judgments implied undistracted observation of the ball's actual movements, the associated hippocampal activation is consistent with the concept that the hippocampus participates in a top-down exerted sensory gating mechanism. In this way, it may play a role in novelty (saliency) detection.

4.4. Data analysis

Image processing and statistical analysis were conducted with Statistical Parametric Mapping (SPM, Friston et al., 1995) version 5. Pre-processing with SPM included realignment and spatial normalization (template of the Montreal Neurological Institute, MNI). Images were smoothed using a Gaussian filter of 8 mm FWHM. Cortical activations were rendered onto the surface of a standard MNI brain. For the projection on brain slices, the same template was used. For the statistical analysis of regional differences in cerebral activation, all conditions (including 5 and 6) were modelled in a blocked design at subject level. To identify the distribution of activations related to cerebral processing beyond primary visuomotor control in the experimental conditions 1–3, each of these three conditions was contrasted to the visuomotor control condition (4) at subject level, after which each contrast was separately analysed at group level using one sample t-tests. To look at the activations that were commonly present in all three conditions, a conjunction analysis was conducted of the three SPM < Z > maps obtained from the t-tests. For the contrasts of the experimental conditions with the visuomotor control condition, an initial threshold of p < 0.001 was used (response height at voxel-level, uncorrected; extent kE of 20 voxels). Differences between conditions 1, 2 and 3 were made by making comparisons at second level using a one-way ANOVA for repeated measures (random effect analysis). The contrasts of the conditions 1,2,3 with the baseline task of passive viewing a stationary ball in the centre of the screen were used in the ANOVA. Conditions were assumed to be dependent and equally variant, whereas subjects were assumed to be independent and equally variant.