Introduction:
Underlying spatial memory and talking about spatial layouts are common cognitive processes (Haun et al. 2005). For example, to locate an object in space it is obligatory to choose a coordinate system called frame of reference in cognition as well as in its verbal expression. Coding space within different frames of reference requires different cognitive processes (e.g. Neggers et al. 2005). In relative frames of reference the origin of the coordinate system is the viewpoint of a person. In intrinsic frames of reference an object is located in relation to another object (Levinson 2003). FMRI data have suggested that different frames of reference show different patterns of neural activation (Burgess et al. 2002; Committeri et al. 2004). However, the number of existing frames of reference and their neural correlates remain controversial. In an event-related fMRI study we investigated whether differential neural networks for relative and intrinsic frames of reference can be isolated.
Methods:
In the present study an implicit sentence picture matching task was used to investigate differential neural correlates for relative and intrinsic frames of reference. Twenty-eight healthy human adults (16 women, 12 men) read a sentence describing a spatial scene followed by a picture, and decided whether the sentence matches the picture or not. Feedback was given either supporting a relative or an intrinsic frame of reference. After half of the trails the feedback switched from one reference frame to the respective other reference frame (Fig.1). Participants were instructed to respond as accurately and as quickly as possible. They responded with their right hand by pressing a key with the index finger for a correct decision and a second key with the middle finger for an incorrect judgment. Two baseline tasks were included (Fig.1): a high level baseline (c5) and a low level baseline (c6).
A 3 Tesla MRI system (Siemens TRIO, Erlangen, Germany) was used to acquire functional images of the whole brain. Using a gradient-echo echo planar scanning sequence 36 axial slices were obtained for each participant (voxel-size 3 x 3 x 3 mm, TR = 2310 ms, field of view = 192, TE = 30 ms, flip angle = 75). All functional images were acquired in one run that lasted for 50 minutes. Following the acquisition of functional images a high-resolution anatomical scan (T1-weighted MP-RAGE, 176 slices) was acquired. FMRI data were analyzed using BrainVoyager QX (Brain Innovation, Maastricht, The Netherlands). Random-effects whole brain group analyses were performed. The statistical threshold at the voxel level was set at p < 0.001, uncorrected for multiple comparisons.
Results:
Intrinsic trials as compared to baseline trials revealed increased activity in the parietal lobe and in the parahippocampal gyrus. Relative as compared to baseline trails revealed a widespread network of activity. Increased activity was observed in occipitotemporal cortices, in the parietal lobe, and in frontal areas.
We focused on the direct comparison between relative and intrinsic trials. Results showed increased activity in the left parahippocampal gyrus only for intrinsic trials as compared to relative trails. An ANOVA of the averaged beta-weights with the within factors Reference frame and Condition and the between factor Block order (relative-intrinsic and intrinsic-relative), obtained for all voxels in the parahippocampal gyrus, showed no main effect of Reference frames and Condition. A significant interaction between the factors Reference frame and Condition was observed (p < 0.05). T-contrasts showed a significant effect for intrinsic (c4) as compared to relative trials (c3; p < 0.001).
Conversely, relative as compared to intrinsic trials showed strong increased activity in the left medial frontal gyrus. An ANOVA of the beta-weights in the brain area showed no main effects. A significant interaction between the factors Reference frame and Condition was observed (p < 0.05). T-contrasts showed a significant effect for intrinsic (c4) as compared to relative trials (c3, p < 0.01).
When comparing all intrinsic and relative conditions together to the baseline we observed increased activity in the right and left frontal eye fields (Fig. 2). An ANOVA of the averaged beta-weights with the within factors Reference frame and the between factor Block order obtained for all voxels in the left frontal eye fields showed a main effect of Block order (p < 0.001) and an trend effect of Reference frame (p = 0.08). An ANOVA of the averaged beta-weights for all voxels in the right frontal eye fields showed a main effect of Block order (p < 0.05) only.
Conclusions:
Using a sentence-picture matching task, we investigated whether differential neural correlates for intrinsic and relative frames of reference can be isolated. Intrinsic trials compared to relative trials showed increased activity in the parahippocampal gyrus whereas relative trails compared to intrinsic trials revealed increased neural activity in the frontal and parietal lobe. Both frames of reference together compared to a baseline show increased activity in the frontal eye fields which was stronger for the second block. This could be related to switching of reference frames (Wallentin et al. 2008). The present results confirm studies which report the parietal lobe to be involved in relative coding (Cohen & Andersen 2002). The neural correlates of intrinsic frames of reference were previously less well investigated. The present results show differential neural networks for both frames of reference that are crucial to spatial language.
References:
Burgess, N. (2002), 'The human hippocampus and spatial and episodic memory', Neuron, vol. 36, pp. 625-641.
Cohen, Y. (2002), 'A common reference frame for movement plans in the posterior parietal cortex', Nature Reviews Neuroscience, vol. 3, pp. 553-562.
Committeri, G. (2004), 'Reference frames for spatial cognition: Different brain areas are involved in viewer-, object-, and landmark centered judgments about object location', Cognitive Neuroscience, vol. 16, pp. 1517-1535.
Haun, D. (2005), 'Bias in spatial memory: a categorical endorsement', Acta Psychologia, vol. 118, pp. 149-170.
Levinson, S. (2003), 'Space in language and cognition: Explorations in cognitive diversity', Cambridge: CUP.
Neggers, S. (2005), 'Quantifing the interactions between allo- and egocentric representation of space', Acta Psychologia, vol. 118, pp. 25-45.
Wallentin, M. (2008), 'Frontal eye fields involved in shifting frames of reference within working memory for scenes', Neuropsychologia, vol. 46, pp. 399-408.