Scenes unseen: the parahippocampal cortex intrinsically subserves contextual associations, not scenes or places per se

Abstract

The parahippocampal cortex (PHC) has been implicated in both place/scene processing and episodic memory. We proposed that this region should instead be seen as intrinsically mediating contextual associations and not place/scene processing or episodic memory exclusively. Given that place/scene processing and episodic memory both rely on associations, this modified framework provides a platform for reconciling what seemed like different roles assigned to the same region. Comparing scenes with scenes, we show here that the PHC responds significantly more strongly to scenes with rich contextual associations compared with scenes of equal visual qualities but less associations. This result adds unequivocal support to the view that the PHC mediates contextual associations in general rather than places or scenes proper, and necessitates a revision of the current view that the PHC contains a dedicated place/scenes "module."

Figures

Figure 1.

A projection of findings from the literature pertaining to the presumed roles of the PHC. In green are studies implicating the PHC in spatial processing (e.g., navigation, places, and scenes) and in red are studies implicating the PHC in episodic memory. This figure is adopted from Aminoff et al. (2007), in which the exact references for all these studies can be found. The black outline represents the boundaries of the PHC, based on the definition of Insausti et al. (1998) and Reber et al. (2002).

Figure 2.

Examples of the stimuli in the two conditions: scenes with strong and weak contextual associations. Overall scene complexity was equated between the scenes in the two conditions. In addition to complexity, both conditions had the same percentage of indoor and outdoor scenes and a comparable spatial frequency content. Therefore, visual and place information was matched, and the consistent difference between the scenes in the two conditions was how strongly associative are the foreground objects.

Figure 3.

Comparison of the activation elicited by scenes with strong and with weak contextual associations, equated across scene/place and visual information. As can be seen, the network of contextual associations (Bar and Aminoff, 2003; Bar, 2004; Bar et al., 2007) is differentially more active for scenes with stronger associations, therefore demonstrating that these regions are sensitive to contextual associations and not to scenes/places per se. A, The statistical parametric maps of significant blood oxygenation level-dependent differences, calculated using a random-effect analysis. Superimposed on these maps are the outlines of the activations elicited by the separate PPA localizer (green indicates the RSC; white indicates the PHC). B, The corresponding signal change from baseline, for strong and weak scenes. *p < 0.05, **p < 0.01. Error bars are 1 SE.