Neural Representations of Self-Motion During Natural Scenes in the Human Brain

Navigating through the environment is one of the important everyday tasks of the visual system. This task relies on processing of at least two visual cues: visual motion, and scene content. Our sense of motion heavily relies on understanding and separating visual cues resulting from object motion and self-motion. Processing and understanding of visual scenes is an equally abundant task we are exposed to in our everyday environment. Together, motion and scene processing allow us to fulfill navigation tasks such as way finding and spatial updating. In terms of neural processing, both, regions involved in motion processing, and regions involved in scene processing have been studied in great detail. However, how motion regions are influenced by scene content and how scene regions are involved in motion processing has barely been addressed. In order to understand how self-motion and scene processing interact in the human brain, I completed a series of studies as part of this thesis. First of all, using planar horizontal motion and visual scenes, the first study of this thesis investigates motion responses of scene regions. The next study investigates whether eye-centered or world-centered reference frames are used during visual motion processing in scene regions, using objective ‘real’ motion and retinal motion during pursuit eye movements and natural scene stimuli. The third study investigates the effect of natural scene content during objective and retinal motion processing in motion regions. The last study investigates how motion speed is represented in motion regions during objective and retinal motion. Since many visual areas are optimized for natural visual stimuli, the speed responses were tested on Fourier scrambles of natural scene images in order to provide natural scene statistics as visual input. I found evidence that scene processing regions parahippocampal place area (PPA) and occipital place area (OPA) are motion responsive while retrosplenial cortex (RSC) is not. In addition, PPA’s motion responses are modulated by scene content. With respect to reference frames, I found that PPA prefers a world-centered reference frame while viewing dynamic scenes. The results from motion regions (MT/V5+, V3A, V6 and cingulate sulcus visual area (CSv)) revealed that motion responses of all of them are enhanced during exposure to scenes compared to Fourier-scramble, whereas only V3A responded also to static scenes. The last study showed that all motion responsive regions tested (MT/V5, MST, V3A, V6 and CSv) are modulated by motion speed but only V3A has a distinctly stronger speed tuning for objective compared to retinal motion. These results reveal that using natural scene stimuli is important while investigating self-motion responses in human brain: many scene regions are modulated by motion and one of them (PPA) even differentiates object motion from retinal motion. Conversely, many motion regions are modulated by scene content and one of them (V3A) is even responsive to still scenes. Moreover, the objective motion preference of V3A is even stronger during higher speeds. These results question a strong separation of ‘where’ and ‘what’ pathways and show that scene region PPA and motion region V3A have similar objective motion and scene preferences

Object Rivalry: Competition Between incompatible Representations of the Same Object

To understand that an object has changed state during an event, we must represent the `before\u27 and `after\u27 states of that object. Because a physical object cannot be in multiple states at any one moment in time, these `before\u27 and `after\u27 object states are mutually exclusive. In the same way that alternative states of a physical object are mutually exclusive, are cognitive representations of alternative object states also incompatible? If so, comprehension of an object state-change involves interference between the constituent object states. Through a series of functional magnetic resonance imaging experiments, we test the hypothesis that comprehension of object state-change requires the cognitive system to resolve conflict between representationally distinct brain states. We discover that (1) comprehension of an object state-change evokes a neural response in prefrontal cortex that is the same as that found for known forms of conflict, (2) the degree to which an object is described as changing in state predicts the strength of the prefrontal cortex conflict response, (3) the dissimilarity of object states predicts the pattern dissimilarity of visual cortex brain states, and (4) visual cortex pattern dissimilarity predicts the strength of the prefrontal cortex conflict response. Results from these experiments suggest that distinct and incompatible representations of an object compete when representing object state-change. The greater the dissimilarity between described object states, the greater the dissimilarity between rival brain states, and the greater the conflict

Refreshing and Integrating Visual Scenes in Scene-selective Cortex

Constructing a rich and coherent visual experience involves maintaining visual information that is not perceptually available in the current view. Recent studies suggest that briefly thinking about a stimulus (refreshing) can modulate activity in category-specific visual areas. Here, we tested the nature of such perceptually refreshed representations in the parahippocampal place area (PPA) and retrosplenial cortex (RSC) using fMRI. We asked whether a refreshed representation is specific to a restricted view of a scene, or more view-invariant. Participants saw a panoramic scene and were asked to think back to (refresh) a part of the scene after it disappeared. In some trials, the refresh cue appeared twice on the same side (e.g., refresh left–refresh left), and other trials, the refresh cue appeared on different sides (e.g., refresh left–refresh right). A control condition presented halves of the scene twice on same sides (e.g., perceive left–perceive left) or different sides (e.g., perceive left–perceive right). When scenes were physically repeated, both the PPA and RSC showed greater activation for the different-side repetition than the same-side repetition, suggesting view-specific representations. When participants refreshed scenes, the PPA showed view-specific activity just as in the physical repeat conditions, whereas RSC showed an equal amount of activation for different- and same-side conditions. This finding suggests that in RSC, refreshed representations were not restricted to a specific view of a scene, but extended beyond the target half into the entire scene. Thus, RSC activity associated with refreshing may provide a mechanism for integrating multiple views in the mind

What is the function of the human retrosplenial cortex?

The retrosplenial cortex (RSC) comprises Brodmann areas 29/30 and is an integral part of a brain system that is engaged by spatial navigation, scene processing, recollection of the past and imagining the future. Damage involving the RSC in humans can result in significant memory and navigation deficits, while the earliest metabolic decline in Alzheimer's disease is centred upon this region. The precise function of the RSC, however, remains elusive. In this thesis I sought to determine the key contribution of the RSC in a series of six studies that each comprised behavioural and functional magnetic resonance imaging (fMRI) experiments. Specifically, I discovered that the RSC is acutely responsive to landmarks in the environment that maintain a fixed, permanent location in space, and moreover is sensitive to the exact number of permanent landmarks in view. Using a virtual reality environment populated with entirely novel ‘alien’ landmarks I then tracked the de novo acquisition of landmark knowledge and observed the selective engagement of the RSC as information about landmark permanence accrued. In three further studies I established the parameters within which the RSC operates by contrasting permanent landmarks in large- and small-scale space, by comparing landmark permanence with orientation value, and by investigating permanence in non-spatial domains. In parallel lines of inquiry, I uncovered evidence that a fully functional RSC may be a prerequisite for successful navigation, while also characterising RSC interactions with other brain regions, such as the hippocampus, that could have importance for constructing reliable representations of the world. Together my findings provide new insights into the role of the RSC in a range of cognitive functions. The RSC’s processing of permanent predictable features may represent a key building block for spatial and scene representations that are central to navigation, recalling past experiences and imagining the future