Abstract
There is a wealth of literature on the role of short-range interactions between low-level orientation-tuned filters in the perception of discontinuous contours. However, little is known about how spatial information is integrated across more distant regions of the visual field in the absence of explicit local orientation cues, a process referred to here as visuospatial interpolation (VSI). To examine the neural correlates of VSI high field functional magnetic resonance imaging was used to study brain activity while observers either judged the alignment of three Gabor patches by a process of interpolation or discriminated the local orientation of the individual patches. Relative to a fixation baseline the two tasks activated a largely over-lapping network of regions within the occipito-temporal, occipito-parietal and frontal cortices. Activated clusters specific to the orientation task (orientation>interpolation) included the caudal intraparietal sulcus, an area whose role in orientation encoding per se has been hotly disputed. Surprisingly, there were few task-specific activations associated with visuospatial interpolation (VSI>orientation) suggesting that largely common cortical loci were activated by the two experimental tasks. These data are consistent with previous studies that suggest higher level grouping processes -putatively involved in VSI- are automatically engaged when the spatial properties of a stimulus (e.g. size, orientation or relative position) are used to make a judgement.
Highlights
Since Hubel and Wiesel [1] discovered that individual cells in the occipital cortex are sensitive to the onset of individual bright or dark bars with a specific orientation much progress has been made in the characterisation of receptive field (RF) anatomy, circuitry and physiology. (See [2] for review)
Anatomical regions are defined in Montreal Neurological Institute (MNI) coordinates, which closely approximates to the space described by Talairach and Tournoux [32]
This study was designed to examine the neural correlates of visuospatial interpolation (VSI), and more generally, to shed light on the mechanisms involved in the encoding of relative position
Summary
Since Hubel and Wiesel [1] discovered that individual cells in the occipital cortex are sensitive to the onset of individual bright or dark bars with a specific orientation much progress has been made in the characterisation of receptive field (RF) anatomy, circuitry and physiology. (See [2] for review). Unless one assumes that relative position is implicitly encoded in patterns of activation within early topographic maps [3] one must hypothesise the existence of a second stage to the computation that either integrates information from low level filters [14,15,9,16,13] or endogenously generates ‘virtual contours’ between the stimulus elements [3,16] Candidate regions for this second stage to the process of VSI are the posterior parietal cortex, which has been implicated in spatial processing [17,18,19,20,21] and visual feature binding [22], and the occipito-temporal cortex, which is thought to underlie global integration of local signals [23,24,25] and the perception of subjective / virtual contours [26,27,28,29]
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