Authors:

CUSACK, R.

Reference:

Year of publication:

2003

CBU number:

5604

Abstract:

Introduction A feature binding process has often been proposed to organise visual input into separate perceived objects. Neuropsychological studies of Bálint's syndrome [1] alongside neuroimaging studies [2] have implicated parietal cortex in this binding process. As it is well established that the parietal lobes play an important part in spatial attention, it has been argued that binding depends heavily on spatial processing [cf. 3]. In the current study, we have attempted to separate binding and spatial roles by using fMRI to examine the neural bases of perceptual binding in the absence of spatial cues, in auditory stimuli spread over time. Method We presented sequences that can perceptually organise in one of two ways. They comprised repeating triplets of tones, and are either heard bound together into a single perceptual stream, or segregated into two streams (Figure 1). A change in grouping leads to a salient change in rhythm. The percept switches pseudo-randomly over time and subjects were asked to rate which they heard throughout. The sequences were 20s long and separated by 10s gaps. Eighteen subjects were scanned at 3T using a sparse imaging protocol (TR=10s, TE=27ms, resolution 128x128x21) on two runs of 98 scans. Standard pre-processing and a random-effects analysis was performed with SPM99 and a toolbox extension to correct for distortions due to magnetic field inhomogeneities [4]. Results As expected, for larger frequency separations, subjects heard greater segregation into two auditory streams. A BOLD contrast between the presentation of sound and performance of a task to the silent baseline, showed bilateral activation in primary and secondary auditory cortices, in right Broca's area, and in the right cerebellum. A contrast of the segregated percept minus the fused percept showed parietal and occipital activation on the bank of the right intraparietal sulcus. This persisted even when frequency separation was entered into the model as a parameter to covary out its effect, showing that the activation reflects a difference in percept rather than an effect of the physical differences in the stimuli. Conclusions Parieto-occipital regions play a role in the perceptual organisation of sounds across time. Its function might be in the segregation process, or representation or selection when multiple streams are present. The finding that the parietal lobes are involved in grouping over time adds to the growing body of evidence that parietal regions have important non-spatial functions [5], and call into question the proposal that spatial attention is responsible for their role in visual feature segregation. We propose that the parietal lobes are important for multi-modal (and probably cross-modal) perceptual organisation. From other recent results in hearing we propose that this takes the form of a hierarchical fragmentation of the currently attended part of the scene, with the features in the attended part progressively more resolved. References[1] Friedmann-Hill, SR, Robertson, LC & Treisman, A (1995). Science 269: 853-855 [2] Shafritz, KM, Gore, JC & Marois, R (2002): PNAS 99: 10917-10922 [3] Treisman, A & Gelade, G (1980): Cognitive Psychology 12: 97-136 [4] Cusack, R, Brett, M & Osswald, K (2003): NeuroImage 18: 127-142 [5] Wojciulik, E & Kanwisher, N (1999): Neuron 23: 747-764