local processing

ABSTRACTS

Forest and trees: Brain systems involved in global/local processing. G.R. FinkS, P.W. HalliganZ,3, J.C. Marshall3, C.D. Frith], R.S.J. Frackowiakl, R.J. Dolanl

1Wellcome Department of Cognitive Neurology, Institute of Neurology, London, UK," eRivermead Rehabilitation Centre, Abingdon Road, Oxford, UK; 3Neuropsychology Unit, University Department of Clinical Neurology, The Radcliffe Infirmary, Oxford, UK Introduction Physiological and neuropsychological studies suggest that there is hemispheric specialization for global and local processing of hierarchically structured stimuli (1) and that this hemispheric global/local asymmetry is due to perceptual/attentional mechanisms with a critical anatomical locus centered in the temporo-parietal regions (2). Recently, studies employing Evoked Related Potentials (ERP) reported the involvement of an early posterior negative component (N250) which suggested that global/local attributes of hierarchically organized structures may be processed by discrete systems located in the early visual system (3). The current study used PET to ascertain the brain systems involved in global/local level processing of hierarchically structured stimuli.

Subjects and

Methods

Ten male right handed normal volunteers (aged 19 - 32 years) were studied using a Siemens/CPS Ecat Exact HR+ (962) with a total field of view of 15 cm. Data were acquired in 3D septa removed mode. Relative rCBF measurements were performed during the presentation of non-congruent hierarchically organized stimuli (i.e. large letters made of small letters, stimulus presentation 300 ms, stimulus-to-stimulus interval 1200 ms). Prior to each rCBF measurement, subjects were required to name either the global or the local aspect of the figures. To control for possible size effects both small and large hierarchical figures were presented to the subjects during alternate rCBF measurements. Data were analyzed using SPM (4, 5).

Results Significant increases (p<0.001) in relative rCBF (Fig. la) were seen in the right lingual gyrus (Brodmann area 18) when subjects were required to attend to the global attribute.When the subjects were required to attend the local attribute this led to relative rCBF increases (Fig. lb) in the left occipital cortex (BA 18) with an additional focus in the left medial occipito-parietal cortex (BA 19/7). No differential rCBF increases during the local/global divided attention tasks were seen in the temporoparietal areas. An effect of figure size was observed in the striate (BA 17) and extrastriate areas (BA 18/19), no such effect was observed in the left medial occipito-parietal cortex. [ '!:~

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Conclusion These findings indicate that global/local processing involves the peristriate cortex (BA 18) and support the ERP evidence for early visual processing in the perceptual/attentional mechanisms involved in global/local processing (3). A right/left hemispheric preponderance during global/local processing was observed supporting the concept of hemispheric specialization. Our data extends previous findings (2) which considered temporoparietal areas to be the critical anatomical locus for the processing of hierarchical stimuli.

References 1. Navon, D. Cogn. Psychol. 1977, 9: 353-383. 2. Robertson, L.C., Lamb, M.R. Cog. Psychol. 1991, 23:299-330. 3. Heinze, H.-J., Mtinte, T.F. Neuropsychol. 1993, 8: 841-852. 4. Friston, K.J., et al. Hum. Brain Map. 1995, 2: 1-25. 5. Friston, K.J., et al. Hum. Brain Map. 1995, 2: 189-210. $178