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Streams of Consciousness Alumit Ishai When investigating the neural correlates of consciousness neuroscientists distinguish between conscious state being awake as opposed to asleep or in a coma which is regulated by brainstem and thalamic nuclei and conscious representation awareness of specific phenomenal experience The advent of functional brain imaging techniques especially fMRI enables the noninvasive inquiries of the mechanisms underlying conscious experience particularly in the visual system To date most experimental paradigms designed to study consciousness contrast the response during conscious visual experience with the response during unconscious visual experience the so called blindsight phenomenon or record patterns of brain activation during binocular rivalry perception of bistable figures and visual mental imagery The current data suggest that activity in high level areas of the ventral visual pathway but not in V1 are necessary for conscious visual experience Moreover visual awareness requires parietal and prefrontal regions for a recent review see Rees Kreiman Koch 2002 The neural correlates of conscious vision therefore parallel the distributed cortical networks that modulate visual attention and visual imagery As most researchers confuse awareness with consciousness the reported differential activity during consciousness is currently indistinguishable from that of other higher cognitive functions In his article Functional fMRI and the Study of Human Consciousness Dan Lloyd uniquely combines a conceptual analysis of consciousness with neuroscientific methods in order to characterize the neural manifestations of consciousness Lloyd 2002 Lloyd adopts Husserl s criteria according to which the phenomenology of consciousness is based on three essential principles intentionality the external world as it is experienced and not as it is superposition sensory and nonsensory properties are present in perception and temporality all objects share perception of present past and anticipated future If indeed these aspects of consciousness are implemented in the brain the empirical evidence should include temporal flux with passing time the multivariate differences between images should increase and superposition images sharing task or stimulus conditions should be similar Lloyd s methodological approach includes three con National Institutes of Health D 2002 Massachusetts Institute of Technology straints First time points in a scan series are considered individually because temporality implies that consciousness at each point in time is distinct from the preceding and the proceeding points Second subjects are considered individually because intersubject averaging could eliminate individual expression of consciousness Finally brain states are considered globally seeking distributed patterns of activation that encompass large cortical areas rather than assuming localized responses To test his predictions Lloyd reanalyzes four data sets Hazeltine Poldrack Gabrieli 2000 Ishai Ungerleider Martin Haxby 2000 Mechelli Friston Price 2000 Postle Berger Taich D Esposito 2000 provided by The fMRI Data Center The studies published in the December 2000 issue of the Journal of Cognitive Neuroscience included a variety of cognitive tasks target tracking passive viewing delayed matching reading and spatial working memory stimuli faces objects words pseudowords 2 D arrays of squares colored circles and motor responses button presses and saccades Needless to say none of the original studies was designed for or aimed at underpinning the neural mechanisms of neurophenomenology Nevertheless Lloyd preprocesses and reanalyzes the raw data to test his predictions about the general structures of consciousness which by their nature are task and stimulus independent Using multivariate distance analysis and artificial neural networks he shows a time distance effect i e as the time series progressed the distance between images increased and that the past and future brain states retention and protention respectively are embedded in present brain states As time passes suggests Lloyd the brain is changing globally incrementally and monotonically Previous fMRI studies of consciousness compared one state of awareness with another assumed localization and ignored the temporal flux Lloyd s original approach proposes methodological and conceptual advantages Traditionally fMRI data analysis focused on two parameters namely the spatial extent of the activation and the amplitude of the response within an activated region The data are usually displayed as statistical maps indicating the location and size of significant activation and graphs or histograms showing the percent fMRI signal change Given the spatial and temporal resolution of the technique extracting temporal information about the tripartite temporality i e the experienced present of Journal of Cognitive Neuroscience 14 6 pp 832 833 an object is influenced by the past and the future is problematic The hemodynamic function presents a serious challenge as each time point is contaminated by the immediately preceding response Lloyd s correction that is excluding 10 seconds before and after each time point is thus necessary Moreover he trains artificial neural networks to reconstruct the temporal information encoded in each time point In all subjects the neural networks succeeded in recovering information about the preceding volume retention and the following volume protention Lloyd assumes distributed patterns of activation Ishai Ungerleider Martin Schouten Haxby 1999 Ishai et al 2000 however his current analysis does not include spatial localization Further investigation is therefore required to determine which brain regions mediate the temporal manifestations of consciousness Lloyd s finding that performance is better when the network recovered information about the past than the future was perhaps not surprising due to the asymmetry between the known immediate past and the unknown future One could assume however that top down effects such as expectation anticipation and attention should modulate future patterns of activation For example Kastner Pinsk De Weerd Desimone Ungerleider 1999 have shown that in the absence of a sensory stimulus anticipation modulated the response within both sensory and control areas Furthermore when subjects are engaged in a block design experiment with predicted context for example while viewing chair stimuli for several seconds


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