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Spatial structure from temporal change
Update Monitor Summaries of recently published papers of interest to cognitive scientists. Readers who would like to contribute to this section, by identifying appropriate papers and writing short summaries, should contact the Editor ([email protected]).
Musical statistics Transitional probabilities are very simple statistics: they merely calculate how likely one particular element will appear given the presence of another particular element. Saffran et al. showed previously that eight-month-old infants are able to use information about transitional probabilities to segment threesyllable pseudowords from a continuous stream of speech1. The same authors have now extended this research outside the linguistic domain by asking whether infants (and adults) can use differences in transitional probabilities to segment three-tone units from a continuous tonal stream2. Following procedures used in previous work, the experimenters presented subjects with an extended sequence of tones, composed of three-tone units. But as there were no pauses between these units, the only cue to their boundaries was the difference in transitional probabilities: withinunit probabilities were higher than the between-unit probabilities, although there was some overlap in the statistical profiles of the two categories. In the
testing stage, both adults and eightmonth-old infants were able to discriminate reliably three-tone sequences that had occurred in the original input stream from three-tone sequences that had never occurred as a unified sequence in the input. Discrimination persisted even when two of the three tones in the novel sequence were the same as in a true unit from the input. The authors conclude that transitional probabilities play a crucial role in infants’ segmentation of the tonal (i.e. musical) domain as well as in the linguistic domain. They argue that these simple statistics reflect an innate constraint on pattern detection that is used to subserve many domains of cognition. References 1 Saffran, J.R., Aslin, R.N. and Newport, E.L. (1996) Statistical learning by 8-month-old infants Science 274, 1926–1928 2 Saffran, J., Johnson, E.K., Aslin, R.N. and Newport, E.L. (1999) Statistical learning of tone sequences by human infants and adults Cognition 70, 27–52
Spatial structure from temporal change To represent the environment meaningfully, human vision must group those parts of the retinal image relating to a single object, and segregate that object from others in the image. Grouping and segregation are often considered in spatial terms, such as continuity and proximity. Lee and Blake now show that grouping can be achieved by purely temporal information1. They created a scattered array of local motion elements that were random in phase, orientation and spatial location, and whose motion directions oscillated back and forth irregularly. An array like this appears highly incoherent. Yet, when the oscillating elements from a given region were made to change direction simultaneously, they grouped into a vivid spatial form that stood out from the surrounding elements. Verifying this observation, observers easily judged the orientation of virtual rectangular regions undergoing synchronized change, even when only a proportion of the elements changed direction synchronously while the remainder changed randomly. This might sound reminiscent of ‘structure from motion’, a phenomenon whereby discontinuities in speed or direction of motion create perceived forms. But Lee and Blake’s demonstration is more subtle. First, the
oscillating elements in the synchronized rectangular region all differed in direction and orientation. Second, an element within this region and one from the surrounding region are indistinguishable: both oscillate irregularly in direction and are assigned location, phase and orientation randomly. This randomization removes spatial cues to the region’s orientation. What, then, makes the directional oscillations within the region group into a clearly seen form? Simply that their changes in direction over time are correlated. A more apt term would be ‘structure from correlated change’. For a real-life example, consider a gusting breeze causing a tree to sway to and fro. The leaves move locally in many directions, but all change together with each gust of wind and a coherently swaying tree is perceived. Similarly, in the laboratory, Lee and Blake have shown that the brain can exploit purely temporal information to create spatial structure2.
MRI of mental time travel The experience of remembering past events carries with it such an intense sensation of past experience that it has been called ‘mental time travel’1. Indeed, it is a quite different experience to ‘remember’ rather than to ‘know’ that something is from the past. Given the strength of this phenomenon, it seems reasonable to expect that neuroimaging studies of memory would reveal changes in brain activity specifically associated with the experience of successfully retrieving information about previously experienced episodes or events. Yet, surprisingly, a study of recognition memory by Henson et al.2 reveals the first clear evidence that fMRI is sensitive to the different experiences associated with mental time travel. That is, it demonstrates the existence of brain activity that distinguishes successful from unsuccessful explicit memory retrieval. It is true that many fMRI studies have revealed neural correlates of memory retrieval. However, no previous study has demonstrated neural correlates that are specifically associated with the subjective experience of remembering (or not remembering) individual test items. Henson et al. employed event-related fMRI procedures, which allow data to be analysed contingent upon subject performance. Subjects studied a series of words, and were then required to judge whether test items were recollected as being old (consciously remembered), known to be old (familiar, but not recollected) or new (not seen at study). The findings revealed that there are fMRI correlates of retrieval success (differences between old and new items) and moreover that the pattern of neural activity is sensitive to differences in the experience of mental time travel that subjects report (i.e. remembering versus knowing). As the authors claim, event-related fMRI procedures, combined with behavioral measures of performance, herald an exciting future for the neuroimaging of human memory. References 1 Wheeler, M.A. et al. (1997) Toward a theory of episodic memory: the frontal lobes and
References
autonoetic consciousness Psychol. Bull. 121,
1 Lee, S-H. and Blake, R. (1999) Visual form created solely from temporal structure Science 284, 1165–1168 2 Demonstrations can be found at: http://www. psy.vanderbilt.edu/faculty/blake/Demos/TS/TS. html
331–354 2 Henson, R.N.A. et al. (1999) Recollection and familiarity in recognition memory: An eventrelated
functional
magnetic
resonance
imaging study J. Neurosci. 99, 3962–3972
1364-6613/99/$ – see front matter © 1999 Elsevier Science. All rights reserved. Trends in Cognitive Sciences – Vol. 3, No. 8,
August 1999
293
Musical statistics Transitional probabilities are very simple statistics: they merely calculate how likely one particular element will appear given the presence of another particular element. Saffran et al. showed previously that eight-month-old infants are able to use information about transitional probabilities to segment threesyllable pseudowords from a continuous stream of speech1. The same authors have now extended this research outside the linguistic domain by asking whether infants (and adults) can use differences in transitional probabilities to segment three-tone units from a continuous tonal stream2. Following procedures used in previous work, the experimenters presented subjects with an extended sequence of tones, composed of three-tone units. But as there were no pauses between these units, the only cue to their boundaries was the difference in transitional probabilities: withinunit probabilities were higher than the between-unit probabilities, although there was some overlap in the statistical profiles of the two categories. In the
testing stage, both adults and eightmonth-old infants were able to discriminate reliably three-tone sequences that had occurred in the original input stream from three-tone sequences that had never occurred as a unified sequence in the input. Discrimination persisted even when two of the three tones in the novel sequence were the same as in a true unit from the input. The authors conclude that transitional probabilities play a crucial role in infants’ segmentation of the tonal (i.e. musical) domain as well as in the linguistic domain. They argue that these simple statistics reflect an innate constraint on pattern detection that is used to subserve many domains of cognition. References 1 Saffran, J.R., Aslin, R.N. and Newport, E.L. (1996) Statistical learning by 8-month-old infants Science 274, 1926–1928 2 Saffran, J., Johnson, E.K., Aslin, R.N. and Newport, E.L. (1999) Statistical learning of tone sequences by human infants and adults Cognition 70, 27–52
Spatial structure from temporal change To represent the environment meaningfully, human vision must group those parts of the retinal image relating to a single object, and segregate that object from others in the image. Grouping and segregation are often considered in spatial terms, such as continuity and proximity. Lee and Blake now show that grouping can be achieved by purely temporal information1. They created a scattered array of local motion elements that were random in phase, orientation and spatial location, and whose motion directions oscillated back and forth irregularly. An array like this appears highly incoherent. Yet, when the oscillating elements from a given region were made to change direction simultaneously, they grouped into a vivid spatial form that stood out from the surrounding elements. Verifying this observation, observers easily judged the orientation of virtual rectangular regions undergoing synchronized change, even when only a proportion of the elements changed direction synchronously while the remainder changed randomly. This might sound reminiscent of ‘structure from motion’, a phenomenon whereby discontinuities in speed or direction of motion create perceived forms. But Lee and Blake’s demonstration is more subtle. First, the
oscillating elements in the synchronized rectangular region all differed in direction and orientation. Second, an element within this region and one from the surrounding region are indistinguishable: both oscillate irregularly in direction and are assigned location, phase and orientation randomly. This randomization removes spatial cues to the region’s orientation. What, then, makes the directional oscillations within the region group into a clearly seen form? Simply that their changes in direction over time are correlated. A more apt term would be ‘structure from correlated change’. For a real-life example, consider a gusting breeze causing a tree to sway to and fro. The leaves move locally in many directions, but all change together with each gust of wind and a coherently swaying tree is perceived. Similarly, in the laboratory, Lee and Blake have shown that the brain can exploit purely temporal information to create spatial structure2.
MRI of mental time travel The experience of remembering past events carries with it such an intense sensation of past experience that it has been called ‘mental time travel’1. Indeed, it is a quite different experience to ‘remember’ rather than to ‘know’ that something is from the past. Given the strength of this phenomenon, it seems reasonable to expect that neuroimaging studies of memory would reveal changes in brain activity specifically associated with the experience of successfully retrieving information about previously experienced episodes or events. Yet, surprisingly, a study of recognition memory by Henson et al.2 reveals the first clear evidence that fMRI is sensitive to the different experiences associated with mental time travel. That is, it demonstrates the existence of brain activity that distinguishes successful from unsuccessful explicit memory retrieval. It is true that many fMRI studies have revealed neural correlates of memory retrieval. However, no previous study has demonstrated neural correlates that are specifically associated with the subjective experience of remembering (or not remembering) individual test items. Henson et al. employed event-related fMRI procedures, which allow data to be analysed contingent upon subject performance. Subjects studied a series of words, and were then required to judge whether test items were recollected as being old (consciously remembered), known to be old (familiar, but not recollected) or new (not seen at study). The findings revealed that there are fMRI correlates of retrieval success (differences between old and new items) and moreover that the pattern of neural activity is sensitive to differences in the experience of mental time travel that subjects report (i.e. remembering versus knowing). As the authors claim, event-related fMRI procedures, combined with behavioral measures of performance, herald an exciting future for the neuroimaging of human memory. References 1 Wheeler, M.A. et al. (1997) Toward a theory of episodic memory: the frontal lobes and
References
autonoetic consciousness Psychol. Bull. 121,
1 Lee, S-H. and Blake, R. (1999) Visual form created solely from temporal structure Science 284, 1165–1168 2 Demonstrations can be found at: http://www. psy.vanderbilt.edu/faculty/blake/Demos/TS/TS. html
331–354 2 Henson, R.N.A. et al. (1999) Recollection and familiarity in recognition memory: An eventrelated
functional
magnetic
resonance
imaging study J. Neurosci. 99, 3962–3972
1364-6613/99/$ – see front matter © 1999 Elsevier Science. All rights reserved. Trends in Cognitive Sciences – Vol. 3, No. 8,
August 1999
293