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How neurons become BOLD?
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News & Comment
TRENDS in Cognitive Sciences Vol.5 No.10 October 2001
How neurons become BOLD? Neuroimaging, particularly fMRI, has become a popular tool for studying brain function in humans. The assumption of the method is that the activity within regional populations of neurons is reflected in the fMRI response, namely the blood oxygen level-dependent or ‘BOLD’ signal. At last, almost a decade after the introduction of fMRI, this assumption has been tested directly. ‘...the fMRI signal largely reflects the input and local processing within an area rather than the spiking output.’ Logothetis et al.1 simultaneously scanned and recorded from cortex in anaesthetized monkeys, using fMRI and electrophysiological recording. The monkeys viewed moving checkerboard patterns while electrodes placed in primary visual cortex measured single- and multiunit neural spiking activity, as well as local
field potentials (LFPs), which reflect the dendro-somatic inputs of the neural population. Simultaneous fMRI scanning was used to define a region of interest near the electrode tip that showed a significant BOLD response to the stimulus. The BOLD response was significantly correlated only with the LFPs, which were stronger and more sustained than the single- and multiunit spiking activity. However, the signal-tonoise ratio of the fMRI signal was at least an order of magnitude weaker than the LFPs. These results indicate that the BOLD signal does indeed reflect an increase in neural activity. However, the difference between what the two techniques measure suggests that caution is warranted in making direct comparisons. Specifically, the correlation between BOLD and LFPs, rather than action potentials, suggest that the fMRI signal largely reflects the input and local processing within an area rather than the spiking output. Thus, it
might be possible to observe a BOLD response in an area that has no single-unit activity measured electrophysiologically, particularly if the input is primarily modulatory. Furthermore, given the weak signal-to-noise ratio of the fMRI signal, a lack of fMRI activity does not necessarily imply an absence of neural activity. Although these caveats limit direct comparisons between physiology and neuroimaging data, the two techniques together offer a more comprehensive picture of the neural activity and circuitry than either can alone. And future imaging studies can proceed with the confidence that what they measure does indeed reflect real changes in neuronal activity.
although many areas of the brain respond to the stimulus of a frequency change, only areas in the right hemisphere code for change of subjective pitch. MW
language influenced priming effects, even though both groups were tested in English. Mandarin speakers were faster to make temporal judgments after a vertical prime, whereas English speakers were faster after a horizontal prime. However, this difference was not absolute. Although bilingual subjects whose first language was Mandarin did show a vertical effect, its size depended on the age at which they learnt English. Furthermore, English subjects who were taught to talk about time as if it were vertical (e.g. ‘Wednesday is lower than Tuesday’) began to show a vertical effect. Therefore, although first language does influence concepts, experience of another language, or another style of talking about time, can change the way we think about time. HJB
1 Logothetis, N.K. et al. (2001) Neurophysiological investigation of the basis of the fMRI signal. Nature 412, 150–157
Jody Culham [email protected]
In Brief
The right hemisphere pitches in In recent years, cognitive neuroscience has been making great efforts to home in on the difference between the physical stimulus and the perceived sensation, hence the attempts to find neural correlates of what we actually perceive. In auditory illusions, the pitch of a sound – its subjective highness or lowness – may not be the same as its physical frequency. In the missing frequency (MF) illusion, for example, people can hear frequencies not present in physical sound waves. Using MF stimuli and magnetoencephalography (MEG), Patel and Balaban recently showed that there is a correlation between the temporal pattern of brain activation in the right cerebral hemisphere and the subjectively perceived pitch [Nature Neurosci. (2001) 4, 839–844]. Using the fact that people differ in their susceptibility to the MF illusion, Patel and Balaban divided their subjects into those who perceived pitch change in the same direction as frequency change, and those who perceived it in the opposite direction. Comparing MEG scans from the two groups of subjects, they found that, http://tics.trends.com
Language shapes thought How much does the language we speak shape the way we think? A study by Lera Boroditsky sheds new light on this long-standing question by comparing concepts of time in English and Mandarin speakers [Cognitive Psychology (2001) 43, 1–22]. In English, time is typically talked about as if it were horizontal whereas in Mandarin it is spoken about as if it were vertical (e.g. words meaning ‘up’ and ‘down’ are used to talk about both space and time). Subjects made true/false judgments on statements about time (e.g. ‘March comes before April’). Before each statement subjects were shown primes – either vertical or horizontal arrays of objects. It was found that subjects’ first
e-body language US programmers are working on a new computer code for conveying non-verbal human communication. The new language, HumanMarkup Language (or HumanML), is being developed by the Organisation for the Advancement of Structured Information Standards (OASIS) in Massachusetts. According to the committee’s website
1364-6613/01/$ – see front matter © 2001 Elsevier Science Ltd. All rights reserved.
News & Comment
TRENDS in Cognitive Sciences Vol.5 No.10 October 2001
How neurons become BOLD? Neuroimaging, particularly fMRI, has become a popular tool for studying brain function in humans. The assumption of the method is that the activity within regional populations of neurons is reflected in the fMRI response, namely the blood oxygen level-dependent or ‘BOLD’ signal. At last, almost a decade after the introduction of fMRI, this assumption has been tested directly. ‘...the fMRI signal largely reflects the input and local processing within an area rather than the spiking output.’ Logothetis et al.1 simultaneously scanned and recorded from cortex in anaesthetized monkeys, using fMRI and electrophysiological recording. The monkeys viewed moving checkerboard patterns while electrodes placed in primary visual cortex measured single- and multiunit neural spiking activity, as well as local
field potentials (LFPs), which reflect the dendro-somatic inputs of the neural population. Simultaneous fMRI scanning was used to define a region of interest near the electrode tip that showed a significant BOLD response to the stimulus. The BOLD response was significantly correlated only with the LFPs, which were stronger and more sustained than the single- and multiunit spiking activity. However, the signal-tonoise ratio of the fMRI signal was at least an order of magnitude weaker than the LFPs. These results indicate that the BOLD signal does indeed reflect an increase in neural activity. However, the difference between what the two techniques measure suggests that caution is warranted in making direct comparisons. Specifically, the correlation between BOLD and LFPs, rather than action potentials, suggest that the fMRI signal largely reflects the input and local processing within an area rather than the spiking output. Thus, it
might be possible to observe a BOLD response in an area that has no single-unit activity measured electrophysiologically, particularly if the input is primarily modulatory. Furthermore, given the weak signal-to-noise ratio of the fMRI signal, a lack of fMRI activity does not necessarily imply an absence of neural activity. Although these caveats limit direct comparisons between physiology and neuroimaging data, the two techniques together offer a more comprehensive picture of the neural activity and circuitry than either can alone. And future imaging studies can proceed with the confidence that what they measure does indeed reflect real changes in neuronal activity.
although many areas of the brain respond to the stimulus of a frequency change, only areas in the right hemisphere code for change of subjective pitch. MW
language influenced priming effects, even though both groups were tested in English. Mandarin speakers were faster to make temporal judgments after a vertical prime, whereas English speakers were faster after a horizontal prime. However, this difference was not absolute. Although bilingual subjects whose first language was Mandarin did show a vertical effect, its size depended on the age at which they learnt English. Furthermore, English subjects who were taught to talk about time as if it were vertical (e.g. ‘Wednesday is lower than Tuesday’) began to show a vertical effect. Therefore, although first language does influence concepts, experience of another language, or another style of talking about time, can change the way we think about time. HJB
1 Logothetis, N.K. et al. (2001) Neurophysiological investigation of the basis of the fMRI signal. Nature 412, 150–157
Jody Culham [email protected]
In Brief
The right hemisphere pitches in In recent years, cognitive neuroscience has been making great efforts to home in on the difference between the physical stimulus and the perceived sensation, hence the attempts to find neural correlates of what we actually perceive. In auditory illusions, the pitch of a sound – its subjective highness or lowness – may not be the same as its physical frequency. In the missing frequency (MF) illusion, for example, people can hear frequencies not present in physical sound waves. Using MF stimuli and magnetoencephalography (MEG), Patel and Balaban recently showed that there is a correlation between the temporal pattern of brain activation in the right cerebral hemisphere and the subjectively perceived pitch [Nature Neurosci. (2001) 4, 839–844]. Using the fact that people differ in their susceptibility to the MF illusion, Patel and Balaban divided their subjects into those who perceived pitch change in the same direction as frequency change, and those who perceived it in the opposite direction. Comparing MEG scans from the two groups of subjects, they found that, http://tics.trends.com
Language shapes thought How much does the language we speak shape the way we think? A study by Lera Boroditsky sheds new light on this long-standing question by comparing concepts of time in English and Mandarin speakers [Cognitive Psychology (2001) 43, 1–22]. In English, time is typically talked about as if it were horizontal whereas in Mandarin it is spoken about as if it were vertical (e.g. words meaning ‘up’ and ‘down’ are used to talk about both space and time). Subjects made true/false judgments on statements about time (e.g. ‘March comes before April’). Before each statement subjects were shown primes – either vertical or horizontal arrays of objects. It was found that subjects’ first
e-body language US programmers are working on a new computer code for conveying non-verbal human communication. The new language, HumanMarkup Language (or HumanML), is being developed by the Organisation for the Advancement of Structured Information Standards (OASIS) in Massachusetts. According to the committee’s website
1364-6613/01/$ – see front matter © 2001 Elsevier Science Ltd. All rights reserved.