ScienceWeek

COGNITIVE SCIENCE: ON fMRI SUBJECT CORRELATIONS IN PERCEPTION

The following points are made by Luiz Pessoa (Science 2004 303:1617):

1) *Functional magnetic resonance imaging (fMRI) allows researchers to noninvasively investigate the organization of the human brain. fMRI is sensitive to blood oxygenation levels and is believed to reflect the integrated activity of large groups of neurons (2). Experiments with fMRI have shown that the human visual cortex is parceled into a set of brain areas that have relatively simple responses to the components of visual stimuli (3) such as patterns of texture. However, other regions of the visual cortex display more complex response behaviors. One such region, situated in what anatomists call the fusiform gyrus, is activated strongly when participants view images of human faces (4,5). Another region is situated in the collateral sulcus and is activated strongly when subjects view images of outdoor scenes, including buildings.

2) In a typical fMRI experiment, participants view a fixed set of controlled stimuli and perform a task. For instance, they may be asked to view a set of photographs of objects presented in a sequence of frames and to indicate every time an object is repeated. fMRI signals are then compared for two conditions, for example, viewing photographs of faces and photographs of buildings. Such comparisons are the basis for generating so-called brain activation maps that assess whether evoked signals are stronger during one condition relative to another. In such maps, activated regions are thought to indicate increased neuronal processing at a specific site in the brain and can be used to assess the degree of functional specialization of a given region.

3) Hasson et al(1) adopted a completely different strategy of fMRI study. Instead of a fixed set of experimental stimuli, participants simply watched an uninterrupted 30-min segment of a film (The Good, the Bad, and the Ugly). The investigators asked whether fMRI signals in one person's brain could predict signals in another person's brain. Remarkably, they found a large degree of correlation between the brains of pairs of participants who watched the same movie segment. On average, close to 30% of the cortical activation of one person's brain could be predicted by the fMRI signals from another individual's brain. To correlate the signals in the brains of two individuals, Hasson et al(1) initially "normalized" each brain into a common coordinate system, a routine procedure in fMRI research. Thus, the locations in one person's brain could be roughly aligned with the same locations in another individual's brain. The remarkable degree of correlation in brain activation among different individuals suggests that individual brains "tick together" in synchronized spatiotemporal patterns when exposed to the same visual environment.

References (abridged):

1. U. Hasson, Y. Nir, I. Levy, G. Fuhrmann, R. Malach, Science 303, 1634 (2004)

2. N. K. Logothetis, J. Pauls, M. Augath, T. Trinath, A. Oeltermann, Nature 412, 150 (2001)

3. B. A. Wandell, Annu. Rev. Neurosci. 22, 145 (1999)

4. J. V. Haxby et al., Science 293, 2425 (2001)

5. N. Kanwisher, J. McDermott, M. M. Chun, J. Neurosci. 17, 4302 (1997)

Science http://www.sciencemag.org

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Notes by ScienceWeek:

functional magnetic resonance imaging: (fMRI) There is a difference between magnetic resonance imaging (MRI) and "functional" magnetic resonance imaging (fMRI) as applied to the brain. The former is essentially a technique for examining morphology, while the latter is a technique for examining activity of brain tissue. Both techniques involve computerized analysis of data. In general, MRI involves magnetic coils producing a static magnetic field parallel to the long axis of the patient or subject, combined with inner concentric magnetic coils producing a static magnetic field perpendicular to the long axis. A radio-frequency coil specifically designed for the head perturbs the static fields to generate a magnetic resonance image. The interaction physics in this technique is that between the magnetic fields and atomic nuclei in brain tissue. "Sliced" views can be obtained from any angle, and the resolution is quite high and on the order of millimeters for current magnetic field strengths of 1.5 tesla. Functional magnetic resonance imaging (fMRI), the variant of MRI discussed here, is based on the fact that oxyhemoglobin, the oxygen-carrying form of hemoglobin, has a different magnetic resonance signal than deoxyhemoglobin, the oxygen-depleted form of hemoglobin. Activated brain areas utilize more oxygen, which transiently decreases the levels of oxyhemoglobin and increases the levels of deoxyhemoglobin, and within seconds the brain microvasculature responds to the local change by increasing the flow of oxygen-rich blood into the active area. This local response thus leads to an increase in the oxyhemoglobin-deoxyhemoglobin ratio, which forms the basis for the fMRI signal in this technique. Because of its high spatial resolution (millimeters) and high temporal resolution (seconds) compared to other imaging techniques, fMRI is now the technology of choice for studies of the functional architecture of the human brain.

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