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Thomas Lauritzen, Alex Wade; Different cortical areas are modulated in different ways by spatial attention in human visual cortex . Journal of Vision 2009;9(14):44. doi: 10.1167/9.14.44.
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Introduction: In primates, sustained spatial attention causes significant increases in single-unit firing rates in areas such as V4 but similar increases are hard to measure in V1. Paradoxically, fMRI measurements of spatial attentional modulation show strong responses in all visual areas, including V1. These fMRI changes are either purely additive  or a mixture of additive and multiplicative mechanisms . It is suggested that attention increases the baseline firing rates or presynaptic activity of all neurons in the attended region irrespective of their tuning . Such a change might be undetectable at the single unit level, but generate a large DC increase in metabolic demand, affecting the fMRI signal. To test this hypothesis, we measured the effect of spatial attention on steady state visually-evoked potentials (SSVEPs) in five retinotopically-defined visual areas using source-imaged EEG.
Methods: SSVEP stimuli consisted of two gratings at 0, 5, 20 or 50% contrast located to the left and right of fixation. Subjects were cued to covertly attend one of the two gratings, detecting small contrast modulations in the grating. Performance was ˜50% correct on all conditions. EEG data were collected with a whole-head, 128-channel EGI Netstation system, and the locations of all electrodes were recorded using a 3D digitizer. Minimum-norm inverses were computed using anatomically-correct headmodels . The time course of the cortical current density was extracted from fMRI-defined visual areas V1, V3A, V4, hMT+ and intraparietal sulcus (IPS). Spectral analysis was used to separate the responses to the two stimulus gratings in all visual areas.
Results: Attention increases the response gain in V1, V3A, hMT+ and IPS, and the contrast gain for V4. Signal-detection analysis reveals a higher response for detected, than for missed targets in areas displaying response gain only.
Conclusion: Attention modulates SSVEPs by a multiplicative, rather than an additive gain function, in all areas. The discrepancies between primate and Human could be due to neural differences in the different spieces, and that attention does not modulate neural signals in primates in as early levels of visual processing as in Humans. A more likely explanation is that EEG/fMRI measures averages over the entire cell population and electrode recordings target specific subgroups of neurons or laminar structures. Other neuronal populations will then respond more similar to the Human results.
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