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Gianluca Campana, Vincent Walsh, Clara Casco, Alan Cowey; Visual area V5/MT “remembers” what, not where. Journal of Vision 2004;4(8):86. doi: 10.1167/4.8.86.
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Repetition of a target's features or spatial position has been shown to facilitate subsequent detection or identification of that target (Maljkovic and Nakayama, 1994, 1996, 2000). This effect, called priming, is likely to rely on those low-level areas functionally specialised for the analysis and representation of simple stimulus attributes (Perceptual Representation System: Tulving and Schacter, 1990). In agreement with the PRS hypothesis, Campana et al. (2002) found that priming for motion direction depends on the functional integrity of the area V5/MT. The retinotopic organisation of this area suggest that this could also be a good candidate as a neural substrate for priming of spatial position, if the stimuli are defined by direction of motion. In order to test this hypothesis we ran two experiments using similar stimuli but different tasks. The stimuli were three sinusoidal moving gratings placed on the right half of a computer screen. Subjects were asked to fixate a cross at the centre of the screen and to perform a visual discrimination task. In the first experiment subjects were required to respond to the spatial position of the target, whereas in the second experiment subjects were required to respond to the motion direction of the target. In both cases the target was defined as the grating moving in a different (opposite) direction respect to distractors. As expected (Maljkovic and Nakayama, 1994, 1996, 2000), we found priming only for the non-responding feature, that is: priming for motion direction in the first experiment and priming for spatial position in the second experiment. Repetitive Transcranial Magnetic Stimulation (rTMS) applied over V5/MT during the inter-stimulus interval (ISI) disrupted priming for motion direction (first experiment) but not priming for spatial position (second experiment), showing specificity of this area for holding the memory representation of motion direction features, but not of spatial position.
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