MEG responses in the human brain during the selection of visual targets

Abstract

It has been previously demonstrated that magnetoencephalography (MEG) is a valuable tool to measure electromagnetic responses in the human brain during a variety of tasks. Here we combine MEG and magnetic resonance imaging (MRI) on healthy humans to study the brain electromagnetic responses during a task that involves selecting a target among different stimuli, sustaining attention on that target and finally the detection of a change in one of the target's attributes. We recorded MEG-evoked responses while subjects were presented with two differently colored moving random dot patterns (RDPs) left and right of a central fixation spot (the target and distractor). Subjects were required to: first, select the target RDP and then identify a transient change either in the direction OR color of this target. Target selection was based on the rule ‘red[[gt]]blue[[gt]]green’ being the target the stimulus with the highest color rank. Preliminary results (n=5) revealed that selection based on this rule was accompanied by contralateral activation of the inferior parietal lobe (IPL) at about 170 ms after target/distractor onset followed about 40 ms later by a lateralized peak of activation in the right IPL. On the other hand, detection of direction changes in the target resulted in contralateral activation of area MT/V5 while color changes activated contralateral extrastriate area V4. The activation latencies were about 190 ms after change onset in both cases. Our findings suggest that target selection in the human brain involves high level areas such as the IPL while change detection mainly involve extrastriate visual areas encoding the changing visual attribute.