Distinct neural correlates of attending speed vs. coherence of motion.

Attention to specific features of moving visual stimuli modulates the activity in human cortical motion sensitive areas. In this study we employed combined event-related electrophysiological, magnetencephalographic (EEG, MEG) and hemodynamic functional magnetic resonance imaging (fMRI) measures of brain activity to investigate the precise time course and the neural correlates of feature-based attention to speed and coherence. Subjects were presented with an aperture of dots randomly moving either slow or fast, at the same time displaying a high or low level of coherence. The task was to attend either the speed or the coherence and press a button upon the high speed or high coherence stimulus respectively. When attention was directed to the speed of motion enhanced neural activity was found in the dorsal visual area V3a and in the IPL, areas previously shown to be specialized for motion processing. In contrast, when attention was directed to the coherence of motion significant hemodynamic activity was observed in the parietal areas fIPS and SPL that are specialized for the processing of complex motion patterns. Concurrent recordings of the event-related electro- and magnetencephalographic responses revealed that the speed-related attentional modulations of activity occurred at an earlier time range (around 240-290 ms), while the coherence-related ones occurred later (around 320-370 ms) post-stimulus. The current results suggest that the attentional selection of motion features modulates neural processing in the lowest-tier regions required to perform the task-critical discrimination.

Repetition suppression versus enhancement--it's quantity that matters.

Upon repetition, certain stimuli induce reduced neural responses (i.e., repetition suppression), whereas others evoke stronger signals (i.e., repetition enhancement). It has been hypothesized that stimulus properties (e.g., visibility) determine the direction of the repetition effect. Here, we show that the very same stimuli can induce both repetition suppression and enhancement, whereby the only determining factor is the number of repetitions. Repeating the same, initially novel low-visible pictures of scenes for up to 5 times enhanced the blood oxygen level-dependent (BOLD) response in scene-selective areas, that is, the parahippocampal place area (PPA) and the transverse occipital sulcus (TOS), presumably reflecting the strengthening of the internal representation. Additional repetitions (6-9) resulted in progressively attenuated neural responses indicating a more efficient representation of the now familiar stimulus. Behaviorally, repetition led to increasingly faster responses and higher visibility ratings. Novel scenes induced the largest BOLD response in the PPA and also higher activity in yet another scene-selective region, the retrospenial cortex (RSC). We propose that 2 separable processes modulate activity in the PPA: one process optimizes the internal stimulus representation and involves TOS and the other differentiates between familiar and novel scenes and involves RSC.