Backward-masking: The effect of the duration of the second stimulus on recognition of the first stimulus

Modulations of temporal perception by consciously and unconsciously perceived stimuli

Temporal processing is of fundamental importance to the understanding of orders and durations of events in daily life. While recent research found that the perception of event durations is modulated by the visibility of perceived stimuli, it is still not known whether consciously and unconsciously perceived stimuli modulate temporal perception of a following stimulus in a similar vein. We investigated this using a temporal-comparison task that requires duration judgments of a standard stimulus and a probe. A prime prior to the standard stimulus reduced the subjective duration of the standard stimulus when observers were aware of the prime, and this effect changed with the temporal distance between the prime and standard. In contrast, a prime increased the subjective duration of the following standard stimulus when observers were unaware of the presence of the prime. Our findings indicate that a temporally neighbouring transient stimulus produces essentially different effects on temporal perception of following events when it is consciously and unconsciously perceived, leading to compression and expansion of subjective time, respectively.

Human brain response to visual stimulus between lower/upper visual fields and cerebral hemispheres

We studied the human brain response to visual stimulation in which a square area was randomly presented in upper and lower visual fields (VFs). Seven normal volunteers carried out a contrast-based visual search task. Magnetic responses were detected in the bilateral parietal regions at 200-250 ms after stimulus onset. We compared the response latencies and strengths of the essential single sensor and root mean square (RMS) of the regions. The former evaluates the strength of neural activity with relatively high spatial resolution, while the latter evaluates the global neural activity. The single sensor and RMS latencies for the lower left VF were significantly longer than that for the upper left (paired t-test, P<0.05). The strengths did not differ between the upper and lower left VFs. There was no significant difference in latency or strength between the upper right and lower right VFs. These findings suggest that only left VF has different response properties in the upper versus lower VF, and that both local and global extrastriate activities are responsible for this anisotropy.

Discrepancy between reaction time and visual evoked magnetic response latency under priming

We examined effects of the interval between first (S1) and second (S2) visual stimuli, stimulus-onset asynchrony (SOA), on the cortical processing of S2 using magnetoencephalography (MEG). S1 (a circle) and S2 (a cross) were presented at the same location. When the SOA was short (33, 50, and 83 ms), the major deflection of the fields evoked in response to S2 (2M) was difficult to distinguish from that evoked by S1 (1M). However, when the SOA was long (350 and 453 ms), paired stimuli clearly evoked 1M and 2M. At a long SOA, the peak latency of 2M (around 200 ms) was significantly longer than that in the control (S2 alone) condition (around 180 ms). However, in contrast, the reaction time to S2 in all SOA conditions was significantly shortened as compared with that in the control condition, suggesting dissociation between processing in the visual cortex and motor processing.