| Abstract No.: | A-D1134 |
| Country: | Canada |
| | |
| Title: | SUBJECTIVE DURATION DISTORTIONS PRODUCED BY MOTION DIRECTION IS POSITIVELY CORRELATED WITH NEURAL ACTIVITY IN AREA MT |
| | |
| Authors/Affiliations: | 1 Navid Sadeghi*; 1 Sameer Apte; 1 Erik Cook;
1 McGill University, Montreal, QC, Canada
|
| | |
| Content: | Objectives: Our subjective perception of the duration of an event does not always match its objective duration. For example, it has been shown that human subjects perceive the duration of an oddball embedded in a train of repeated stimuli to be longer. Since lower-level sensory areas might respond less to a repeated stimulus (adaptation or habituation) or respond more to a novel stimulus (dishabituation), we wanted to know whether there is a relationship between durations perceived for an event and the activity of sensory cortex. In contrast to previous studies that used complex visual shapes making it difficult to pinpoint a specific cortical area, we used linear motion for which Middle Temporal (MT) cortex is known to play a pivotal perceptual role.
Methods: A random number of motion pulses made from random dot kinematograms (75% coherent motion pulses of 200 msec duration, separated by 400 msec of random motion) were presented in each trial. Manipulating the direction of motion allowed us to create three conditions, which were randomly interleaved in a block of trials: having the same direction for all pulses (“familiar” condition), 180 degrees opposite the repeated ones in one pulse (“novel” condition), or a random direction in each pulse (“neutral” condition). For human psychophysics, the last pulse varied in duration and the subjects responded whether the last pulse had been longer or shorter than the other pulses observed in that trial. The oddball in human “novel” condition occurred on the last pulse. For electrophysiology, we recorded from area MT of one Macaque monkey as the monkey observed the three conditions in a similar stimulus, while fixating and attending to the dot patch to detect a noise-related signal. The direction of the oddball pulse in the monkey’s “novel” condition was matched to the cell’s preferred direction. We were therefore able to compare the responses of MT neurons to the preferred direction when it has been preceded by preferred direction repetitions (“familiar” condition), null direction repetitions (“novel” condition), or random motion directions (“neutral” condition). For both experiments, the “neutral” condition served as an important control for quantifying the responses to “familiar” and “novel” conditions.
Results: Compared to the “neutral” control condition, psychometric function curves from six subjects were shifted to the right for the “familiar” condition and shifted to the left for the “novel” condition, indicating duration contraction and duration expansion, respectively. Mirroring those changes, the response of MT neurons was greater when the preferred direction was presented as an oddball and less when it was presented repeatedly, compared to when preceded by random directions. In addition, there was an increase of the effects in time both for duration distortions found in human subjects and the firing rate changes observed in electrophysiology.
Conclusion: Humans can experience both subjective contraction and expansion of time using a simple motion stimulus. Importantly, our electrophysiological results suggest a direct relationship between the activity of a sensory cortical area that encodes low-level visual motion properties and subjective perception of time. |
| | |
| Back |
|
