Your brain has a remarkable ability to extract and process biological cues from the deluge of visual information. It is highly sensitive to the movements of living things, especially those of other people – so much so that it conjures the illusion of movement from a picture of a moving body. Although static, such pictures trigger dynamic representations of the body, ‘motor images’ containing information about movement kinematics and timing. Researchers at the Institute of Cognitive Neuroscience in London now show that biological motion is processed unconsciously, and that the speed of apparent motion alters the perception of time.
Play around with this point light display and you’ll see that your brain has specialized mechanisms dedicated to recognizing and visually processing the human body and its movements. The demo shows that the brain is adept at inferring structure from motion, so thatwe readily perceive biological motion even when a minimum amount of information about the body is available. It also shows that the motions of the body contain information about sex, weight, emotional state and even some personality traits, and that we can extract this information effortlessly.
On the other hand, the brain also infers motion from structure. Static pictures produce the illusion of movement by activating regions of the visual cortex that are sensitive to motion. In studies of apparent motion, participants shown two static images showing the initial and final position experience seeing an intermediate position linking the two, as long as the interval between them is consistent with the duration of the movement. If, however, the interval between the initial and final positions is very short, they see an impossible movement linking the two.
Motion-from-structure studies suggest that our perceptions of biological motion contain information about the timing of motions, and that apparent motion is linked to the subjecive experience of passage of time. Guido Orgs and his colleagues set out to investigate this. They captured a small number of professionally choreographed dance moves on video and produced a series of static pictures of each one, representing the initial, intermediate and final position of the dancer.
The researchers manipulated the apparent movement paths in some of the sequences by changing the order of the pictures. In the ‘short path’ picture sequences, the intermediate posture of the movement was in the middle position. In the ‘long path’ sequences (bottom), it was placed at the end, so that it produced an apparent movement path that is about one and a half times longer than those in the short path sequences.
18 undergraduates were shown five of these picture sequences interspersed with sequences of control pictures consisting of the same pictures with all biological cues removed. During each of 240 trials, the participants viewed five body movement picture sequences and five control picture sequences, in random order on a computer screen. The pictures in each sequence were shown one after the other, for one-tenth of a second each, with an interval of between 50 and 300 thousandths of a second, which was varied randomly between trials.
The pictures were surrounded by a white rectangle, which remained visible for the same duration as the sequence in the trial. Participants were told that the rectangle would be displayed for varying durations, and asked to judge how long it stayed on the screen in each trial, by pressing keys to indicate whether the duration of the rectangle was relatively longer or shorter than the one in the previous trial.
If the participants’ judgements were based on the apparent movement paths, we would expect them to perceive the rectangles presented with the long path sequences as being visible for longer. The sequences would produce a dilation of subjective time, because they would take longer to execute. But the opposite was found to be the case: the long path sequences were perceived to take less time than the short path sequences, showing that the subjective experience of the durations for which they were displayed had been compressed.
In a second experiment, the participants were shown the same picture sequences and asked to judge the velocity of the apparent movements. This time the apparent movements in the long path sequences were perceived as being faster than the apprent movements in the short path sequences. Analysis of the data revealed that the apparent speed of long path sequences was perceived on average to be one and a half times faster than that of the short path sequences.
Thus, manipulating the apparent movement paths produced directly proportional changes in the perceived speed of the movements and inversely proportional changes in time perception. That is, sequences implying the longer movement paths produced faster apparent movements and a compression of subjective time, whereas sequences implying the shorter paths produced slower apparent movements and dilated the sequence duration.
These effects were far more pronounced for sequences of body pictures than for the control sequences or for picture sequences of the body in upside-down postures, suggesting that the brain mechanisms involved are specific for realistic biomotion. And the finding that the body posture pictures affected time perception despite being unrelated to the tasks being performed shows that the brain extracts biomotion information implicitly and automatically.
The researchers suggest a possible ‘top-down’ mechanism whereby sequences of body posture pictures are automatically merged into a continuous, dynamic representation of movement, which includes information about the timing of the movements. The mechanism produces propotional changes in the apparent duration and speed of the movements, contracting time as the apparent movement path increases, and vice versa. Their findings further suggest that the mechanism only processes static biological images when they are in the correct configuration, but does not rely on conscious attempts to “see” them.
Orgs, G., et al. (2011). From Body Form to Biological Motion: The Apparent Velocity of Human Movement Biases Subjective Time. Psych. Sci. DOI: 10.1177/0956797611406446
Grosjean, M., et al. (2007). Fitts’s Law Holds for Action Perception. Psych. Sci. 18: 95-99 [PDF]
Blake, R. & Schiffrar, M. (2007). Perception of human motion. Annu. Rev. Psychol. 58: 47-73 [PDF]
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