While humans complete this process with ease, from a computational neuroscience perspective there is still much to be learned as to how this complex perceptual problem is solved.
One tool which many research studies in this area use is a display stimuli called a point light walker.
Research in this area seeks to identify the specific brain regions or circuits responsible for processing the information which the visual system perceives in the world.
While single-cell recording is not conducted on humans, this research uses neuroimaging methods such as fMRI, PET, EEG/ERP to collect information on what brain areas become active when executing biological motion perception tasks, such as viewing point light walker stimuli.
[5][6][7][8][9][10] Valuable information can also be learned from cases where a patient has suffered from some sort of neurological damage and consequently loses certain functionalities of neural processing.
One patient with bilateral lesions that included the human homologue of area MT, lost their ability to see biological motion when the stimulus was embedded in noise, a task which the average observer is able to complete.
Another study on stroke patients sustaining lesions to their superior temporal and premotor frontal areas showed deficits in their processing of biological motion stimuli, thereby implicating these areas as important to that perception process.
A case study conducted on a patient with bilateral lesions involving the posterior visual pathways and effecting the lateral parietal-temporal-occipital cortex struggled with early motion tasks, and yet was able to perceive the biological motion of a point light walker, a higher-order task.
Previous research has not untangled the circumstances under which local motion cues are needed or only additive.
This model looks at how form-only cues can replicate psychophysical results of biological motion perception.
Each dot in a given stimulus frame is compared to the nearest limb location on a template and these combined, weighted distances are outputted by the function: where
The best fitting template was then selected by a winner-takes-all mechanism and entered into a leaky integrator: where
This model shows the possibility of the use of form cues, but can be criticized for a lack of ecological validity.
An experiment was conducted to test the validity of this model, in which subjects are presented moving point-light and stick-figure walking stimuli.
Each frame of the walking stimulus is matched to a posture template, the progression of which is recorded on a 2D posture–time plot that implies motion recognition.
[16] 3D coordinates of the twelve major joints (feet, knees, hips, hands, elbows, and shoulders) were tracked and interpolated between to generate limb motion.
Five sets of 2D projections were created: leftward, frontward, rightward, and the two 45° intermediate orientations.
Finally, projections of the nine walkers were normalized for walking speed (1.39 seconds at 100 frames per cycle), height, and hip location in posture space.
Template Matching Template matching is computed by simulating posture selective neurons as described by [17] A neuron is excited by similarity to a static frame of the walker stimulus.
Response Simulation The neuron most closely resembling the posture of the walking stimulus changes over time.
These following assumptions were brought about from results of both statistical analysis and psychophysical experiments.
The analysis found that dominant local optic flow features are very similar in both full body 2d stimuli and point light walkers (Figure 1).
Through psychophysical experiments, it was found that subjects could recognize biological motion using a CFS stimulus which contained opponent motion in the horizontal direction but randomly moving dots in the horizontal direction (Figure 2).
[22] Because of the movement of the dots, this stimulus could not be fit to a human skeleton model suggesting that biological motion recognition may not heavily rely on form as a critical feature.
Also, the psychophysical experiments showed that subjects similarly recognize biological motion for both the CFS stimulus and SPS, a stimulus in which dots of the point light walker were reassigned to different positions within the human body shape for every nth frame thereby highlights the importance of form vs the motion (Fig.1.).
is the rectangular speed tuning function such that The direction-tuning of motion energy detectors are given by where
These detectors were modeled after neurons sensitive to opponent motion such as the ones in MT and medial superior temporal (MST).
is a diagonal matrix which contains elements that have been set during training and correspond to vector u.
The activity of the optic flow pattern neuron is modeled by the following equation of in which
equals the activity of optic flow pattern detector at kth frame in sequence l. Using correct determination of walking direction of both the CFS and SPS stimulus, the model was able to replicate similar results as the psychophysical experiments.