Optical Illusions and Human Visual System: Can we reveal more? CIS Micro-Grant Executive Report and Summary of Results

Transcription

1 Optical Illusions and Human Visual System: Can we reveal more? CIS Micro-Grant 2011 Executive Report and Summary of Results Prepared By: Principal Investigator: Siddharth Khullar 1, Ph.D. Candidate 1. Chester F. Carlson Center for Imaging Science Submitted to: Prof. John Schott, Director, CIS, RIT. 1

2 1. Abstract Illusions may be described as the stimuli that exist at the extremes of our visual system s evolution profile, making them difficult to handle. There is no unique cause for optical illusions in the visual system; they may be a result of assumptions made by the visual system or interestingly, represent an active recalibration. We examined the low-level and behavioral response of the visual system to perceiving a particular type of optical illusion known as the motion after effect (MAE). Preliminary data was collected from a group of 15 healthy subjects with normal (or corrected vision) using a SMI eye-tracker setup. Each subject was presented with the illusory stimuli on a computer screen and was asked to compare the perceived effect to a group of control stimuli. The subjects were presented the illusion as multiple trials of unguided and guided eye movements. Each subject s response was recorded as behavioral and eye-tracking data. We analyzed the behavioral data for particular patterns based on the unguided and variants of guided eye movements. The eye-tracking data was used to create the stimulus for the guided trials. Various hypothesis and the results of our experiment are presented in this executive report in addition to the limitations associated with experiment design and future work for improvement. 1. Background The Waterfall illusion can be explained by this theory where objects seem to move upward in an illusory sense, after the observer has looked at a waterfall for considerable amount of time. This illusion is a consequent result of increased firing rate of competing neurons, which produces the illusion of stationary things to crawl upwards. First findings of this fatigue explanation were reported in [1] in 1963 and later confirmed by PET and fmri studies by [2], [3] respectively. Although this explanation plausible, but not necessarily the only reason for aftereffect illusion. Wohlgemuth [4] showed that even if the eyes are closed after viewing the waterfall [1], and then opened after a while, the effect can be still be well observed. In 1965, McCollough discovered how viewing colored stripes at specific orientations can result in aftereffect illusions that last for days, which further raised questions about the fatigue theory. Conclusively, it can be said that the motion aftereffect is still not properly understood and its proper reasoning is yet to be established. A comprehensive history and studies related to this particular illusion can be found in [5] 2. Hypothesis In this study, we initially proposed to investigate the underlying phenomenology that causes the MAE illusion using a multi-modal setup comprised of a 64-channel EEG machine and an eye-tracker. However, in the best interest of this project, we first validated our initial experiment design by conducting a pilot-study. This study was run using the eye-tracker only in order to avoid complexity by using two modalities together. Furthermore, our perceived goal through this study was revised in order to exploit the illusion itself. In order to identify if there occurs a lapse in severity of its effect when 2

3 viewed simultaneously with another real moving stimulus, subjects were presented with another moving stimulus at the instant they experienced the static illusion. We hypothesize that there may exist a rivalry between the attention and visual networks of the brain if an illusory and a real moving stimulus are presented simultaneously. The ultimate goal of our experiment was to investigate if by adding a real moving stimulus, the perceived effect of the illusory stimulus gets alleviated. Using the eye-tracker, we were able to record each subject s eye-movements in response to viewing the MAE illusion. The co-ordinates of these gaze patterns were then used to present a moving circular stimulus. We recorded behavioral results where each subject compared their personal experience of illusion s severity to ten different control stimuli created to depict the static illusion s motion patterns. 3. Experiment Design The experiment comprised five different paradigms for each subject. There were 4 trials for every paradigm where a trial comprised calibration, stimulus presentation, and lastly behavioral response. In our experiment, a trial further is defined as a combination of the following three events (1) Rotating trizonal plates presented for T s secs, (2) Static image presented for T im seconds, and (3) Neutral grey screen (rest period) presented for T r seconds. Figure 1: Experiment Design. The visual stimuli were presented in a block sequence as shown in Fig. 1. Each paradigm consisted of 4 trials with T s є (15s, 30s, 45s, 60s), T im є (6s, 8s, 10s, 12s), and T r є (6s, 8s, 10s, 12s) as presented in Fig. 1(a). The lengths for T s were picked arbitrarily whereas the other two were estimated through a calibration experiment on 10 separate subjects. We estimated that on average, the period that the MAE illusion lasted for varies linearly with length of the trizonal plate presentation time (T s ) by a factor of 2. We took advantage of this fact, and presented two static images- one of a rock and another of grey screen over this period. This was done in order to record the eye-tracking patterns 3

4 when the subjects view a real-world image as well as when they are just observing a grey screen. We were interested in investigating the eye-movements during the neutral grey screen viewing while the illusion was still under play. This process is illustrated in Fig. 1 (b). As mentioned before, we utilized 5 different paradigms to investigate our hypothesis that are briefly explained here and illustrated in Fig. 2. (A) Unguided: subject was asked to view and experience the MAE illusion naturally without looking away from the screen. (B) Fixated: subject fixated at a static red dot placed in the center of the static rock image. (C) Guided Rock: The saccade and gaze patterns were extracted from the T im time-frame (while they viewed the rock image) from (a) and utilized as motion vectors for a circular black dot. The moving black dot was then overlaid the static rock image and presented during the new T im. (D) Guided Rest: Same as (c) except the motion vectors used here were extracted from saccade/gaze patterns from the T r time frame. (E) Guided Random: Same as (c) except the motion vectors used here were extracted from participants other than the subject population. 4. Results Figure 2: Paradigm Design. This study was conducted with a few clearly defined goals and few future directions under consideration. As mentioned before, one of the primary goals we attempted to address in this study was to validate and optimize an experiment design that may be suitable for the future directions of this work. We analyzed the behavioral responses of 14 subjects and did not find any specific patterns that were statistically significant (p < 0.05). We expected to see reduced effect of the illusion for paradigms (C), (D) and (E) in no specific order as presented in Fig. 3. However, the actual measurements were extremely noisy and the behavioral results varied from person-to-person. In some cases, they were completely counter-intuitive. These observations provided us with constructive ideas about the design of various control stimuli and possibly signaling towards a need for a control experiment in order to 4

5 rank the control stimuli prior to using them in an actual experiment setting. This will be done for the future work. Figure 3: Expected results Overall, we were able to achieve the following milestones through this research study: 1. A strong understanding of the MAE illusion in conjunction to eye-movement patterns for a group of 15 subjects. 2. Identify loop-holes in our experiment design and other aspects of designing a subject-based study. 3. For the PI, valuable leadership skills through mentoring two undergraduate students by stimulating interest in functional brain imaging that is a new area for research at CIS. 5. Future Directions For the next experiment, we are interested in utilizing multiple modalities such as eyetracking, EEG, and fmri in order to acquire data with high temporal and spatial resolution as well as eye-movement patterns in response to the new experiment design. Unfortunately, we were unable to acquire an EEG machine for the purpose of this pilot study after repeated attempts. The work done in the course of the project will be made available to RIT and CIS students in form of a technical report or a white paper. This research will be presented at various group meetings such as the CIS weekly seminar and the weekly IS&T meetings. This will help gain interest and support for writing grants to external funding agencies such as NSF and NIH for further extension of research. If feasible, this research will be presented at the prestigious Imagine RIT innovation festival. We plan to achieve the following milestones in the future: 5

6 4. Identifying unique patterns that may be strongly associated with perceiving illusory motion across a range of subjects and patterns that are unique to the whole group. 5. Submit a full-scale conference paper highlighting the findings of this pilot study in addition to our multi-modal experiment to a prestigious neuroscience conference such as Annual meeting Society for Neuroscience (SfN) or Organization for the Human brain Mapping 6. Make the recorded EEG, eye-tracking and fmri data available openly for free use by researchers in the field of imaging and signal processing. The long term goal, as mentioned before, is being able to develop a model that is representative of the reverse process involved in perceiving illusions. Such a visual stimulus/process could be finally used to alleviate the whirling or spinning sensations experienced by a vertigo patient. Such a stimulus could be packaged in form of a mobile phone application and distributed easily as an open source product. We truly believe that there is a possible connection between how such illusions are perceived and some of the visual processing conditions such as motion sickness and vertigo. Note that, we are not interested in handling conditions that are caused due to vestibular fluid s displacement and deviations. 4. Summary of Funds $300 used for paying subjects. 6 weeks of stipend for two undergraduate students ($3500). 5. References 1. Barlow, H.B. and Hill, R.M. (1963) Evidence for a physiological explanation for the waterfall phenomenon and figural aftereffects. Nature 200, Hautzel H, The motion after effect: more than area V5/MT? J. Brain Research, 892, pp ,( 2001). 3. Tootell, R.B.H. et al. (1995) Visual motion aftereffect in human cortical area MT revealed by functional magnetic resonance imaging Nature 375, Wohlgemuth, A. (1911) On the aftereffect of seen movement. Br. J.Psychol. Monogr. (Suppl.) 1, Antis, A. (1998) The motion aftereffect. Trends in Cog. Sci,2(3),