Context Effect in Spatial Vision

Our visual performance to a stimulus can be affected by the presence of other visual stimuli. This phenomenon is called context effect. We have focused on two types of context effect. The first one, called masking in the literature, has a target stimulus overlap with a context stimulus. The other, called long-range interaction in the literature, has the context placed in a different location in the visual field from the target stimulus. In either case, the psychophysical studies involve measuring the target detection threshold in the presence of the context. By systematically changing the context and observing how the target threshold varied with the context, we are able to estimate how the context affects our visual behavior. We also study the context effect with fMRI and VEP. In these paradigms, we measure how the brain activity changes with the presence of a spatial context.

Psychophyics of masking

In a typical masking experiment, the task of an observer is to detect a designate target in the presence of a context stimulus, or a mask. As shown in the figure below. Suppose that the target is a vertical oriented period pattern. the upper row of Figure 1 illustrate the situation where the mask is a horizontal oriented pattern. In a forced choice experiment, we ask the observer whether they can tell the difference between the mask alone and the target plus mask. The minimum target contrast for the observer to tell the difference between the two (with certain probability of success) is the target threshold. The mask can be made identical with the target in every spatiotemporal properties except contrast, as show in the lower row of Figure 1. In this case, the observer is practically doing contrast discrimination Figure 1.

If one systematic measures the target threshold at various mask contrasts, one can get a target threshold vs. mask contrast (TvC) function as shown in Figure 2. This TvC function reflects the internal response function of the target detection mechanism. In order for the observer to tell the difference between the mask and mask-plus-target, the target should have enough contrast to increase the mechanism response by certain amount. We can define this amount of response increment as one unit. When there is no mask, the response to the mask alone is zero. Hence, the target absolute threshold Ct0 is the contrast the produces one unit of response in the mechanism. Suppose that the mechanisms response function has a sigmoid shape. At a low mask contrast C1, the response function is in its acceleration phase. It takes a target contrast ?C1, which is less than Ct0, to increase the response by 1. Thus the target threshold at low mask is smaller than the absolute threshold (facilitation). On the other hand, given a sufficient high mask C2, the response function is in its deceleration phase. It requires a target contrast ?C2, which is greater than Ct0, to increase response by one unit. Thus, a masking effect is observed at high masker contrast. Figure 2.

The masking paradigm has been a well established tools to explore internal properties of the visual system. I this lab, we used masking paradigm extensively on any aspects of spatial vision. Recently, we extended the masking paradigm to study long-range interactions. That is, how the visual behavior to a target stimulus be affected by stimuli projected onto different retina locations. We developed a dual masking paradigm where the target threshold is measured in the presence of a pedestal (a mask that is overlapped with the target in space) and a flank (a mask that is presented at different spatial location from the target). We showed that a flank has a very different effect on target detection from a pedestal. Beyond Psychophysics We also made effort to study the neural basis of the context effect. Corroborate with Dr. Takuji Kasamatsu at the Smith-Kettlwell Eye Research Institute, we showed that the long-range interaction among V1 neurons are contrast dependent. We also developed a computational model to explain the long-range interaction. Corroborate with Dr. Christopher Tyler at the Smith-Kettlwell Eye Research Institute, with fMRI, we showed that BOLD activation in the occipital cortex can be increased at unstimulated areas provide a lateral context is available. Currently, the neuroimaging studies of spatial context effects is the main focus of this lab. For more info about our research on this issue, please check the following references.

References

Articles

Chen, C. C., Tyler, C. W., Liu, C. L. & Wang, Y. H. (2005). Lateral modulation of BOLD activation in unstimulated regions of the human visual cortex. Neuroimage, 24:802-9.

Chen, C. C. & Foley, J. M. (2004). Pattern detection: Interactions between oriented and concentric patterns. Vision Research, 44 915-924.

Chen, C. C. Tyler, C. W. (2002). Lateral modulation of contrast discrimination: Flanker orientation effects. Journal of Vision, 2, 530-538.

Chen, C. C., Kasamatsu, T., Polat, U. & Norcia, A. M. (2001). . Neuroreport, 12, 655-661.

Chen, C. C. & Tyler, C. W. (2001). Lateral sensitivity modulation explains the flanker effect in contrast discrimination. The Proceedings of the Royal Society (London) Series B, 268, 509-516.

Chen, C. C., Foley, J. M. & Brainard, D. H. (2000). Detection of chromoluminance patterns on chromoluminance pedestals I: Threshold measurements. Vision Research, 40, 773-788.

Chen, C. C., Foley, J. M. & Brainard, D. H. (2000). Detection of chromoluminance patterns on chromoluminance pedestals II: Model. Vision Research, 40, 789-803.

Chen, C. C. & Tyler, C. W. (2000). Spatial long-range modulation of contrast discrimination. SPIE Proceedings, 4080, 87-93.

Chen, C. C., Foley, J. M. & Brainard, D. H. (1999). A divisive inhibition model for chromoluminance pattern discrimination. SPIE proceedings, 3644, 78-87.

Foley, J. M. & Chen C.C. (1999). Pattern detection in the presence of maskers that differ in spatial phase and temporal offset: Threshold measurements and a model. Vision Research, 39, 3855-3872.

Chen, C. C., Foley, J. M. and Brainard, D. H. (1997). Detecting chromatic patterns on chromatic pattern pedestals. IS&T Proceedings: Optics and Imaging in the Information Age, 19-24.

Foley, J. M. & Chen C.C. (1997). Analysis of the effect of pattern adaptation on pattern pedestal effect: A two-process model. Vision Research, 37, 2779-2788.

Conference presentations

Chen, C.-C., Chang, H.-C., Liu, C.-L., Chen, C.-F., & Han, H.-Y. (2004). The human brain responses to Glass patterns: The effects of signal to noise ratio. Journal of Vision, 4(8), 714a.

Chen, C. C. & Tyler, C. W. (2003). Mapping psychophysical non-classic receptive field with dual masking. Paper presented in the 3nd Annual Meeting of the Vision Sciences Society, Sarasota, FL.

Tyler, C. W., Chen, C. C., Liu, C. L. & Wang, Y. H (2003). FMRI Reveals Rebound Activation Lateral to Stimulated Regions of Retinotopic V1. Investigative Ophthalmology & Vision Science (suppl.), 44, s4201.

Chen, C. C. & Tyler, C. W. (2002). Lateral modulation of BOLD activation in unstimulated regions in the retinotopic area V1. Paper presented in the Society for Neuroscience annual meeting.

Chen, C. C. & Tyler, C. W. (2002). Lateral masking with chromoluminance patterns. Paper presented in OSA vision meeting, San Francisco, CA.

Chen, C. C. & Tyler, C. W. (2002). Lateral modulation of contrast discrimination: Flanker orientation and location effects. Paper presented in the 2nd Annual Meeting of the Vision Sciences Society, Sarasota, FL.

Chen, C. C. & Tyler, C. W. (2001). Lateral modulation of contrast discrimination: Flanker orientation effects. Paper presented in OSA/UCI conference Oct. 2001, Irving, CA.

Chen, C. C., Tyler, C. W. & Norcia, A.M. (2000). The time course of long-range modulation on contrast discrimination. Investigative Ophthalmology & Vision Science (suppl.), 41, 803.

Chen, C. C. & Tyler, (1999). Long-range modulation of contrast discrimination: flanker contrast effect. Paper presented in 1999 Optical Society of America Annual Meeting, Santa Clara, CA.

Chen, C. C. & Kasamatsu, T. (1998). A sensitivity modulation model for lateral interaction among striate cells in cat visual cortex. Abstract of Society for Neuroscience, 24, 1979.

Chen, C. C. & Tyler, C. W. (1998). Lateral long-range modulation of spatial orientation tuning functions. Paper presented in 1998 Optical Society of America Annual Meeting, Baltimore, MD.

Chen, C. C., & Foley, J. M. (1997). Contrast pattern masking with oriented and concentric patterns. Paper presented in 1997 Optical Society of America Annual Meeting, Long Beach, CA.

Chen, C. C., Foley, J. M. & Brainard, D. H. (1997). A three-mechanism model of chromo-luminance pattern masking. Investigative Ophthalmology and Visual Science. Investigative Ophthalmology & Vision Science (suppl.), 38, 255.

Chen, C. C., Foley, J. M. & Brainard, D. H. (1996). Detecting chromatic patterns on chromatic pattern pedestals. Paper presented in 1996 Optical Society of America Annual Meeting, Rochester, NY.

Chen, C. C., Foley, J. M. & Brainard, D. H. (1996). A masking analysis of the chromatic properties of pattern detection mechanisms. Investigative Ophthalmology & Vision Science (suppl.), 37, 1064.

Chen, C. C. & Foley, J. M. (1995). Effect of pattern adaptation on pattern masking: An experiment and a model. Investigative Ophthalmology & Vision Science (suppl.), 36, 465.

Foley, J. M., Greenlee, M. W. & Chen C. C. (1995). Effect of pattern adaptation on pattern masking and pattern matching: Adaptation reduces contrast gain. Paper presented in1995 Optical Society of America Annual Meeting, Portland, OR.