Consciousness

What is sight?

Should you trust what you see?

Does the brain process visual information outside of awareness?

These are the questions we seek to answer in this project. We are conducting multiple studies that are concerned with how visual information is processed both within and outside of conscious awareness, and how this information may be perceived objectively, closely reflecting the properties of the physical world, or subjectively, reflecting our own personal biases and experiences, leading to illusory perceptual experiences. There are multiple techniques we use.

One technique is continuous flash suppression. Using mirrors and a computer monitor, as shown in the diagram below, one can present images subliminally without the participant being consciously aware of them. Specifically, this is done by presenting two different images (A and B) to each eye (Left and Right). The brain is not used to receiving such conflicting information and resolves the conflict by bringing into conscious awareness image B while suppressing the other image A from entering consciousness.

Courtesy of Joshua Sherman, 4th year honour student.
Courtesy of Joshua Sherman, 4th year honour student in 2015.

Another technique we use are illusions. An illusion is a perceptual departure from reality where the discrepancy between what is perceived and physically correct is not recognised. We favour a Bayesian account for explaining illusions. Under this model, the brain is a machine that derives a percept from computing an optimal combination between the existing sensory evidence (e.g., what is captured by the retina) and our knowledge acquired from previous experiences. It then follows that our perceptions are highly dependent on the rules that the brain has learned about the world. One of these rules would be that the physical properties of an object are always constant. Namely, their size, shape, colour, and location are always the same regardless of changes in retinal information that arise from changes in viewing distance, angle of perspective, and lighting conditions.

To study these mechanisms, one can trick the brain into perceiving something different by providing misleading contextual information so that it perceptually rescales a stimulus in a way that it should not. For example, in the Ponzo illusion, shown below, the converging railroad tracks in the background indicates to the brain that the upper line is farther away in the scene than the bottom line. Ordinarily, objects of the same size will have a smaller retinal image size when they are further away. Because of these perspective cues, the brain interprets the upper line as being further away and therefore perceptually rescales it as being longer. After all, under normal circumstances, size–distance scaling is applied to counteract variations in retinal image size. The use of illusions allows us to deconstruct how the brain enables people to perceptually rescale objects.

ponzoThe size-weight (SW) illusion is one of several illusions that we find particularly interesting. In this illusion, people perceive the smaller of two equally weighted objects as heavier when those objects are lifted in succession. Remarkably, these false perceptions of weight occur independently of our ability to lift them. Namely, people quickly learn to scale their fingertip forces to the actual weight of objects, even though they fail to ever learn perceptually that both objects had exactly the same weight. In other words, SW illusions persisted despite the fact that fingertip forces were exquisitely matched to the weight of the objects. The lab is equipped with force transducers that enable us to record fingertip forces that are applied during object lifting so that we can compare and dissociate what we “perceive” as being weight versus the motor commands we project to the arm muscles to “lift” weight.

For additional information underlying the conceptual framework of this project:

Sperandio, I., & Chouinard, P. A. (2015). The mechanisms of size constancy. Multisensory Research, 28(3-4), 253-283.

Chouinard, P. A., Large, M. E., Chang, E. C., & Goodale, M. A. (2009). Dissociable neural mechanisms for determining the perceived heaviness of objects and the predicted weight of objects during lifting: An fMRI investigation of the size–weight illusion. Neuroimage, 44(1), 200-212.

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