Neural basis of conscious visual perception
How is retinal information processed?
How does the brain enable us to recognise objects?
How does the brain guide our limbs to manually make use of objects?
These are some of the questions we seek to answer in Philippe Chouinard's Vision and Action Lab (Bendigo and Bundoora).
In our research, we carry out magnetic resonance imaging (MRI).
- MRI is an imaging technique that uses a powerful magnetic field and radio-frequencies to obtain detailed images of the brain.
- Functional MRI is an application of MRI that measures brain activity by detecting associated changes in blood flow and deoxygenated haemoglobin – as indexed by the so-called BOLD (blood-oxygen-level dependent) signal. When an area of the brain is in use, blood flows to that area and releases oxygen. By presenting a stimulus to the participant, we can determine whether or not a particular brain area reveals a response in the BOLD signal. If it does, we can then infer that this area plays a role in processing that particular stimulus.
We use fMRI to study: 1) how the retina is represented in the brain, and 2) how the brain uses vision for perception and action.
As light enters the eye, it gets refracted through the cornea and the lens such that an inverted (upside down) image is projected onto the retina. This topographical representation of the inverted image on the retina is maintained throughout the visual system as it is relayed to the thalamus and then to various cortical structures.
We map how the retina is represented in both the thalamus and the cortex using the "phase-encoding approach" developed by Marty Sereno and colleagues (1995).
The approach is based on the principle that when a stimulus is presented in a cyclical manner, the fMRI blood-oxygen-level dependent (BOLD) signal will follow a similar cyclical profile (see part A in the figure 1. A movie of a checkerboard stimulus moving across different parts of the visual field is played to the participant in a repeating loop (see parts B & C in figure 1). The BOLD signal is then analysed to determine where in the visual field the response is the highest. These experiments consistently give rise (at the participant level) to retinotopic maps similar to those shown in parts D & E in the figure 1 – each colour superimposed on the brain represents a different part of the visual field.
Goodale and Milner's Two Stream Hypothesis
Much of the conceptual framework for our fMRI research is guided by Melvyn Goodale and David Milner's two-stream hypothesis (1992). According to their model, the ventral stream from the primary visual cortex to the temporal cortex (purple pathway shown in figure 2) analyses visual information for the purposes of perception while the dorsal stream from the primary visual cortex to the parietal cortex (green pathway shown in figure 2) is used for the online visual control of limb movements.
Some of the strongest evidence for this theory arises from patients with brain damage.
- Damage to the ventral stream results in visual agnosia, in which people are unable to recognise objects visually.
- In contrast, damage to the dorsal stream results in optic ataxia, in which people are unable to use vision to guide their limb in space towards an end-point, such as moving their hand towards a three-dimensional object for the purposes of grasping it.
- Equally important, damage to the ventral stream does not result in optic ataxia and damage to the dorsal stream does not lead to classical forms of visual agnosia.
The neural basis for selecting actions based on concepts
There are many instances in which people select actions in response to visual stimuli that do not spatially relate to the actions that they specify – such as our limb movements in response to traffic lights while driving a car or the selection of a functional hand posture on a tool. Actions associated with these stimuli are learned and require the brain to first recognise and retrieve the conceptual meaning of the stimulus, which requires the ventral stream, before an action can be executed, which requires the dorsal stream. According to the two stream model described above, these types of goal-directed actions must require an interaction between the ventral and dorsal streams of visual processing – neither of the two streams can accomplish these actions on their own.
Our fMRI studies focus more on how the two streams interact than how they are dissociable. Specifically, we are most interested in studying the neural mechanisms that underlie the execution of a number of skills that depend on learned associations between a visual stimulus and an action (e.g. playing the piano by sight-reading; driving a car while responding to traffic signals; selecting a functional hand posture on a tool; applying the appropriate fingertip forces for lifting an object with a known weight).
- Goodale, M. A., & Milner, A. D. (1992). Separate visual pathways for perception and action. Trends in neurosciences, 15(1), 20-25.
- Sereno, M. I., Dale, A. M., Reppas, J. B., Kwong, K. K., Belliveau, J. W., Brady, T. J., ... & Tootell, R. B. (1995). Borders of multiple visual areas in humans revealed by functional magnetic resonance imaging. Science, 268(5212), 889-893.
- Sperandio, I., & Chouinard, P. A. (2015). The mechanisms of size constancy. Multisensory Research, 28, 253-283.