• The underlying neuronal mechanisms of visual perception
• Neuronal plasticity, sensory substitution, and residual vision
• Normal and physiopathalogical aging

Nowadays, imaging techniques such as visual evoked potentials, positron emission, tomography and functional magnetic resonance allow us to visualize cerebral functions during visual discrimination tasks in healthy subjects as well as in subjects suffering from cerebral pathologies.

The brain and perception axis team decided to develop the imaging section since it plays a key role in the way that we understand the basic mechanisms of normal and pathological visual processing in human subjects. The research projects described bellow rely on these modern investigative techniques as well as on electrophysiological, psychophysical, pharmacological, behavioral and biochemical techniques.

An
optimal drifting sinewave grating
(1st order) for a linear model of receptive fields (the limits of
the receptive
field are superimposed).

The underlying neuronal mechanisms
of visual perception

Since movement is such an important part of everyday life, the brain and perception axis team decided to investigate which nervous pathways and which structures analyze movement as perceived during locomotion. Analysis of the optic flow created during forward movement is essential for normal visio-motor behavior and autonomy in human subjects. Members of the team are also interested in second order mechanisms that allow the coherent perception of surrounding objects using local signs at the cortical and sub-cortical levels. Research is also being done in order to understand how the parvo and magnocellular systems work in normal human subjects and subjects suffering from amblyopia and strabismus.

A second
order stimulus
that cannot
excite a linear
receptive field.

Neuronal plasticity, sensory substitution
and residual vision

We now know that following a trauma; the visual brain can reorganize itself allowing the subject to recover some sensory functions. This plasticity is present in children and to a lesser extent in adults. Research is being done to determine the nature of trauma resistant neurons and visual pathways and to evaluate the impact of neurotro-phins on the survival of damaged brain neurons. At the moment, retinal ganglion cells that survived a lesion to the primary visual cortex are being studied. A neuroanatomic approach will allow us to determine the action of neurotrophins on the survival rate, the cell type and the nature of surviving retinofugal pathways of retinal neurons. An electrophysiological approach will allow us to determine the functional state of these neurons and of their associated pathways. This research will enable us to identify the neurotrophic factors responsible for the establishment of new connections.

Team members are also pursuing their research on the underlying mechanisms of sensory substitution. Recently, they demonstrated that young animals with lesions to the visual cortex, hence theoretically blind, were able to visually navigate their way through a labyrinth when they reached adulthood.

< Retinothalamic projections in "rewired" adult hamsters. Video micrographs showing retino-
MG projections
(A and C) and retino-LP projections
(B and D)
for two animals.

A selective surgical procedure reoriented the nervous circuits extending from the retina to the auditory cortex! Other research in human subjects showed that blind subjects were better able to locate sounds in space. It is quite likely that the visual cortex of these subjects intervenes in this task. Hence, it is now clear that brain zones that do not receive their normal signals can reorganize and participate in other sensory tasks.

< Hamster performing
a visual discrimination
task.

Finally, members are pursuing their evaluation of residual visual function in brain-damaged subjects. Psychophysical approaches will be used to determine the nature of the surviving visual functions based on age, the extent of damage and the location of the cortical lesions. Cerebral imaging based research will be used to determine the nervous pathways implicated in the preservation of visual function, the regions responsible for ocular movements associated with residual vision and to document oculomotor troubles associated with certain therapeutic lesions (i.e. diabetic subjects).

Normal and physiopathalogical aging

An important decrease in visual capacity often occurs in aging subjects. These sensory problems can considerably reduce a subject’s autonomy hence requiring some type of intervention from both society and family at significant costs. It appears important, if not essential, to understand the underlying nervous mechanisms of normal and physiopathalogical aging of the visual system (and by extension, the brain) in order to develop the proper diagnostic tools and therapeutical strategies.
The visual system of normal aging subjects is evaluated with psychophysical approaches, functional imagery and color, shape and movement discrimination (especially optic flow, since locomotion is an important factor in autonomy). Visual function in subjects suffering from certain pathologies, particularly those that cause attentional dysfunctions and visual negligence are also evaluated This research should help us develop visual learning and behavior rehabilitation tools.

Finally, research is underway to determine the role played by neurotransmitters and neuromodulators in the visual function and their impact on degenerative diseases such as Parkinson’s disease and Alzheimer’s disease. The incidence of neurodegenarative diseases is ever increasing within today’s aging population, it is essential that we understand the mechanisms that provoke sensory losses in aging subjects in order to develop therapeutic tools that will increase their quality of life.

< The visual
system may be studied by neuro anatomical and electro-
physiological techniques. This photograph shows dopaminergic amacrine cells
in the retina. An
action potentiel, that is to say an electrical impulse generated by
the neuron, is superimposed on the photograph.