Sensory Plasticity and Perceptual Learning in AAL Environments

When wearing head-mounting displays (HMD), subjects have to learn to cope with a changed visual input, requiring perceptual learning. ‘Learning’ has been defined as ‘a modification of observable behaviour as a result of preceding experience’, and we define perceptual learning as any change of responses to sensory stimulation following after (extended) training of a perceptual task or a different type of input, such as wearing an HMD. We investigated those parts of the learning process that seem to be relatively independent from conscious, or declarative forms of learning (such as learning a poem) but that rely on rather ‘low-level’ modifications of the central nervous system and that have some resemblance with procedural forms of learning (such as learning to ride a bike). These forms of learning occur also when we wear a new pair of glasses with a changed refraction (changing the size of the retinal image) and when wearing prism goggles in adaptation experiments.

We investigate the extent to which training alters visual performance employing so-called hyperacuity tasks as sensitive probes where observers reach thresholds far below the diameter (and spacing) of foveal photoreceptors, and the adaptation of eye-hand coordination using prism goggles. We performed both psychophysical and sum-potential methods in healthy human subjects. Through training, performance in some hyperacuity tasks improves by a factor of two or more within an hour. This improvement is specific for a number of rather low-level characteristics of the stimulus such as its exact orientation: improvement of thresholds does not transfer across a change in stimulus orientation of 10 degrees or more. The improvement is also at least partly specific for the eye used during monocular training, for the exact task trained, with no transfer between a stimulus consisting of three (almost) aligned dots where the observer has to discriminate between a lateral offset of the middle point relative to an imaginary line through the endpoints (= vernier discrimination) and (almost) the same stimulus but where the observer has to discriminate between an offset towards one or the other of the endpoints, along the imaginary line between them. There is also no transfer between different positions in the visual field, that is, improvement through learning is specific for visual field position. This specificity would argue for a relatively early location, along the visual pathways, of the neuronal changes underlying perceptual learning, and there are electrophysiological results both in primates and psychophysical results in humans indicating changes in neuronal activity in the primary visual cortex. In line with this hypothesis, we find significant improvement through training in simple visual tasks even in patients suffering from amnesic syndromes - even if the patients have no memory whatsoever that they ever trained the task, their performance improves and stays at the higher level for at least several weeks. On the other hand, we find a strong influence of error feedback and attention on perceptual learning, indicating that the adult primary visual cortex is much more plastic than previously thought. Therefore, training might improve performance not only in normal observers, but also in patients suffering from circumscribed brain damage from different causes.

Our results indicate that the notion, generally accepted until a few years ago, that the early visual cortices in adults lacks plasticity, must be wrong. Perceptual learning differs from other forms of learning in that it involves structural and/or functional changes at least partly in primary sensory cortices, and may account for complex phenomena, including some that are often thought to be cognitive. This perceptual plasticity allows humans to adapt fast to wearing HMDs.

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