Department of Brain and Cognitive Science, MIT, USA
Friday 10 June 2005
Seminar Room B10 (Basement)
Alexandra House, 17 Queen Square, London, WC1N 3AR
Plasticity and Dynamics of Visual Cortex Networks
A key emergent property of the primary visual cortex (V1) is the orientation selectivity of its neurons. I will describe recent experiments from our laboratory that demonstrate remarkable bottom-up and top-down plasticity in orientation networks of the adult cortex. Together, these studies demonstrate that vision is inference, and its basis is continually recalibrated even at the earliest stages of cortical processing.
The orientation preference of neurons in adult V1 changes systematically after short-term exposure to one stimulus orientation. Such reversible physiological shifts in orientation preference parallel the orientation tilt aftereffect observed psychophysically. The orientation change is caused by a decrease in response at the adapting orientation and an increase in response at a new preferred orientation that is shifted away from the adapting orientation. Thus, orientation plasticity is an active time-dependent process that involves both response depression and enhancement and likely involves intracortical interactions.
Orientation plasticity is seen primarily at specific foci in V1. V1 neurons are clustered according to their orientation preference in iso-orientation domains that converge at singularities or pinwheel centers. Neurons at or near pinwheel centers show pronounced changes in orientation preference after adaptation with an oriented stimulus, while neurons in iso-orientation domains show minimal changes. Intracellular measurements of excitatory and inhibitory synaptic conductances reveal that excitation and inhibition balance each other at all locations in the cortex, despite a wide diversity of local orientations. This balance is particularly critical at pinwheels, where small changes in synaptic drive can alter orientation tuning quite significantly.
Neurons in V1 of alert, behaving monkeys also exhibit short-term orientation plasticity after very brief adaptation with an oriented stimulus, on the time scale of visual fixation. Adaptation with stimuli that are orthogonal to a neuronís preferred orientation does not alter the preferred orientation but sharpens orientation tuning. Thus, successive fixation on dissimilar image patches, as happens during natural vision, combined with mechanisms of rapid cortical plasticity, actually improves orientation discrimination.
Finally, natural vision involves judgements about where to look next, based on an internal model of the visual world. Experiments in behaving monkeys in which information about future stimulus locations can be acquired in one set of trials but not in another demonstrate that V1 neurons signal the acquisition of internal representations. Such Bayesian updating of responses based on statistical learning is fundamental for higher-level vision, and in particular for deriving inferences about the structure of the visual world.
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