Abstract
Vision research has the potential to reveal fundamental mechanisms underlying sensory experience. Causal experimental approaches, such as electrical microstimulation, provide a unique opportunity to test the direct contributions of visual cortical neurons to perception and behaviour. But in spite of their importance, causal methods constitute a minority of the experiments used to investigate the visual cortex to date. We reconsider the function and organization of visual cortex according to results obtained from stimulation techniques, with a special emphasis on electrical stimulation of small groups of cells in awake subjects who can report their visual experience. We compare findings from humans and monkeys, striate and extrastriate cortex, and superficial versus deep cortical layers, and identify a number of revealing gaps in the ‘causal map′ of visual cortex. Integrating results from different methods and species, we provide a critical overview of the ways in which causal approaches have been used to further our understanding of circuitry, plasticity and information integration in visual cortex. Electrical stimulation not only elucidates the contributions of different visual areas to perception, but also contributes to our understanding of neuronal mechanisms underlying memory, attention and decision-making.
Highlights
The visual cortex is perhaps the most thoroughly investigated of any brain system in mammals
Prominent methods currently used to investigate the function of visual cortex are often correlational and include neuroimaging, such as functional magnetic resonance imaging, and electrophysiological techniques, such as single cell neurophysiology. fMRI is very useful for measuring changes in activity throughout the entire human brain that can be correlated with perception and behaviour
Its spatial and temporal resolution is too low to reveal functional properties of individual neurons or small groups of cells, and it measures blood oxygenation leveldependent (BOLD) responses which are only indirectly coupled to neuronal firing [3,4,5]
Summary
The visual cortex is perhaps the most thoroughly investigated of any brain system in mammals. An electrode is inserted into the brain to directly measure the firing of neurons, individually or in small groups [6] This activity is correlated with simultaneous visual stimulation or perceptual reports to infer the information represented by neuronal firing. Lesion studies involve observing the effect of removing a particular brain area These can provide a causal link between cortical areas and specific visual functions, but are irreversible and results can vary between individuals and change over time. We consider the perceptual functions of visual cortex in primates as revealed by causal intervention methods with a special emphasis on the direct activation of small groups of visual cortical neurons. This leads to new questions and insights about the interaction between visual cortical activity, causal experimental approaches and perception
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