Training improves performance on most visual tasks. Such perceptual learning can modify how information is read out from, and represented in, later visual areas, but effects on early visual cortex are controversial. In particular, it remains unknown whether learning can reshape neural response properties in early visual areas independent from feedback arising in later cortical areas. Here, we tested whether learning can modify feedforward signals in early visual cortex as measured by the human electroencephalogram. Fourteen subjects were trained for Ͼ24 d to detect a diagonal grating pattern in one quadrant of the visual field. Training improved performance, reducing the contrast needed for reliable detection, and also reliably increased the amplitude of the earliest component of the visual evoked potential, the C1. Control orientations and locations showed smaller effects of training. Because the C1 arises rapidly and has a source in early visual cortex, our results suggest that learning can increase early visual area response through local receptive field changes without feedback from later areas.

To optimize perception, neurons in the visual system adapt to the current environment. What determines the durability of this plasticity? Longer exposures to an environment produce longer-lasting effects, which could be due to either (i) a single mechanism controlling adaptation that gains strength over time, or (ii) long-term mechanisms that become active after long-term exposure. Using recently developed technology, we tested adaptation durations an order of magnitude greater that those tested previously, and used a "deadaptation" procedure to reveal effects of a unique long-term mechanism in the longest adaptation periods. After 4 h of contrast adaptation, human observers were exposed to natural images for 15 min, which completely cancelled perceptual aftereffects of adaptation. Strikingly, during continued testing this deadaptation faded, and the original adaptation effects reappeared. This pattern strongly suggests that adaptation was maintained in a distinct long-term mechanism, whereas deadaptation affected a short-term mechanism.orientation | deprivation | adult neuroplasticity T he visual system continually adapts to the current environment to improve its function. Neurons in the retina and visual cortex, for example, decrease their sensitivity after prolonged exposure to a high contrast environment (for reviews, see refs. 1 and 2). Such adaptation is hypothesized to increase the efficiency of neural coding by bringing responses down from ceiling levels and allowing neurons to respond differentially to the high contrast values that are the most likely stimuli in that environment. Because adaptation effects are both large and common in the natural world, understanding them is a key component of any account of visual function (3).Effects of contrast adaptation are evident almost immediately after onset of an adapting stimulus but get stronger and longerlasting as the adaptation period grows in duration. This duration scaling law has been observed for both firing rates of retinal ganglion cells and contrast detection measurements of human observers (4, 5).How the visual system produces these temporal dynamics remains the subject of debate (Fig. 1). One theory proposes that a single neural mechanism controls contrast adaptation at multiple time-scales (putting aside extremely rapid effects that occur within 100-200 ms; refs. 5 and 6). As the adapting period grows longer, this mechanism increases its confidence in its estimates of the current environment. Increased confidence, in turn, produces stronger adaptation effects and also increases the evidence required to convince the system that the environment has changed back to its original state, yielding longer-lasting aftereffects.Alternatively, contrast adaptation might be controlled by multiple neural mechanisms, each tuned to a different preferred timescale (7,8). As the adapting period grows longer, mechanisms tuned to longer timescales exert increased control over adaptation. The dynamics of such mechanisms would most naturally operate at similar long t...

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What happens to neurons in visual cortex when they are deprived of their preferred stimuli? Long-term deprivation during development, spanning weeks, reduces the number of neurons selective for the deprived orientation [1-4]. In contrast, short-term deprivation in adults, for periods of seconds, can increase neural sensitivity relative to a stimulated baseline [5]. Effects over intermediate timescales remain largely unexplored, however. Here we introduce a new method for manipulating the visual environment of adult humans and report effects of four hours of orientation-specific deprivation. Subjects wore a head-mounted video camera that fed into a laptop computer that drove a head-mounted display. Software filtered the video stream in real time, allowing subjects to interact with the world while being deprived of visual input at a specified orientation. Four hours in this environment increased sensitivity to the deprived orientation, which likely reflected an increase in responsiveness of neurons in early visual cortex. Our results help distinguish between two theories of neural adaptation: the response increase optimized the responses of individual neurons, rather than increasing the efficiency of the population code. Our method should be able to produce a wide range of environmental manipulations useful for studying many topics in perception.

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