Neuronal adaptation to visual motion in area MT of the macaque

Coding and binding of color and form in visual cortex

The processing of color and form is largely segregated within the visual brain. But there is also evidence to suggest that these features are coded in combination early in visual processing. Here, we combined high-resolution functional magnetic resonance imaging (fMRI) together with multivariate pattern classification to examine where in the visual cortex specific color form "conjunctions" are represented. Human subjects viewed visual displays containing colored spiral patterns. The spiral patterns could be red or green, and oriented either clockwise or counterclockwise, leading to 4 possible stimulus configurations. Two additional displays combined 2 of the above single color-form pairings, leading to double conjunctions. We applied linear classifiers to voxel activation patterns obtained while subjects viewed such displays. Our findings not only show that color and form information is coded across retinotopically defined visual areas, but also that the 2 double-conjunction stimuli can be distinguished. The voxels most informative about conjunctions were distinct from those most informative about color or form alone. Our results indicate that conjunctions of form and color may be coded by separate functional units as early as primary visual cortex. The results of this study have implications for theories concerning the segregation and binding of color and form information.

The effect of orientation adaptation on responses of lateral geniculate nucleus neurons with high orientation bias in cats

Adaptation to stimulus orientation is assumed to have a cortical basis, but few studies have addressed whether it affects the activity of subcortical neurons. Using single-unit recording, we studied the effects of orientation adaptation on the responses of lateral geniculate nucleus (LGN) neurons with high orientation bias (OB) in anesthetized and paralyzed cats. Following adaptation to one stimulus orientation, the response at the adapting orientation was decreased, and the preferred orientation was shifted away from the adapting orientation. This phenomenon was similar to the effects observed for orientation adaptation in the primary visual cortex (V1), and was obvious when the adapting orientation was at an appropriate location relative to the original preferred orientation. Moreover, when the V1 was inactivated, the response at the adapting orientation was also decreased but the preferred orientation did not show a systematic shift after orientation adaptation in LGN. This result indicates that cortical feedback contributes to the effect of orientation adaptation on LGN neurons, which have a high OB. These data provide an example of how the corticothalamic loop modulates the processing of visual information, and suggest that the LGN is not only a simply passive relay but also a modulator of visual information.

Dynamics of spatial distortions reveal multiple time scales of motion adaptation

Prolonged exposure to consistent visual motion can significantly alter the perceived direction and speed of subsequently viewed objects. These perceptual aftereffects have provided invaluable tools with which to study the mechanisms of motion adaptation and draw inferences about the properties of underlying neural populations. Behavioral studies of the time course of motion aftereffects typically reveal a gradual process of adaptation spanning a period of multiple seconds. In contrast, neurophysiological studies have documented multiple motion adaptation effects operating over similar, or substantially faster (i.e., sub-second) time scales. Here we investigated motion adaptation by measuring time-dependent changes in the ability of moving stimuli to distort the perceived position of briefly presented static objects. The temporal dynamics of these motion-induced spatial distortions reveal the operation of two dissociable mechanisms of motion adaptation with differing properties. The first is rapid (subsecond), acts to limit the distortions induced by continuing motion, but is not sufficient to produce an aftereffect once the motion signal disappears. The second gradually accumulates over a period of seconds, does not modulate the size of distortions produced by continuing motion, and produces repulsive aftereffects after motion offset. These results provide new psychophysical evidence for the operation of multiple mechanisms of motion adaptation operating over distinct time scales.

Cerebellar contributions to adaptive control of saccades in humans

The cerebellum may monitor motor commands and through internal feedback correct for anticipated errors. Saccades provide a test of this idea because these movements are completed too quickly for sensory feedback to be useful. Earlier, we reported that motor commands that accelerate the eyes toward a constant amplitude target showed variability. Here, we demonstrate that this variability is not random noise, but is due to the cognitive state of the subject. Healthy people showed within-saccade compensation for this variability with commands that arrived later in the same saccade. However, in people with cerebellar damage, the same variability resulted in dysmetria. This ability to correct for variability in the motor commands that initiated a saccade was a predictor of each subject's ability to learn from endpoint errors. In a paradigm in which a target on the horizontal meridian jumped vertically during the saccade (resulting in an endpoint error), the adaptive response exhibited two timescales: a fast timescale that learned quickly from endpoint error but had poor retention, and a slow timescale that learned slowly but had strong retention. With cortical cerebellar damage, the fast timescale of adaptation was effectively absent, but the slow timescale was less impaired. Therefore, the cerebellum corrects for variability in the motor commands that initiate saccades within the same movement via an adaptive response that not only exhibits strong sensitivity to previous endpoint errors, but also rapid forgetting.

Temporal whitening: transient noise perceptually equalizes the 1/f temporal amplitude spectrum

Naturally occurring luminance distributions are approximately 1/f in their spatial and temporal amplitude spectra. By systematically varying the spatio-temporal profile of broadband noise stimuli, we demonstrate that humans invariably overestimate the proportion of high spatial and temporal frequency energy. Critically, we find that that the strength of this bias is of a magnitude that predicts a perceptually equalized response to the spatio-temporal fall off in the natural amplitude spectrum. This interpretation is supported by our finding that the magnitude of this transient response bias, while evident across a broad range of narrowband spatial frequencies (0.25-8 cycles/deg), decreases above 2 cycles/deg, which itself compensates for the increase in temporal frequency energy previously observed at high spatial frequencies as a consequence of small fixational eye movements (M. Rucci, R. Iovin, M. Poletti, & F. Santini, 2007). Additional temporal masking and adaptation experiments reveal a transiently biased asymmetry. Whereas temporal frequencies >4 Hz mask and adapt 1- and 15-Hz targets, lower masking and adaptation frequencies have much less effect on sensitivity to 15-Hz compared with 1-Hz targets. These results imply that the visual system over-represents its transient input to an extent that predicts an equalized temporal channel response to the low-frequency-biased structure of natural scenes.

Estimating instantaneous irregularity of neuronal firing

Cortical neurons in vivo had been regarded as Poisson spike generators that convey no information other than the rate of random firing. Recently, using a metric for analyzing local variation of interspike intervals, researchers have found that individual neurons express specific patterns in generating spikes, which may symbolically be termed regular, random, or bursty, rather invariantly in time. In order to study the dynamics of firing patterns in greater detail, we propose here a Bayesian method for estimating firing irregularity and the firing rate simultaneously for a given spike sequence, and we implement an algorithm that may render the empirical Bayesian estimation practicable for data comprising a large number of spikes. Application of this method to electrophysiological data revealed a subtle correlation between the degree of firing irregularity and the firing rate for individual neurons. Irregularity of firing did not deviate greatly around the low degree of dependence on the firing rate and remained practically unchanged for individual neurons in the cortical areas V1 and MT, whereas it fluctuated greatly in the lateral geniculate nucleus of the thalamus. This indicates the presence and absence of autocontrolling mechanisms for maintaining patterns of firing in the cortex and thalamus, respectively.

Adaptation accentuates responses of fly motion-sensitive visual neurons to sudden stimulus changes

Adaptation in sensory and neuronal systems usually leads to reduced responses to persistent or frequently presented stimuli. In contrast to simple fatigue, adapted neurons often retain their ability to encode changes in stimulus intensity and to respond when novel stimuli appear. We investigated how the level of adaptation of a fly visual motion-sensitive neuron affects its responses to discontinuities in the stimulus, i.e. sudden brief changes in one of the stimulus parameters (velocity, contrast, grating orientation and spatial frequency). Although the neuron's overall response decreased gradually during ongoing motion stimulation, the response transients elicited by stimulus discontinuities were preserved or even enhanced with adaptation. Moreover, the enhanced sensitivity to velocity changes by adaptation was not restricted to a certain velocity range, but was present regardless of whether the neuron was adapted to a baseline velocity below or above its steady-state velocity optimum. Our results suggest that motion adaptation helps motion-sensitive neurons to preserve their sensitivity to novel stimuli even in the presence of strong tonic stimulation, for example during self-motion.

How reliable is the pattern adaptation technique? A modeling study

Upon prolonged viewing of a sinusoidal grating, the visual system is selectively desensitized to the spatial frequency of the grating, while the sensitivity to other spatial frequencies remains largely unaffected. This technique, known as pattern adaptation, has been so central to the psychophysical study of the mechanisms of spatial vision that it is sometimes referred to as the "psychologist's microelectrode." While this approach implicitly assumes that the adaptation behavior of the system is diagnostic of the corresponding underlying neural mechanisms, this assumption has never been explicitly tested. We tested this assumption using adaptation bandwidth, or the range of spatial frequencies affected by adaptation, as a representative measure of adaptation. We constructed an intentionally simple neuronal ensemble model of spatial frequency processing and examined the extent to which the adaptation bandwidth at the system level reflected the bandwidth at the neuronal level. We find that the adaptation bandwidth could vary widely even when all spatial frequency tuning parameters were held constant. Conversely, different spatial frequency tuning parameters were able to elicit similar adaptation bandwidths from the neuronal ensemble. Thus, the tuning properties of the underlying units did not reliably reflect the adaptation bandwidth at the system level, and vice versa. Furthermore, depending on the noisiness of adaptation at the neural level, the same neuronal ensemble was able to produce selective or nonselective adaptation at the system level, indicating that a lack of selective adaptation at the system level cannot be taken to mean a lack of tuned mechanisms at the neural level. Together, our results indicate that pattern adaptation cannot be used to reliably estimate the tuning properties of the underlying units, and imply, more generally, that pattern adaptation is not a reliable tool for studying the neural mechanisms of pattern analysis.

Motion fading and the motion aftereffect share a common process of neural adaptation

After prolonged viewing of a slowly drifting or rotating pattern under strict fixation, the pattern appears to slow down and then momentarily stop. Here, we show that this motion fading occurs not only for slowly moving stimuli, but also for stimuli moving at high speed; after prolonged viewing of high-speed stimuli, the stimuli appear to slow down but not to stop. We report psychophysical evidence that the same neural adaptation process likely gives rise to motion fading and to the motion aftereffect.

The motion after-effect: local and global contributions to contrast sensitivity

Motion adaptation is a widespread phenomenon analogous to peripheral sensory adaptation, presumed to play a role in matching responses to prevailing current stimulus parameters and thus to maximize efficiency of motion coding. While several components of motion adaptation (contrast gain reduction, output range reduction and motion after-effect) have been described, previous work is inconclusive as to whether these are separable phenomena and whether they are locally generated. We used intracellular recordings from single horizontal system neurons in the fly to test the effect of local adaptation on the full contrast-response function for stimuli at an unadapted location. We show that contrast gain and output range reductions are primarily local phenomena and are probably associated with spatially distinct synaptic changes, while the antagonistic after-potential operates globally by transferring to previously unadapted locations. Using noise analysis and signal processing techniques to remove 'spikelets', we also characterize a previously undescribed alternating current component of adaptation that can explain several phenomena observed in earlier studies.

Functional magnetic resonance adaptation in visual neuroscience

Functional magnetic resonance imaging (fMRI) is a powerful non-invasive tool to investigate neuronal processing. In the last ten years a new methodological approach in the field of fMRI has been developed: fMRI adaptation. It has been found that the repetition of a stimulus leads to a decrease of the fMRI signal in the brain region that processes this stimulus. The phenomenon has been related to neuronal adaptation effects found in single-cell recordings. Since the first experiments that observed fMRI-adaptation effects, the method has been applied extensively to study various visual phenomena, such as the perception of motion, shape, objects, and orientation. The great advantage of fMRI adaptation is that it allows assessing the functional response profile of a brain region at a subvoxel level. The purpose of the current review is to evaluate the different experimental approaches used to elicit fMRI-adaptation effects. We discuss papers published in the domain of visual neuroscience that made use of fMRI-adaptation paradigms. In doing so, we focus on methodological considerations concerning experimental design, stimulus presentation and influencing factors such as awareness and attention. In the course of this review, we show that different fMRI-adaptation designs capture heterogeneous neuronal adaptation effects. As the picture of the mechanisms underlying neuronal adaptation changes from simple synaptic fatigue to complex network interactions, the concept of fMRI adaptation has to be redefined.

Visual adaptation to convexity in macaque area V4

Aftereffects are perceptual illusions caused by visual adaptation to one or more stimulus attribute, such as orientation, motion, or shape. Neurophysiological studies seeking to understand the basis of visual adaptation have observed firing rate reduction and changes in tuning of stimulus-selective neurons following periods of prolonged visual stimulation. In the domain of shape, recent psychophysical work has shown that adaptation to a convex pattern induces a subsequently seen rectangle to appear slightly concave. In the present study, we investigate the possible contribution of V4 neurons of rhesus monkeys, which are thought to be involved in the coding of convexity, to shape-specific adaptation. Visually responsive neurons were monitored during the brief presentation of simple shapes varying in their convexity level. Each test presentation was preceded by either a blank period or several seconds of adaptation to a convex or concave stimulus, presented in two different sizes. Adaptation consistently shifted the tuning of neurons away from the convex or concave adapter, including shifting response to the neutral rectangle in the direction of the opposite convexity. This repulsive shift resembled the known perceptual distortion associated with adaptation to such stimuli. In addition, adaptation caused a nonspecific response decrease, as well as a specific decrease for repeated stimuli. The latter effects were observed whether or not the adapting and test stimuli matched closely in their size. Taken together, these results provide evidence for shape-specific adaptation of neurons in area V4, which may contribute to the perception of the convexity aftereffect.

Transformation Direction Influences Shape-Similarity Judgments

Three experiments provide evidence that the perceived similarity between two images is systematically affected by the inherent direction of a transformation that links the two. Participants were shown short animations morphing one object into another from the same basic category. They were then asked to make directional similarity judgments (“How similar is object A to object B?”) for two stationary images drawn from the morph continuum. Across three experiments, similarity ratings for identical comparisons were higher when the reference object, B, had appeared before the comparison object, A, in the preceding morph sequence. This response to dynamic transformational sequences is in accordance with the view that similarity depends on the ease of transformation between object representations and that transformations between objects in categorization and object recognition are psychologically real.

Adaptive changes in visual cortex following prolonged contrast reduction

How does prolonged reduction in retinal-image contrast affect visual-contrast coding? Recent evidence indicates that some forms of long-term visual deprivation result in compensatory perceptual and neural changes in the adult visual pathway. It has not been established whether changes due to contrast adaptation are best characterized as "contrast gain" or "response gain." We present a theoretical rationale for predicting that adaptation to long-term contrast reduction should result in response gain. To test this hypothesis, normally sighted subjects adapted for four hours by viewing their environment through contrast-reducing goggles. During the adaptation period, the subjects went about their usual daily activities. Subjects' contrast-discrimination thresholds and fMRI BOLD responses in cortical areas V1 and V2 were obtained before and after adaptation. Following adaptation, we observed a significant decrease in contrast-discrimination thresholds, and significant increase in BOLD responses in V1 and V2. The observed interocular transfer of the adaptation effect suggests that the adaptation has a cortical origin. These results reveal a new kind of adaptability of the adult visual cortex, an adjustment in the gain of the contrast-response in the presence of a reduced range of stimulus contrasts, which is consistent with a response-gain mechanism. The adaptation appears to be compensatory, such that the precision of contrast coding is improved for low retinal-image contrasts.

Relationship between adapted neural population responses in MT and motion adaptation in speed and direction of smooth-pursuit eye movements

We have asked how sensory adaptation is represented in the response of a population of visual motion neurons and whether the neural adaptation could drive behavioral adaptation. Our approach was to evaluate the effects of about 10 s of motion adaptation on both smooth-pursuit eye movements and the responses of neuron populations in extrastriate middle temporal visual area (MT) in awake monkeys. Stimuli for neural recordings consisted of patches of 100% correlated dot textures. There was a wide range of effects across neurons, but on average adaptation reduced the amplitude and width of the direction tuning curves of MT neurons, without large changes in the preferred direction. The effects were greatest when the direction of the adapting stimulus corresponded to the preferred direction of the MT neuron under study. Adaptation also reduced the amplitude of speed-tuning curves, again with the greatest effect when the adapting speed was equal to the preferred speed. The adapted tuning curves were shifted toward lower preferred speeds as the adapting speed increased. We constructed populations of model MT neurons based on our experimental sample and showed that the effects of adaptation on the direction and speed of pursuit eye movements were predicted when a variant of vector averaging decoded the responses of a subset of the neural population. We conclude that the effects of motion adaptation on the responses of MT neurons can support behavioral adaptation in pursuit eye movements.

The motion aftereffect reloaded

The motion aftereffect is a robust illusion of visual motion resulting from exposure to a moving pattern. There is a widely accepted explanation of it in terms of changes in the response of cortical direction-selective neurons. Research has distinguished several variants of the effect. Converging recent evidence from different experimental techniques (psychophysics, single-unit recording, brain imaging, transcranial magnetic stimulation, visual evoked potentials and magnetoencephalography) reveals that adaptation is not confined to one or even two cortical areas, but occurs at multiple levels of processing involved in visual motion analysis. A tentative motion-processing framework is described, based on motion aftereffect research. Recent ideas on the function of adaptation see it as a form of gain control that maximises the efficiency of information transmission at multiple levels of the visual pathway.

Visual cells remember earlier applied target: plasticity of orientation selectivity

BACKGROUND

A canonical proposition states that, in mature brain, neurons responsive to sensory stimuli are tuned to specific properties installed shortly after birth. It is amply demonstrated that that neurons in adult visual cortex of cats are orientation-selective that is they respond with the highest firing rates to preferred oriented stimuli.

METHODOLOGY/PRINCIPAL FINDINGS

In anesthetized cats, prepared in a conventional fashion for single cell recordings, the present investigation shows that presenting a stimulus uninterruptedly at a non-preferred orientation for twelve minutes induces changes in orientation preference. Across all conditions orientation tuning curves were investigated using a trial by trial method. Contrary to what has been previously reported with shorter adaptation duration, twelve minutes of adaptation induces mostly attractive shifts, i.e. toward the adapter. After a recovery period allowing neurons to restore their original orientation tuning curves, we carried out a second adaptation which produced three major results: (1) more frequent attractive shifts, (2) an increase of their magnitude, and (3) an additional enhancement of responses at the new or acquired preferred orientation. Additionally, we also show that the direction of shifts depends on the duration of the adaptation: shorter adaptation in most cases produces repulsive shifts, whereas adaptation exceeding nine minutes results in attractive shifts, in the same unit. Consequently, shifts in preferred orientation depend on the duration of adaptation.

CONCLUSION/SIGNIFICANCE

The supplementary response improvements indicate that neurons in area 17 keep a memory trace of the previous stimulus properties, thereby upgrading cellular performance. It also highlights the dynamic nature of basic neuronal properties in adult cortex since repeated adaptations modified both the orientation tuning selectivity and the response strength to the preferred orientation. These enhanced neuronal responses suggest that the range of neuronal plasticity available to the visual system is broader than anticipated.

Time course and stimulus dependence of repetition-induced response suppression in inferotemporal cortex

Neural responses throughout the sensory system are affected by stimulus history. In the inferotemporal cortex (IT)--an area important for processing information about object shape--there is a substantially reduced response to the second presentation of an image. Understanding the mechanisms underlying repetition suppression may provide important insights into the circuitry that generates responses in IT. In addition, repetition suppression may have important perceptual consequences. The characteristics of repetition suppression in IT are poorly understood, and the details, including the interaction between the content of the first and second stimulus and the time course of suppression, are not clear. Here, we examined the time course of suppression in IT by varying both the duration and stimulus content of two stimuli presented in sequence. The data show that the degree of suppression does not depend directly on the response evoked by the first stimulus in the recorded neuron. Repetition suppression was also limited in duration, peaking at approximately 200 ms after the onset of the second (test) image and disappearing before the end of the response. Neural selectivity to a continuum of related images was enhanced if the first stimulus produced a weak response in the cell. The dynamics of the response suggests that different parts of the input and recurrent circuitry that gives rise to neural responses in IT are differentially modulated by repetition suppression. The selectivity of the sustained response was preserved in spite of substantial suppression of the early part of the response. The data suggest that suppression in IT is a property of the input and recurrent circuitry in IT and is not directly related to the degree of response in the recorded neuron itself.

Investigating visual motion perception using the transcranial magnetic stimulation-adaptation paradigm

The state-dependency approach of transcranial magnetic stimulation (TMS) enables differential stimulation of functionally distinct neural populations within the affected region of cortex. Here we tested the validity of a paradigm based on state-dependency, the TMS-adaptation paradigm, in the context of visual motion perception. Visual adaptation was used to induce an activity imbalance in direction-selective neurons in the visual cortex, after which participants performed a motion direction discrimination task. When TMS was applied over the motion-selective area V5/MT before each experimental trial, the detection of the direction encoded by the adapted neurons was facilitated relative to the direction encoded by the nonadapted neurons. This finding demonstrates, in the domain of visual motion detection, the state-dependency of TMS effects and the validity of the TMS-adaptation paradigm.

Adaptation and selective information transmission in the cricket auditory neuron AN2

Sensory systems adapt their neural code to changes in the sensory environment, often on multiple time scales. Here, we report a new form of adaptation in a first-order auditory interneuron (AN2) of crickets. We characterize the response of the AN2 neuron to amplitude-modulated sound stimuli and find that adaptation shifts the stimulus–response curves toward higher stimulus intensities, with a time constant of 1.5 s for adaptation and recovery. The spike responses were thus reduced for low-intensity sounds. We then address the question whether adaptation leads to an improvement of the signal's representation and compare the experimental results with the predictions of two competing hypotheses: infomax, which predicts that information conveyed about the entire signal range should be maximized, and selective coding, which predicts that “foreground” signals should be enhanced while “background” signals should be selectively suppressed. We test how adaptation changes the input–response curve when presenting signals with two or three peaks in their amplitude distributions, for which selective coding and infomax predict conflicting changes. By means of Bayesian data analysis, we quantify the shifts of the measured response curves and also find a slight reduction of their slopes. These decreases in slopes are smaller, and the absolute response thresholds are higher than those predicted by infomax. Most remarkably, and in contrast to the infomax principle, adaptation actually reduces the amount of encoded information when considering the whole range of input signals. The response curve changes are also not consistent with the selective coding hypothesis, because the amount of information conveyed about the loudest part of the signal does not increase as predicted but remains nearly constant. Less information is transmitted about signals with lower intensity.

Sensory systems have the ability to adapt to changes in the environment. In a quiet room, the nervous system is very responsive, so that even a whisper can be easily understood. In contrast, the perceived loudness on a crowded street will be reduced to prevent an overload of the nervous system. Two different hypotheses have been proposed to explain how the nervous system achieves this adaptation. According to one idea, all present sensory signals are equally enhanced, so that the whole range of input signals is reliably represented. On the other hand, the aim of the nervous system may be to extract the most important parts of the acoustic signal, for example, an approaching car, and thus abolish the irrelevant rest. To address which of these two principles is implemented in the auditory system of the cricket, we investigated the responses of a single auditory neuron, called interneuron AN2, to different sound signals. We found that adaptation actually reduces the amount of encoded information when considering the whole range of input signals. However, the changes were also not in agreement with the idea that only the most important signal is transmitted, because the amount of information conveyed about the loudest part of the signal does not increase. Thus, we here report the unusual case of a reduction of information transfer by adaptation, while in most other systems reported of so far adaptation actually enhances coding of sensory information.

State-dependency of transcranial magnetic stimulation

Transcranial magnetic stimulation (TMS), a tool that allows noninvasive modulation of cortical neural activity, has become an important tool in cognitive neuroscience and is being increasingly explored in neurotherapeutics. Amongst the factors that are likely to influence its efficacy, the importance of the baseline cortical activation state on the impact of TMS has not received much attention. However, this state-dependency is important as the neural impact of any external stimulus represents an interaction with the ongoing brain activity at the time of stimulation. The effects of any external stimulus are therefore not only determined by the properties of that stimulus but also by the activation state of the brain. Here we review the existing evidence on the state-dependency of TMS and propose how its systematic study can provide unique insights into brain function and significantly enhance the effectiveness of TMS in investigations on the neural basis of perception and cognition. We also describe novel approaches based on this state-dependency which can be used to investigate the properties of distinct neural subpopulations within the stimulated region. Furthermore, we discuss how state-dependency can explain the functional mechanisms through which TMS impairs perception and behavior.

Aging affects contrast response functions and adaptation of middle temporal visual area neurons in rhesus monkeys

In the present study we studied the effects of aging on the coding of contrast in area V1 (primary visual cortex) and MT (middle temporal visual area) of the macaque monkey using single-neuron in vivo electrophysiology. Our results show that both MT and V1 neurons in old monkeys are less sensitive to contrast than those in young monkeys. Generally, contrast sensitivity is affected by aging more severely in MT cells than in V1 cells. Specifically, MT cells were affected more severely than motion direction selective V1 cells. Particularly, we found that MT neurons in old monkeys exhibited enhanced maximum visual responses, higher levels of spontaneous activity and decreased signal-to-noise ratios. In addition, we also found age-related changes in neuronal adaptation to visual motion in MT. Compared with young animals, the contrast gain of MT neurons in old monkeys is less affected, but the response gain by adaptation of MT neurons is more affected. Our results suggest that there may be an anomalous visual processing in both the magnocellular and parvocellular pathways. The neural changes described here are consistent with an age-related degeneration of intracortical inhibition and could underlie some deficits in visual function during normal aging.

fMRI and its interpretations: an illustration on directional selectivity in area V5/MT

fMRI is a tool to study brain function noninvasively that can reliably identify sites of neural involvement for a given task. However, to what extent can fMRI signals be related to measures obtained in electrophysiology? Can the blood-oxygen-level-dependent signal be interpreted as spatially pooled spiking activity? Here we combine knowledge from neurovascular coupling, functional imaging and neurophysiology to discuss whether fMRI has succeeded in demonstrating one of the most established functional properties in the visual brain, namely directional selectivity in the motion-processing region V5/MT+. We also discuss differences of fMRI and electrophysiology in their sensitivity to distinct physiological processes. We conclude that fMRI constitutes a complement, not a poor-resolution substitute, to invasive techniques, and that it deserves interpretations that acknowledge its stand as a separate signal.

Retinotopic encoding of the direction aftereffect

Kohn and Movshon [Kohn, A., & Movshon, J. (2003). Neuronal adaptation to visual motion in area MT of the macaque. Neuron, 39, 681-691; Kohn, A., & Movshon, J. A. (2004). Adaptation changes the direction tuning of macaque MT neurons. Nature Neuroscience, 7(7), 764-772] measured the contrast response functions of single neurons in MT (V5) before and after adaptation to high contrast gratings. They found that when gratings were smaller than the MT receptive field, so that adapting and test regions could be either co-localised or non-overlapping, adaptation was spatially specific. This led to the hypothesis that grating adaptation occurs in V1, where receptive fields are small and retinotopically organized, and that MT merely inherits this adaptation. We predicted that spatial specificity would be less for dot stimuli that probably adapt MT cells directly. Also, given recent contradictory claims that hMT primarily exhibits both spatiotopy [d'Avossa, G., Tosetti, M., Crespi, S., Biagi, L., Burr, D., & Morrone, M. (2006). Spatiotopic selectivity of BOLD responses to visual motion in human area MT. Nature Neuroscience, 10, 249-255] and retinotopy [Gardner, J. L., Merriam, E. P., Movshon, J. A., & Heeger, D. J. (2008). Maps of visual space in human occipital cortex are retinotopic, not spatiotopic. The Journal of Neuroscience, 28, 3988-3999], we were interested in producing relevant psychophysical evidence using the direction aftereffect. In three experiments, we measured direction aftereffects (DAEs) induced and tested either with drifting gratings or drifting dots when stimulus location was changed both retinotopically and spatiotopically between adaptation and test; when retinotopic location only was changed; and when spatiotopic location only was changed. We predicted and found that spatial specificity was greater for gratings than for dots. We also found very small spatiotopic effects that call into question some recent claims that area MT exhibits a high degree of spatiotopicity.

Adaptation changes the spatial frequency tuning of adult cat visual cortex neurons

The modular layout of striate cortex is arguably a hallmark of all cortical organization. Neurons of a given module or domain respond optimally to very few specific properties, such as orientation or direction. However, it is possible, under appropriate conditions, to compel a neuron to respond preferentially to a different optimal property. In anesthetized cats, prepared for electrophysiological recordings in the visual cortex, we applied a spatial frequency (SF) that differs (by 0.25-3.0 octaves) from the optimal one for 7-13 min without interruption. This application shifted the tuning curve of the cell mainly in the direction of the imposed SF. Indeed, results indicate an attractive push occurring more frequently (50%) than a repulsive (30%) shift in cortical cells. The increase of responsivity is band-limited and is around the imposed SF, while flanked responses remained unmodified in all conditions. We hypothesize that the observed reversible plasticity is obtained by a modulation of the balance between the strengths of the respective synaptic inputs. These changes in preferred original optimal spatial frequencies may allow a dynamic reaction of cortex to a new environment and particularly to ''zoom'' cellular activity toward persistent stimuli in spite of the tuning inherited from genetic programming of response properties and environmental conditions during critical periods in new born animals.

Adaptive control of saccades via internal feedback

Ballistic movements like saccades require the brain to generate motor commands without the benefit of sensory feedback. Despite this, saccades are remarkably accurate. Theory suggests that this accuracy arises because the brain relies on an internal forward model that monitors the motor commands, predicts their sensory consequences, and corrects eye trajectory midflight. If control of saccades relies on a forward model, then the forward model should adapt whenever its predictions fail to match sensory feedback at the end of the movement. Using optimal feedback control theory, we predicted how this adaptation should alter saccade trajectories. We trained subjects on a paradigm in which the horizontal target jumped vertically during the saccade. With training, the final position of the saccade moved toward the second target. However, saccades became increasingly curved, i.e., suboptimal, as oculomotor commands were corrected on-line to steer the eye toward the second target. The adaptive response had two components: (1) the motor commands that initiated the saccades changed slowly, aiming the saccade closer to the jumped target. The adaptation of these earliest motor commands displayed little forgetting during the rest periods. (2) Late in saccade trajectory, another adaptive response steered it still closer to the jumped target, producing curvature. Adaptation of these late motor commands showed near-complete forgetting during the rest periods. The two components adapted at different timescales, with the late-acting component displaying much faster rates. It appears that in controlling saccades, the brain relies on an internal feedback that has the characteristics of a fast-adapting forward model.

Dynamics of distributed 1D and 2D motion representations for short-latency ocular following

Integrating information is essential to measure the physical 2D motion of a surface from both ambiguous local 1D motion of its elongated edges and non-ambiguous 2D motion of its features such as corners or texture elements. The dynamics of this motion integration shows a complex time course as read from tracking eye movements: first, local 1D motion signals are extracted and pooled to initiate ocular responses, then 2D motion signals are integrated to adjust the tracking direction until it matches the surface motion direction. The nature of these 1D and 2D motion computations are still unclear. One hypothesis is that their different dynamics may be explained from different contrast sensitivities. To test this, we measured contrast-response functions of early, 1D-driven and late, 2D-driven components of ocular following responses to different motion stimuli: gratings, plaids and barberpoles. We found that contrast dynamics of 1D-driven responses are nearly identical across the different stimuli. On the contrary, late 2D-driven components with either plaids or barberpoles have similar latencies but different contrast dynamics. Temporal dynamics of both 1D- and 2D-driven responses demonstrates that the different contrast gains are set very early during the response time course. Running a Bayesian model of motion integration, we show that a large family of contrast-response functions can be predicted from the probability distributions of 1D and 2D motion signals for each stimulus and by the shape of the prior distribution. However, the pure delay (i.e. largely independent upon contrast) observed between 1D- and 2D-motion supports the fact that 1D and 2D probability distributions are computed independently. This two-pathway Bayesian model supports the idea that 1D and 2D mechanisms represent edges and features motion in parallel.

Dynamics of lactate concentration and blood oxygen level-dependent effect in the human visual cortex during repeated identical stimuli

In vivo (1)H NMR spectroscopy at 7 T was utilized to measure the changes in lactate concentration upon repeated identical visual stimuli, each lasting for 2 min. The average amplitude of these increases was found to be reduced over time (P < 0.01), from 0.13 +/- 0.02 micromol/g during the first half of the stimulation paradigm, to 0.06 +/- 0.02 micromol/g during the second half of the stimulation paradigm. In contrast, the blood oxygen level-dependent (BOLD) effect on the fMRI water signal and on the height of the total creatine signal at 3.03 ppm was persistent during the whole observation period. This finding may suggest a differential adaptation of cortical output that is not reflected at the level of the global excitation-inhibition activity of the cortical canonical circuits. Alternative possibilities that could account for an adaptation of [Lac] changes are also discussed.

Fractal rotation isolates mechanisms for form-dependent motion in human vision

Here, we describe a motion stimulus in which the quality of rotation is fractal. This makes its motion unavailable to the translation-based motion analysis known to underlie much of our motion perception. In contrast, normal rotation can be extracted through the aggregation of the outputs of translational mechanisms. Neural adaptation of these translation-based motion mechanisms is thought to drive the motion after-effect, a phenomenon in which prolonged viewing of motion in one direction leads to a percept of motion in the opposite direction. We measured the motion after-effects induced in static and moving stimuli by fractal rotation. The after-effects found were an order of magnitude smaller than those elicited by normal rotation. Our findings suggest that the analysis of fractal rotation involves different neural processes than those for standard translational motion. Given that the percept of motion elicited by fractal rotation is a clear example of motion derived from form analysis, we propose that the extraction of fractal rotation may reflect the operation of a general mechanism for inferring motion from changes in form.

Gamma oscillations underlying the visual motion aftereffect

After having been exposed to strong visual motion in one direction, a subsequently presented stationary visual scene seems to move in the opposite direction. This motion aftereffect (MAE) is usually ascribed to short-term functional changes in cortical areas involved in visual motion analysis akin to adaptation. Using magnetoencephalography (MEG), we show increased global field activity due to the MAE which could mostly be explained by a dipole located near the putative location of human area MT+. We further demonstrate that the induced MAE is accompanied by a significant increase in gamma-band activity (GBA) recorded from parietooccipital cortex contralateral to the visual motion stimulus. This gamma oscillation most likely reflects an increase in neuronal response coherence due to decreased inhibition of a group of neurons with similar preferred direction, namely the direction opposite to the adapted one. A second focal GBA response was picked up by the most posterior sensors ipsilateral to the side of the stimulus, reflecting the size of the MAE, whose source could not be reliably located.

Recent history of stimulus speeds affects the speed tuning of neurons in area MT

Visual motion processing plays a key role in enabling primates' successful interaction with their dynamic environments. Although in natural environments the speed of visual stimuli continuously varies, speed tuning of neurons in the prototypical motion area MT has traditionally been assessed with stimuli that moved at constant speeds. We investigated whether the representation of speed in a continuously varying stimulus context differs from the representation of constant speeds. We recorded from individual MT neurons of fixating macaques while stimuli moved either at a constant speed or in a linearly accelerating or decelerating manner. We found clear speed tuning even when the stimulus consisted of visual motion with gradual speed changes. There were, however, important differences with the speed tuning as measured with constant stimuli: the stimulus context affected neuronal preferred speed as well as the associated tuning width of the speed tuning curves. These acceleration-dependent changes in response lead to an accurate representation of the acceleration of these stimuli in the MT cells. To elucidate the mechanistic basis of this signal, we constructed a stochastic firing rate model based on the constant speed response profiles. This model incorporated each cell's speed tuning and response adaptation dynamics and accurately predicted the response to constant speeds as well as accelerating and decelerating stimuli. Because the response of the model neurons had no explicit acceleration dependence, we conclude that speed-dependent adaptation creates a strong influence of temporal context on the MT response and thereby results in the representation of acceleration signals.

Visual adaptation: neural, psychological and computational aspects

The term 'visual adaptation' describes the processes by which the visual system alters its operating properties in response to changes in the environment. These continual adjustments in sensory processing are diagnostic as to the computational principles underlying the neural coding of information and can have profound consequences for our perceptual experience. New physiological and psychophysical data, along with emerging statistical and computational models, make this an opportune time to bring together experimental and theoretical perspectives. Here, we discuss functional ideas about adaptation in the light of recent data and identify exciting directions for future research.

Fast-scale adaptive changes of directional tuning in fly tangential cells are explained by a static nonlinearity

The response of vertebrate motion-sensitive neurons to a directional stimulus is affected by the direction of the stimulus that immediately preceded it. These nonlinear effects are also observed for orientation tuning and are typically interpreted as fast-scale adaptive changes. We verified that similar effects are observed for spiking tangential cells in the fly lobula plate. We also investigated the spatial selectivity of these effects by presenting multiple patches at different positions within the receptive field,and found that the effects are strictly local.

We modelled the data using elementary operators (linear filters and threshold nonlinearities). A satisfactory account of the results is obtained when an early static nonlinearity acts on the outputs of multiple front-end filters that are subsequently pooled in a spatially restricted manner by the tangential cell. In line with recent studies, these findings emphasize the importance of testing simple nonlinear models before attempting more elaborate interpretations of fast-scale adaptive phenomena in single neurons. We discuss a potential neural implementation of the model based on medullar projections to the lobula plate.

Benefits of contrast normalization demonstrated in neurons and model cells

The large dynamic range of natural stimuli poses a challenge for neural coding: how is a neuron to encode large differences at high contrast while remaining sensitive to small differences at low contrast? Many sensory neurons exhibit contrast normalization: gain depends on the range of stimuli presented, such that firing-rate modulation is not proportional to contrast. However, coding depends strongly on the precision of spike timing and the reliability of spike number, neither of which can be predicted from neural gain. The presumption that contrast normalization is associated with maintained coding efficiency remained untested. We report that, as contrast decreases, responses are more variable and encode less information, as expected. Nevertheless, these changes can be small, and information transmission is even better preserved across contrasts than rate modulation. The extent of contrast normalization is correlated with the extent to which information transmission is preserved across contrasts. Specifically, normalization is associated with maintaining the bits of information per spike rather than bits per second. Finally, we show that a nonadapting model can exhibit both contrast normalization and the associated information preservation.

Attraction of flashes to moving dots

Motion is known to distort visual space, producing illusory mislocalizations for flashed objects. Previously, it has been shown that when a stationary bar is flashed in the proximity of a moving stimulus, the position of the flashed bar appears to be shifted in the direction of nearby motion. A model consisting of predictive projections from the sub-system that processes motion information onto the sub-system that processes position information can explain this illusory position shift of a stationary flashed bar in the direction of motion. Based on this model of motion-position interactions, we predict that the perceived position of a flashed stimulus should also be attracted towards a nearby moving stimulus. In the first experiment, observers judged the perceived vertical position of a flash with respect to two horizontally moving dots of unequal contrast. The results of this experiment were in agreement with our prediction of attraction towards the high contrast dot. We obtained similar findings when the moving dots were replaced by drifting gratings of unequal contrast. In control experiments, we found that neither attention nor eye movements can account for this illusion. We propose that the visual system uses predictive influences from the motion processing sub-system on the position processing sub-system to overcome the temporal limitations of the position processing system.

Adaptation to Real Motion Reveals Direction-selective Interactions between Real and Implied Motion Processing

Viewing static pictures of running humans evokes neural activity in the dorsal motion-sensitive cortex. To establish whether this response arises from direction-selective neurons that are also involved in real motion processing, we measured the visually evoked potential to implied motion following adaptation to static or moving random dot patterns. The implied motion response was defined as the difference between evoked potentials to pictures with and without implied motion. Interaction between real and implied motion was found as a modulation of this difference response by the preceding motion adaptation. The amplitude of the implied motion response was significantly reduced after adaptation to motion in the same direction as the implied motion, compared to motion in the opposite direction. At 280 msec after stimulus onset, the average difference in amplitude reduction between opposite and same adapted direction was 0.5 μV on an average implied motion amplitude of 2.0 μV. These results indicate that the response to implied motion arises from direction-selective motion-sensitive neurons. This is consistent with interactions between real and implied motion processing at a neuronal level.

The timecourse of higher-level face aftereffects

Perceptual aftereffects for simple visual attributes processed early in the cortical hierarchy increase logarithmically with adapting duration and decay exponentially with test duration. This classic timecourse has been reported recently for a face identity aftereffect [Leopold, D. A., Rhodes, G., Müller, K.-M., & Jeffery, L. (2005). The dynamics of visual adaptation to faces. Proceedings of the Royal Society of London, Series B, 272, 897-904], suggesting that the dynamics of visual adaptation may be similar throughout the visual system. An alternative interpretation, however, is that the classic timecourse is a flow-on effect of adaptation of a low-level, retinotopic component of the face identity aftereffect. Here, we examined the timecourse of the higher-level (size-invariant) components of two face aftereffects, the face identity aftereffect and the figural face aftereffect. Both showed the classic pattern of logarithmic build-up and exponential decay. These results indicate that the classic timecourse of face aftereffects is not a flow-on effect of low-level retinotopic adaptation, and support the hypothesis that dynamics of visual adaptation are similar at higher and lower levels of the cortical visual hierarchy. They also reinforce the perceptual nature of face aftereffects, ruling out demand characteristics and other post-perceptual factors as plausible accounts.

Brain areas selective for both observed and executed movements

When observing a particular movement a subset of movement-selective visual and visuomotor neurons are active in the observer's brain, forming a representation of the observed movement. Similarly, when executing a movement a subset of movement-selective motor and visuomotor neurons are active, forming a representation of the executed movement. In this study we used an fMRI-adaptation protocol to assess cortical response selectivity to observed and executed movements simultaneously. Subjects freely played the rock-paper-scissors game against a videotaped opponent, sometimes repeatedly observing or executing the same movement on subsequent trials. Numerous brain areas exhibited adaptation (repetition suppression) during either repeated observations or repeated executions of the same movement. A subset of areas exhibited an overlap of both effects, containing neurons with selective responses for both executed and observed movements. We describe the function of these movement representation areas in the context of the human mirror system, which is expected to respond selectively to both observed and executed movements.

Contrast adaptation in a nonadapting LGN model

Sensory neurons appear to adapt their gain to match the variance of signals along the dimension they encode, a property we shall call "contrast normalization." Contrast normalization has been the subject of extensive physiological and theoretical study. We previously found that neurons in the lateral geniculate nucleus (LGN) exhibit contrast normalization in their responses to full-field flickering white-noise stimuli, and that neurons with the strongest contrast normalization best preserved information transmission across a range of contrasts. We have also shown that both of these properties could be reproduced by nonadapting model cells. Here we present a detailed comparison of this nonadapting model to physiological data from the LGN. First, the model cells recapitulated other contrast dependencies of LGN responses: decreasing stimulus contrast resulted in an increase in spike-timing jitter and spike-number variability. Second, we find that the extent of contrast normalization in this model depends on model parameters related to refractoriness and to noise. Third, we show that the model cells exhibit rapid, transient changes in firing rate just after changes in contrast, and that this is sufficient to produce the transient changes in information transmission that have been reported in other neurons. It is known that intrinsic properties of neurons change during contrast adaptation. Nevertheless the model demonstrates that the spiking nonlinearity of neurons can produce many of the temporal aspects of contrast gain control, including normalization to input variance and transient effects of contrast change.

Effects of spatial attention and salience cues on chromatic and achromatic motion processing

While several previous psychophysical and neurophysiological studies have demonstrated chromatic (red/green) input to motion processing, the nature of this input is still a matter of debate. In particular, there exists controversy as to whether chromatic motion processing is mediated by low-level motion mechanisms versus higher-level, attention- or salience-based mechanisms. To address the role of attention, in Experiment 1, we asked whether spatial attention exerts larger effects on chromatic (red/green), as compared to achromatic, motion. To this end, we employed a motion after-effect (MAE) paradigm, and measured attention effects by comparing MAE duration between conditions where subjects attended to the adapting moving grating stimulus versus ignored that stimulus because they were required to perform an attentionally demanding vowel detection task at the center of gaze. The results from these experiments revealed equal effects of spatial attention on chromatic and achromatic motion processing, which were essentially constant (roughly 1.4-fold) across a wide range of stimulus contrasts (3.2-25% cone contrast). These findings suggest that chromatic motion processing is not affected disproportionally by higher-level spatial attention mechanisms. To address the role of salience, in Experiment 2, we investigated the effects of bottom-up salience cues on the strength of chromatic and achromatic motion, as measured with the MAE. Salience was manipulated by varying the relationship between the moving gratings and the background color. The results of these experiments revealed small and insignificant effects of salience cues on chromatic and achromatic motion processing. These findings suggest that mechanisms sensitive to feature salience do not influence low-level chromatic motion mechanisms mediating the motion after-effect.

Visual adaptation: physiology, mechanisms, and functional benefits

Recent sensory experience affects both perception and the response properties of visual neurons. Here I review a rapid form of experience-dependent plasticity that follows adaptation, the presentation of a particular stimulus or ensemble of stimuli for periods ranging from tens of milliseconds to minutes. Adaptation has a rich history in psychophysics, where it is often used as a tool for dissecting the perceptual mechanisms of vision. Although we know comparatively little about the neurophysiological effects of adaptation, work in the last decade has revealed a rich repertoire of effects. This review focuses on this recent physiological work, the cellular and biophysical mechanisms that may underlie the observed effects, and the functional benefit that they may afford. I conclude with a brief discussion of some important open questions in the field.

The lingering effects of an artificial blind spot

Background

When steady fixation is maintained on the centre of a large patch of texture, holes in the periphery of the texture rapidly fade from awareness, producing artificial scotomata (i.e., invisible areas of reduced vision, like the natural ‘blind spot’). There has been considerable controversy about whether this apparent ‘filling in’ depends on a low-level or high-level visual process. Evidence for an active process is that when the texture around the scotomata is suddenly removed, phantasms of the texture appear within the previous scotomata.

Methodology

To see if these phantasms were equivalent to real low-level signals, we measured contrast discrimination for real dynamic texture patches presented on top of the phantasms.

Principal Findings

Phantasm intensity varied with adapting contrast. Contrast discrimination depended on both (real) pedestal contrast and phantasm intensity, in a manner indicative of a common sensory threshold. The phantasms showed inter-ocular transfer, proving that their effects are cortical rather than retinal.

Conclusions

We show that this effect is consistent with a tonic spreading of the adapting texture into the scotomata, coupled with some overall loss of sensitivity. Our results support the view that ‘filling in’ happens at an early stage of visual processing, quite possibly in primary visual cortex (V1).

Subtractive and divisive adaptation in visual motion computations

Models of visual motion processing that introduce priors for low speed through Bayesian computations are sometimes treated with scepticism by empirical researchers because of the convenient way in which parameters of the Bayesian priors have been chosen. Using the effects of motion adaptation on motion perception to illustrate, we show that the Bayesian prior, far from being convenient, may be estimated on-line and therefore represents a useful tool by which visual motion processes may be optimized in order to extract the motion signals commonly encountered in every day experience. The prescription for optimization, when combined with system constraints on the transmission of visual information, may lead to an exaggeration of perceptual bias through the process of adaptation. Our approach extends the Bayesian model of visual motion proposed byWeiss et al. [Weiss Y., Simoncelli, E., & Adelson, E. (2002). Motion illusions as optimal perception Nature Neuroscience, 5:598-604.], in suggesting that perceptual bias reflects a compromise taken by a rational system in the face of uncertain signals and system constraints.

FMRI adaptation reveals separate mechanisms for first-order and second-order motion

A key unresolved debate in human vision concerns whether we have two different low-level mechanisms for encoding image motion. Separate neural mechanisms have been suggested for first-order (luminance modulation) and second-order (e.g., contrast modulation) motion in the retinal image but a single mechanism could handle both. Human functional magnetic resonance imaging (fMRI) has not so far convincingly revealed separate anatomical substrates. To examine whether two separate but co-localized mechanisms might exist, we used the technique of fast fMRI adaptation. We found direction-selective adaptation independently for each type of motion in the motion area V5/MT+ of the human brain. However, there was a total absence of cross-adaptation between first-order and second-order motion stimuli. This was true in both of the two subcomponents of MT+ (MT and MST) and similar results were found in V3A. This pattern of adaptation was consistent with psychophysical measurements of detection thresholds in similar stimulus sequences. The results provide strong evidence for separate neural populations that are responsible for detecting first- and second-order motion.

How MT cells analyze the motion of visual patterns

Neurons in area MT (V5) are selective for the direction of visual motion. In addition, many are selective for the motion of complex patterns independent of the orientation of their components, a behavior not seen in earlier visual areas. We show that the responses of MT cells can be captured by a linear-nonlinear model that operates not on the visual stimulus, but on the afferent responses of a population of nonlinear V1 cells. We fit this cascade model to responses of individual MT neurons and show that it robustly predicts the separately measured responses to gratings and plaids. The model captures the full range of pattern motion selectivity found in MT. Cells that signal pattern motion are distinguished by having convergent excitatory input from V1 cells with a wide range of preferred directions, strong motion opponent suppression and a tuned normalization that may reflect suppressive input from the surround of V1 cells.

Cross-fixation transfer of motion aftereffects with expansion motion

It has been shown that motion aftereffect (MAE) not only is present at the adapted location but also partially transfers to nearby non-adapted locations. However, it is not clear whether MAE transfers across the fixation point. Since cells in area MSTd have receptive fields that cover both sides of the fixation point and since many MSTd cells, but not cells in earlier visual areas, prefer complex motion patterns such as expansion, we tested cross-fixation transfer of MAE induced by expanding random-dots stimuli. We also used rightward translational motion for comparison. Subjects adapted to motion patterns on a fixed side of the fixation point. Dynamic MAE was then measured with a nulling procedure at both the adapted site and the mirror site across the fixation point. Subjects' eye fixation during stimulus presentation was monitored with an infrared eye tracker. At the adapted site, both the expansion and the translation patterns generated strong MAEs, as expected. However, only the expansion pattern, but not translation pattern, generated significant MAE at the mirror site. This remained true even after we adjusted stimulus parameters to equate the strengths of the expansion MAE and translation MAE at the adapted site. We conclude that there is cross-fixation transfer of MAE for expansion motion but not for translational motion.

Response adaptation to broadband sounds in primary auditory cortex of the awake ferret

Driven by previous reports of adaptation to persistent stimuli in other brain regions, we investigated adaptive effects in the Primary Auditory Cortex of awake non-behaving ferrets (Mustela putorius furo). Electrophysiological data was obtained in response to the presentation of auditory gratings with a structured spectro-temporal envelope of varying bandwidth which had repeated transitions between low and high modulation depths. The responses were analyzed in terms of the evoked spike rates and in terms of the degree of phase locking to the modulation. We found two populations of cells, both of which showed adaptation in the traditional sense. For one population, we also found a second order of adaptation--i.e., adaptation of the adaptation. This suggests the existence of at least two coding strategies which differ in the weight placed on sensory context.

Linking dynamical perceptual decisions at different levels of description in motion pattern formation: computational simulations

A two-level dynamical model of motion pattern formation is developed in which local motion/ nonmotion perceptual decisions are based on inhibitory competition between area V1 detectors responsive to motion-specifying versus motion-independent stimulus information, and pattern-level perceptual decisions are based on inhibitory competition between area MT motion detectors with orthogonal directional selectivity. The model accounts for the effects of luminance perturbations on the relative size of the pattern-level hysteresis effects reported by Hock and Ploeger (2006) and also accounts for related experimental results reported by Hock, Kelso, and Schöner (1993). Single-trial simulations demonstrated the crucial role of local motion/nonmotion bistability and activation-dependent future-shaping interactions in stabilizing perceived global motion patterns. Such interactions maintain currently perceived motion patterns by inhibiting the soon-to-be-stimulated motion detectors that otherwise would be the basis for the perception of an alternative pattern.

Duration-Dependent fMRI Adaptation and Distributed Viewer-Centered Face Representation in Human Visual Cortex

Two functional magnetic resonance imaging (fMRI) face viewpoint adaptation experiments were conducted to investigate whether fMRI adaptation in high-level visual cortex depends on the duration of adaptation and how different views of a face are represented in the human visual system. We found adaptation effects in multiple face-selective areas, which suggest a distributed, viewer-centered representation of faces in the human visual system. However, the nature of the adaptation effects was dependent on the length of adaptation. With long adaptation durations, face-selective areas along the hierarchy of the visual system gradually exhibited viewpoint-tuned adaptation. As the angular difference between the adapter and test stimulus increased, the blood oxygen level-dependent (BOLD) signal evoked by the test stimulus gradually increased as a function of the amount of 3-dimensional (3D) rotation. With short adaptation durations, however, face-selective areas in the ventral pathway, including the lateral occipital cortex and right fusiform area, exhibited viewpoint-sensitive adaptation. These areas showed an increase in the BOLD signal with a 3D rotation, but this signal increase was independent of the amount of rotation. Further, the right superior temporal sulcus showed little or very weak viewpoint adaptation with short adaptation durations. Our findings suggest that long- and short-term fMRI adaptations may reflect selective properties of different neuronal mechanisms.