Contralateral visual hemifield representations in the human pulvinar nucleus

The pulvinar is a major nucleus of the thalamus. Macaque pulvinar includes two subregions that are connected to the visual cortex and are retinotopically organized, but the organizing principles of the visual portions of the human pulvinar are unknown. We employed two tasks to address the question of whether human pulvinar exhibits spatial organization using event-related functional magnetic resonance imaging. The first was a global-motion discrimination with a rich visual stimulus and the second a luminance-discrimination task of similar difficulty that used a minimal visual stimulus. Both tasks required central fixation and covert peripheral attention. A group analysis of blood-oxygen-level-dependent responses elicited in the motion-discrimination task revealed activity bilaterally in the ventral pulvinar (z = 2 in Talairach space). Clear position specificity was observed with activity elicited only by contralateral stimuli. Ipsilateral stimuli caused suppression. This locus of activity is distinct from the more dorsal (z = 10) region of the pulvinar that has previously been reported to be visually responsive but not retinotopic. In the luminance-discrimination task, similar activity was seen, but it was weaker and detectable only in the left pulvinar. In additional experiments with no task, passively viewed global-motion stimuli also activated the ventral pulvinar bilaterally. Our results show for the first time a distinct, bilateral visual representation in human inferior pulvinar that appears to be contralaterally organized.     

Occipital network for figure/ground organization

To study the cortical mechanism of figure/ground categorization in the human brain, we employed fMRI and the temporal-asynchrony paradigm. This paradigm is able to eliminate any differential activation for local stimulus features, and thus to identify only global perceptual interactions. Strong segmentation of the image into different spatial configurations was generated solely from temporal asynchronies between zones of homogeneous dynamic noise. The figure/ground configuration was a single geometric figure enclosed in a larger surround region. In a control condition, the figure/ground organization was eliminated by segmenting the noise field into many identical temporal-asynchrony stripes. The manipulation of the type of perceptual organization triggered dramatic reorganization in the cortical activation pattern. The figure/ground configuration generated suppression of the ground representation (limited to early retinotopic visual cortex, V1 and V2) and strong activation in the motion complex hMT+/V5+; conversely, both responses were abolished when the figure/ground organization was eliminated. These results suggest that figure/ground processing is mediated by top-down suppression of the ground representation in the earliest visual areas V1/V2 through a signal arising in the motion complex. We propose a model of a recurrent cortical architecture incorporating suppressive feedback that operates in a topographic manner, forming a figure/ground categorization network distinct from that for “pure” scene segmentation and thus underlying the perceptual organization of dynamic scenes into cognitively relevant components. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Rapid and precise retinotopic mapping of the visual cortex obtained by voltage-sensitive dye imaging in the behaving monkey

Retinotopy is a fundamental organizing principle of the visual cortex. Over the years, a variety of techniques have been used to examine it. None of these techniques, however, provides a way to rapidly characterize retinotopy, at the submillimeter range, in alert, behaving subjects. Voltage-sensitive dye imaging (VSDI) can be used to monitor neuronal population activity at high spatial and temporal resolutions. Here we present a VSDI protocol for rapid and precise retinotopic mapping in the behaving monkey. Two monkeys performed a fixation task while thin visual stimuli swept periodically at a high speed in one of two possible directions through a small region of visual space. Because visual space is represented systematically across the cortical surface, each moving stimulus produced a traveling wave of activity in the cortex that could be precisely measured with VSDI. The time at which the peak of the traveling wave reached each location in the cortex linked this location with its retinotopic representation. We obtained detailed retinotopic maps from a region of about 1 cm(2) over the dorsal portion of areas V1 and V2. Retinotopy obtained during <4 min of imaging had a spatial precision of 0.11-0.19 mm, was consistent across experiments, and reliably predicted the locations of the response to small localized stimuli. The ability to rapidly obtain precise retinotopic maps in behaving monkeys opens the door for detailed analysis of the relationship between spatiotemporal dynamics of population responses in the visual cortex and perceptually guided behavior.     

Scene segmentation and attention in primate cortical areas V1 and V2

The responses of many neurons in primary visual cortex are modulated by stimuli outside the classical receptive field in ways that may contribute to integrative processes like scene segmentation. To explore this issue, single-unit neuronal responses were recorded in monkey cortical areas V1 and V2 to visual stimuli containing either a figure or a background pattern over the receptive field. Figures were defined either by orientation contrast or by illusory contours. In all conditions, the stimulation over the RF and its nearby surround was identical. Both figure types enhanced the average population response in V1 and V2. For orientation contrast figures, enhancement averaged 50% in V2 and 30% in V1; for illusory contour figures, the enhancement averaged 24% in V2 and 18% in V1. These differences were statistically significant for figure type but not for visual area. In V2, the latency of enhancement to illusory contour-defined figures was longer than that to orientation-defined figures. Neuronal responses were recorded while the monkey performed a directed-attention task. Enhancement to both figure types was observed even when attention was directed away from the figure. Attention slightly enhanced responses in V2, independent of figure type, but did not affect responses in V1. There was no discernible effect of attention on background firing rate in either V1 or V2. These results suggest that scene segmentation is a distributed process, in which neuronal signals at successive stages of the visual hierarchy and over time increasingly reflect the global structure of the image. This process occurs independent of directed visual attention.     

Visual, saccade-related, and cognitive activation of single neurons in monkey extrastriate area V3A

Area V3A is an extrastriate visual area that provides a major input to parietal cortex. To identify the sensory, saccade-related, and cognitive signals carried by V3A neurons, we recorded from single units in alert monkeys during performance of fixation and memory guided saccade tasks. We found that visual responses to stationary stimuli in area V3A were affected by the behavioral relevance of the stimulus. The amplitude of the visual response differed between the memory-guided saccade task, in which the monkey had to use the information provided by the stimulus to guide its behavior, and the fixation task. For 18% (29/163) of V3A neurons, the response was significantly enhanced in the memory-guided saccade task as compared with that in the fixation task. For 8% (13/163) of V3A neurons, the amplitude of response in the memory-guided saccade task was significantly suppressed. We also observed task-related modulation of activity prior to stimulus onset. Among the V3A neurons (37/163) that showed significant differences between tasks in prestimulus activity, the majority (89%; 33/37) showed greater prestimulus activity in the memory-guided saccade task. Task-related increases in prestimulus activity in the memory-guided saccade task were not always matched by increases in the sensory response, indicating that visual responses and prestimulus activity can be modulated independently. Activity in the memory period was suppressed compared with prestimulus activity for 83% (49/59) of the V3A neurons that showed a significant difference in activity (59/197) between these two epochs. For some neurons, memory-period activity dropped even below the baseline level in the fixation task, indicating that there may be an active suppression mechanism. Many V3A neurons (75%, 148/197) also had activity in the saccade epoch. This activity was most prominent immediately after the saccade. Postsaccadic activity was observed even when testing was carried out in total darkness, indicating that this activity reflects, at least in part, extraretinal signals and is not simply a response to visual reafference. These results indicate that several kinds of signals are carried by single neurons in extrastriate area V3A. Moreover, activity in V3A is subject to modulation by extraretinal factors, including attention, anticipation, memory, and saccadic eye movements.     

Differential human brain activation by vertical and horizontal global visual textures

Mid-level visual processes which integrate local orientation information for the detection of global structure can be investigated using global form stimuli of varying complexity. Several lines of evidence suggest that the identification of concentric and parallel organisations relies on different underlying neural substrates. The current study measured brain activation by concentric, horizontal parallel, and vertical parallel arrays of short line segments, compared to arrays of randomly oriented segments. Six subjects were scanned in a blocked design functional magnetic resonance imaging experiment. We compared percentage BOLD signal change during the concentric, horizontal and vertical blocks within early retinotopic areas, the fusiform face area and the lateral occipital complex. Unexpectedly, we found that vertical and horizontal parallel forms differentially activated visual cortical areas beyond V1, but in general, activations to concentric and parallel forms did not differ. Vertical patterns produced the highest percentage signal change overall and only area V3A showed a significant difference between concentric and parallel (horizontal) stimuli, with the former better activating this area. These data suggest that the difference in brain activation to vertical and horizontal forms arises at intermediate or global levels of visual representation since the differential activity was found in mid-level retinotopic areas V2 and V3 but not in V1. This may explain why earlier studies—using methods that emphasised responses to local orientation—did not discover this vertical–horizontal anisotropy.     

Uncertainty and invariance in the human visual cortex

The way in which input noise perturbs the behavior of a system depends on the internal processing structure of the system. In visual psychophysics, there is a long tradition of using external noise methods (i.e., adding noise to visual stimuli) as tools for system identification. Here, we demonstrate that external noise affects processing of visual scenes at different cortical areas along the human ventral visual pathway, from retinotopic regions to higher occipitotemporal areas implicated in visual shape processing. We found that when the contrast of the stimulus was held constant, the further away from the retinal input a cortical area was the more its activity, as measured with functional magnetic resonance imaging (fMRI), depended on the signal-to-noise ratio (SNR) of the visual stimulus. A similar pattern of results was observed when trials with correct and incorrect responses were analyzed separately. We interpret these findings by extending signal detection theory to fMRI data analysis. This approach reveals the sequential ordering of decision stages in the cortex by exploiting the relation between fMRI response and stimulus SNR. In particular, our findings provide novel evidence that occipitotemporal areas in the ventral visual pathway form a cascade of decision stages with increasing degree of signal uncertainty and feature invariance.     

Spatiotopic temporal integration of visual motion across saccadic eye movements

Saccadic eye movements pose many challenges for stable and continuous vision, such as how information from successive fixations is amalgamated into a single precept. Here we show in humans that motion signals are temporally integrated across separate fixations, but only when the motion stimulus falls either on the same retinal region (retinotopic integration) or on different retinal positions that correspond to the same external spatial coordinates (spatiotopic integration). We used individual motion signals that were below detection threshold, implicating spatiotopic trans-saccadic integration in relatively early stages of visual processing such as the middle temporal area (MT) or V5 of visual cortex. The trans-saccadic buildup of important congruent visual information while irrelevant non-congruent information fades could provide a simple and robust strategy to stabilize perception during eye movements.     

Feedback to V1: a reverse hierarchy in vision

We examined the involvement of visual area V1 in visual detection to assess the role of feedback connections to V1 as proposed in reverse hierarchy theory (Ahissar and Hochstein 2000). In Experiment 1, signal detection was decreased in feature and conjunction detection tasks by repetitive pulse TMS (rTMS) over V1 for 500 ms after stimulus onset. In Experiment 2, rTMS was delayed to allow uninterrupted signal processing for 100 ms after visual stimulus onset. TMS for the subsequent 500 ms now disrupted detection of conjunction but not feature targets. In Experiment 3 we applied double pulse TMS at varying intervals to assess the timing of V1 involvement in these tasks. Single feature detection involved V1 only at some point between 40 and 100 ms after visual array onset; detection of targets defined by conjunctions of features involved V1 throughout the first 100 ms and also between 200 and 240 ms after visual stimulus onset. We suggest that the early effects in the conjunction task are due to repeated sampling of the visual array to extract the signal from external noise. The later effects in conjunction search are attributed to the return projections from secondary visual areas back to V1, consistent with the reverse hierarchy theory. The effects in both tasks are consistent with early and repeated iterations of feed forward and feedback loops as hypothesised in recent neurophysiological experiments (see Foxe and Simpson 2002). Electronic Publication     

Neuronal correlates of perception in early visual cortex

We used functional magnetic resonance imaging (fMRI) to measure activity in human early visual cortex (areas V1, V2 and V3) during a challenging contrast-detection task. Subjects attempted to detect the presence of slight contrast increments added to two kinds of background patterns. Behavioral responses were recorded so that the corresponding cortical activity could be grouped into the usual signal detection categories: hits, false alarms, misses and correct rejects. For both kinds of background patterns, the measured cortical activity was retinotopically specific. Hits and false alarms were associated with significantly more cortical activity than were correct rejects and misses. That false alarms evoked more activity than misses indicates that activity in early visual cortex corresponded to the subjects' percepts, rather than to the physically presented stimulus.     

Visual responses of the human superior colliculus: a high-resolution functional magnetic resonance imaging study

The superior colliculus (SC) is a multimodal laminar structure located on the roof of the brain stem. The SC is a key structure in a distributed network of areas that mediate saccadic eye movements and shifts of attention across the visual field and has been extensively studied in nonhuman primates. In humans, it has proven difficult to study the SC with functional MRI (fMRI) because of its small size, deep location, and proximity to pulsating vascular structures. Here, we performed a series of high-resolution fMRI studies at 3 T to investigate basic visual response properties of the SC. The retinotopic organization of the SC was determined using the traveling wave method with flickering checkerboard stimuli presented at different polar angles and eccentricities. SC activations were confined to stimulation of the contralateral hemifield. Although a detailed retinotopic map was not observed, across subjects, the upper and lower visual fields were represented medially and laterally, respectively. Responses were dominantly evoked by stimuli presented along the horizontal meridian of the visual field. We also measured the sensitivity of the SC to luminance contrast, which has not been previously reported in primates. SC responses were nearly saturated by low contrast stimuli and showed only small response modulation with higher contrast stimuli, indicating high sensitivity to stimulus contrast. Responsiveness to stimulus motion in the SC was shown by robust activations evoked by moving versus static dot stimuli that could not be attributed to eye movements. The responses to contrast and motion stimuli were compared with those in the human lateral geniculate nucleus. Our results provide first insights into basic visual responses of the human SC and show the feasibility of studying subcortical structures using high-resolution fMRI.     

Temporal dynamics of shape analysis in macaque visual area V2

The firing rate of visual cortical neurons typically changes substantially during a sustained visual stimulus. To assess whether, and to what extent, the information about shape conveyed by neurons in visual area V2 changes over the course of the response, we recorded the responses of V2 neurons in awake, fixating monkeys while presenting a diverse set of static shape stimuli within the classical receptive field. We analyzed the time course of various measures of responsiveness and stimulus-related response modulation at the level of individual cells and of the population. For a majority of V2 cells, the response modulation was maximal during the initial transient response (40-80 ms after stimulus onset). During the same period, the population response was relatively correlated, in that V2 cells tended to respond similarly to specific subsets of stimuli. Over the ensuing 80-100 ms, the signal-to-noise ratio of individual cells generally declined, but to a lesser degree than the evoked-response rate during the corresponding time bins, and the response profiles became decorrelated for many individual cells. Concomitantly, the population response became substantially decorrelated. Our results indicate that the information about stimulus shape evolves dynamically and relatively rapidly in V2 during static visual stimulation in ways that may contribute to form discrimination.     

Effect of attentive fixation in macaque thalamus and cortex

Attentional modulation of neuronal responsiveness is common in many areas of visual cortex. We examined whether attentional modulation in the visual thalamus was quantitatively similar to that in cortex. Identical procedures and apparatus were used to compare attentional modulation of single neurons in seven different areas of the visual system: the lateral geniculate, three visual subdivisions of the pulvinar [inferior, lateral, dorsomedial part of lateral pulvinar (Pdm)], and three areas of extrastriate cortex representing early, intermediate, and late stages of cortical processing (V2, V4/PM, area 7a). A simple fixation task controlled transitions among three attentive states. The animal waited for a fixation point to appear (ready state), fixated the point until it dimmed (fixation state), and then waited idly to begin the next trial (idle state). Attentional modulation was estimated by flashing an identical, irrelevant stimulus in a neuron's receptive field during each of the three states; the three responses defined a "response vector" whose deviation from the line of equal response in all three states (the main diagonal) indicated the character and magnitude of attentional modulation. Attentional modulation was present in all visual areas except the lateral geniculate, indicating that modulation was of central origin. Prevalence of modulation was modest (26%) in pulvinar, and increased from 21% in V2 to 43% in 7a. Modulation had a push-pull character (as many cells facilitated as suppressed) with respect to the fixation state in all areas except Pdm where all cells were suppressed during fixation. The absolute magnitude of attentional modulation, measured by the angle between response vector and main diagonal expressed as a percent of the maximum possible angle, differed among brain areas. Magnitude of modulation was modest in the pulvinar (19-26%), and increased from 22% in V2 to 41% in 7a. However, average trial-to-trial variability of response, measured by the coefficient of variation, also increased across brain areas so that its difference among areas accounted for more than 90% of the difference in modulation magnitude among areas. We also measured attentional modulation by the ratio of cell discharge due to attention divided by discharge variability. The resulting signal-to-noise ratio of attention was small and constant, 1.3 +/- 10%, across all areas of pulvinar and cortex. We conclude that the pulvinar, but not the lateral geniculate, is as strongly affected by attentional state as any area of visual cortex we studied and that attentional modulation amplitude is closely tied to intrinsic variability of response.     

Short-term visual deprivation alters neural processing of tactile form

Blindness is known to alter the responsiveness of visual cortex. Recently, reversible visual deprivation by blindfolding has been shown to affect non-visual abilities as well as visual cortical function. Here we investigated the effect of 2 h of blindfolding on cerebral cortical activation patterns during tactile form perception, using functional magnetic resonance imaging. Two form tasks were used, one requiring discrimination of global stimulus form and the other, detection of a gap in a bar. Blindfolded subjects showed significant deactivation during these tasks in regions that are intermediate in the hierarchy of visual shape processing: probable V3A and ventral intraparietal sulcus (vIPS). These regions lacked signal changes in controls. There were also task-specific increases in activation in blindfolded relative to control subjects, favoring the form over the gap task, along the IPS and in regions of frontal and temporal cortex. We also found alterations of functional connectivity that corresponded to the activity differences, with the emergence of correlated activity between the vIPS and V3A in blindfolded subjects. We conclude that blindfolding sighted individuals for a 2-h period induces significant changes in the neural processing of tactile form, probably reflecting short-term neural plasticity.     

A comparison of magnification functions in area 19 and the lateral suprasylvian visual area in the cat

A retinotopic map can be described by a magnification function that relates magnification factor to visual field eccentricity. Magnification factor for primary visual cortex (VI) in both the cat and the macaque monkey is directly proportional to retinal ganglion cell density. However, among those extrastriate areas for which a magnification function has been described, this is often not the case. Deviations from the pattern established in V1 are of considerable interest because they may provide insight into an extrastriate area's role in visual processing. The present study explored the magnification function for the lateral suprasylvian area (LS) in the cat. Because of its complex retinotopic organization, magnification was calculated indirectly using the known magnification function for area 19. Small tracer injections were made in area 17, and the extent of anterograde label in LS and in area 19 was measured. Using the ratio of cortical area labeled in LS to that in area 19, and the known magnification factor for area 19 at the corresponding retinotopic location, we were able to calculate magnification factor for LS. We found that the magnification function for LS differed substantially from that for area 19: central visual field was expanded, and peripheral field compressed in LS compared with area 19. Additionally, we found that the lower vertical meridian's representation was compressed relative to that of the horizontal meridian. We also examined receptive field size in areas 17, 19, and LS and found that, for all three areas, receptive field size was inversely proportional to magnification factor.     

Neuroimaging of direction-selective mechanisms for second-order motion

Psychophysical findings have revealed a functional segregation of processing for 1st-order motion (movement of luminance modulation) and 2nd-order motion (e.g., movement of contrast modulation). However neural correlates of this psychophysical distinction remain controversial. To test for a corresponding anatomical segregation, we conducted a new functional magnetic resonance imaging (fMRI) study to localize direction-selective cortical mechanisms for 1st- and 2nd-order motion stimuli, by measuring direction-contingent response changes induced by motion adaptation, with deliberate control of attention. The 2nd-order motion stimulus generated direction-selective adaptation in a wide range of visual cortical areas, including areas V1, V2, V3, VP, V3A, V4v, and MT+. Moreover, the pattern of activity was similar to that obtained with 1st-order motion stimuli. Contrary to expectations from psychophysics, these results suggest that in the human visual cortex, the direction of 2nd-order motion is represented as early as V1. In addition, we found no obvious anatomical segregation in the neural substrates for 1st- and 2nd-order motion processing that can be resolved using standard fMRI.     

Pulvinar nuclei of the behaving rhesus monkey: visual responses and their modulation

We have examined the properties of neurons in three subdivisions of the pulvinar of alert, trained rhesus monkeys 1) an inferior, retinotopically mapped area (PI), 2) a lateral, retinotopically organized region (PL), and 3) a dorsomedial visual portion of the lateral pulvinar (Pdm), which has a crude retinotopic organization. We tested the neurons for visual responses to stationary and moving stimuli and for changes in these responses produced by behavioral manipulations. All areas contain cells sensitive to stimulus orientation as well as neurons selective for the direction of stimulus movement; however, the majority of cells in all three regions are either broadly tuned or nonselective for these attributes. Nearly all cells respond to stimulus onset, a significant number also give a response to stimulus termination, and rarely a cell gives only off responses. Nearly all cells increase their discharge rate to visual stimuli. Receptive fields in the two retinotopically mapped regions, PI and PL, have well-defined borders. The sizes of these receptive fields show a positive correlation with the eccentricity of the receptive fields. The receptive fields in the remaining region, Pdm, are frequently very large, but with these large fields excluded, show a similar correlation with eccentricity. All pulvinar cells tested (n = 20) were mapped in retinal coordinates; the receptive fields are positioned in relation to the retina. We found no cells with gaze-gated characteristics (2), nor cells mapped in a spatial coordinate system. The response latencies in PI and PL are shorter and less variable than the latencies in Pdm. Active use of a stimulus can produce an enhancement or attenuation of the visual response. Eye-movement modulation was found in all three subdivisions in about equal frequencies. Attentional modulation was common in Pdm and was rare in PI and PL. The modulation is spatially selective in Pdm and nonselective in PI for a small number of tested cells. These data demonstrate functional differences between Pdm and the other two areas and suggest that Pdm plays a role in selective visual attention, whereas PI and PL probably contribute to other aspects of visual perception.     

Impaired attention modulation of the blink reflex R3 component in Parkinson's disease: a non-task warning paradigm study.

PURPOSE: the aim of this experimental study was to evaluate the attention modulating actions on the polysynaptic component of blink reflex responses and especially of the R3 component in patients suffering from Parkinson's Disease (PD). To this end, a non-task warning paradigm was adopted. METHODS: attention processing was evaluated by means of a non-task paradigm in 55 patients suffering from PD. Subjects were presented with a visual 'warning' prestimulus and the blink reflex (BR) analyzed with special regard for any modulation of its polysynaptic components (R2-R3). RESULTS: The mean amplitude of the post-warning R3 component (PW-R3c) of 'de novo' PD patients was 62% of the corresponding component following unannounced stimuli, a figure which differs significantly from both treated PD patients (18.9%) and control subjects (15.4%). De novo patients subsequently started on L-dopa therapy exhibited a more pronounced inhibition of the R3 component after warning stimulus, as the PW-R3c percentage decreased. Inversely, treated patients whose therapy was withheld showed decreased inhibition of this component. Regarding R2, the mean PW-R2c in the de novo patients differed slightly from that of the treated patients (P<0.05), but not from that of the control subjects. Such a finding may be attributable to a specific effects on the excitability of the polysynaptic responses. CONCLUSIONS: Attention disorders in PD have been well documented by means of this kind of non-task warning paradigm, which appears to probe the modulation of the BR R3 component, even if the interpretation of this R3 changes suggesting a specific alteration of attention processing must be put forward extremely carefully, because something similar, but less evident, appears also for R2.     

Orientation-selective adaptation to first- and second-order patterns in human visual cortex

Second-order textures-patterns that cannot be detected by mechanisms sensitive only to luminance changes-are ubiquitous in visual scenes, but the neuronal mechanisms mediating perception of such stimuli are not well understood. We used an adaptation protocol to measure neural activity in the human brain selective for the orientation of second-order textures. Functional MRI (fMRI) responses were measured in three subjects to presentations of first- and second-order probe gratings after adapting to a high-contrast first- or second-order grating that was either parallel or orthogonal to the probe gratings. First-order (LM) stimuli were generated by modulating the stimulus luminance. Second-order stimuli were generated by modulating the contrast (CM) or orientation (OM) of a first-order carrier. We used four combinations of adapter and probe stimuli: LM:LM, CM:CM, OM:OM, and LM:OM. The fourth condition tested for cross-modal adaptation with first-order adapter and second-order probe stimuli. Attention was diverted from the stimulus by a demanding task at fixation. Both first- and second-order stimuli elicited orientation-selective adaptation in multiple cortical visual areas, including V1, V2, V3, V3A/B, a newly identified visual area anterior to dorsal V3 that we have termed LO1, hV4, and VO1. For first-order stimuli (condition LM:LM), the adaptation was no larger in extrastriate areas than in V1, implying that the orientation-selective first-order (luminance) adaptation originated in V1. For second-order stimuli (conditions CM:CM and OM:OM), the magnitude of adaptation, relative to the absolute response magnitude, was significantly larger in VO1 (and for condition CM:CM, also in V3A/B and LO1) than in V1, suggesting that second-order stimulus orientation was extracted by additional processing after V1. There was little difference in the amplitude of adaptation between the second-order conditions. No consistent effect of adaptation was found in the cross-modal condition LM:OM, in agreement with psychophysical evidence for weak interactions between first- and second-order stimuli and computational models of separate mechanisms for first- and second-order visual processing.     

Perceptual learning and top-down influences in primary visual cortex

Neuronal responses at early stages in visual cortical processing, including those in primary visual cortex (V1), are subject to the influences of visual context, experience and attention. Here we show that for monkeys trained in a shape discrimination task, V1 neurons took on novel functional properties related to the attributes of the trained shapes. Furthermore, these properties depended on the perceptual task being performed; neurons responded very differently to an identical visual stimulus under different visual discrimination tasks. These top-down influences were seen from the very beginning and throughout the entire time course of the neural responses. Information theoretic analysis showed that neurons carried more information about a stimulus attribute when the animals were performing a task related to that attribute. Our findings suggest that the output from V1 reflects both sensory and behavioral context.