Retinotopic Specificity Research Articles

Constraining the Possible Mechanisms Underlying Performance Improvements Following Peripheral Vision Training

AbstractSome training paradigms can improve performance specific to the trained portion of the visual field. Similarly, experience with both pathological and simulated central vision loss can result in compensatory improvement in performance specific to a spared retinal location. The mechanisms underlying these improvements are still debated. Modifications in the capacity to allocate attention to trained locations might explain some of the changes in performance after learning, especially given similarities between behavioral improvements due to training and improvements due to changes in attention. Using a gaze-contingent simulated scotoma paradigm which simulates central vision loss, we tested whether training to use peripheral vision influenced three aspects of visual attention: habitual attention, dynamic exogenous attention, and dynamic endogenous attention. After training, performance improvements were consistent with increased habitual attention to the trained location. Conversely, measures of dynamic shifts of attention (exogenous and endogenous attention) improved similarly in both trained and untrained locations. The lack of retinotopic specificity in dynamic attention improvements suggests that retinotopically specific perceptual improvements following simulated central vision loss are not mediated by dynamic attention changes. On the other hand, habitual attention did change retinotopically, leaving the possibility open that this aspect of attention may drive some retinotopically specific training effects. This work constrains the range of mechanisms that could underlie performance improvements after peripheral vision training, suggesting that habitual attention and dynamic attention are affected independently.

Visual experience reduces the spatial redundancy between cortical feedback inputs and primary visual cortex neurons

The role of experience in the organization of cortical feedback (FB) remains unknown. We measured the effects of manipulating visual experience on the retinotopic specificity of supragranular and infragranular projections from the lateromedial (LM) visual area to layer (L)1 of the mouse primary visual cortex (V1). LM inputs were, on average, retinotopically matched with V1 neurons in normally and dark-reared mice, but visual exposure reduced the fraction of spatially overlapping inputs to V1. FB inputs from L5 conveyed more surround information to V1 than those from L2/3. The organization of LM inputs from L5 depended on their orientation preference and was disrupted by dark rearing. These observations were recapitulated by a model where visual experience minimizes receptive field overlap between LM inputs and V1 neurons. Our results provide a mechanism for the dependency of surround modulations on visual experience and suggest how expected interarea coactivation patterns are learned in cortical circuits.

Low-Level Visual Information Is Maintained across Saccades, Allowing for a Postsaccadic Handoff between Visual Areas.

Experience seems continuous and detailed despite saccadic eye movements changing retinal input several times per second. There is debate whether neural signals related to updating across saccades contain information about stimulus features, or only location pointers without visual details. We investigated the time course of low-level visual information processing across saccades by decoding the spatial frequency of a stationary stimulus that changed from one visual hemifield to the other because of a horizontal saccadic eye movement. We recorded magnetoencephalography while human subjects (both sexes) monitored the orientation of a grating stimulus, making spatial frequency task irrelevant. Separate trials, in which subjects maintained fixation, were used to train a classifier, whose performance was then tested on saccade trials. Decoding performance showed that spatial frequency information of the presaccadic stimulus remained present for ∼200 ms after the saccade, transcending retinotopic specificity. Postsaccadic information ramped up rapidly after saccade offset. There was an overlap of over 100 ms during which decoding was significant from both presaccadic and postsaccadic processing areas. This suggests that the apparent richness of perception across saccades may be supported by the continuous availability of low-level information with a “soft handoff” of information during the initial processing sweep of the new fixation.SIGNIFICANCE STATEMENT Saccades create frequent discontinuities in visual input, yet perception appears stable and continuous. How is this discontinuous input processed resulting in visual stability? Previous studies have focused on presaccadic remapping. Here we examined the time course of processing of low-level visual information (spatial frequency) across saccades with magnetoencephalography. The results suggest that spatial frequency information is not predictively remapped but also is not discarded. Instead, they suggest a soft handoff over time between different visual areas, making this information continuously available across the saccade. Information about the presaccadic stimulus remains available, while the information about the postsaccadic stimulus has also become available. The simultaneous availability of both the presaccadic and postsaccadic information could enable rich and continuous perception across saccades.

Visuocortical changes during a freezing-like state in humans

An adaptive response to threat requires optimized detection of critical sensory cues. This optimization is thought to be aided by freezing - an evolutionarily preserved defensive state of immobility characterized by parasympathetically mediated fear bradycardia and regulated by the amygdala-periaqueductal grey (PAG) circuit. Behavioral observations in humans and animals have suggested that freezing is also a state of enhanced visual sensitivity, particularly for coarse visual information, but the underlying neural mechanisms remain unclear. We induced a freezing-like state in healthy volunteers using threat of electrical shock and measured threat-related changes in both stimulus-independent (baseline) and stimulus-evoked visuocortical activity to low-vs. high-spatial frequency gratings, using functional MRI. As measuring immobility is not feasible in MRI environments, we used fear bradycardia and amygdala-PAG coupling in inferring a freezing-like state. An independent functional localizer and retinotopic mapping were used to assess the retinotopic specificity of visuocortical modulations. We found a threat-induced increase in baseline (stimulus-independent) visuocortical activity that was retinotopically nonspecific, which was accompanied by increased connectivity with the amygdala. A positive correlation between visuocortical activity and fear bradycardia (while controlling for sympathetic activation), and a concomitant increase in amygdala-PAG connectivity, confirmed the specificity of these findings for the parasympathetically dominated freezing-like state. Visuocortical responses to gratings were retinotopically specific, but did not differ between threat and safe conditions across participants. However, individuals who exhibited better discrimination of low-spatial frequency stimuli showed reduced stimulus-evoked V1 responses under threat. Our findings suggest that a defensive state of freezing involves an integration of preparatory defensive and perceptual changes which may be regulated by a common mechanism involving the amygdala.

Performance-monitoring integrated reweighting model of perceptual learning

Perceptual learning (PL) has been traditionally thought of as highly specific to stimulus properties, task and retinotopic position. This view is being progressively challenged, with accumulating evidence that learning can generalize (transfer) across various parameters under certain conditions. For example, retinotopic specificity can be diminished when the proportion of easy to hard trials is high, such as when multiple short staircases, instead of a single long one, are used during training. To date, there is a paucity of mechanistic explanations of what conditions affect transfer of learning. Here we present a model based on the popular Integrated Reweighting Theory model of PL but departing from its one-layer architecture by including a novel key feature: dynamic weighting of retinotopic-location-specific vs location-independent representations based on internal performance estimates of these representations. This dynamic weighting is closely related to gating in a mixture-of-experts architecture. Our dynamic performance-monitoring model (DPMM) unifies a variety of psychophysical data on transfer of PL, such as the short-vs-long staircase effect, as well as several findings from the double-training literature. Furthermore, the DPMM makes testable predictions and ultimately helps understand the mechanisms of generalization of PL, with potential applications to vision rehabilitation and enhancement.

The functional organization of cortical feedback inputs to primary visual cortex

Cortical feedback is thought to mediate cognitive processes like attention, prediction, and awareness. Understanding its function requires identifying the organizational logic of feedback axons relaying different signals. We measured retinotopic specificity in inputs from the lateromedial visual area in mouse primary visual cortex (V1) by mapping receptive fields in feedback boutons and relating them to those of neurons in their vicinity. Lateromedial visual area inputs in layer 1 targeted, on average, retinotopically matched locations in V1, but many of them relayed distal visual information. Orientation-selective axons overspread around the retinotopically matched location perpendicularly to their preferred orientation. Direction-selective axons were biased to visual areas shifted from the retinotopically matched position along the angle of their antipreferred direction. Our results show that feedback inputs show tuning-dependent retinotopic specificity. By targeting locations that would be activated by stimuli orthogonal to or opposite to a cell's own tuning, feedback could potentially enhance visual representations in time and space.

Retinotopic Specificity of Face Encoding in Neurotypicals and Developmental Prosopagnosics

Retinotopic Specificity of Face Encoding in Neurotypicals and Developmental Prosopagnosics

0126 NEUROIMAGING OF CIRCUIT-SPECIFIC PROTEIN SYNTHESIS IN HUMAN SUBJECTS DURING SLEEP-DEPENDENT MEMORY CONSOLIDATION

The notion that memory consolidation depends on protein synthesis is based on demonstrations that protein synthesis inhibitors prevent consolidation. We sought to demonstrate directly that protein synthesis is affected during sleep-dependent memory consolidation. We used L-[1-(11)C]leucine positron emission tomography (PET) to measure circuit-specific brain protein synthesis during a daytime nap opportunity. Subjects completed the texture discrimination task (TDT) before and after the nap opportunity. We randomized 34 subjects to either wakefulness or sleep and counterbalanced training to either left or right visual field. In accord with the retinotopic specificity of the TDT, we considered ipsilateral V1 as the untrained within-subject control. Following training on the TDT, subjects underwent the PET scan with simultaneous polysomnography. V1 was drawn on magnetic resonance volumes and transferred to corresponding PET volumes to compute average rates of protein synthesis in left and right V1. If we disregard visual field trained (left, right), the Condition (wakefulness, sleep) x Local State (untrained, trained) interaction was not statistically significant (F[1,32]=0.07,p=0.79,f^2=0.002). Consideration of visual field trained indicates that the Condition x Visual Field Trained x Local State interaction was statistically significant (F[1,30]=9.08,p=0.005,f^2=0.30). Side-to-side differences were statistically significant only in the sleep condition: protein synthesis in the trained hemisphere was significantly higher if training was in left visual field and significantly lower if training was in right visual field. Another way to view these results is protein synthesis was higher in the right V1 during sleep-dependent memory consolidation regardless of the location of training. If we disregard visual field trained, our study could indicate sleep-dependent memory consolidation depends on processes other than protein synthesis such as synaptic renormalization. Consideration of visual field trained, however, may indicate that each hemisphere has inherent properties requiring either reduced or elevated protein synthesis for memory consolidation to occur during sleep. Intramural Research Program of the National Institute of Mental Health, Intramural Research Program of the National Institute of Neurological Disorders and Stroke, and Military Operational Medicine Research Program of the United States Army Medical Research and Materiel Command.

LSD alters eyes-closed functional connectivity within the early visual cortex in a retinotopic fashion

The question of how spatially organized activity in the visual cortex behaves during eyes-closed, lysergic acid diethylamide (LSD)-induced (e.g., visions of geometric patterns and more complex phenomena) has never been empirically addressed, although it has been proposed that under psychedelics, with eyes-closed, the brain may function there is visual input when there is none. In this work, resting-state functional connectivity (RSFC) data was analyzed from 10 healthy subjects under the influence of LSD and, separately, placebo. It was suspected that eyes-closed psychedelic imagery might involve transient local retinotopic activation, of the sort typically associated with visual stimulation. To test this, it was hypothesized that, under LSD, patches of the visual cortex with congruent retinotopic representations would show greater RSFC than incongruent patches. Using a retinotopic localizer performed during a nondrug baseline condition, nonadjacent patches of V1 and V3 that represent the vertical or the horizontal meridians of the visual field were identified. Subsequently, RSFC between V1 and V3 was measured with respect to these a priori identified patches. Consistent with our prior hypothesis, the difference between RSFC of patches with congruent retinotopic specificity (horizontal-horizontal and vertical-vertical) and those with incongruent specificity (horizontal-vertical and vertical-horizontal) increased significantly under LSD relative to placebo, suggesting that activity within the visual cortex becomes more dependent on its intrinsic retinotopic organization in the drug condition. This result may indicate that under LSD, with eyes-closed, the early visual system behaves as if it were seeing spatially localized visual inputs. Hum Brain Mapp 37:3031-3040, 2016. © 2016 Wiley Periodicals, Inc.

Distinct effects of trial-driven and task Set-related control in primary visual cortex

Task sets are task-specific configurations of cognitive processes that facilitate task-appropriate reactions to stimuli. While it is established that the trial-by-trial deployment of visual attention to expected stimuli influences neural responses in primary visual cortex (V1) in a retinotopically specific manner, it is not clear whether the mechanisms that help maintain a task set over many trials also operate with similar retinotopic specificity. Here, we address this question by using BOLD fMRI to characterize how portions of V1 that are specialized for different eccentricities respond during distinct components of an attention-demanding discrimination task: cue-driven preparation for a trial, trial-driven processing, task-initiation at the beginning of a block of trials, and task-maintenance throughout a block of trials. Tasks required either unimodal attention to an auditory or a visual stimulus or selective intermodal attention to the visual or auditory component of simultaneously presented visual and auditory stimuli. We found that while the retinotopic patterns of trial-driven and cue-driven activity depended on the attended stimulus, the retinotopic patterns of task-initiation and task-maintenance activity did not. Further, only the retinotopic patterns of trial-driven activity were found to depend on the presence of inter-modal distraction. Participants who performed well on the intermodal selective attention tasks showed strong task-specific modulations of both trial-driven and task-maintenance activity. Importantly, task-related modulations of trial-driven and task-maintenance activity were in opposite directions. Together, these results confirm that there are (at least) two different processes for top-down control of V1: One, working trial-by-trial, differently modulates activity across different eccentricity sectors – portions of V1 corresponding to different visual eccentricities. The second process works across longer epochs of task performance, and does not differ among eccentricity sectors. These results are discussed in the context of previous literature examining top-down control of visual cortical areas.

Sleep Enhancement of Texture Discrimination Performance is Dependent on Training Paradigm

Previous studies indicate that performance on visual discrimination tasks is enhanced after a delay involving sleep, a result that has been interpreted as reflecting sleep consolidation (Karni, et al., 1994). In the motor domain, however, sleep gains are eliminated when fatigue-reducing spaced practice is used (Rickard, et al., 2008). We applied analogous methodology to test sleep enhancement in a classic texture discrimination task in which subjects indicate whether three slanted lines embedded in an array of lines have a vertical or horizontal orientation. Improvement in this task is measured by a decreased stimulus-to-mask onset asynchrony (SOA), here estimated using a staircase algorithm. Subjects trained on a TDT in the evening and were retested after a 24-hour delay involving normal sleep. Fifteen subjects trained using a standard massed training paradigm involving 192 trials with no breaks. Another 15 trained using an optimized training paradigm. In that condition there were 20s break after every 12 trials to reduce fatigue, and the 192 training trials were randomly interspersed with dummy trials which have been shown to mitigate adaptation effects thought to impair learning (Harris, Gliksberg & Sagi, 2012). Performance in the standard training paradigm was indeed improved on retest. The average SOA (109.6 ms) from the first 8 blocks of Session 2 was significantly decreased compared to the average SOA (137.7 ms) from the last 8 blocks of Session 1, t(1) = 3.45, p = .0014. However, in the optimized training group we found no evidence of enhancement after sleep, t(14) = 0.18, p= .52. The group by session interaction was highly significant, F(1, 28) = 8.4, p = .007. Retinotopic specificity was observed for both groups, indicating that training was sufficient to induce neural changes. These findings invite a theoretical reinterpretation of prior results demonstrating enhanced visual discrimination performance following sleep. Meeting abstract presented at VSS 2014

Prolonged training at threshold promotes robust retinotopic specificity in perceptual learning.

Human perceptual learning is classically thought to be highly specific to trained stimuli's retinal location. Together with evidence that specific learning effects can result in corresponding changes in early visual cortex, researchers have theorized that specificity implies regionalization of learning in the brain. However, other research suggests that specificity can arise from learning readout in decision areas or through top-down processes. Notably, recent research using a novel double-training paradigm reveals dramatic generalization of perceptual learning to untrained locations when multiple stimuli are trained. These data provoked significant controversy in the field and challenged extant models of perceptual learning. To resolve this controversy, we investigated mechanisms that account for retinotopic specificity in perceptual learning. We replicated findings of transfer after double training; however, we show that prolonged training at threshold, which leads to a greater number of difficult trials during training, preserves location specificity when double training occurred at the same location or sequentially at different locations. Likewise, we find that prolonged training at threshold determines the degree of transfer in single training of a peripheral orientation discrimination task. Together, these data show that retinotopic specificity depends highly upon particularities of the training procedure. We suggest that perceptual learning can arise from decision rules, attention learning, or representational changes, and small differences in the training approach can emphasize some of these over the others.

Spatial specificity of working memory representations in the early visual cortex

Recent fMRI decoding studies have demonstrated that early retinotopic visual areas exhibit similar patterns of activity during the perception of a stimulus and during the maintenance of that stimulus in working memory. These findings provide support for the sensory recruitment hypothesis that the mechanisms underlying perception serve as a foundation for visual working memory. However, a recent study by Ester, Serences, and Awh (2009) found that the orientation of a peripheral grating maintained in working memory could be classified from both the contralateral and ipsilateral regions of the primary visual cortex (V1), implying that, unlike perception, feature-specific information was maintained in a nonretinotopic manner. Here, we evaluated the hypothesis that early visual areas can maintain information in a spatially specific manner and will do so if the task encourages the binding of feature information to a specific location. To encourage reliance on spatially specific memory, our experiment required observers to retain the orientations of two laterally presented gratings. Multivariate pattern analysis revealed that the orientation of each remembered grating was classified more accurately based on activity patterns in the contralateral than in the ipsilateral regions of V1 and V2. In contrast, higher extrastriate areas exhibited similar levels of performance across the two hemispheres. A time-resolved analysis further indicated that the retinotopic specificity of the working memory representation in V1 and V2 was maintained throughout the retention interval. Our results suggest that early visual areas provide a cortical basis for actively maintaining information about the features and locations of stimuli in visual working memory.

Non-Retinotopic Motor-Visual Recalibration to Temporal Lag

Temporal order judgment (TOJ) between the voluntary motor action and its perceptual feedback is important in distinguishing between a sensory feedback which is caused by observer’s own action and other stimulus, which are irrelevant to that action. Prolonged exposure to fixed temporal lag between motor action and visual feedback recalibrates motor-visual temporal relationship, and consequently shifts the point of subjective simultaneity (PSS). Previous studies on the audio-visual temporal recalibration without voluntary action revealed that the low level processing is involved. However, it is not clear how the low and high level processings affect the recalibration to constant temporal lag between voluntary action and visual feedback. This study examined retinotopic specificity of the motor-visual temporal recalibration. During the adaptation phase, observers repeatedly pressed a key, and visual stimulus was presented in left or right visual field with a fixed temporal lag (0 or 200 ms). In the test phase, observers performed a TOJ for observer’s voluntary keypress and test stimulus, which was presented in the same as or opposite to the visual field in which the stimulus was presented in the adaptation phase. We found that the PSS was shifted toward the exposed lag in both visual fields. These results suggest that the low visual processing, which is retinotopically specific, has minor contribution to the motor-visual temporal lag adaptation, and that the adaptation to shift the PSS mainly depends upon the high level processing such as attention to specific properties of the stimulus.

Rapid Plasticity of Visual Responses in the Adult Lateral Geniculate Nucleus

Compared to the developing visual system, where neuronal plasticity has been well characterized at multiple levels, little is known about plasticity in the adult, particularly within subcortical structures. We made intraocular injections of 2-amino-4-phosphonobutyric acid (APB) in adult cats to block visual responses in On-center retinal ganglion cells and examined the consequences on visual responses in the lateral geniculate nucleus (LGN) of the thalamus. In contrast to current views of retinogeniculate organization, which hold that On-center LGN neurons should become silent with APB, we find that ∼50% of On-center neurons rapidly develop Off-center responses. The time course of these emergent responses and the actions of APB in the retina indicate the plasticity occurs within the LGN. These results suggest there is greater divergence of retinogeniculate connections than previously recognized and that functionally silent, nonspecific retinal inputs can serve as a substrate for rapid plasticity in the adult.

REM sleep prevents interference in the texture discrimination task

Improvement in the texture discrimination task (TDT) depends on sleep, shows feature and retinotopic specificity, varies with stimulus exposure, and is vulnerable to interference. Using a classic interference paradigm, Yotsumoto et al (2009) showed that two-part training on competing background orientations blocked learning of the initial stimulus set. We examined the role of sleep in perceptual interference on a short version of the TDT (to avoid deterioration effects). Interference was tested using two-part training with vertical or horizontal background orientations, which were presented in either the same or different retinotopic locations (lower left and upper right). We examined three nap conditions: naps (with and without rapid eye movement (REM) sleep) compared with quiet rest. We found that quiet rest showed no improvement in the interference condition, whereas naps with REM sleep eliminated interference effects. In fact, REM naps showed more than double the magnitude of learning in the interference condition compared with the non-interference condition. Learning in the interference condition was significantly correlated with the amount of REM sleep in the nap, no other sleep stage was related to performance changes. Interestingly, quiet rest produced perceptual learning to the same degree as sleep in the non-interference conditions. In conclusion, when the brain is serially presented with competing information, such as discriminating a target embedded within two different background orientations, REM sleep appears to enhance memory for information presented first. Thus, the memory trace becomes resilient to interference from subsequent competing targets. Furthermore, although prior studies have compared sleep to active waking controls, we find that quiet rest maybe as effective for learning as REM sleep.

Simultaneous constraints on pre- and post-synaptic cells couple cortical feature maps in a 2D geometric model of orientation preference

The most prominent feature of mammalian striate cortex (V1) is the spatial organization of response preferences for the position and orientation of elementary visual stimuli. Models for the formation of cortical maps of orientation and 'retinotopic' position typically rely on a combination of Hebbian or correlation-based synaptic plasticity, and constraints on the distribution of synaptic weights. We consider a simplified model of orientation and retinotopic specificity based on the geometry of the feed-forward synaptic weight distribution from an 'unoriented' layer of cells to a first weakly oriented layer. We model the feed-forward weight distribution as a system of planar Gaussian receptive fields each elongated in the direction matching the preferred orientation of the postsynaptic cell. Under the constraint of presynaptic weight normalization (each cell in the oriented layer receives the same net synaptic weight) and a uniform retinotopic map (displacement of centres of mass of receptive fields in the unoriented layer is strictly proportional to the displacement of the corresponding cells in the oriented layer), we find that imposing a pattern of orientation preference forces the system to violate postsynaptic weight normalization (each cell in the unoriented layer no longer sends forth the same net synaptic weight). We study this deviation from uniformity of the postsynaptic weight, and find that the deviation has a distinct form in the vicinity of the 'pinwheel' singularities of the orientation map. We show that uniform synaptic coverage of the unoriented layer can be restored by introducing a distortion in the retinotopic locations of the receptive fields. We calculate, to first order in the relative elongation of the receptive fields, the retinotopic distortion vector field. Both the pattern of postsynaptic weight non-uniformity and the corrective retinotopic distortion vector field fail to possess the reflection symmetry commonly assumed to relate orientation singularities with topological index +/- pi. Hence, we show that 'right-handed' and 'left-handed' orientation singularities are funda-mentally distinct anatomical structures when full 2D synaptic architecture is taken into account. Finally, we predict specific patterns of retinotopic distortion that should obtain in the vicinity of +/- pi-fold orientation singularities, if uniform pre- and post-synaptic weight constraints are strongly enforced.

Take afternoon naps to improve perceptual learning

All subjects were trained in a standard visual texture discrimination task at 09:00 and tested at 19:00 the same day and again at 9:00 the next morning. At about 14:00 on the day of the learning the task subjects either had no nap ðn 1⁄4 28Þ or naps of about 60 ðn 1⁄4 26Þ or 90 min ðn 1⁄4 19Þ durations depending on the group to which they were assigned. The process for assigning subjects to groups was not specified. Data were analyzed for each nap condition and then also for nap groups divided not by length of nap but by whether or not REM sleep occurred. The amount of nocturnal sleep for the night before the test and the next night after the nap was also recorded for each subject, but the methods for this (sleep log, questionnaire the next day, activity or home recordings, polysomnogram) were not specified. The learning task involved detection of whether diagonal bars in the lower left visual quadrant of a stimulus were in a horizontal row or vertical column in a stimulus presented and then followed by a masking stimulus of both horizontal and diagonal bars. The dependent measure of minimum inter-stimulus interval for 80% accuracy generally shows initial improvement over a few minutes of training, deterioration during the wake time and improvement after night’s sleep with performance actually better the next morning than immediately after learning the task. Learning was evaluated by presenting the test stimuli in the same visual quadrant as that during the training, but as a control, the test stimuli were also presented in visual quadrants other than that for the original learning to check the retinotopic specificity of the effects on learning.

Physiological correlates of perceptual learning in monkey V1 and V2.

Performance in visual discrimination tasks improves with practice. Although the psychophysical parameters of these improvements have suggested the involvement of early areas in visual cortex, there has been little direct study of the physiological correlates of such perceptual learning at the level of individual neurons. To examine how neuronal response properties in the early visual system may change with practice, we trained monkeys for more than 6 mo in an orientation discrimination task in which behaviorally relevant stimuli were restricted to a particular retinal location and oriented around a specific orientation. During training the monkeys' discrimination thresholds gradually improved to much better than those of naive monkeys or humans. Although this improvement was specific to the trained orientation, it showed little retinotopic specificity. The receptive field properties of single neurons from regions representing the trained location and a location in the opposite visual hemifield were measured in V1 and V2. In most respects the receptive field properties in the representations of the trained and untrained regions were indistinguishable. However, in the regions of V1 and V2 representing the trained location, there were slightly fewer neurons whose optimal orientation was near the trained orientation. This resulted in a small but significant decrease in the V1 population response to the trained orientation at the trained location. Consequently, the observed neuronal populations did not exhibit any orientation-specific biases sufficient to explain the orientation specificity of the behavioral improvement. Pooling models suggest that the behavioral improvement was accomplished with a task-dependent and orientation-selective pooling of unaltered signals from early visual neurons. These data suggest that, even for training with stimuli suited to the selectivities found in early areas of visual cortex, behavioral improvements can occur in the absence of pronounced changes in the physiology of those areas.

Early Determination of Nasal-Temporal Retinotopic Specificity in the Eye Anlage of the Chick Embryo

The retinotectal projection of the chick is established between Embryonic Days 3 and 13 (E3 to E13). Fate mappings of the eye anlage by local injections of the fluorescent dyes DiI and DiA revealed that the anteroposterior axis of the optic vesicle corresponds to the nasotemporal axis of the retina. To investigate possible alterations in retinotopic specificity after ablating parts of the early eye anlage, we resected either most of the presumptive temporal or a large part of the presumptive nasal half of the eye anlage around stage 11 of the Hamburger-Hamilton scale (40-45 hr). After such treatment, the axes are restored in the healed optic vesicle. In the healing process the wound is closed by cells moving in from surrounding areas. After early posterior (i.e., temporal) ablation, the projection from the restored temporal half-retina onto the optic tectum was examined in embryos (E13 to E17) and juvenile chicken (P16) by retrograde and anterograde labeling of ganglion cells and their axons with DiI and DiASP. Normally, only a small fraction of ganglion cells from the temporal retina (between 6.4% on E13 and 0.08% on P16) projects onto the caudal part of the tectum. In experimental embryos and juvenile chicken this fraction is significantly increased (up to 80%). Retrograde double-labeling from the rostral and the caudal tectum reveals that temporal cells project onto either the rostral or the caudal tectum, but not via collaterals upon both areas. The ganglion cells with "displaced nasal" identity within the temporal retina that were backlabeled from the caudal tectum were to a large extent segregated into distinct clusters, indicating their derivation from few or possibly even single progenitor cells. Likewise, ablation of the anterior half of the optic vesicle led to clusters of rostrally projecting cells of "displaced temporal" identity within the restored nasal retina. In these experiments the dorsal-ventral retinotectal relationship remained intact. The simplest, though not exclusive interpretation of the findings is that positional information is imposed as a temporal-to-nasal step function already on the multipotent progenitors of the ganglion cells in the eye anlage by stage 11 or earlier. The positional values remain stable when during the healing of the optic vesicle cells of one specification become surrounded by cells of another specification.