Activity-dependent receptive field changes in the surround of adult cat visual cortex lesions

Placebo effect after visual restitution training: no eye-tracking controlled perimetric improvement after visual border stimulation in late subacute and chronic visual field defects after stroke

Introduction

A significant number of Restitution Training (RT) paradigms claim to ameliorate visual field loss after stroke by re-activating neuronal connections in the residual visual cortex due to repeated bright light-stimulation at the border of the blind and intact fields. However, the effectiveness of RT has been considered controversial both in science and clinical practice for years. The main points of the controversy are (1) the reliability of perimetric results which may be affected by compensatory eye movements and (2) heterogeneous samples consisting of patients with visual field defects and/or visuospatial neglect.

Methods

By means of our newly developed and validated Virtual Reality goggles Salzburg Visual Field Trainer (SVFT) 16 stroke patients performed RT on a regular basis for 5  months. By means of our newly developed and validated Eye Tracking Based Visual Field Analysis (EFA), we conducted a first-time full eye-movement-controlled perimetric pre-post intervention study. Additionally, patients subjectively rated the size of their intact visual field.

Results

Analysis showed that patients' mean self-assessment of their subjective visual field size indicated statistically significant improvement while, in contrast, objective eye tracking controlled perimetric results revealed no statistically significant effect.

Discussion

Bright-light detection RT at the blind-field border solely induced a placebo effect and did not lead to training-induced neuroplasticity in the visual cortex of the type needed to ameliorate the visual field size of stroke patients.

Spared perilesional V1 activity underlies training-induced recovery of luminance detection sensitivity in cortically-blind patients

Damage to the primary visual cortex (V1) causes homonymous visual-field loss long considered intractable. Multiple studies now show that perceptual training can restore visual functions in chronic cortically-induced blindness (CB). A popular hypothesis is that training can harness residual visual functions by recruiting intact extrageniculostriate pathways. Training may also induce plastic changes within spared regions of the damaged V1. Here, we link changes in luminance detection sensitivity with retinotopic fMRI activity before and after visual discrimination training in eleven patients with chronic, stroke-induced CB. We show that spared V1 activity representing perimetrically-blind locations prior to training predicts the amount of training-induced recovery of luminance detection sensitivity. Additionally, training results in an enlargement of population receptive fields in perilesional V1, which increases blind-field coverage and may support further recovery with subsequent training. These findings uncover fundamental changes in perilesional V1 cortex underlying training-induced restoration of conscious luminance detection sensitivity in CB.

In humans, stroke damage to V1 causes large visual field defects. Spared V1 activity prior to training predicts the amount of training-induced recovery in luminance detection sensitivity. Moreover, visual training changes population receptive field properties within residual V1 circuits.

Salzburg Visual Field Trainer (SVFT): A virtual reality device for (the evaluation of) neuropsychological rehabilitation

OBJECTIVE

"Visual Restitution Therapies" (VRT) claim to ameliorate visual field defects of neurological patients by repeated visual light stimulation, leading to training-related neuroplasticity and resulting in reconnection of lesioned neurons in early cortical areas. Because existing systems are stationary, uncomfortable, and unreliable, we developed a training instrument based on virtual reality goggles. The goal of the "Salzburg Visual Field Trainer" (SVFT) is twofold: (1) The device facilitates the clinical evaluation of established neuropsychological rehabilitation approaches, such as VRT. (2) The device enables patients to independently perform VRT based (or other) neuropsychological training methodologies flexibly and comfortably.

METHODS AND ANALYSIS

The SVFT was developed on the principles of VRT. Individual configuration of the SVFT is based on perimetric data of the respective patient's visual field. To validate the utmost important aspect of neuropsychological rehabilitation methodologies-that is displaying stimuli precisely in desired locations in the user's visual field-two steps were conducted in this proof-of-concept study: First, we assessed the individual "blind spots" location and extent of 40 healthy, normal sighted participants. This was done with the help of our recently developed perimetric methodology "Eye Tracking Based Visual Field Analysis" (EFA). Second, depending on the individual characteristics of every participant's blind spots, we displayed-by means of the SVFT-15 stimuli in the respective locations of every participants' blind spots and 85 stimuli in the surrounding, intact visual area. The ratio between visible and non-visible stimuli, which is reflected in the behavioral responses (clicks on a remote control) of the 40 participants, provides insight into the accuracy of the SVFT to display training stimuli in areas desired by the investigator. As the blind spot is a naturally occurring, absolute scotoma, we utilized this blind area as an objective criterion and a "simulated" visual field defect to evaluate the theoretical applicability of the SVFT.

RESULTS

Outcomes indicate that the SVFT is highly accurate in displaying training stimuli in the desired areas of the user's visual field with an accuracy of 99.0%. Data analysis further showed a sensitivity of .98, specificity of .99, a positive predictive value of .96, a negative predictive value of .996, a hit rate of .99, a random hit rate of .74 and a RATZ-Index of .98. This translates to 14.7% correct non-reactions, 0.7% false non-reactions, 0.3% false reactions and 84.3% correct reactions to displayed test stimuli during the evaluation study. Reports from participants further indicate that the SVFT is comfortable to wear and intuitive to use.

CONCLUSIONS

The SVFT can help to investigate the true effects of VRT based methodologies (or other neuropsychological approaches) and the underlying mechanisms of training-related neuroplasticity in the visual cortex in neurological patients suffering from visual field defects.

Altered Sensitivity to Motion of Area MT Neurons Following Long-Term V1 Lesions

Primates with primary visual cortex (V1) damage often retain residual motion sensitivity, which is hypothesized to be mediated by middle temporal area (MT). MT neurons continue to respond to stimuli shortly after V1 lesions; however, experimental and clinical studies of lesion-induced plasticity have shown that lesion effects can take several months to stabilize. It is unknown what physiological changes occur in MT and whether neural responses persist long after V1 damage. We recorded neuronal responses in MT to moving dot patterns in adult marmoset monkeys 6-12 months after unilateral V1 lesions. In contrast to results obtained shortly after V1 lesions, we found that fewer MT neurons were direction selective, including neurons expected to still receive projections from remaining parts of V1. The firing rates of most cells increased with increases in motion strength, regardless of stimulus direction. Furthermore, firing rates were higher and more variable than in control MT cells. To test whether these observations could be mechanistically explained by underlying changes in neural circuitry, we created a network model of MT. We found that a local imbalance of inhibition and excitation explained the observed firing rate changes. These results provide the first insights into functional implications of long-term plasticity in MT following V1 lesions.

Eye-tracking-based visual field analysis (EFA): a reliable and precise perimetric methodology for the assessment of visual field defects

OBJECTIVE

Several studies report evidence for training-related neuroplasticity in the visual cortex, while other studies suggest that improvements simply reflect inadequate eye fixation control during perimetric prediagnostics and postdiagnostics.

METHODS AND ANALYSIS

To improve diagnostics, a new eye-tracking-based methodology for visual field analysis (eye-tracking-based visual field analysis (EFA)) was developed. The EFA is based on static automated perimetry and additionally takes individual eye movements in real time into account and compensates for them. In the present study, an evaluation of the EFA with the help of blind spots of 58 healthy participants and the individual visual field defects of 23 clinical patients is provided. With the help of the EFA, optical coherence tomography, Goldmann perimetry and a Humphrey field analyser, these natural and acquired scotomas were diagnosed and the results were compared accordingly.

RESULTS

The EFA provides a SE of measurement of 0.38° for the right eye (OD) and 0.50° for the left eye (OS), leading to 0.44° of visual angle for both eyes (OU). Based on participants' individual results, the EFA provides disattenuated correlation (validity) of 1.00 for both OD and OS. Results from patients suffering from cortical lesions and glaucoma further indicate that the EFA is capable of diagnosing acquired scotoma validly and is applicable for clinical use.

CONCLUSION

Outcomes indicate that the EFA is highly reliable and precise in diagnosing individual shape and location of scotoma and capable of recording changes of visual field defects (after intervention) with unprecedented precision. Test duration is comparable to established instruments and due to the high customisability of the EFA, assessment duration can be shortened by adapting the diagnostic procedure to the patients' individual visual field characteristics. Therefore, the saccade-compensating methodology enables researchers and healthcare professionals to rule out eye movements as a source of inaccuracies in pre-, post-, and follow-up assessments.

Robust Visual Responses and Normal Retinotopy in Primate Lateral Geniculate Nucleus following Long-term Lesions of Striate Cortex

Lesions of striate cortex (V1) trigger massive retrograde degeneration of neurons in the LGN. In primates, these lesions also lead to scotomas, within which conscious vision is abolished. Mediation of residual visual capacity within these regions (blindsight) has been traditionally attributed to an indirect visual pathway to the extrastriate cortex, which involves the superior colliculus and pulvinar complex. However, recent studies have suggested that preservation of the LGN is critical for behavioral evidence of blindsight, raising the question of what type of visual information is channeled by remaining neurons in this structure. A possible contribution of LGN neurons to blindsight is predicated on two conditions: that the neurons that survive degeneration remain visually responsive, and that their receptive fields continue to represent the region of the visual field inside the scotoma. We tested these conditions in male and female marmoset monkeys (Callithrix jacchus) with partial V1 lesions at three developmental stages (early postnatal life, young adulthood, old age), followed by long recovery periods. In all cases, recordings from the degenerated LGN revealed neurons with well-formed receptive fields throughout the scotoma. The responses were consistent and robust, and followed the expected eye dominance and retinotopy observed in the normal LGN. The responses had short latencies and preceded those of neurons recorded in the extrastriate middle temporal area. These findings suggest that the pathway that links LGN neurons to the extrastriate cortex is physiologically viable and can support residual vision in animals with V1 lesions incurred at various ages.SIGNIFICANCE STATEMENT Patients with a lesion of the primary visual cortex (V1) can retain certain visually mediated behaviors, particularly if the lesion occurs early in life. This phenomenon ("blindsight") not only sheds light on the nature of consciousness, but also has implications for studies of brain circuitry, development, and plasticity. However, the pathways that mediate blindsight have been the subject of debate. Recent studies suggest that projections from the LGN might be critical, but this finding is puzzling given that the lesions causes severe cell death in the LGN. Here we demonstrate in monkeys that the surviving LGN neurons retain a remarkable level of visual function and could therefore be the source of the visual information that supports blindsight.

Population receptive field analysis of the primary visual cortex complements perimetry in patients with homonymous visual field defects

Injury to the primary visual cortex (V1) typically leads to loss of conscious vision in the corresponding, homonymous region of the contralateral visual hemifield (scotoma). Several studies suggest that V1 is highly plastic after injury to the visual pathways, whereas others have called this conclusion into question. We used functional magnetic resonance imaging (fMRI) to measure area V1 population receptive field (pRF) properties in five patients with partial or complete quadrantic visual field loss as a result of partial V1+ or optic radiation lesions. Comparisons were made with healthy controls deprived of visual stimulation in one quadrant ["artificial scotoma" (AS)]. We observed no large-scale changes in spared-V1 topography as the V1/V2 border remained stable, and pRF eccentricity versus cortical-distance plots were similar to those of controls. Interestingly, three observations suggest limited reorganization: (i) the distribution of pRF centers in spared-V1 was shifted slightly toward the scotoma border in 2 of 5 patients compared with AS controls; (ii) pRF size in spared-V1 was slightly increased in patients near the scotoma border; and (iii) pRF size in the contralesional hemisphere was slightly increased compared with AS controls. Importantly, pRF measurements yield information about the functional properties of spared-V1 cortex not provided by standard perimetry mapping. In three patients, spared-V1 pRF maps overlapped significantly with dense regions of the perimetric scotoma, suggesting that pRF analysis may help identify visual field locations amenable to rehabilitation. Conversely, in the remaining two patients, spared-V1 pRF maps failed to cover sighted locations in the perimetric map, indicating the existence of V1-bypassing pathways able to mediate useful vision.

A randomized controlled trial comparing 2 interventions for visual field loss with standard occupational therapy during inpatient stroke rehabilitation

BACKGROUND AND PURPOSE

Compensatory and restorative treatments have been developed to improve visual field defects after stroke. However, no controlled trials have compared these interventions with standard occupational therapy (OT).

METHODS

A total of 45 stroke participants with visual field defect admitted for inpatient rehabilitation were randomized to restorative computerized training (RT) using computer-based stimulation of border areas of their visual field defects or to a computer-based compensatory therapy (CT) teaching a visual search strategy. OT, in which different compensation strategies were used to train for activities of daily living, served as standard treatment for the active control group. Each treatment group received 15 single sessions of 30 minutes distributed over 3 weeks. The primary outcome measures were visual field expansion for RT, visual search performance for CT, and reading performance for both treatments. Visual conjunction search, alertness, and the Barthel Index were secondary outcomes.

RESULTS

Compared with OT, CT resulted in a better visual search performance, and RT did not result in a larger expansion of the visual field. Intragroup pre-post comparisons demonstrated that CT improved all defined outcome parameters and RT several, whereas OT only improved one.

CONCLUSIONS

CT improved functional deficits after visual field loss compared with standard OT and may be the intervention of choice during inpatient rehabilitation. A larger trial that includes lesion location in the analysis is recommended.

A new taxonomy for perceptual filling-in

Perceptual filling-in occurs when structures of the visual system interpolate information across regions of visual space where that information is physically absent. It is a ubiquitous and heterogeneous phenomenon, which takes place in different forms almost every time we view the world around us, such as when objects are occluded by other objects or when they fall behind the blind spot. Yet, to date, there is no clear framework for relating these various forms of perceptual filling-in. Similarly, whether these and other forms of filling-in share common mechanisms is not yet known. Here we present a new taxonomy to categorize the different forms of perceptual filling-in. We then examine experimental evidence for the processes involved in each type of perceptual filling-in. Finally, we use established theories of general surface perception to show how contextualizing filling-in using this framework broadens our understanding of the possible shared mechanisms underlying perceptual filling-in. In particular, we consider the importance of the presence of boundaries in determining the phenomenal experience of perceptual filling-in.

Visual field expansion after visual restoration therapy

Objective: To determine whether visual field expansion occurs with visual restoration therapy (VRT), using fundus-controlled microperimetry to assess visual fields.

Design: This longitudinal cohort analysis assesses patients’ visual fields before and after visual restoration therapy using microperimetry and standard high-resolution perimetry.

Subjects: Seven patients with stroke-induced homonymous field cuts were studied.

Intervention: Visual restoration therapy is a computerized, home-based treatment aimed at reducing the size of the visual field defect of stroke patients with hemianopia through repetitive stimulation of the visual borderzone adjacent to the blind field. During twice-daily therapy for three months patients maintain central fixation while responding to eccentrically placed stimuli in the visual borderzone. The programme is adjusted monthly to changes in the patient’s visual field. Controversy exists as to whether expansion of visual fields measured at home with high-resolution perimetry is due to inadvertent eye movements and therefore would overrepresent the treatment’s effect.

Main measures: Microperimentry uses an infrared camera to track retinal vessels so that any shift or movement between the reference image and the real-time fundus image corrects the stimulus position, thus delivering stimuli to known retinal locations, and allowing accurate assessment of visual fields independent of eye movements.

Results: There was an average improvement in stimulus detection rate by microperimetry of 12.5% (range —1.4% to 38.9%, P =0.033). Six of 7 patients had ≥ 3% improvement in stimulus detection by home-based perimetry.

Conclusion: Our results demonstrate modest but real expansion in visual fields following visual restoration therapy which is not due to eye movements.

Perceptual distortion in homonymous paracentral scotomas

BACKGROUND

Cortical remapping after peripheral or central visual deafferentation alters visual perception, but it is unclear whether such a phenomenon impinges on areas remote from a scotoma. To investigate this question, we studied variations of perceptual spatial distortion in the visual field of patients with homonymous paracentral scotoma.

METHODS

Two patients with right inferior homonymous paracentral scotoma were asked to describe their perception of a series of figures showing two isometric vertical lines symmetrically located on either side of a fixation point. In each figure, the fixation point varied by steps of 2 degrees along a hypothetical vertical line equidistant between the test lines. The lines subtended 20 degrees of visual angle, and the right line passed through the scotoma in both cases. Time for spatial distortion to manifest was recorded.

RESULTS

Both subjects reported that the right line was perceived as shorter than the left one. The line shortening varied in magnitude with the distance of the fixation point from the end of the line and was more pronounced when the distance increased. Moreover, perceptual line shortening appeared 5-10 seconds after steady fixation, but values of shortening varied during the following 10 seconds. In addition, the right line appeared uninterrupted or slightly blurred in the scotoma region.

CONCLUSIONS

These observations reflect long-range cortical reorganization after brain damage. Larger receptive fields in the periphery of the visual map could explain why perceptual shortening is more pronounced with increased eccentricity.

Remapping the somatosensory cortex after stroke: insight from imaging the synapse to network

Together, thousands of neurons with similar function make up topographically oriented sensory cortex maps that represent contralateral body parts. Although this is an accepted model for the adult cortex, whether these same rules hold after stroke-induced damage is unclear. After stroke, sensory representations damaged by stroke remap onto nearby surviving neurons. Here, we review the process of sensory remapping after stroke at multiple levels ranging from the initial damage to synapses, to their rewiring and function in intact sensory circuits. We introduce a new approach using in vivo 2-photon calcium imaging to determine how the response properties of individual somatosensory cortex neurons are altered during remapping. One month after forelimb-area stroke, normally highly limb-selective neurons in surviving peri-infarct areas exhibit remarkable flexibility and begin to process sensory stimuli from multiple limbs as remapping proceeds. Two months after stroke, neurons within remapped regions develop a stronger response preference. Thus, remapping is initiated by surviving neurons adopting new roles in addition to their usual function. Later in recovery, these remapped forelimb-responsive neurons become more selective, but their new topographical representation may encroach on map territories of neurons that process sensory stimuli from other body parts. Neurons responding to multiple limbs may reflect a transitory phase in the progression from their involvement in one sensorimotor function to a new function that replaces processing lost due to stroke.

Recovery of visual fields in brain-lesioned patients by reaction perimetry treatment

BACKGROUND

The efficacy of treatment in hemianopic patients to restore missing vision is controversial. So far, successful techniques require laborious stimulus presentation or restrict improvements to selected visual field areas. Due to the large number of brain-damaged patients suffering from visual field defects, there is a need for an efficient automated treatment of the total visual field.

METHODS

A customized treatment was developed for the reaction perimeter, permitting a time-saving adaptive-stimulus presentation under conditions of maximum attention. Twenty hemianopic patients, without visual neglect, were treated twice weekly for an average of 8.2 months starting 24.2 months after the insult. Each treatment session averaged 45 min in duration.

RESULTS

In 17 out of 20 patients a significant and stable increase of the visual field size (average 11.3 degrees +/- 8.1) was observed as well as improvement of the detection rate in the defective visual field (average 18.6% +/- 13.5). A two-factor cluster analysis demonstrated that binocular treatment was in general more effective in augmenting the visual detection rate than monocular. Four out of five patients with a visual field increase larger than 10 degrees suffered from hemorrhage, whereas all seven patients with an increase of 5 degrees or less suffered from infarction. Most patients reported that visual field restoration correlated with improvement of visual-related activities of daily living.

CONCLUSION

Rehabilitation treatment with the Lubeck Reaction Perimeter is a new and efficient method to restore part of the visual field in hemianopia. Since successful transfer of treatment effects to the occluded eye is achieved under monocular treatment conditions, it is hypothesized that the damaged visual cortex itself is the structure in which recovery takes place.

Brain activity associated with stimulation therapy of the visual borderzone in hemianopic stroke patients

BACKGROUND AND OBJECTIVE

Visual restoration therapy is a home-based treatment program intended to expand visual fields of hemianopic patients through repetitive stimulation of the borderzone adjacent to the blind field. We hypothesized that the training itself would induce visual field location-specific changes in the brain's response to stimuli, a phenomenon demonstrated in animal experiments but never in humans with brain injury.

METHODS

Six chronic right hemianopic patients underwent functional magnetic resonance imaging (fMRI)--responding to stimuli in the trained visual borderzone versus the nontrained seeing field before and after 1 month of visual restoration therapy. Spatially normalized fMRI time-series data were analyzed in a fixed-effects group analysis comparing blood oxygen level dependent (BOLD) activity in the borderzone versus seeing location at baseline and at 1 month. Percent BOLD change was measured to determine each condition's contribution to the time-by-condition interaction.

RESULTS

There was a significant time by condition interaction manifested as increased BOLD activity for borderzone detection relative to seeing detection after the first month of therapy, which correlated with a relative improvement in response times in the borderzone location out-of-scanner. The right inferior and lateral temporal, right dorsolateral frontal, bilateral anterior cingulate, and bilateral basal ganglia showed the greatest response.

CONCLUSION

Visual restoration therapy appears to induce an alteration in brain activity associated with a shift of attention from the nontrained seeing field to the trained borderzone. The effect appears to be mediated by the anterior cingulate and dorsolateral frontal cortex in conjunction with other higher order visual areas in the occipitotemporal and middle temporal regions. Demonstration of a visual field-specific training effect on brain activity provides an important starting point for understanding the potential for visual therapy in hemianopia.

Role of substantia innominata in cerebral blood flow autoregulation

Ascending projections from the substantia innominata (SI) may have an important role in the regulation of cerebral blood flow (CBF). However, several reports have suggested that unilateral lesion of the SI does not affect CBF autoregulation. On the other hand, it is also reported that several cortical and subcortical functions may be regulated not only by ipsilateral SI, but also by contralateral SI. Thus, the objective of this study is to test the hypothesis that bilateral lesions of the SI affect CBF autoregulation. Experiments were performed on anesthetized male Sprague-Dawley rats. Ibotenic acid or physiological saline was microinjected into bilateral SI. Rats were classified into four groups as follows: bilateral SI lesion rats (ibotenic acid was injected bilaterally), left or right SI lesion rats (ibotenic acid was injected into the unilateral SI and saline into the contralateral SI), and control rats (saline was injected bilaterally). Ten days after injection, CBF in the left frontal cortex was measured by laser-Doppler flowmetry during stepwise controlled hemorrhagic hypotension. In bilateral SI lesion rats, CBF was started to decrease significantly at 80 mm Hg (p<0.01). In the other three groups, CBF was well maintained until 50 mm Hg. Changes in CBF through stepwise hypotension in bilateral SI lesion rats were significantly different from the other groups (p<0.01). These results suggest that bilateral SI regulates cortical vasodilator mechanisms during hemorrhagic hypotension. Under unilateral SI lesion, some compensatory effects from the contralateral SI may maintain CBF autoregulation.

Acquiring long-term representations of visual classes following extensive extrastriate damage

Different areas of human visual cortex are thought to play different roles in the learning of visual information: whereas in low/intermediate cortical areas, plasticity may be manifested by enhanced selectivity to learned visual features, in higher-level areas, plasticity may result in generalization and development of tolerance to degraded versions of the learned stimuli. The most effective tolerance to degraded information is presumably achieved in the case of cooperation between the different forms of plasticity. Whether this tolerance to degraded information also applies when the visual input is degraded as a result of a lesion to lower levels of the visual system remains an open question. To address this, we studied visual classification learning in a patient with an extensive bilateral lesion affecting intermediate/low-level visual areas but sparing higher-level areas. Despite difficulty in perceiving the stimuli, the patient learned to classify them, albeit not as quickly as control participants. Moreover, the patient's learning was maintained over the long term and was accompanied by improved discrimination of individual stimuli. These findings demonstrate that degraded output from lesioned, lower areas can be exploited in the service of a new visual task and the results likely implicate a combination of bottom-up and top-down processing during visual learning.

Advanced optics in a jellyfish eye

Cubozoans, or box jellyfish, differ from all other cnidarians by an active fish-like behaviour and an elaborate sensory apparatus. Each of the four sides of the animal carries a conspicuous sensory club (the rhopalium), which has evolved into a bizarre cluster of different eyes. Two of the eyes on each rhopalium have long been known to resemble eyes of higher animals, but the function and performance of these eyes have remained unknown. Here we show that box-jellyfish lenses contain a finely tuned refractive index gradient producing nearly aberration-free imaging. This demonstrates that even simple animals have been able to evolve the sophisticated visual optics previously known only from a few advanced bilaterian phyla. However, the position of the retina does not coincide with the sharp image, leading to very wide and complex receptive fields in individual photoreceptors. We argue that this may be useful in eyes serving a single visual task. The findings indicate that tailoring of complex receptive fields might have been one of the original driving forces in the evolution of animal lenses.

Perceptual learning of line orientation modifies the effects of transcranial magnetic stimulation of visual cortex

Perceptual learning may be accompanied by physiological changes in early visual cortex. We used transcranial magnetic stimulation (TMS) to test the postulate that perceptual learning of a visual task initially performed at 60-65% accuracy strengthens visual processing in early visual cortex. Single pulse TMS was delivered to human occipital cortex at time delays of 70-154 ms after the onset of an odd-element, line orientation discrimination task. When TMS was delivered at a delay of 84 ms or later the accuracy of visual discrimination was transiently degraded in ten subjects. As visual performance in control trials without TMS improved with training, the absolute magnitude of TMS suppression of performance decreased in parallel. This result occurred both when TMS was delivered to broad areas of occipital cortex and when TMS was optimally delivered to early occipital cortex. No change in TMS suppression was observed when three new subjects were given feedback during an odd-element task that did not require substantial perceptual learning. Thus, perceptual learning improved visual performance and reduced TMS suppression of early visual cortex in parallel.

Functional reorganization of visual cortex maps after ischemic lesions is accompanied by changes in expression of cytoskeletal proteins and NMDA and GABA(A) receptor subunits

Reorganization of cortical representations after focal visual cortex lesions has been documented. It has been suggested that functional reorganization may rely on cellular mechanisms involving modifications in the excitatory/inhibitory neurotransmission balance and on morphological changes of neurons peripheral to the lesion. We explored functional reorganization of cortical retinotopic maps after a focal ischemic lesion in primary visual cortex of kittens using optical imaging of intrinsic signals. After 1, 2, and 5 weeks postlesion (wPL), we addressed whether functional reorganization correlated in time with changes in the expression of MAP-2, GAP-43, GFAP, GABA(A) receptor subunit alpha1 (GABA(A)alpha1), subunit 1 of the NMDA receptor (NMDAR1), and in neurotransmitter levels at the border of the lesion. Our results show that: (1) retinotopic maps reorganize with time after an ischemic lesion; (2) MAP-2 levels increase gradually from 1wPL to 5wPL; (3) MAP-2 upregulation is associated with an increase in dendritic-like structures surrounding the lesion and a decrease in GFAP-positive cells; (4) GAP-43 levels reach the highest point at 2wPL; (5) NMDAR1 and glutamate contents increase in parallel from 1wPL to 5wPL; (6) GABA(A)alpha1 levels increase from 1wPL to 2wPL but do not change after this time point; and (7) GABA contents remain low from 1wPL to 5wPL. This is a comprehensive study showing for the first time that functional reorganization correlates in time with dendritic sprouting and with changes in the excitatory/inhibitory neurotransmission systems previously proposed to participate in cortical remodeling and suggests mechanisms by which plasticity of cortical representations may occur.

Reorganization of visual cortical maps after focal ischemic lesions

Plasticity after central lesions may result in the reorganization of cortical representations of the sensory input. Visual cortex reorganization has been extensively studied after peripheral (retinal) lesions, but focal cortical lesions have received less attention. In this study, we investigated the organization of retinotopic and orientation preference maps at different time points after a focal ischemic lesion in the primary visual cortex (V1). We induced a focal photochemical lesion in V1 of kittens and assessed, through optical imaging of intrinsic signals, the functional cortical layout immediately afterwards and at 4, 13, 33, and 40 days after lesion. We analyzed histologic sections and evaluated temporal changes of functional maps. Histological analysis showed a clear lesion at all time points, which shrank over time. Imaging results showed that the retinotopic and orientation preference maps reorganize to some extent after the lesion. Near the lesion, the cortical retinotopic representation of one degree of visual space expands over time, while at the same time the area of some orientation domains also increases. These results show that different cortical representations can reorganize after a lesion process and suggest a mechanism through which filling-in of a cortical scotoma can occur in cortically damaged patients.

Spatio-temporal plasticity of cortical receptive fields in response to repetitive visual stimulation in the adult cat

Many psychophysical experiments on perceptual learning in humans show increases of performance that are most probably based on functions of early visual cortical areas. Long-term plasticity of the primary visual cortex has so far been shown in vivo with the use of visual stimuli paired with electrical or pharmacological stimulation at the cellular level. Here, we report that plasticity in the adult visual cortex can be achieved by repetitive visual stimulation. First, spatial receptive field profiles of single units (n=38) in area 17 or 18 of the anesthetized cat were determined with optimally oriented flashing light bars. Then a conditioning protocol was applied to induce associative synaptic plasticity. The receptive field center and an unresponsive region just outside the excitatory receptive field were synchronously stimulated ('costimulation', repetition rate 1 Hz; for 10-75 min). After costimulation the receptive field and its adjacent regions were mapped again. We observed specific increases of the receptive field size, changes of the receptive field subfield structure as well as shifts in response latency. In 37% of the cells the receptive field size increased specifically towards the stimulated side but not towards the non-stimulated opposite side of the receptive field. In addition, changes in the relative strength and size of the on and off subfield regions were observed. These specific alterations were dependent on the level of neuronal activity during costimulation. During recovery, the new responses dropped down to 120% of the preconditioning value on average within 103 min; however, the decay times significantly depended on the response magnitude after costimulation. In the temporal domain, the latency of new responses appeared to be strongly influenced by the latency of the response during costimulation.Twenty-nine percent of the units displayed no receptive field enlargement, most likely because the activity during costimulation was significantly lower than in the cases with enlarged receptive fields. An unspecific receptive field enlargement towards both the stimulated and non-stimulated side was observed in 34% of the tested cells. In contrast to the cells with specifically enlarged receptive fields, the unspecific increase of receptive field size was always accompanied by a strong increase of the general activity level. We conclude that the receptive field changes presumably took place by strengthening of synaptic inputs at the recorded cells in a Hebbian way as previously shown in the visual cortex in vitro and in vivo. The observed receptive field changes may be related to preattentive perceptual learning and could represent a basis of the 'filling in' of cortical scotomas obtained with specific training procedures in human patients suffering from visual cortex lesions.