Recovery of pattern discrimination ability in rats receiving serial or one-stage visual cortex lesions

Successive bilateral frontal controlled cortical impact injuries show behavioral savings

Traumatic brain injuries (TBIs) affect millions of people each year. Research investigating repeated or serial damage in the form of lesions indicates that behavioral deficits are reduced in animals given sequential lesions separated by a sufficient period of recovery. In the lesion literature, this phenomenon is known as the serial lesion effect (SLE). Although the SLE phenomenon is established in the lesion literature, it has not been thoroughly investigated under current models of brain injury. In the current study, a controlled cortical impact of the bilateral frontal cortex was performed in either a single procedure or a serial procedure separated by two weeks. Rats were tested on the Morris water maze, bilateral tactile adhesive removal task, rotarod and Barnes maze task to determine behavioral deficits. Histology was performed to determine lesion size and astrocyte and microglial response. A serial lesion effect was demonstrated across a majority of the behavioral tasks. However, histological analyses did not suggest a clear mechanistic link to the behavioral phenomena. This is the first study to demonstrate the SLE in a model of TBI, suggesting that behavioral deficits may actually be reduced in repeated head injuries, given an adequate time window between injuries.

A discrimination task used as a novel method of testing decision-making behavior following traumatic brain injury

Traumatic brain injury (TBI) results in a multitude of deficits following injury. Some of the most pervasive in humans are the changes that affect frontally-mediated cognitive functioning, such as decision making. The assessment of decision-making behavior in rodents has been extensively tested in the field of the experimental analysis of behavior. However, due to the narrow therapeutic window following TBI, time-intensive operant paradigms are rarely incorporated into the battery of tests traditionally used, the majority of which assess motor and sensory functioning. The cognitive measures that are used are frequently limited to memory and do not account for changes in decision-making behavior. The purpose of the present study was to develop a simplified discrimination task that can assess deficits in decision-making behavior in rodents. For the task, rats were required to dig in cocoa-scented sand (versus unscented sand) for a reinforcer. Rats were given 12 sessions per day until a criterion level of 80% accuracy for 3 days straight was reached. Once the criterion was achieved, cortical contusion injuries were induced (frontal, parietal, or sham). Following a recovery period, the rats were re-tested on cocoa versus unscented sand. Upon reaching criterion, a reversal discrimination was evaluated in which the reinforcer was placed in unscented sand. Finally, a novel scent discrimination (basil versus coffee with basil reinforced), and a reversal (coffee) were evaluated. The results indicated that the Dig task is a simple experimental preparation that can be used to assess deficits in decision-making behavior following TBI.

Vision restoration after brain and retina damage: the "residual vision activation theory"

Vision loss after retinal or cerebral visual injury (CVI) was long considered to be irreversible. However, there is considerable potential for vision restoration and recovery even in adulthood. Here, we propose the "residual vision activation theory" of how visual functions can be reactivated and restored. CVI is usually not complete, but some structures are typically spared by the damage. They include (i) areas of partial damage at the visual field border, (ii) "islands" of surviving tissue inside the blind field, (iii) extrastriate pathways unaffected by the damage, and (iv) downstream, higher-level neuronal networks. However, residual structures have a triple handicap to be fully functional: (i) fewer neurons, (ii) lack of sufficient attentional resources because of the dominant intact hemisphere caused by excitation/inhibition dysbalance, and (iii) disturbance in their temporal processing. Because of this resulting activation loss, residual structures are unable to contribute much to everyday vision, and their "non-use" further impairs synaptic strength. However, residual structures can be reactivated by engaging them in repetitive stimulation by different means: (i) visual experience, (ii) visual training, or (iii) noninvasive electrical brain current stimulation. These methods lead to strengthening of synaptic transmission and synchronization of partially damaged structures (within-systems plasticity) and downstream neuronal networks (network plasticity). Just as in normal perceptual learning, synaptic plasticity can improve vision and lead to vision restoration. This can be induced at any time after the lesion, at all ages and in all types of visual field impairments after retinal or brain damage (stroke, neurotrauma, glaucoma, amblyopia, age-related macular degeneration). If and to what extent vision restoration can be achieved is a function of the amount of residual tissue and its activation state. However, sustained improvements require repetitive stimulation which, depending on the method, may take days (noninvasive brain stimulation) or months (behavioral training). By becoming again engaged in everyday vision, (re)activation of areas of residual vision outlasts the stimulation period, thus contributing to lasting vision restoration and improvements in quality of life.

Visual pathways following cerebral hemispherectomy

The anatomical consequences of unilateral cerebral hemispherectomy in some animal models are reviewed. We have shown that the retinogenigulate pathway undergoes severe degenerative changes in hemispherectomized monkeys, greater than those shown in cats and we proposed that remaining retinal terminals to the dorsal lateral geniculate nucleus have little potential for conveying visual information any further. All subdivisions of the pulvinar undergo severe degeneration following hemispherectomy showing that the ascending tectofugal pathway is also shut off. On the other hand, the retina subserving the blind field is not depleted of ganglion cells which still send normal appearing terminals to the midbrain pretectum and superior colliculus. Visual information from the blind hemifield can thus gain access to the brain and could potentially reach the contralateral cerebral cortex through the midbrain commissure and possibly through thalamic commissural cells.

Effects of monocular enucleation at birth upon learning of a vertical-horizontal discrimination in hooded rats

Previously we have demonstrated that adult albino rats with one eye removed at birth (OEB) relearn a black-white discrimination faster than those monocularly enucleated at maturity (OET) when relearning is conducted after lesioning of the visual cortex contralateral to the remaining eye (Type A experiment). This faster relearning phenomenon is considered to be one behavioral expression of the functioning of the expanded uncrossed visual pathways (expanded UXVPs) resulting from monocular enucleation at birth. However, neither OEBs nor OETs were able to master the discrimination when the experiment was conducted without previous learning following the same surgical treatment (Type B experiment). We hypothesized that this occurs because the cues to discriminate might be close to the threshold of discrimination for either the normal UXVPs or the expanded UXVPs. In order to gain insight into the hypothesis, the present study was undertaken using hooded rats as subjects which genetically possess larger and presumably more efficient functioning UXVPs. The questions addressed were as follows: 1) Whether or not the UXVPs can mediate a vertical-horizontal discrimination in OEBs and OETs. 2) If they can, is there any difference in the upper limit of discrimination capacity between the normal UXVPs and the expanded UXVPs? Three experiments were carried out. In the Type A experiment OEBs relearned discrimination of the 10-mm stripes [0.44 cycles/degree (c/d)] faster than OETs (Experiment 1), yet in the Type B experiment neither OEBs nor OETs were capable of acquiring that discrimination (Experiment 2). However, they could originally master the discrimination equally well when the width of stripes was broadened to 30 mm (0.15 c/d). And when the width of stripes was systematically reduced thereafter, the width of the smallest stripes for the expanded UXVPs to discriminate was found to be 6 mm (0.73 c/d) and that for the normal UXVPs 10 mm (0.44 c/d) [Experiment 3]. These findings were discussed in relation to the hypothesis advanced on our previous data in albino rats.

Neurotrophic and behavioral effects of occipital cortex transplants in newborn rats

Cell suspensions of embryonic occipital cortex were transplanted into newborn rats with large unilateral visual cortex lesions. When the animals were adults, they were tested on a difficult visual discrimination, and subsequently their brains were analyzed for possible neurotrophic effects of the transplants on nonvisual cortical areas which normally form connections with the occipital cortex. Behaviorally, animals with lesions and transplants learn to discriminate between columns and rows of squares at a rate which is identical to normal rats while animals with lesions and no transplants are impaired. Volume and cell-density measures show that the transplants also rescue neurons in cortical area 8 that would normally degenerate following the cortical lesion. No such neurotrophic effect of the transplants is found in cortical area 24 or area 17 contralateral to the lesion. In rats with lesions and no transplants, there is a significant correlation between the amount of area 8 remaining after the lesion and trials to criterion on the columns-rows discrimination, a relationship that does not exist in transplant animals because of their normal learning curve and the consistent sparing of area 8. Injections of HRP into the visual cortex contralateral to the lesion result in variable numbers of labeled cells within the transplant. However, there is no consistent relationship between the number of transplant cells which project to the opposite hemisphere and learning of the discrimination. It is suggested that the learning deficit following the lesion is largely attentional and that the sparing of cortical area 8 (which in rats may include the analog of the frontal eye fields present in the primate cortex) contributes to the sparing of function.

Visual discrimination in the absence of visual cortex

Normal rats and rats with devascularization lesions ranging from subtotal removals of striate cortex (Area 17) to complete removal of neocortex were trained in a horizontal/vertical stripe discrimination for a liquid reinforcer. Subgroups of animals were identified on the basis of size and location of lesion (with particular reference to striate cortex) as Subtotal, Striate, Posterior and Decorticate. Some animals in all of the lesion groups were able to acquire the discrimination, but there was a direct relationship between lesion size and number of training trials. Those animals which reached criterion on the original discrimination were trained on a second horizontal/vertical discrimination under either transfer or reversal conditions using 'rotated obliques' stimuli. Performance on this second discrimination indicated that animals from all lesion groups had been using visual stimuli based on stripe orientation in the original problem. Members of all lesion groups solved the rotated obliques problem under the transfer condition, though the speed and completeness with which they did so was again inversely related to lesion size. These data show high levels of visual competence in the absence of visual cortex even when stimuli thought to detect form discrimination are used and thus reinforce the view that superior colliculus may be a more significant visual area for the rat than was previously assumed. They also support other observations that animals do not use residual visual capacities without extensive experience and appropriate motivation.

The effect of devascularization of the visual cortex on visual function in the rabbit

The role of the visual cortex in brightness and pattern vision was re-examined in the rabbit. Animals were trained on both a brightness and a horizontal-vertical striation discrimination. Bilateral removal of the dura mater overlying the entire visual cortex produced no impairments in either brightness or pattern vision. When the visual cortex was devascularized by removal of the pia mater bilaterally, total loss of pattern vision was found with no impairment of brightness discrimination ability. The significance of the finding that the effects of slowly developing degenerative lesions are the same as those of surgical ablation was discussed.

A comparison of lateral peristriate and striate neocortical ablations in the rat

The effects of striate, lateral peristriate and sham ablations upon the post-operative reacquisition of visual discriminations were studied in two experiments. The results of Experiment 1 indicated that lesions restricted to the lateral peristriate neocortex rendered the rat incapable of resolving an oblique stripe discrimination. Seven of the 9 rats with striate ablations were unable to form the discrimination. The histological findings indicated that the deficit following lateral peristriate injury was not due to interruption of the geniculostriate system. However, the behavioral impairment in destriate rats corresponded to the degree of unintentional injury in lateral peristriate neocortex, but not medial peristriate tissue. In Experiment 2, the effects of these lesions were studied upon the post-operative reacquisition of either a non-reversal or reversal of a preoperatively acquired brightness discrimination. Animals with striate damage demonstrated considerable savings on the non-reversed brightness discrimination, and they acquired the reversal at a rate equal to that seen in animals with sham ablations. Rats with peristriate ablations showed no savings on the non-reversed brightness habit, and they required more extensive training than both the striate and sham groups on the reversal. These findings suggest that the frequently reported loss of a preoperative brightness habit following large posterior neocortex ablations is due to damage in the lateral peristriate neocortex rather than damage of the geniculostriate system. These results, taken with other literature, suggest an important role of the lateral peristriate neocortex in visuospatial sensory function.

Visual discrimination learning in rats with lesions of superior colliculus: door-push and approach errors in modified jumping stand

It has been suggested that, for some species, lesions of the superior colliculus affect visual discrimination learning, but only in certain conditions: (a) when problems are first learnt only after operation, or (b) when discriminanda require detailed scanning, or (c) when “approach” responses to the discriminanda are measured, rather than the response of actually touching them. These suggestions were examined in rats learning visual discriminations in a modified jumping-stand apparatus, after sustaining large lesions of the superior colliculus (and in some cases also of the pretectum). The lesions produced open-field hyperactivity and reduced exploration, indicating effective tectal damage, but the rats learnt a series of difficult discriminations in a door-push task as fast as normal rats, and they did not make more approach errors. Their main abnormality in the discrimination apparatus was that they looked less often between the stimulus doors before stepping across to one of them from the central platform. It is suggested that in rats, as in other animals, lesions of the superior colliculus disrupt the control of scanning head and eye movements; in rats, however, such disruption need not affect discrimination learning (at least in some kinds of apparatus), possibly because the retina of the rat has a relatively poorly developed area centralis.

Visual acuity in hooded rats: effects of superior collicular or posterior neocortical lesions

The visual resolution acuity of hooded rats was measured with an avoidance technique, using large, high contrast square-wave gratings of high mean luminance. Measurements were taken before and after ablation of either posterior cortex or the superior colliculus. The cortical lesions included both striate and temporal cortex, and caused retrograde degeneration throughout the dorsal lateral geniculate nucleus. Neither group showed signs of detecting even coarse square-wave gratings when first tested after operation. The animals with collicular lesions quickly relearnt, and their acuity was unaltered. After extensive training 3 out of 4 cortical animals relearnt to detect gratings, and their acuity was reduced to about one-third of its preoperative value. It seems likely that in rats the geniculocortical pathway carries sufficient information for the normal detection of high spatial frequencies. Whether a pathway from superior colliculus to neocortex via a thalamic relay also carries this information is uncertain.

Anatomical and neurobehavioral investigations concerning the thalamo-cortical organization of the rat's visual system

The organization of thalamic afferents to the rat's visual cortex was investigated autoradiographically and through the retrograde transport of horseradish peroxidase (HRP) following infections into striate and peristriate cortex. The results revealed that Nucleus lateralis posterior (NLP) projects to a large peristriate cortical field that includes areas 18A, 7, and the anterior portion of area 18, and to a circumscribed temporal area corresponding to Krieg's ('46a,b) area 20. The dorsal lateral geniculate nucleus (LGNd) was shown to project to two spatially discontinuous cortical areas. The largest geniculate receiving area is partially coextensive with Krieg's area 17, but an extension of this projection posterior and medial to the striate cortex was found. In addition, a geniculate projection to a restricted field located in the lateral peristriate cortex was identified. Concurrent investigations were designed to assess the pattern discrimination abilities of rats prepared with striate cortical ablations, lesions in NLP and combined striate-cortical and thalamic ablations. Comparison of these animals with normal control subjects revealed that the striate cortex in the rat (as in the cat [Doty, '71; Sprague et al., '77] and the tree shrew [Killackey and Diamond, '71; Ware et al., '74]) is not necessary for successful pattern discrimination, and that the geniculo-striate and NLP-extra-striate projection systems are both involved in mediating the visual discriminative abilities of the rat. The results add species generality to the concept that the central connections to the visual cortex are characterized by parallel-conducting thalamic channels and contribute to the growing number of demonstrations that the extra-striate cortex and associated thalamic cell groups contribute significantly to the process of visual-pattern recognition.

Effects of posterior neocortical lesions on wavelength, light/dark and stripe orientation discrimination in ground squirrels

Thirteen-lined ground squirrels (Citellus tridecemlineatus) were trained on three two-choice visual discrimination problems: light/dark, color and stripe orientation. After posterior neocortical lesions in one or two stages, they were tested on all three discriminations. The results demonstrate that animals with large posterior neocortical lesions which produced retrograde changes throughout the dorsal lateral geniculate (LGNd) were capable of light/dark and wavelength discrimination. These animals were not able to discriminate stripe orientation. It is proposed that wavelength discrimination depends on extrageniculostriate mechanisms in posterior neodecorticates of this species.