Unilateral lesions of the anteromedial cortex in the rat impair approach to contralateral visual cues

The Visual Cortex in Context

In this article, we review the anatomical inputs and outputs to the mouse primary visual cortex, area V1. Our survey of data from the Allen Institute Mouse Connectivity project indicates that mouse V1 is highly interconnected with both cortical and subcortical brain areas. This pattern of innervation allows for computations that depend on the state of the animal and on behavioral goals, which contrasts with simple feedforward, hierarchical models of visual processing. Thus, to have an accurate description of the function of V1 during mouse behavior, its involvement with the rest of the brain circuitry has to be considered. Finally, it remains an open question whether the primary visual cortex of higher mammals displays the same degree of sensorimotor integration in the early visual system.

Parallel pathways from whisker and visual sensory cortices to distinct frontal regions of mouse neocortex

The spatial organization of mouse frontal cortex is poorly understood. Here, we used voltage-sensitive dye to image electrical activity in the dorsal cortex of awake head-restrained mice. Whisker-deflection evoked the earliest sensory response in a localized region of primary somatosensory cortex and visual stimulation evoked the earliest responses in a localized region of primary visual cortex. Over the next milliseconds, the initial sensory response spread within the respective primary sensory cortex and into the surrounding higher order sensory cortices. In addition, secondary hotspots in the frontal cortex were evoked by whisker and visual stimulation, with the frontal hotspot for whisker deflection being more anterior and lateral compared to the frontal hotspot evoked by visual stimulation. Investigating axonal projections, we found that the somatosensory whisker cortex and the visual cortex directly innervated frontal cortex, with visual cortex axons innervating a region medial and posterior to the innervation from somatosensory cortex, consistent with the location of sensory responses in frontal cortex. In turn, the axonal outputs of these two frontal cortical areas innervate distinct regions of striatum, superior colliculus, and brainstem. Sensory input, therefore, appears to map onto modality-specific regions of frontal cortex, perhaps participating in distinct sensorimotor transformations, and directing distinct motor outputs.

Fractionating dead reckoning: role of the compass, odometer, logbook, and home base establishment in spatial orientation

Rats use multiple sources of information to maintain spatial orientation. Although previous work has focused on rats' use of environmental cues, a growing number of studies have demonstrated that rats also use self-movement cues to organize navigation. This review examines the extent that kinematic analysis of naturally occurring behavior has provided insight into processes that mediate dead-reckoning-based navigation. This work supports a role for separate systems in processing self-movement cues that converge on the hippocampus. The compass system is involved in deriving directional information from self-movement cues; whereas, the odometer system is involved in deriving distance information from self-movement cues. The hippocampus functions similar to a logbook in that outward path unique information from the compass and odometer is used to derive the direction and distance of a path to the point at which movement was initiated. Finally, home base establishment may function to reset this system after each excursion and anchor environmental cues to self-movement cues. The combination of natural behaviors and kinematic analysis has proven to be a robust paradigm to investigate the neural basis of spatial orientation.

Organization of anterior cingulate and frontal cortical projections to the anterior and laterodorsal thalamic nuclei in the rat

The anterior and laterodorsal thalamic nuclei provide massive projections to the anterior cingulate and frontal cortices in the rat. However, the organization of reciprocal corticothalamic projections has not yet been studied comprehensively. In the present study, we clarified the organization of anterior cingulate and frontal cortical projections to the anterior and laterodorsal thalamic nuclei, using retrograde and anterograde axonal transport methods. The anteromedial nucleus (AM) receives mainly ipsilateral projections from the prelimbic and medial orbital cortices and bilateral projections from the anterior cingulate and secondary motor cortices. The projections from the anterior cingulate cortex are organized such that the rostrocaudal axis of the AM corresponds to the rostrocaudal axis of the cortex, whereas those from the secondary motor cortex are organized such that the rostrocaudal axis of the AM corresponds to the caudorostral axis of the cortex. The ventromedial part of the anteroventral nucleus receives ipsilateral projections from the anterior cingulate cortex and bilateral projections from the secondary motor cortex, in a topographic manner similar to the projections to the AM. The ventromedial part of the laterodorsal nucleus (LD) receives ipsilateral projections from the anterior cingulate and secondary motor cortices. The projections are roughly organized such that more dorsal and ventral regions within the ventromedial LD receive projections preferentially from the anterior cingulate cortex. The difference in anterior cingulate and frontal cortical projections to the anterior and laterodorsal nuclei may suggest that each thalamic nucleus plays a different functional role in spatial memory processing.

Phenobarbital administration directed against kindled seizures delays functional recovery following brain insult

Anti-convulsant drug administration or recurrent seizures can impact functional recovery following brain insult. The nature of that impact depends on a variety of factors, including timing of drug administration and drug mechanism of action, as well as seizure number, timing, and severity. The objective of this study was to determine the functional consequences of anti-convulsant administration directed against seizure activity in brain-damaged animals. To this end, phenobarbital was coupled with daily electrical kindling of the amygdala beginning 48 h after a unilateral anteromedial cortex lesion. Recovery from somatosensory deficits was assessed, as was regional atrophy and basic fibroblast growth factor (bFGF) expression. Animals receiving phenobarbital prior to daily kindling failed to recover within 2 months of testing. In contrast, animals receiving saline prior to kindling as well as phenobarbital-treated non-kindled animals recovered within 2 months after the lesion. Though the exact mechanisms underlying these behavioral phenomena remain uncertain, patterns of bFGF expression among the groups provide some insight. Taken together, results from the present study suggest that anti-convulsant drug administration directed against subclinical seizure activity can be more detrimental to functional recovery than seizures alone or anti-convulsant drug treatment after seizure activity has occurred.

Associational projections of the anterior midline cortex in the rat: intracingulate and retrosplenial connections

Past studies indicate that distinct areas of anterior midline cortex in the rat contribute to diverse functions, such as autonomic nervous system regulation and learning, but the anatomical substrate for these functions has not been fully elucidated. The present study characterizes the associational connections within the midline cortex of the rat by using the anterograde transport of Phaseolus vulgaris leucoagglutinin and Fluororuby. The prelimbic area and the rostral part of the anterior cingulate area (both dorsal and ventral subdivisions) are extensively interconnected with each other. In addition, the caudal half of anterior cingulate cortex has extensive projections to precentral medial cortex and caudally directed projections to retrosplenial cortex. Other cortical areas within anterior midline cortex have relatively limited cortical-cortical projections. The infralimbic, dorsal peduncular, and medial precentral cortices have dense intrinsic projections, but have either very limited or no projections to other areas in the anterior midline cortex. Although it has been suggested that cortical-cortical projections from anterior cingulate cortex and prelimbic cortex to infralimbic cortex may be important for linking learning processes with an autonomic nervous system response, the paucity of direct projections between these areas calls this hypothesis into question. Conversely, the results suggest that the anterior midline cortex contains two regions that are functionally and connectionally distinct.

Comparisons of hemi-inattention produced by unilateral lesions of the posterior parietal cortex or medial agranular prefrontal cortex in rats: neglect, extinction, and the role of stimulus distance

Neglect in human and non-human primates has been demonstrated following unilateral lesions of both posterior parietal and prefrontal areas. While it has now been well established that a unilateral lesion of the rodent analog of dorsolateral prefrontal cortex, medial agranular cortex (AGm), results in neglect, the effects of unilateral damage restricted to rodent posterior parietal cortex (PPC) have not been examined in detail. The current study assessed rats with unilateral lesions of PPC or AGm on their ability to orient to unilateral and bilateral stimulation. Since it has been proposed in both the primate and rodent literatures that frontal areas may be responsible for the perception of near space while parietal areas may be responsible for far space, stimuli were presented at two different distances. Lesions of PPC and AGm resulted in severe neglect relative to control operates, with both PPC and AGm operates manifesting severe hemi-inattention and allesthesia relative to control operates. After behavioral recovery from neglect there was no evidence of extinction to bilateral simultaneous stimulation. While neglect to visual stimuli predominated in unilateral PPC operates, unilateral AGm operates had severe neglect in all modalities. In addition, while both left and right PPC operates showed contralesional neglect, AGm operates demonstrated the lateralized differences in neglect reported in previous studies. All groups demonstrated an approximately equivalent level of neglect to stimuli presented at the two different distances, and thus failed to support the suggestion of a peripersonal-extrapersonal dichotomy between frontal and parietal areas in rodents.

Subcortical deterioration after cortical damage: effects of diazepam and relation to recovery of function

Agents which enhance the activity of gamma-aminobutyric acid (GABA) can severely disrupt behavioral recovery in rats following damage to the neocortex if delivered during a sensitive postoperative period. The mechanisms of this disruption have not been found. It has been suggested previously that the ipsilateral striatum and related structures may be transiently disabled after cortical lesions and that diazepam may interfere with restoration of function in these areas. In the present experiment, the subcortical anatomical effects of chronic (3 weeks) administration of diazepam, an indirect GABAergic agonist, were assessed following unilateral lesions of the anteromedial cortex (AMC) or the sensorimotor cortex (SMC) in rats. Atrophic and degenerative changes were examined in the striatum, substantia nigra and thalamus. Following either AMC or SMC lesions, there was a reduction in the size of the ipsilateral striatum and thalamus and a loss of neurons in the ipsilateral substantia nigra pars reticulata (SNr). After AMC lesions, striatal atrophy and neuron loss in the SNr were increased by the diazepam regimen relative to vehicle-treated controls. In addition, diazepam interfered with the behavioral recovery from somatic-sensorimotor asymmetries in AMC-lesioned rats. After SMC lesions, the sites of striatal and thalamic atrophy were different from that observed after AMC lesions, and the extent of atrophy and neuron loss was not exaggerated by diazepam treatment. Consistent with these data, diazepam did not significantly affect recovery from SMC lesions. These findings suggest that the long-term disruptive effects of diazepam on recovery of function after AMC lesions may be related to an augmentation of lesion-induced degeneration.

A further characterization of the spatial problem-solving deficit induced by lesions of the medial frontal cortex in the rat

Two experiments were conducted to investigate the basis of the spatial impairment displayed by rats with lesions to the medial frontal cortex, using a three-table Y-shaped apparatus. In both experiments, animals were first given an exploratory experience of the maze, followed by a short feeding experience on one of the tables, and were then required to return to the location where they had just been fed. In Expt. 1, a spatial working memory procedure was used in which the location of the goal table was varied from day to day. When compared to normal animals frontal rats showed a marked impairment, despite the addition of (a) distinctive visual cues on the tables and their associated runways, or (b) a conspicuous visual pattern placed directly above the goal. Expt. 2 used a spatial learning procedure, in which the spatial location of the goal table remained constant over days. However, the whole apparatus was daily rotated so that animals could not learn to associate the goal table with specific cues located behind it. This procedure did not prevent frontal animals from learning the consistent location of the food by using the spatial relationships of the environment. These results, together with previous ones, suggest that frontal animals suffer from a specific (though not restricted to the domain of spatial information) working memory deficit, and their spatial reference memory is not impaired.

Functional subdivisions of the rat somatic sensorimotor cortex

The behavioural impairments and subsequent recovery were studied in rats with circumscribed unilateral lesions in the somatic sensorimotor cortex (SMC). Lesions were made in the caudal forelimb region (CFL), the rostral forelimb region (RFL), the anteromedial cortex (AMC) or the hindlimb area. Rats with damage in the CFL produced a deficit in placing the forelimb contralateral to the lesion during exploratory locomotion on a grid surface. Rats with AMC damage circled in the direction ipsilateral to the lesion. Lesions in the CFL or AMC produced an ipsilateral somatosensorimotor asymmetry on the bilateral-stimulation test (responding to adhesive patches placed on the contralateral forelimb was slower) that recovered in 7 days following AMC lesions or 28 days following CFL lesions. Finally, RFL lesions produced an ipsilateral asymmetry on the bilateral-stimulation task that was more severe and enduring (recovery in 60 days). After behavioral recovery, the effects of an additional lesion placed in the homotopic contralateral cortex were examined (two-stage bilateral lesion). Rats receiving two-stage bilateral lesions in the RFL or CFL responded slower to tactile stimulation of the forelimb contralateral to the second lesion. In the case of CFL-damaged rats, placing deficits also appeared contralateral to the most recent injury. In contrast, rats receiving two-stage bilateral AMC lesions did not exhibit behavioral asymmetries following the second lesion. These results provide evidence to suggest that subdivisions of the rat SMC can be distinguished with lesion/behavioral experiments. Moreover, a comparison of the effects of unilateral and two-stage bilateral lesions may help in the parcellation of the rat SMC into functionally distinct subareas and provide a basis for studying the processes of recovery and maintenance of function following brain damage.

Septum and medial frontal cortex contribution to spatial problem-solving

An attempt was made to contrast the effects of lesions to the medial frontal cortex and septum in two spatial tasks. In the fixed-goal (FG) task, the food was located on the same table throughout testing, and the start table was randomly varied from day to day. In the variable-goal (VG) task, the start table remained constant but the food was randomly distributed on one or the other of the two remaining tables. In both tasks, normal animals performed better than frontal and septal rats whose performance, however, improved over days in the FG, but not in the VG, task. In both tasks, significant improvement within days was found in medial frontal animals, but not in septal animals. Additional analyses revealed that septal animals had a general pattern of disrupted exploration and a tendency to use a response strategy (i.e. to repeat the same response both within and between days) which decreased over days in the FG task. In contrast, medial frontal animals did not demonstrate disrupted exploration nor any response tendency. It is concluded that both septal and medial frontal cortical damage produce a common spatial working memory impairment. However, there is some evidence to suggest that this common memory impairment could result from disruption of distinct mechanisms in septal and frontal animals. It is proposed that medial frontal lesions could affect some specific mechanism related either to attentional processes or to the ability to anticipate future events, whereas septal damage would interfere with the building of comprehensive and flexible spatial memories.

The effect of unilateral anteromedial cortex lesions on prey-catching and spatio-motor behaviour in the rat

The neglect which follows a unilateral lesion of anteromedial cortex in rat was examined using two tasks. In the first task animals had to press the left bar of a two-bar Skinner box for food reward. In the second task animals captured live locusts presented in either their right or left visual field. The results showed that in the Skinner box there was a transitory spatial neglect contralateral to the lesion, which disappeared after 2-3 days. In the prey-catching task animals failed to target or track locusts when they were in the field contralateral to the lesion throughout the 4-day testing period. Capture and consummatory responses were normal when the locust was presented (or moved independently) into the ipsilateral (non-neglected) field. 60 days later the asymmetrical response to locusts in the contralateral field had disappeared.

Neglect following unilateral ablation of the caudal but not the rostral portion of medial agranular cortex of the rat and the therapeutic effect of apomorphine

The medial agranular cortex (AGm) of the rat is often considered analogous to the frontal eye field (FEF) of the monkey. However, recent anatomical, physiological, and behavioral evidence indicates that, while the caudal portion of AGm may indeed be an analog of the primate FEF, the rostral portion of AGm may be more similar to the primate supplementary motor area. The current study examined the effects of unilateral ablation of both the rostral and caudal components of AGm on the ability to orient to unilaterally presented stimuli. As expected, lesions of caudal but not rostral AGm resulted in a severe unilateral neglect of stimuli characteristic of rats with lesions of the entire rostral-caudal extent of AGm and of monkeys with FEF lesions. Caudal AGm operates also had more severe neglect than a group of rats from a previous study with larger AGm lesions which damaged the caudal portion of AGm to varying degrees. In addition, a second study showed the dopamine agonist apomorphine to have an acute dose-dependent therapeutic effect on the neglect resulting from caudal AGm ablation similar to that seen in animals with ablation of the entire rostral-caudal extent of AGm.

The effects of anteromedial cortex lesions on pattern discrimination learning in rat

The visual angular difference threshold of rats with unilateral anteromedial cortex lesions was measured. The stereotyped running pattern and preoperatively preferred side of approach in a two-choice visual discrimination apparatus was recorded for each animal. Lesions in the hemisphere ipsilateral to the preferred approach side did not affect either the running pattern or the angular difference threshold. However, there was a general reduction in the detection of the positive stimulus throughout the scale. Lesions in the contralateral hemisphere produced a transient reversal of the side preference with an overall loss of S + detection in the field contralateral to the side approached.

Multiple reward systems and the prefrontal cortex

Electrical stimulation of the major divisions of the prefrontal cortex, the mediodorsal and sulcal areas, can serve as a reinforcing stimulus. Studies of self-stimulation of the prefrontal cortex have produced behavioral, anatomical and pharmacological evidence that the substrate of these rewarding effects can be dissociated from that subserving self-stimulation of ventral diencephalic sites such as the lateral hypothalamus. Other studies indicate that within the prefrontal cortex itself, self-stimulation of the medial and sulcal divisions can be attributed to dissociable processes. These observations suggest the existence of multiple, largely autonomous prefrontal subsystems involved in reinforcement. This raises the question of the functional significance of such systems, and of their organization. An approach to this problem is to consider the relationship between the behavioral functions of the prefrontal divisions and the characteristics of stimulation-induced reward obtained at each site. Studies of the effects of restricted prefrontal lesions indicate that the medial and sulcal divisions can be dissociated according to their involvement in the control of distinct types of sensory and motor events. Further experiments indicate that damage to each division causes selective deficits in the learning of stimulus-reinforcer and response-reinforcer relations, depending in part on the nature of the reinforcing event. Conditioning experiments further show that the rewarding effects produced by stimulation of these areas are preferentially associated to sensory events which correspond to the functional specialization of each division. These data are interpreted to suggest that different rewarding events and/or different attributes of rewarding stimuli are processed by distinct systems which are reflected by the organization of dissociable self-stimulation pathways.

Co-administration of Ro 15-1788 prevents diazepam-induced retardation of recovery of function

Following unilateral lesions of the anteromedial cortex, recovery from somatosensory asymmetry reliably occurs within about 10 days. Chronic exposure to diazepam significantly delays this recovery. In the present study, co-administration of Ro 15-1788, a benzodiazepine antagonist (i.e. it blocks the negative and positive allosteric modulation of GABA), prevented diazepam-induced retardation of recovery from somatosensory asymmetry. Nocturnal ambulatory (motor) activity was not different between rats receiving diazepam-alone and those receiving Ro 15-1788 in combination with diazepam. These data suggest that the benzodiazepine receptor is importantly involved in the detrimental effects of diazepam on recovery, and that non-specific behavior sedation plays little or no role.

Seizures and recovery from experimental brain damage

The effects of the gamma-aminobutyric acid antagonist, pentylenetetrazol (PTZ), on recovery from somatosensory and motor asymmetries after unilateral sensorimotor cortex lesions were investigated. Behavior was assessed using a bilateral tactile stimulation test and a measure of forelimb motor coordination. Immediately after surgery, the PTZ-treated and saline (SAL) control groups both exhibited severe ipsilateral behavioral asymmetries. Rats receiving PTZ recovered significantly faster from somatosensory asymmetry than those receiving SAL. Recovery was complete in the PTZ group within 3 postoperative weeks, while the SAL group failed to reach a comparable level until 2 months after surgery. There was no difference between PTZ and SAL groups on recovery of forelimb motor coordination. No difference in lesion size between the SAL and the PTZ groups could be found. These data are consistent with the hypothesis that post-traumatic neuronal depression may contribute to the severity of sensorimotor deficits observed after brain damage.

Food wrenching and dodging: use of action patterns for the analysis of sensorimotor and social behavior in the rat

Developments of a procedure to study two movements, food wrenching (stealing food from a conspecific) and dodging (escaping with food from a conspecific), used in the competition for food by rats is described. These include, (A) procedures for adaptation, (B) procedures for filming and scoring, and (C) procedures for measuring dimensions of movements. The character of the movements have features of action patterns in the sense that the term is used by ethologists. It is suggested that they can be used to study the neural basis of complex sequencing of behavior as well as to study the neural basis of sensorimotor behavior and sensorimotor asymmetries.

Unilateral and bilateral lesions of the anteromedial cortex increase perseverative head movements of the rat

Lesions of the anteromedial cortex were made in rats trained to orient their heads to 9 positions located laterally and centrally in upper, middle and lower regions of space. The water-deprived subjects were tested in sessions of 20 trials in each of which all positions were baited once with a small amount of water. Training continued until no position was consistently missed and the number of returns to positions already selected on that trial (perseverative responses) was low and stable. In Expt. 1, rats with unilateral anteromedial lesions tested with 3 days recovery did not miss any position or significantly change the order in which correct positions were selected but they made more perseverative responses to all positions. Bilateral anteromedial lesions moderately increased misses but markedly increased perseverations. Visual cortex lesions comparable in size produced no changes. In Expt. 2, unilateral anteromedial lesions tested with 7 days' recovery did not increase misses and only slightly increased perseverations. In Expt. 3, unilateral anteromedial lesions tested within 1 day increased perseverations more than lesions tested with 6 days' recovery. Neither group increased misses or changed selection order. The perseverations appeared on a trial after 6 of the 9 correct selections and were directed to both recently and remotely selected positions. Lesions of the anteromedial cortex appear to produce a deficit in the suppressive control of head orienting.

Circling elicited from the anteromedial cortex and medial pons: refractory periods and summation

Contraversive circling is evoked by stimulating the anteromedial cortex (AMC) of rats, and ipsiversive circling is evoked by stimulating the medial pons (PONS). During AMC circling, lateral and vertical head movements and vibrissae movements were exhibited. During PONS circling, although lateral head movements were exhibited, vertical head movements and vibrissae movements were not exhibited. Refractory periods were estimated by delivering trains of paired pulses and measuring the frequency thresholds for circling at various intrapair intervals. Refractory periods at AMC circling sites were much longer (range 1.4-3.3 ms) than at PONS circling sites (range 0.5-1.0 ms). To determine the degree of summation between the AMC and contralateral PONS, the two sites were stimulated concurrently. Summation of 95-100% was observed for AMC and PONS circling. No collision was observed at short intrapair intervals of paired pulses. Thus, the AMC and PONS are not connected axonally but are related, perhaps serially, for the production of circling.

Recovery of function after brain damage: severe and chronic disruption by diazepam

Following unilateral damage to the anterior-medial region of the neocortex (AMC) in rats a sensory asymmetry appeared, but recovered within a week. In a separate group of rats with AMC lesions daily 3-week exposure to diazepam (Valium, 5 mg/kg) beginning 12 h after surgery caused recovery to be delayed indefinitely. The efficiency and speed (as opposed to symmetry) of behavior was not impaired. More than 9 weeks after discontinuation of diazepam (12 weeks postsurgery), recovery was still not apparent. Postmortem analysis ruled out lesion size as a contributing factor. In a second experiment undrugged animals with AMC lesions were allowed to recover for at least 3 weeks before being exposed to diazepam. These animals showed only a transient (2-day) reinstatement of asymmetry despite continuous drug treatment. We conclude that important mechanisms serving recovery of function may be vulnerable during a short period soon after brain damage.

Neglect after section of a left telencephalotectal tract in pigeons

The effects of unilateral and bilateral lesions of the telencephalotectal fibres of the tractus occipitomesencephalicus (OM) were studied in pigeons in a successive visual pattern discrimination. On alternate sessions the animals performed the task with both eyes open or sight restricted to one or the other eye. Unilateral lesions of the left OM led to severe deficits in the total number of pecking responses emitted under both monocular and binocular conditions. Pigeons with lesions of the right OM showed no deficits and the animals with bilateral lesions had an impaired performance only in the binocular condition. The percent correct discrimination performance of all experimental pigeons were at the same level as that of the control group. The results are discussed in the context of visual lateralization in birds. It is suggested that the deficits reflect a sensory neglect through deactivation of neurons in the deep tectal laminae which are known to have projections to reticular nuclei of the brainstem.

Convergence of intra- and interhemispheric cortical afferents: lack of collateralization and evidence for a subrhinal cell group projecting heterotopically

The distribution of corticocortical projecting neurons in the rat's brain was investigated with fluorescent dyes and the retrograde transport of horseradish peroxidase. Although the fluorescent techniques especially revealed the existence of a considerable number of neurons interconnecting the limbic areas (sub- and perirhinal cortex, prefrontal, cingulate, and retrosplenial cortex) both intra- and interhemispherically, only a negligibly small number of neurons with collateralized axons could be detected. In the rat's anterior dorsolateral cortex an area is described whose neurons are organized in a columnlike fashion and project intra- and interhemispherically to the limbic areas examined. The density of efferent connections differed between areas, with an especially high density found in a small region of the subrhinal cortex. Injections of horseradish peroxidase into different regions of the cingulate and retrosplenial cortex confirmed the existence of widespread heterotopic interhemispheric connections originating from this defined subrhinal area, though the number of retrogradely labeled cells remained consistently smaller than that obtained following the injection of fluorescent dyes. Among the regions studied with horseradish peroxidase injections, those into the retrosplenial cortex showed the highest density of labeled cells within this subrhinal area. A more detailed examination of the subrhinal region containing the densely labeled neurons (projecting to the contralateral hemisphere) made use of Nissl stains and revealed a morphologically separable area which was characterized by medium-sized, dark-staining neurons whose long axons were oriented mediolaterally. The region includes portions of the insular-perirhinal, entorhinal, and piriform cortex. It is suggested that the corticocortical projections are basically noncollateralized in the rat. However, there apparently is a dense interhemispheric interconnectivity between the limbic areas. Functional evidence for the defined subrhinal area suggests a prominent role of its neurons in cognitive information processing. The present evidence for considerable interhemispheric cortical projections may provide a new impetus for studying the intercommunication of the two sides of the brain with both anatomical and behavioral methods.

Spatial behaviour of cats in cue-controlled environments

Two groups of six cats were subjected to a spatial learning task in cross-mazes that differed only in the spatial relationship between the goal and the single available visual cue. When this cue was remote from the goal (“mapping situation”), the cats did not need more testing days to reach the learning criterion than when it was above the goal (“guidance situation”). Additional behavioural data gave evidence of two information-gathering strategies within each group: the first consisted of taking the relevant information from the starting point; the second was to choose the correct path from the central choice-point. These strategies were discussed in the light of O'Keefe and Nadel's theory (1978) concerning two spatial behaviours, mapping and route-following.

The results of a second experiment showed that the cats subjected to the mapping situation were markedly less affected than those subjected to the guidance situation by a delay introduced between the moment when the relevant cue was still available and the moment when the subjects were allowed to run to the goal.

In a third experiment, it was shown that only cats that were seen as choosing their path from the starting point during learning (Experiment 1) succeeded in using short-cuts: subjects that chose from the central choicepoint needed to pass through this place when a shorter path was available.

These results show that the processes involved in spatial behaviours may be characterized by taking into account some of the behavioural components. Furthermore, the properties of these processes can be assessed with additional experiments, such as delayed-reaction tasks and short-cutting tests.

Distractibility following simultaneous bilateral lesions of the superior colliculus or medial frontal cortex in the rat

Hooded rats with bilateral lesions of the superior colliculus or medial frontal cortex were compared with controls for locomotor guidance in shuttling back and forth between goal-doors at two opposite ends of a large arena. Colliculectomized rats accomplished this with great accuracy. When flashing distractor lights were introduced midway down the runway, frontal corticals and controls were severely disrupted but colliculars continued to run normally. This result was obtained both when all training occurred postoperatively (Experiment 1) and when runway performance had been stabilized preoperatively (Experiment 2), thus after a long or short postoperative recovery interval. The results offer support for previous studies with rats which have demonstrated sensory 'neglect' but good locomotor guidance after collicular ablation. Frontal corticals differed from controls only in terms of their elevated rate of repeat door-pressing upon postoperative resumption of testing in Experiment 2. Despite the similarity between effects reported elsewhere of collicular and frontal lesions made unilaterally, bilateral deficits clearly demonstrable after collicular ablation were absent here after frontal lesions. The results imply that the functional responsibilities of superior colliculus and frontal cortex in the rat are separable; at least, they have different rates of functional recovery.