Paola Guandalini

The efferent connections to the thalamus and brainstem of the physiologically defined eye field in the rat medial frontal cortex

The anterograde tracer Phaseolus vulgaris leucoagglutinin (PHA-L) was injected into sites of the rat frontal eye field (FEF) located in the medial frontal cortex. After a single iontophoretic injection of PHA-L into a FEF site where intracortical microstimulation elicited eye movements, anterogradely labelled fibres and terminal-like elements were found in the thalamus in the anterior nuclei, intralaminar nuclei, lateral portion of the mediodorsal nucleus and posterior nuclear group. In the midbrain and pons, labelled fibres were located in the anterior pretectal area, Darkschewitsch nucleus, superior colliculus and dorsolateral portion of the central gray. When the tracer was injected at the FEF periphery, at a site the stimulation of which evoked both eye and whisker movements, labelling distribution in the thalamus differed from that observed after FEF injections, while a similar distribution was observed in the brainstem. In the thalamus, anterograde labelling was observed in these latter cases in the anterior nuclei, ventral nuclei, medial portion of the laterodorsal nucleus. The present findings point out that the FEF and FEF periphery are connected with numerous subcortical structures of the thalamus and brainstem. In addition, the connections of FEF and FEF periphery with the thalamus differ, whereas the midbrain and pons connections of the two subdivisions share common targets.

The corticocortical projections of the physiologically defined eye field in the rat medial frontal cortex

This study investigated in the rat the corticocortical projections of the frontal eye field (FEF), which is located within the medial frontal cortex. The experiments were carried out on Wistar rats. Seven animals received a single iontophoretic injection of Phaseolus vulgaris leucoagglutinin in an FEF site within the medial frontal cortex where intracortical microstimulation elicited eye movements. In these cases, anterogradely labeled fibers and terminal-like elements were found in both hemispheres. The densest labeling was seen in the injected hemisphere, where labeled fibers prevailed in the visual cortex and their laminar distribution differed between the primary and secondary visual cortices. Dense labeled fibers were also seen in the frontal and retrosplenial cortex, whereas a columnar arrangement of terminal-like elements was detected in a restricted part of area 1 of the somatosensory cortex. Contralaterally to the injection site, labeled fibers were distributed mainly in the homotopic region. In two animals, the tracer was injected in a site at the FEF border whose stimulation evoked eye and whisker movements. In these animals, a different distribution of labeling was observed with respect to the other rats in which the tracer was deposited within the FEF, and anterograde labeling was observed in areas 1 and 2 of the parietal cortex of both hemispheres; in addition, no labeling was observed in these cases in the primary visual cortex. These findings suggest that cortical sites confined within the rat FEF are implicated in the control of orienting and exploring behaviors in addition to the control of eye movement.

Somatic receptive-field properties of single fibres in the rostral portion of the corpus callosum in awake cats

SummaryIn fifteen awake, chronic cats single-unit recordings were obtained from 316 fibres isolated in the rostral portion of the corpus callosum (CC). Altogether, 304 units were reactive to peripheral stimuli. They were fired by hair bending, light touch or light pressure (S units; 79.3%) or by gentle rotation of joints and/or by pressure on muscle bellies or tendons (D units; 20.7%). All the reactive units were endowed with small and unilateral receptive fields (RFs) located in trigeminal (49.7%) or segmentai (50.3%) regions. Trigeminal and forepaw units had the smallest RFs. All the trigeminal units were of the S type. Their RFs were located in either the ophthalmic, maxillar, and mandibular face districts or in the oral vestible. The vast majority of segmental units (146 out 153 fibres) had RFs in the forelimb. Very few units were fired by stimulation of the trunk (6 fibres), and only one had its RF in the tail. Almost half of the forelimb units (69 fibres) were fired by stimulation of the most proximal parts of the forelimb and of the shoulder; about one third (57 fibres) exhibited RFs located in the forepaw; the remaining units (20 fibres) had their RFs in the intermediate region of the forelimb. Neither the trigeminal nor segmental RFs ever extended across the midline. The distribution of the fibres within the CC conformed to a somatotopic pattern. The representations of the trigeminal and segmental regions were largely coextensive. Along the rostro-caudal axis of the CC, units with RFs in the mandibular, maxillar and ophthalmic divisions of the trigeminal region tended to lie in this order in the rostralmost 4 mm. Segmental representation extended over the rostralmost 6 mm. Shoulder fibres were mainly found in the rostral half, whereas forepaw units were segregated in the caudal half.

Low threshold unilateral and bilateral facial movements evoked by motor cortex stimulation in cats

Organizational features of the ipsilateral representation in the cat face motor cortex were investigated by using the technique of intracortical microstimulation (less than 30 microA). In 4 intact animals, 61 efferent zones controlling facial muscles were identified. They were devoted to contralateral muscles (contralateral efferent zones; n = 35), ipsilateral muscles (ipsilateral efferent zones; n = 8) or symmetrical muscles of both sides (bilateral efferent zones; n = 18). Contralateral efferent zones were found within both the rostral part of the coronal gyrus and the lateral bank of the presylvian sulcus, whereas ipsilateral and bilateral efferent zones were exclusively localized to the rostromedial region of the face motor cortex. Movement thresholds proved to be lowest at the contralateral efferent zones and highest at the ipsilateral efferent zones, with intermediate values at the bilateral efferent zones. Latencies of suprathreshold EMG responses evoked from contralateral and ipsilateral efferent zones were the shortest and the longest, respectively; intermediate values were found upon stimulation of sites from bilateral efferent zones. The efferent zones identified in 3 lesioned animals (two cats with contralateral motor cortex ablation and one cat with transection of the rostral two thirds of the corpus callosum) were contralateral (n = 14), bilateral (n = 13), and ipsilateral (n = 9). Mean thresholds of effective sites from bilateral and ipsilateral efferent zones in lesioned preparations were not significantly higher than those in intact animals. This would thus suggest that extracallosal pathways may account for ipsilateral responses.

The efferent connections of the pupillary constriction area in the rat medial frontal cortex

This study investigated, in the rat, the efferent projections of the pupillary constriction area, which is located within the medial frontal cortex. In order to identify the location of the pupillary constriction area, in preliminary experiments the medial frontal cortex was microstimulated. Intracortical microstimulation elicited pupillary constriction in a thin strip of cortex near the interhemispheric fissure and bordering the frontal eye field and vibrissae area of the somatomotor cortex. Seven animals received a single iontophoretic injection of Phaseolus vulgaris leucoagglutinin in the pupillary constriction area. In these cases, anterogradely labelled fibres and terminal-like elements were found in both hemispheres. The densest labeling was seen in several areas of the injected hemisphere, where labeled fibers prevailed in the secondary visual cortex. Dense labeled fibers were also found in the retrosplenial and cingulate cortex. In the thalamus, labeled fibers were seen in the intralaminar nuclei and posterior nuclear group. In the midbrain and pons, labeled fibers were located in the anterior pretectal area, superior colliculus and in the dorsolateral portion of the central gray. Contralaterally to the injection site, labeled fibers were distributed in the homotopic region. These findings led us to assume that, in the medial frontal cortex of the rat, besides controlling pupillary constriction, the pupillary constriction area may also be involved in controlling orientation and exploring behavior.

The vibrissal motor output following severing and repair of the facial nerve in the newborn rat reorganises less than in the adult.

This study examined the ability of facial motoneurons and motor cortex to reorganise their relationship with the somatic musculature following the severing and repair of the facial nerve in rats at birth. In each adult rat, the organisation of the facial nucleus and the cortical motor output corresponding to the normal side were compared with those corresponding to the reinnervated side. Labelling was used to reveal reinnervation‐induced long‐term changes in the motoneuron pool supplying vibrissal muscles. Cortical motor output was assessed by mapping the vibrissal movement area extension and thresholds evoked by intracortical microstimulation. After facial nerve reinnervation: (i) the proportion of labelled cell profiles decreased by 85.2% of that in the control side and cortical representation of vibrissal movement decreased by 66.3% of that in control hemispheres; (ii) the reorganised vibrissal representation was shrunken to the medialmost portion of the normal vibrissal representation and there was a medial extension of the forelimb representation, and a more modest lateral extension of eye representation, into the vibrissal territory; (iii) the normal pattern of contralateral vibrissal movement was observed in only 10% of the vibrissal sites, whereas ipsilateral vibrissal movement was found in 53% of the vibrissal sites; (iv) there was an increase in the mean threshold required to evoke contralateral vibrissal movement (32.5 ± 11.1 vs. 20.5 ± 6.9 µA). Thresholds to evoke other types of movement were similar to normal. These changes indicate that an incomplete motor axon regeneration at birth does not restore normal innervation and normal cortical control over the vibrissal muscles.

Motor responses mediated by orthodromic and antidromic activation of the rostral portion of the cat corpus callosum

SummaryThe effects of microstimulation of the rostral portion of the corpus callosum (CC) were examined in seven chronic cats submitted to either unilateral motor cortex ablation (5 preparations) or transection of the rostral two thirds of the CC (2 preparations) in order to identify the routes (ortho-or antidromic) followed by callosal impulses to provoke the motor effects. As in intact animals, motor responses in lesioned preparations consisted of very localized contractions of shoulder, whisker, or eyelid muscles, according to the stimulated sites. Unlike intact animals in which motor responses upon CC microstimulation were bilateral and symmetrical (Spidalieri and Guandalini 1983), in lesioned preparations they appeared contralaterally to the emitting hemisphere, i.e., they were contralateral to the stimulated callosal stump (split-brain preparations) and ipsilateral to the side of the cortical lesion (preparations with unilateral motor cortex ablation), regardless of the current intensity applied (up to a maximum of 50 μA). The unilateral motor responses occurred by the first day after lesion and persisted for the duration of the experiments which lasted to a month or more. Since orthograde degeneration of callosal fibres deprived of their somata has been shown by previous anatomical studies to be complete within 11 days after lesion, these results indicate that selective antidromic activation of callosal fibres is capable of eliciting motor responses. Thresholds for the motor effects in lesioned preparations proved to be from 1.3 to 3.9 (mean, ¯x = 2.4±0.7 SD) times higher than those found before motor cortex ablation. By 18 days after lesion a decrease of threshold currents for the motor responses was observed ranging from 6 to 37% (mean, ¯x = 24.2±13.6 SD), depending on the stimulated sites, relative to values previously found. The shortest train duration and the lowest frequency for minimum threshold were longer (40 vs. 30 ms) and higher (400 vs. 300 Hz), respectively in lesioned preparations than in intact controls. Moreover, a decrease in train duration or frequency provoked larger threshold increases in lesioned preparations than those observed in intact animals. As a whole, these results suggest that in intact animals the motor effects are also mediated by orthodromic callosal volleys.

Evidence for a facilitatory role of callosal afferents to the cat motor cortex in the initiation of conditioned bilateral movements

The effects of selective transection of the rostralmost portion of the corpus callosum, which contains fibres interconnecting the motor cortices of the two hemispheres, on frequency of occurrence and latency of conditioned responses (CRs) in both eyes were examined in seven cats trained to blink in response to a 500-ms tone. A 100-ms air-puff delivered to one eye only (ipsilateral eye) 400 ms after tone onset was used as an unconditioned stimulus. Both before and after callosal lesion, bilateral CRs were the most frequent response pattern. Following callosal lesion, a statistically significant reduction in the percentage of CRs in at least one eye was observed in only two cats. In all seven animals, both before and after callosal lesion, the mean CR latencies of the ipsilateral eye were significantly shorter than those of the contralateral eye. Callosal lesion caused a significant increase in the mean CR latencies of both eyes in all subjects. These results provide evidence that the two hemispheres influence each other in controlling conditioned bilateral blinking by reciprocally exchanging facilitatory signals contributing to initiation of CRs in both eyes.

The functional development of input-output relationships in the rostral portion of the corpus callosum in the kitten

SummaryMicrostimulation of the rostral portion of the corpus callosum (CC) was carried out on 21 awake kittens ranging in age from 45 to 105 days to determine the age at which motor responses first appeared and that at which they assumed functional adult-like properties. Motor responses to microstimulation first appeared over an interval ranging from 78–86 days postnatally. As in adults, they consisted of discrete, well-localized contractions of shoulder, whisker, and eyelid muscles according to the stimulated sites. In the first days after their appearance, motor responses differed markedly from those in adults because: (a) they exhibited higher thresholds; (b) they did not faithfully follow pulse trains delivered at 10 s intervals; (c) they had variable and longer latencies. Thereafter, motor responses gradually became stable, faithfully followed suprathreshold stimulation delivered at 0.1/s frequency, and acquired lower thresholds and shorter latencies, until they exhibited adult-like properties at 93–100 days of age. Single-unit recordings were obtained from 138 fibres isolated in the same callosal region submitted to microstimulation in order to study the response properties of the callosal fibres to somatic stimuli in immature animals. On the basis of their reactivity to peripheral stimulation, fibres were classified into three main types: (1) unreactive units (58 fibres), which could not be driven by somatic stimuli. (2) Adult-like units (55 fibres), which were readily driven by somatic stimuli and were endowed with fixed and small receptive fields (RFs) indistinguishable from those of adults. (3) Immature units (25 fibres), which were unsteadily driven by somatic stimuli applied over large areas at the periphery. Neither the RFs nor the adequate stimuli could be reliably determined. This type of units was not found in the adult cat (Spidalieri et al. 1985). The proportion of unreactive units was the highest before the appearance of motor responses and gradually decreased, approaching the adult level after attaining adult-like motor responses. Conversely, the proportion of adult-like units was lowest before the appearance of motor responses and gradually increased, approaching the adult level after motor responses had acquired adult-like properties.

Bilateral coupling in learned blinking: side superiority, synchrony and temporal coordination in normal cats

In order to study interocular temporal coupling in the initiation of learned symmetrical blinking, experiments were carried out on cats trained to blink in response to a 500-ms tone paired with 100-ms airpuffs randomly delivered to either eye (alternate airpuff; 6 animals) or simultaneously directed to both eyes (bilateral airpuff; 4 animals) 400 ms after tone onset. In spite of the fact that differences in conditioned response (CR) latencies of the right and left eye varied in a wide range between positive and negative values in all subjects, a statistically significant difference between the mean CR latencies of the two eyes (further called side superiority) was found in 6 animals, of which 4 were trained by alternate airpuff and 2 by bilateral airpuff. Superiority of the right eye was found in 3 animals and the opposite was observed in the other 3. Analysis of the differences between CR latencies of the two eyes showed that side superiority was not due to the ability of one eye to give CRs consistently shorter than those of the other eye, but it crucially depended on the higher proportion of trials in which the superior eye led. The ability to give simultaneous CRs by the two eyes was found inversely related to the mean CR latency per session and subject. Regression analysis showed that power equations best described these relationships. In all animals, the frequency distribution of simultaneous CRs paralleled the frequency distribution of all CRs. In spite of a considerable trial-by-trial variability in the temporal relationships between CR latencies of the two eyes, clear-cut linear correlations were found by plotting the mean CR latencies of the right and left eye per both session and subject. The results reviewed in this paper are best accounted for by suggesting that blink onset of the two eyes is independently controlled by two distinct command signals and is modulated by bilaterally-balanced and lateralized influences.