G. Spidalieri

Anatomical and functional aspects of the associative projections from somatic area SI to SII

Summary1.Electrophysiological and morphological (retrograde axonal transport of horseradish peroxidase, HRP) experiments have been carried out in the cat in order to study the associative projections from area SI to ipsilateral SII.2.Microelectrode recordings were performed in the forepaw focus of SII both in normal (64 units) and in SI-undercut (51 units) cats. 29.6% of the neurons recorded in the unoperated and 29.4% of those collected in the operated cats were excited by electric stimulation of the ipsilateral SI (forepaw focus). In both preparations almost all such units were endowed with large (either contra- or bilateral) receptive fields (RF). Cell population recorded in the SI-undercut cats showed no significant impairment to peripheral stimuli and/or changes in the size of the RFs.3.From the forepaw focus of SI, 150 units have been recorded and tested by stimulation of the homologous focus of the ipsilateral SII. Eight of them were fired antidromically and thus identified as association cells. Their RFs were very small and located only in the digits of the contralateral forepaw.4.Both single or multiple HRP injections were performed in SII. Retrogradely labelled cells were found in the ipsilateral SI. The great majority of association cells are pyramids and dwell mainly in layer III. In spite of the large diffusion of the exogeneous reaction product in the injected SII and of the presence of retrogradely labelled cells anywhere in the ipsilateral thalamic VB complex, the distribution of association cells is unequal throughout SI since they strongly predominate in the digit zone of the forepaw representation.

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.

Motor representation in the rostral portion of the cat corpus callosum as evidenced by microstimulation

SummaryIn six chronic cats experiments were carried out to examine the motor effects elicited by microstimulation of the rostral portion of the corpus callosum (CC) which contains fibres interconnecting the motor cortices of the two hemispheres. Serial dorso-ventral penetrations were stereotaxically performed along the rostro-caudal extent of the CC at 0.25–0.5 mm intervals. Motor responses, consisting of very discrete contractions of shoulder, whisker and eyelid muscles, were obtained upon stimulation of about the most rostral 4 mm at intensities lower than 50 μA. At threshold the responses appeared in only one body region and were often unilateral. A light increase in current gave rise to symmetrical bilateral effects. Thresholds were the lowest in the middle of CC and gradually rose towards its dorsal and ventral surfaces. In the course of a penetration an effect once elicited persisted until either the threshold became higher than 50 μA or the ventral edge was reached. All motor effects had thresholds higher than 10 μA. Contractions of shoulder, whisker and eyelid muscles at a threshold lower than 20 μA were obtained in 62.7%, 25.7% and 54.3% of penetrations, respectively. Shoulder and eyelid muscles were represented in the rostral and caudal half of the effective zone, respectively, whereas the representation of whisker muscles overlapped with the other two in the rostral third or the middle part of the effective zone.Single fibres driven by palpation of muscles responding to microstimulation or by movements of the joint mobilized by microstimulation were found at the most rostral sites, whereas units responding to hair displacement over or around the affected muscles were isolated mainly at the most caudal sites.It was inferred that functionally homogeneous callosal fibres are clustered in bundles arranged in dorso-ventrally oriented laminae. The somatotopic representation of the motor effects elicited by microstimulation along the rostro-caudal extent of the effective zone should be the consequence of the serial arrangement of these partially overlapping laminae.

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.

The head midline as a reliable reference frame for encoding head-on-body orientation.

Twelve right-handed male subjects were asked to use each hand to point accurately toward the center of their forehead, determining the correct position according to their mental representation, while the head was either aligned with the trunk or tilted 30 degrees to the right or left. Analysis of end-positions of pointing revealed that the right hand exhibited a slight leftward bias with respect to the putative head midline passing through the center of the glabella and the center of the fissure between the two upper central incisors, regardless of the head-on-body orientation, whereas the left hand proved very accurate when the head was aligned with the trunk but, when the head was tilted either way, it deviated to the opposite side. These results lead to the conclusion that the head midline appears to act as a reliable reference frame for encoding head-on-body orientation when pointing is carried out by the dominant hand.

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.

Gravitational cues contribute to accurate localisation of mentally represented cutaneous targets.

Abstract. The purpose of this research was to ascertain whether gravitational cues contribute to localisation ability of mentally represented cutaneous targets. Eight right-handed male subjects were asked to use each hand to point accurately toward four points in the anterior trunk midline solely on the basis of the mental representation of their position along the trunk while standing upright and while lying supine. It was found that subjects were more accurate when performing in the upright posture. However, statistical analysis of the mean constant transverse errors of pointing showed that neither posture nor target factors had any significant effect on pointing accuracy. On the contrary, analysis of the mean constant longitudinal errors showed that both the posture and target factors were significant. Subjects pointed caudally to the target locations and made the largest errors, which were on the order of centimetres, when performing in the horizontal posture. These findings indicate that gravitational cues are critical in accurately localising mentally represented cutaneous targets.

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.

Bilateral coupling in learned blinking: effects of lesion aimed at callosal disconnection of the cat motor cortex

The effects of selective transection of the rostralmost portion of the corpus callosum, which contains fibers interconnecting the motor cortices of the two hemispheres, on interocular temporal coupling in the initiation of learned symmetrical blinking were examined in 5 cats trained to blink in response to a 500-ms tone paired with 100-ms airpuffs randomly delivered to either eye (alternate airpuff; 3 animals) or simultaneously directed to both eyes (bilateral airpuff; 2 animals) 400 ms after tone onset. Lesioning had no effect on ability to respond, but did cause a significant increase in the mean conditioned response (CR) latencies of both eyes in all subjects. In the intact cats, a statistically significant difference between the mean CR latencies of the two eyes (hereafter called side superiority) was found in three animals, two of which exhibited superiority of the left eye. After lesioning, it was found that the right eye proved superior to the left eye in four subjects. Both before and after lesioning, 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. A linear correlation between CR latencies of the right and left eye was found in all animals both before and after lesioning. It is suggested that subsequent to lesioning aimed at callosal disconnection of the motor cortex, the effects on initiation of learned symmetrical blinking are the consequences of withdrawal of both bilaterally balanced modulatory influences to both eyes and lateralized modulatory influences to the left eye.