Afferent connections of the medial frontal cortex of the rat. II. Cortical and subcortical afferents.

In order to compare the frontal cortex of rat and macaque monkey, cortical and subcortical afferents to subdivisions of the medial frontal cortex (MFC) in the rat were analyzed with fluorescent retrograde tracers. In addition to afferent inputs common to the whole MFC, each subdivision of the MFC has a specific pattern of afferent connections. The dorsally situated precentral medial area (PrCm) was the only area to receive inputs from the somatosensory cortex. The specific pattern of afferents common to the ventrally situated prelimbic (PL) and infralimbic (IL) areas included projections from the agranular insular cortex, the entorhinal and piriform cortices, the CA1-CA2 fields of the hippocampus, the subiculum, the endopiriform nucleus, the amygdalopiriform transition, the amygdalohippocampal area, the lateral tegmentum, and the parabrachial nucleus. In all these structures, the number of retrogradely labeled cells was larger when the injection site was located in area IL. The dorsal part of the anterior cingulate area (ACd) seemed to be connectionally intermediate between the adjacent areas PrCm and PL; it receives neither the somatosensory inputs characteristic of area PrCm nor the afferents characteristic of areas PL and IL, with the exception of the afferents from the caudal part of the retrosplenial cortex. A comparison of the pattern of afferent and efferent connections of the rat MFC with the pattern of macaque prefrontal cortex suggests that PrCm and ACd areas share some properties with the macaque premotor cortex, whereas PL and IL areas may have characteristics in common with the cingulate or with medial areas 24, 25, and 32 and with orbital areas 12, 13, and 14 of macaques.

Afferent connections of the medial frontal cortex of the rat. A study using retrograde transport of fluorescent dyes. I. Thalamic afferents.

The present study of the medial frontal cortex of the rat was undertaken with two objectives. First, to compare the pattern of afferent thalamic neurons for each of the three subdivisions of the medial frontal cortex: the medial precentral (PrCm), dorsal anterior cingulate (ACd) and prelimbic (PL) areas. Second, to provide a firmer basis for anatomical comparisons of cortical regions between rat and monkey. Focal injections of retrogradely transported fluorescent tracers, true blue and diamidino yellow, were placed in different regions of the medial frontal cortex, to reveal the organization of afferent thalamic neurons. The PL area can be readily distinguished from PrCm and ACd areas because it receives afferents from a large number of neurons from both the medial and the lateral parts of the mediodorsal nucleus (MD) whereas only a few neurons, from the lateral MD exclusively, project to PrCm and ACd areas. Moreover, the paratenial and the paraventricular thalamic nuclei project only to the PL area, and the central medial nucleus projects mostly to the PL area. The ventrolateral nucleus projects only to the dorsal part of the medial frontal cortex. The rhomboid, reuniens, ventromedial, intralaminar, posterior and laterodorsal nuclei project to the whole medial frontal cortex. On the basis of these findings, the pattern of thalamic afferents to the PL area was compared to the pattern of thalamic afferents to cingulate and retrosplenial cortices in rat. The conclusion is that the PL area has a pattern of thalamic afferents which is different not only from those of PrCm and ACd areas but also from those of cingulate and retrosplenial cortices. On the basis of its rich innervation from the mediodorsal nucleus, the prelimbic area could very likely be a part of the prefrontal cortex of rat.

Visual field in dark-reared cats after an extended period of recovery.

The visual field of dark-reared cats was behaviourally measured after several years of recovery in a normal environment. A reduction of the visual field was observed and affected the contralateral field as well as the ipsilateral field when tested in monocular viewing. The longer the deprivation period, the more reduced was the visual field. Our results suggest that binocular deprivation might have stabilized the visual system in an immature state.

Visual localization and discrimination after ibotenic lesion of the cat orbito-insular cortex.

Behavioral tasks were used to investigate how the orbito-insular cortex (OIC) of the cat is involved in complex operations such as the orienting reaction towards a novel stimulus. Six cats were trained preoperatively on a perimetry test to assess their ability to orient the head and eyes to objects presented in restricted regions of the visual field, and on brightness, pattern and form discrimination tasks for food reward in a two-choice discrimination apparatus. Two animals then underwent unilateral chemical lesion of the OIC using injections of ibotenic acid, two others received bilateral lesions of this same area, and the remaining two cats were used as normal controls. Postoperative performance of brightness, pattern and form discrimination was normal following OIC lesions, and no lack of retention was observed. In contrast, the cats with OIC lesions had significant deficits in their visually guided behavior. The cats ignored objects presented in the monocular segment of both sides of the visual field, even after unilateral lesion, and there was an effect on the ability to attend and fixate the central preconditioned stimulus.

Visual properties of neurons in the suprageniculate nucleus of the cat.

Visual response properties were studied in 83 single units recorded in the cat suprageniculate nucleus (Sg). About 70% of the cells were visually driven preferentially by the contralateral eye and triggered by moving stimuli without directional selectivity. Receptive fields were usually of a large size (greater than 20 degrees) and for half the cells, extended into both contralateral and ipsilateral fields of vision. No retinotopy nor functional clustering within the nucleus could be demonstrated. These properties of the visual neurons in the Sg nucleus are discussed in relation to the afferent and efferent connections of this nucleus.