Relative sizes of cortical visual areas in marmosets: functional and phylogenetic implications

Visual areas V1, V2 and MT (V5) were identified through myeloarchitectonic criteria and their sizes estimated in a flattened map of caudal cerebral cortex in the marmoset Callithrix penicillata. The ratio MT/V1 in this species is similar to values reported for other species of primates, but the ratio V2/V1 in Callithrix penicillata is smaller than that in macaques and larger than that in Aotus. The possible implications of these results are discussed.

Absence of tectotectal pathway in the rabbit: an anatomical and electrophysiological study

Absence of tectotectal pathway in rabbits is demonstrated by means of anatomical (retrograde axonal transport of horseradish peroxidase (HRP)) and of physiological (electrical stimulation of the superior colliculus (SC)) methods. Despite the absence of that pathway it was still possible to verify the occurrence of electrophysiological correlates of Sprague's phenomenon. It is suggested that, in rabbits at least, reciprocal inhibitory influences between the colliculi can be mediated by pathway(s) other than the tectotectal projection. It is also hypothesized that the absence of tectotectal pathway could possibly be functionally correlated to a small degree of binocular vision and to a small representation of the ipsilateral visual hemifield in the superior colliculus.

Horseradish peroxidase labeling of cat tectotectal cells

By means of unilateral intracollicular injections of horseradish peroxidase (HRP), the cells of origin of the tectotectal projection in cats were detected in the opposite colliculus. The 9 injected animals were divided into two groups in accordance to the location of their injections along the collicular rostrocaudal axis. The majority of the labeled cells were obtained from two injections performed at the level of the commissure of the superior colliculus (231 cells) whereas 5 injections performed at lower levels led to labeling of 49 cells. In two animals, the injections did not cause labeling of cells, a fact that was attributed to the superficiality of the injections. Concerning the distribution of the labeled cells over the collicular layers and along the collicular rostrocaudal axis, there was no difference between the two groups of animals. Among the 280 labeled cells, 1 cell (0.35%) was found in stratum griseum superficiale, 83 cells (29.64%) in stratum opticum, 182 cells (65.00%) in stratum griseum intermediale and 14 cells (5.00%) in stratum griseum profundum. Ninety-two per cent of the cells were located in the rostral portion of the colliculus, in the region between the collicular rostral tip and the collicular rostral plane that corresponds to the vertical meridian representation. Eight per cent of the cells were concentrated in a zone comprising the lower half of stratum opticum and the upper half of stratum griseum intermediale, a location that corresponds to the transition between the visual-sensory and the visual-motor colliculus. On the basis of laminar segregation of collicular functions, the possibility that the tectotectal projection could be related to visual orienting, following and attentive collicular functions through the suppression of activity (a non-enhancement effect?) of homologous cells in the opposite colliculus is discussed.

Sensory and motor representation in the cerebral cortex of the three-toed sloth (Bradypus tridactylus)

Cortical sensory receiving areas were studied in 32 specimens of the three-toed sloth, Bradypus tridactylus, using the evoked response technique and barbiturate anaesthesia. Somatotopic organization in the somesthetic first area (S-I) was shown to be similar to that reported in higher mammals. The area devoted to the representation of the forelimb was considerably larger than that for the remaining body parts. The second somatosensory area (S-II), showing bilateral representation without a precise topographical organization, was identified in the rostral portion of the ectosylvian gyrus. Visual and auditory projections, occupying restricted areas, were found along the caudal banks of the ectosylvian fissure. Electrically excitable cortical motor area was explored in 8 animals anesthetized with diallyl barbituric acid in urethane. The motor representation of the various body parts was shown to coincide with the sensory projections, demonstrating the existence in a Eutherian mammal of a sensorimotor amalgam, identical to that described in marsupials. These findings suggest that sloths display a primitive pattern of neural organization, and that the sensory motor amalgam is a generalized form of cortical organization of primitive Therian mammals.