The middle temporal area (MT) of the macaque monkey is a region of extrastriate cortex involved in the analysis of visual motion. MT receives strong projections from striate cortex and from area V2, which is dependent on striate for visual responsiveness. Accordingly, the visual properties of MT neurons have been thought to reflect the further processing of its input from striate cortex. We examined the dependence of MT activity on pathways deriving from striate cortex by recording from MT neurons following removal of their striate input. Repeated recordings in area MT were made in 4 hemispheres of anesthetized macaques following either partial or total ablations of striate cortex. Cells in MT were tested for responsiveness, selectivity for direction of motion and direction tuning, and ocular dominance. Receptive fields were also plotted. In an additional animal, we recorded from MT neurons during reversible cooling of the central representation in striate cortex. We found that striate cortex removal or inactivation did not abolish the visual responsiveness of the majority of MT cells. Although the residual responses were generally much weaker than in the intact animal, direction selectivity and binocularity were still present. Moreover, receptive field size and overall topography appeared unaltered.

The middle temporal visual area (MT) in macaque extrastriate cortex is characterized by a high proportion of neurons selective for the direction of stimulus motion, and is thus thought to play an important role in motion perception. Previous studies identified a population of cells in MT that appeared capable of coding the motion of whole visual patterns independent of the motions of contours within them (Gizzi et al. 1983; Movshon et al. 1985). These "pattern-motion selective" neurons are unlike motion sensitive cells that have been observed at earlier stages of the visual system. Using very different criteria, we have also previously identified an apparently functionally distinct group of MT neurons (Albright 1984). We predicted that these "Type II" neurons correspond to the pattern-motion neurons. In the present study, we have applied both sets of criteria to individual neurons in MT and found that these two differently defined sets of cells actually form the same population. These results support the idea that MT contributes to a specialized type of motion processing which reflects the integrity of normal perception.

In a previous study (Rodman et al., 1989), we found that many neurons in the middle temporal area (MT) of the macaque monkey remain visually responsive and directionally selective after striate cortex lesions or cooling. In the present study, we examined the effects of superior colliculus (SC) lesions and combined lesions of striate cortex and the SC on the visual properties of MT neurons. Removal of the SC alone had no effect on the proportion of visually responsive cells, strength of direction selectivity and direction tuning, orientation tuning, receptive field size, or binocularity in MT. There was, however, a slight increase in response strength to both stationary and moving slit stimuli. In contrast to the minor effects of SC lesions alone, addition of an SC lesion to striate cortex damage abolished all visual responsiveness in area MT. The results indicate that pathways damaged by the SC lesion are not necessary for most of the properties of MT neurons found in the intact animal, although these pathways are capable of sustaining considerable visual responsiveness and direction selectivity when striate input is removed.

No abstract

The retinofugal pathways in the California ground squirrel, Spermophilus beecheyi, were mapped after intravitreal injections of cholera toxin B-subunit. The results of the current study are consistent with work in other mammals and provide new details relevant to the organization and evolution of the visual system. All retinorecipient nuclei received bilateral input, with a contralateral predominance. The suprachiasmatic nucleus is heavily innervated, and sparse terminals were noted in other hypothalamic areas. In addition to the interstitial, medial, lateral, and dorsal terminal nuclei, a few fibers of the accessory optic tract may enter the ventral lateral geniculate and the nucleus of the optic tract, though this innervation may not derive from the same ganglion cells innervating the accessory optic nuclei. Retinal terminals are found in the intergeniculate leaflet and the "dorsal cap" of the ventral lateral geniculate. Retinal fibers pass rostrally from the dorsal cap toward the anterodorsal thalamus, confirming a projection described in the tree shrew and monkeys. Retinal termination patterns in the dorsal lateral geniculate reveal a hexilaminate organization of alternating ipsilateral and contralateral input. Variations in terminal morphology suggest that sublayers receive input from distinct ganglion cell types and that laminar comparisons can be made with primates. Finally, terminal patterns in the superior colliculus reveal a dense, highly ordered columnar organization supporting functional properties of tectal receptive fields. All the visual structures in the ground squirrel are large and well differentiated, making the sciurid visual system an accessible rodent model for comparing visual processing with that in other diurnal vertebrates.