In macaque monkeys, lesions involving the posterior portion of the inferior temporal cortex, cytoarchitectonic area TEO, produce a severe impairment in visual pattern discrimination. Recently, this area has been shown to contain a complete, though coarse, representation of the contralateral visual field (Boussaoud, Desimone, and Ungerleider: J. Comp. Neurol. 306:554-575, '91). Because the inputs and outputs of area TEO have not yet been fully described, we injected a variety of retrograde and anterograde tracers into 11 physiologically identified sites within TEO of seven rhesus monkeys and analyzed the areal and laminar distribution of its cortical connections. Our results show that TEO receives feedforward, topographically organized inputs from prestriate areas V2, V3, and V4. Additional sparser feedforward inputs arise from areas V3A, V4t, and MT. Each of these inputs is reciprocated by a feedback projection from TEO. TEO was also found to have reciprocal intermediate-type connections with the fundus of the superior temporal area (area FST), cortex in the most posteromedial portion of the superior temporal sulcus (the posterior parietal sulcal zone [area PP]), cortex in the intraparietal sulcus (including the lateral intraparietal area [area LIP]), the frontal eye field, and area TF on the parahippocampal gyrus. The connections with V3A, V4t, and PP were found only after injections in the peripheral field representations of TEO. Finally, TEO was found to project in a feedforward pattern to area TE and to areas anterior to FST on the lateral bank and floor of the superior temporal sulcus (areas TEm, TEa, and IPa, Seltzer and Pandya: Brain Res. 149:1-24, '78), all of which send feedback projections to TEO. Feedback projections also arise from parahippocampal area TH, and areas TG, 36, and possibly 35. These are complemented by only sparse feedforward projections to TG from central field representations in TEO and to TH from peripheral field representations. The results thus indicate that TEO forms an important link in the occipitotemporal pathway for object recognition, sending visual information forward from V1 and prestriate relays in V2-V4 to anterior inferior temporal area TE.

We studied the effect of eye position on visual and pursuit-related activity in neurons in the superior temporal sulcus of the macaque monkey. Altogether, 109 neurons from the middle temporal area (area MT) and the medial superior temporal area (area MST) were tested for influence of eye position on their stimulus-driven response in a fixation paradigm. In this paradigm the monitored eye position signal was superimposed onto the stimulus control signal while the monkey fixated at different locations on a screen. This setup guaranteed that an optimized stimulus was moved across the receptive field at the same retinal location for all fixation locations. For 61% of the MT neurons and 82% of the MST neurons the stimulus-induced response was modulated by the position of the eyes in the orbit. Directional selectivity was not influenced by eye position. One hundred sixty-eight neurons exhibited direction-specific responses during smooth tracking eye movements and were tested in a pursuit paradigm. Here the monkey had to track a target that started to move in the preferred direction with constant speed from five different locations on the screen in random order. Pursuit-related activity was modulated by eye position in 78% of the MT neurons as well as in 80% of the MST neurons tested. Neuronal activity varied linearly as a function of both horizontal and vertical eye position for most of the neurons tested in both areas, i.e., two-dimensional regression planes could be approximated to the responses of most of the neurons. The directions of the gradients of these regression planes correlated neither with the preferred stimulus direction tested in the fixation paradigm nor with the preferred tracking direction in the pursuit paradigm. Eighty-six neurons were tested with both the fixation and the pursuit paradigms. The directions of the gradients of the regression planes fit to the responses in both paradigms tended to correlate with each other, i.e., for more than two thirds of the neurons the angular difference between both directions was less than +/-90 degrees. The modulatory effect of the position of the eyes in the orbit proved to balance out at the population level for neurons in areas MT and MST, tested with the fixation as well as the pursuit paradigm. Results are discussed in light of the hypothesis of an ongoing coordinate transformation of the incoming sensory signals into a nonretinocentric representation of the visual field.

We studied the effect of eye position on pursuit-related discharges and activity during fixation in darkness for neurons of monkey visual cortical areas (lateral intraparietal area) LIP and 7A. In a first step, neurons were tested for direction-specific activity related to pursuit eye movements while the monkey tracked a moving target. In consecutive trials the pursuit target moved in random order in one of four directions on a translucent screen. For 39% of the neurons, located mostly in a dorsoposterior region of area LIP, as well as 42% of the neurons tested in area 7A, a direction-specific pursuit-related activity could be found. To test whether responsiveness of these neurons was modulated by eye position, we employed a pursuit paradigm. In this paradigm, the monkey had to track a target that started to move in the preferred direction with constant speed from five different locations on the screen in random order. For the majority of cells in both areas, pursuit-related activity was modulated by eye position. Most of the neurons tested also revealed an influence of eye position on their spontaneous activity during fixation in darkness (fixation paradigm). For the majority of cells (> 50%) recorded in both areas, two-dimensional regression planes could be approximated significantly (P < 0.05) or nearly significantly (P < 0.1) to the neuronal discharges observed on the fixation paradigm and pursuit paradigm. For 79% of the LIP neurons and 83% of the 7A neurons tested in both experimental paradigms, the directions of the gradients of the regression planes pointed into the same hemifield, suggesting a common neuronal mechanism mediating the eye position effect regardless of the behavioral task the monkey was performing. The observed effects very much resemble the effects of eye position on light-sensitive and saccade-related responses already described for areas LIP and 7A. Regarding also our results observed for the middle temporal and medial superior temporal areas, it is suggested that the observed modulatory effect of eye position on neuronal activity is a common phenomenon in the macaque visual cortical system subserving an internal representation of the external space in a nonretinocentric frame of reference.

Highlights d Primate superior colliculus (SC) is highly sensitive to foveal visual input d SC visually sensitive neurons sample foveolar visual space non-uniformly d SC magnifies foveal visual images in neural tissue as much as primary visual cortex d Tiny foveal stimuli activate large SC tissue area due to fixational eye movements

Eight monoclonal antibodies were used to label Müller cells in four mammals that have vascular retinae (cats, dogs, humans, and rats) and in three with avascular retinae (echidnas, guinea pigs, and rabbits). Müller cells were found to have a fairly uniform retinal distribution in seven species, with a mean density of 8,000-13,000 cells mm-2. Müller cells in avascular retinae differ from their vascular counterparts in four respects. First, they are shorter than those in vascular retinae. This difference is mainly due to a reduction in the thickness of the outer nuclear layer. Second, the trunks of Müller cells in avascular retinae tend to be thicker, although those in echidnas are an exception to this trend. Third, Müller cell rootlets in avascular retinae follow a more tortuous course than those in vascular retinae, reflecting the fact that photoreceptor nuclei in the two types of retina have different shapes and stacking patterns. Fourth, due to a reduction in the density of photoreceptors in avascular retinae, there are fewer neurones per Müller cell. Although these four features may enable Müller cells to assist the nutrition of neurones in the inner layers of avascular retinae, they are unlikely to be morphological specializations that have evolved for that purpose. Rather, these features appear to be a direct consequence of the fact that avascular retinae are thinner and have a differently organised outer nuclear layer. These features aside, Müller cells in avascular retinae closely resemble their counterparts in vascular retinae.

In this paper, for the first time a quantitative description of the morphology and distribution of Müller cells in the macaque monkey retina using immunohistochemistry and high resolution confocal laser scanning microscopy is given. By their morphological features Müller cells are ideally adapted to their neuronal environment in the various retinal layers, with a dense network of horizontal processes, especially in the inner plexiform layer, and close contacts to neuronal somata especially in the outer nuclear layer and ganglion cell layer. Morphology varies with retinal eccentricity. The thickness of the inner trunk increases significantly with increasing retinal eccentricity. According to the overall thickness of the retina, Müller cells in central retina are longer than in peripheral regions. In the parafoveal region, the outer trunks of Müller cells in the outer plexiform layer are immensely elongated. These Müller fibres can reach lengths of several hundred micrometers as they travel through the outer plexiform layer from the foveal centre towards the foveal border where they enter the inner nuclear layer. Müller cell density varies between 6000 cells/mm2 in far peripheral and peak densities of > 30,000 cells/mm2 in the parafoveal retina. There is a close spatial relationship between Müller cells and blood vessels in the monkey retina, suggesting a role of Müller cells in the formation of the blood-retinal barrier, in the uptake of nutrients and the disposal of metabolites.

Physiological and anatomical criteria were used to clearly establish the existence of a pretectal relay of visual information to the ipsilateral inferior olive in the macaque monkey. After injection of horseradish peroxidase into the inferior olivary nucleus, retrogradely labelled neurons were found in the nucleus of the optic tract (NOT) and the dorsal terminal nucleus of the accessory optic tract (DTN). The labelled cells were distributed in a sparse band arching below the margin of the brachium of the superior colliculus between the dorsal and lateral borders of the brainstem at the caudal edge of the pulvinar. Various types of cells could be distinguished. More superficially the cells were extremely spindle shaped, cells deeper within the midbrain had more compact somata. NOT-DTN neurons in the same region were also found to respond with short latencies to electrical stimulation of both the inferior olive and the optic chiasm. All neurons in the NOT-DTN which were antidromically activated from the inferior olive were also found to have direction specific binocular visual responses. Such neurons were excited by ipsiversive motion and suppressed by contraversive motion, regardless of whether large area random dot stimuli moved across the visual field or small single dots moved across the fovea. Direct retinal input to these neurons was via slowly conducting fibers (3-9 m/s) from the monkey's optic tract conduction velocity spectrum. As shown previously for non-primates, NOT-DTN cells may also in the monkey carry a signal representing the velocity error between stimulus and retina (retinal slip), and relay this signal into the circuitry mediating the optokinetic reflex.

Top-down attention increases coding abilities by altering firing rates and rate variability. In the frontal eye field (FEF), a key area enabling top-down attention, attention induced firing rate changes are profound, but its effect on different cell types is unknown. Moreover, FEF is the only cortical area investigated in which attention does not affect rate variability, as assessed by the Fano factor, suggesting that task engagement affects cortical state nonuniformly. We show that putative interneurons in FEF of Macaca mulatta show stronger attentional rate modulation than putative pyramidal cells. Partitioning rate variability reveals that both cell types reduce rate variability with attention, but more strongly so in narrow-spiking cells. The effects are captured by a model in which attention stabilizes neuronal excitability, thereby reducing the expansive nonlinearity that links firing rate and variance. These results show that the effect of attention on different cell classes and different coding properties are consistent across the cortical hierarchy, acting through increased and stabilized neuronal excitability.