Rogelio Perez

Location of neurons projecting to the retina in mammals.

In this study, horseradish peroxidase (HRP) was used as a retrograde tracer in order to investigate the existence of centrifugal pathways to the retina in mammals and to locate the somas of the retinopetal neurons. After the application of HRP to the stump of the cut optic nerve in monkeys, cats, guinea-pigs and rabbits, labeled neurons were located in various areas. The largest number of labeled neurons was found bilaterally in the hypothalamus in the premammillary area, and some neurons were also found slightly more rostral and dorsally towards the posterior hypothalamic area. At the mesencephalic-metencephalic junction, a few labeled neurons were observed in the dorsal raphe nucleus and more ventrally in the tegmental area situated between the medial longitudinal fascicle and the superior cerebellar peduncle. Furthermore, labeled neurons were found in the contralateral medial pretectal area in guinea-pigs, in the ipsilateral tegmental mesencephalic reticular formation in monkeys, in the laterodorsal tegmental nucleus in rabbits, and in the dorsal hypothalamic area near the nucleus of the anterior commissure in monkeys. We were unable to find labeled cells in the dorsal raphe nucleus or in the hypothalamus in rabbits or in the above-mentioned tegmental area in guinea-pigs. These divergences may be due to species differences or may simply be false-negative results due to the known difficulty of using the currently available tracers to label retinopetal neurons.

Influence of layer V of area 18 of the cat visual cortex on responses of cells in layer V of area 17 to stimuli of high velocity.

Focal blockade of restricted regions in layer V of area 18 was used to assess the contribution of this region to the responses to high-velocity stimuli of cells in retinotopically matched, layer V in area 17. In 40% of cases, blockade within area 18 revealed responses of area 17 cells to high-velocity stimuli to which they previously showed only poor responses. Stimulus specificity of the cells in area 17 was otherwise unaltered. All effects were reversible and repeatable. We suggest that a component of the output of layer V from area 18 normally suppresses the responses of retinotopically matched cells within area 17 to stimuli of high velocity, thereby enhancing the specificity of those cells to stimuli of low velocity

Cell responses to vertical and horizontal retinal disparities in the monkey visual cortex

Because of the horizontal separation of both ocular globes, the projection angles are slightly different. These differences are commonly termed retinal disparities. Vertical and horizontal retinal disparities occur constantly in normal life. We have investigated the responses of single cells in cortical areas V1 and V2 of behaving Macaca mulatta monkeys to retinal disparities by using dynamic random dot stereograms. Our findings show that cortical visual cells are sensitive to both vertical and horizontal disparities. To calculate the distance between two objects in a three-dimensional space from horizontal disparities, it is necessary to know the fixation distance. It has been suggested that the horizontal gradient of vertical disparity contains information to estimate the fixation distance and therefore to scale horizontal disparities. We suggest that these cells sensitive to horizontal and vertical disparities represent a neural mechanism that provides information to the visual system in order to achieve a correct eye alignment and depth perception.

Lateral-posterior and pulvinar reaching cells—comparison with parietal area 5a: a study in behaving Macaca nemestrina monkeys

SummaryIn a previous study we have demonstrated the existence of pulvinar (puv) cells which were optimally activated when a monkey executed reaching movements with his limbs (Acuña et al 1983). We now describe further observations in four Macaca nemestrina monkeys trained to perform goal directed reaching movements aimed at four different positions in space. Extracellular unit activity in the lateralis posterior (lp) and puv nuclei, together with electrooculograms were recorded during the execution of the task. Seven hundred and sixty neurons were studied in the lp-puv complex. One hundred and twenty three cells (16%) showed changes in activity related to the reaching movements. Reaching related cells fell into two categories: goal direction sensitive (28/123 = 23%) and pandirectional (95/123 = 77%). Goal direction sensitive cells showed different responses depending on the direction of the goal relative to the starting point of the movement. The responses of the pandirectional cells were independent of goal direction. The activity of the remaining cells (637/760) could not be correlated with reaching movements. In a smaller number of area 5a (PE) cells (n = 109) studied in one monkey, 82 (75%) were classified as reaching related cells. Of these, 76% (62/82) were goal direction sensitive and 24% (20/82) pandirectional. The lp-puv cells were more dependent on the intentionality of movement than area 5a cells, and not reliably activated by passive manipulation of the limb. After injection of HRP-WGA in area 5a, where the reaching cells were recorded, labeled cells and terminals were located in the lp-puv zones where reaching cells were also found. The data shown here suggest that the lp-puv reaching related cells are both anatomically and functionally linked to cortical zones concerned with information regarding directional movement. Cortical areas might be involved in encoding the direction of movement, whereas the lp-puv might participate by redirecting attention during aimed movements.

Orientational influences of layer V of visual area 18 upon cells in layer V of area 17 in the cat cortex

We examined the orientation tuning curves of 86 cells located in layer V of area 17, before, during, and after focal blockade of a small (300-μm diameter) region of near-retinotopic register in layer V of area 18 of quantitatively established orientation preference. Such focal blockade revealed three distinct populations of area 17 layer V cells-cells with decreased responses to stimuli of some orientations (21%), cells with increased responses to stimuli of some orientations (43%), and cells unaffected by the focal blockade (36%). These effects were clearcut, reproducible, and generally directly related to the known receptive field properties of the cell recorded in area 18 at the center of the zone of blockade. These effects were also analyzed in terms of alterations in orientation bandwidth in the cells in area 17 as a result of the blockade-bandwidth increases (22%) and decreases (24%) were found; however, these changes were essentially unrelated to the measured receptive field properties. Inhibitory and excitatory effects were most pronounced when the regions in areas 17 and 18 were of like ocular dominance and were of similar orientation preference. Inhibitory effects (suggesting a normally excitatory input) were most dependent upon the similarity of receptive fields; excitatory effects (suggesting a normally inhibitory input) were less heavily dependent.

Teleophthalmology link between a primary health care centre and a reference hospital

We have evaluated a teleophthalmology service linking a primary health care centre and an eye clinic at a reference hospital. General practitioners at the primary care centre serving a population of 15,000 and ophthalmologists at the reference hospital participated in this study. Eye fundus digital images were taken from 278 eye fundi of 139 consecutive patients with clinical conditions that could potentially produce fundus alterations. Fundus images were obtained with a non-mydriatic fundus camera (Canon CR6-45M) and were electronically sent reference hospital where ophthalmologist inspected the images and returned the diagnosis. In 18 patients (13%) the images did not have good enough quality to exclude eye fundus lesions. In 24 patients (17%) clear eye fundus alterations were found in at least one eye. In 14 patients (10%) there were image features suggesting retinal alterations that could not be confirmed by image inspection. Media opacity (13 eyes, 5%, seven patients, 5%) was the most common cause of poor image quality. The most difficult assessment was the evaluation of optic nerve head cupping. Retinal oedema was not observable in the digital images. In our experience teleopthalmology services seem to be an effective alternative for eye fundus diagnosis and patient follow-up.We have evaluated a teleophthalmology service linking a primary health care centre and an eye clinic at a reference hospital. General practitioners at the primary care centre serving a population of 15,000 and ophthalmologists at the reference hospital participated in this study. Eye fundus digital images were taken from 278 eye fundi of 139 consecutive patients with clinical conditions that could potentially produce fundus alterations. Fundus images were obtained with a non-mydriatic fundus camera (Canon CR6-45M) and were electronically sent to a reference hospital where an ophthalmologist inspected the images and returned the diagnosis. In 18 patients (13%) the images did not have good enough quality to exclude eye fundus lesions. In 24 patients (17%) clear eye fundus alterations were found in at least one eye. In 14 patients (10%) there were image features suggesting retinal alterations that could not be confirmed by image inspection. Media opacity (13 eyes, 5%, seven patients, 5%) was the most common cause of poor image quality. The most difficult assessment was the evaluation of optic nerve head cupping. Retinal oedema was not observable in the digital images. In our experience teleopthalmology services seem to be an effective alternative for eye fundus diagnosis and patient follow-up.

Modulation of cell responses to horizontal disparities by ocular vergence in the visual cortex of the awake macaca mulatta monkey

Horizontal retinal disparity is the most important cue for stereopsis. However, accurate stereoscopic perception requires additional information on fixation distance. The ocular vergence angle may provide information on fixation distance and therefore may be used to calibrate horizontal disparities. We studied the responses of cells from cortical visual area V1 of one macaca mulatta monkey to dynamic random dot stereograms at different ocular vergence angles. We observed that in about half of cells sensitive to horizontal disparity the vergence angle modifies the cell responses to horizontal disparities. These results suggest that vergence angle may be used to calibrate horizontal disparities for fixation distance.

Putamen neurons process both sensory and motor information during a complex task

The putamen has classically been considered to be primarily a motor structure. It is involved in a broad range of roles and its neurons have been postulated to function as pattern classifiers of behaviourally significant events. However, its specific role in motor and sensory processing is still unclear. For the purpose of better categorizing putamen neurons, we trained two rhesus monkeys to perform multisensory operant tasks by using complex stimuli such as short videoclips. Trials involved image or soundtrack or both. Some stimuli required a motor response associated to reward, whereas others did not require response and produced no reward. We found that neurons in the putamen showed pure visual responses, action-related activity, and reward responses. Insofar as action-related activity, preparation of movement, movement execution, and withholding of movement involved three different putamen neuron populations. Moreover, our data suggest an involvement of putamen neurons in processing primary rewards and visual events in a complex task, which may contribute to reinforcement learning through stimulus-reward association.

Activity of neurons in the caudate and putamen during a visuomotor task.

Evidence supporting a role of the caudate and putamen nuclei in associative learning is present. We recorded the activity of 21 caudate and 26 putamen cells in one macaque monkey while performing a visuomotor task, which involved a visual stimulus and the execution of a motor response. Ninety-one percent of caudate cells and 65% of putamen cells showed changes in activity while the monkey was performing the task. Approximately half of the caudate cells and one third of the putamen cells showed changes in activity without a motor response. Our results show that caudate and putamen cells are activated regardless of the presence or absence of a motor action. These findings are consistent with the idea that these nuclei may play a role in associative learning.

Response latencies to visual stimulation and disparity sensitivity in single cells of the awake Macaca mulatta visual cortex.

The onset response latencies to dynamic random dot figures (solid figures) and dynamic random dot stereograms were measured in single units recorded from areas V1 and V2 of two awake Macaca mulatta monkeys. We studied 56 cells, 39 from V1 and 17 from V2. In 14 disparity sensitive and 13 disparity unsensitive cells from V1 the median latencies to solid figures were 59.8 and 73.6 ms, respectively, which were statistically different. In 26 disparity sensitive cells from V1 and 17 from V2 the median latencies to stereofigures were 85.6 and 97.9 ms, respectively, which were statistically different. These results indicate that V1 disparity sensitive cells may have shorter integration time than disparity unsensitive cells and that there is a transferring delay for disparity information between areas V1 and V2.