Maria C. Romero

Time Course of Cold Cataract Development in Anesthetized Mice

Purpose: The aim of the present study was to study the time-course development of cold cataract in mice under general anesthesia. Methods: We anesthetized five groups of 10 mice (12 weeks old) with 400 mg/Kg intraperitoneal injections of chloral hydrate and exposed them to 0, 7, 15, 23, and 37°C for 1 hr. Cataract development was assessed and graded as no cataract, mild, medium, or severe at 10, 20, 30, 45, and 60 min after the exposure started. For quantification purposes, a value from 0 to 3 was assigned to each cataract grade, and the median value was calculated for each group and time point (cataract index, CI). Results: The CI for each temperature fitted a negative exponential equation. We found that four mice of the 37°C group, nine of the 23°C group, and all animals of the 15, 7, and 0°C groups developed cataract. The cataract started at 10 min after exposure to 0°C and at 20 min when exposed to 7, 15, and 23°C. The speed of development and CI significantly increased with lower temperatures. Similar results were observed when the procedure was repeated 48 hr later in the 15, 23, and 37°C groups. In all instances the cataract was reversible. Conclusion: Our findings show that cold cataract development is temperature dependent and that cataract formation starts between 10 and 20 min after exposure to low temperature. This finding is relevant for those experimental settings in which clear ocular media are required.

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.

Hemifield dependence of responses to colour in human fusiform gyrus.

To investigate the hemifield dependence of visually evoked responses to colour in the human fusiform gyrus we recorded evoked potentials from subdural electrodes in a patient suffering from occipital epilepsy. The responses in the fusiform gyrus show a strong hemifield dependence and discriminate the onset from the offset of the stimulus. Additionally, we found responses to squares made of random dots, whereas no responses were found to squares with a homogeneous bright surface. Our findings further support the idea that the fusiform gyrus is related to colour and pattern perception. However, the hemifield dependence we found may indicate that further processing is required in order to combine information from both visual hemifields.

Spatial frequency components influence cell activity in the inferotemporal cortex.

We studied the correlation between the spatial frequency of complex stimuli and neuronal activity in the monkey inferotemporal (IT) cortex while performing a task that required visual recognition. Single-cell activity was recorded from the right IT cortex. The frequency components of the images used as stimuli were analyzed by using a fast Fourier transform, and a modulus was obtained for 40 spatial frequency ranges from 0.3 to 11.1 cycles/deg. We recorded 82 cells showing statistically significant responses (analysis of variance, P < 0.05) to at least one of the images used as a stimulus. Seventy-eight percent of these cells (n = 64) showed significant responses to at least three images, and in two thirds of them (n = 42), we found a statistically significant correlation (P < 0.05) between cell response and the modulus amplitude of at least one frequency range present in the images. Our results suggest that information about spatial frequency of the visual images is present in the IT cortex.

Eye dominance and response latency in area V1 of the monkey

We measured the latency of 35 cells from V1 in two rhesus monkeys, to dynamic random dot stimuli monocular and binocularly presented. Mean latencies after non-dominant eye stimulation (97.9 ms) were longer than those for dominant eye (78.2 ms) and binocular (70.7 ms) stimulation. Differences between latencies for dominant eye and binocular stimulation were not statistically significant. For dominant eye, there was a significant statistical correlation between dominance strength and latency (R = -0.36; p = 0.03). We failed to find significant statistical differences between latencies for cells with temporal and nasal dominant receptive-field. We conclude that, in V1, the response latency is largely determined by the dominant eye, whereas interocular interactions do not seem to play a relevant role regarding response latency.

Sensitivity to direction and orientation of random dot stereobars in the monkey visual cortex.

We are able to judge the direction of movement and orientation of objects because they have contrast‐defined edges. However, we are also able to perceive the orientation and direction of movement of stereobars made of random dot stereograms in the absence of contrast‐defined edges. We recorded 207 disparity‐sensitive cells from visual areas V1 and V2 of two Macaca mulatta monkeys while performing an attentive fixation task. Luminance defined bars and random‐dot stereo‐defined bars were used to assess direction and orientation selectivity of these cells. Orientation and direction preference for luminance bars and for stereobars showed a statistically significant relationship (r = 0.83, P < 0.01 for direction; r = 0.63, P < 0.01 for orientation). However, disparity‐sensitive cells from these areas seem to be more sensitive to luminance than to stereobars regarding orientation and direction of movement. Similar results were obtained when the two areas were considered separately. Our results show that cells in areas V1 and V2 of the monkey visual cortex are able to detect the orientation and direction of movement of stereobars in a manner similar to those of luminance‐defined bars. This finding is relevant because to detect the direction and orientation of stereobars a comparison between left and right eye inputs is required.

Temporal characteristics of visual receptive fields in primary visual cortex and medial superior temporal cortex areas

We mapped the receptive fields of 49 cells from primary visual cortex and 19 cells from medial superior temporal cortex in two awake monkeys. The receptive field structures we obtained lasted a mean time of 32.7 ms in primary visual cortex and 38.4 ms in medial superior temporal cortex, showing no statistical difference. This result suggests that both areas have the same time requirements for processing visual information. In primary visual cortex, 100% of cells had conformed the receptive field structure at 65 ms pre-spike, whereas in medial superior temporal cortex it occurred at 150 ms. In both areas, cells with shorter response latencies had receptive field structures with longer durations. This may indicate that cells tend to synchronize their output to other areas.

Visual Perception in Anterior Temporal Lobectomy

PURPOSE To study visual perception in patients with anterior temporal lobectomy. MATERIALS AND METHODS We explored some aspects of visual perception and compared the results obtained from 14 control subjects and 14 patients with unilateral anterior temporal lobectomy. Each group included 7 men and 7 women and the same age distribution (patients and controls: age range 27-48 years; mean 37 years). All subjects underwent a conventional ophthalmic examination and were tested for color perception, stereopsis, texture perception, face recognition, and visual illusions. To quantify color, stereoscopic, and texture perception they performed a visuomotor task that required a rapid response to a visual stimulus. Reaction times were measured under several conditions. RESULTS Mild visual field defects involving the superior quadrant contralateral to the lobectomy were found in five patients; two other patients presented more severe defects. Lobectomized patients showed a lower number of correct trials than normal subjects when performing tasks involving color and texture perception. These patients also had longer reaction times for color, stereoscopic, and texture stimulus detection. Face recognition and perception of illusory images were preserved after unilateral anterior temporal lobectomy. CONCLUSION Our data indicate that patients with anterior temporal lobectomy show moderate deficits in color, stereo, and texture perception, with no impairment in complex visual stimuli perception.

Orientation preference and horizontal disparity sensitivity in the monkey visual cortex

Purpose:  Disparity sensitivity may be explained by interocular positional differences of the receptive fields (RF) of visual cortical cells or by interocular shifts of the On and Off RF subregions. Since this latter model assumes shifts are orthogonal to the orientation of the RF, cells with disparity sensitivity should be oriented. The objective of the present study is to test this assumption.