Biomedical subjects

Tanya Orlov

Publications and source records attributed to Tanya Orlov.

4 recordsLinked to original sources

PubMed2006

Normal and abnormal fMRI activation patterns in the visual cortex after recovery from optic neuritis.

Recovery to normal or near normal visual acuity after an optic neuritis episode is common, despite frequent persistence of conduction abnormalities, evident in prolonged visual evoked potential (VEP) latencies. Improvement of visual function is commonly attributed to peripheral nerve recovery. However, central reorganization processes may also be involved. To assess this, we compared the patterns of fMRI activation, elicited by stimulation of the affected and the normal eye, along the visual cortical hierarchy. Activation was assessed in 8 subjects, which recovered clinically from an episode of optic neuritis but still had prolonged VEP latencies. In all patients, reduced fMRI activation was seen in V1 during stimulation of the affected eye, compared to the normal eye. The fMRI signal difference decreased in magnitude with progression along the visual hierarchy, and in some regions within the lateral occipital complex even showed the opposite preference (for the affected eye). These results may indicate a built-in robustness of the object-related areas to disruption of the visual input. Alternatively, it could reflect an adaptive functional reorganization of the cortical response to an abnormal input.

Adult↗

PubMed2006

Memory of ordinal number categories in macaque monkeys.

What mechanism underlies serial order memory? Studying preverbal serial memory shows that macaque monkeys reproducing a sequence of items can acquire knowledge of item ordinal position. In our previous experiment, macaques were repeatedly presented with image lists (first shown sequentially and then simultaneously on a touch screen together with a distractor chosen randomly from other lists). The task was to touch list images in the correct order. The monkeys' natural tendency was to categorize images by their ordinal position or number because their most common error was touching the distractor when it had the same ordinal number (in its own list) as the correct image. Item-to-item associations were used to complete the categorization strategy. Proposing a dynamic image-salience hypothesis for serial recall (based on category-to-image influence and a salience computation for identifying touch targets), we now study the category label characteristics in the context of this hypothesis. We found that these category labels are absolute, ordinal-number-based categories (first, second, etc.), not relative memorized as relative distance from the beginning and the end of the list, and not based on fixed ranking of reward contingency/image familiarity. Even isolated from item-item associations, the categories demonstrate category tuning (as well as the corresponding overlap of adjacent ordinal number codes). Moreover, monkeys choose images by proximity of their category to the current touch number, irrespective of the accuracy of the preceding choice. Category tuning itself is symmetric relative to correct ordinal position, but is skewed by other factors (reward, etc.). Tuning width increases with list length, with a concurrent increased use of item-to-item associations for determining touch order.

Animals↗

PubMed2004

Multi-item working memory--a behavioral study.

Macaque monkeys were trained to recognize the repetition of one of the images already seen in a sequence of random length. On average, performance decreased with sequence length. However, this was due to a complex combination of factors, as follows: performance was found to decrease with the separation in the sequence of the test (repetition image) from the cue (its first appearance in the sequence), for trials with sequences of fixed length. In contrast, performance improved as a function of sequence length, for equal cue-test separations. Reaction times followed a complementary trend: they increased with cue-test separation and decreased with sequence length. The frequency of false positives (FPs) indicates that images are not always removed from working memory between successive trials, and that the monkeys rarely confuse different images. The probability of miss errors depends on number of intervening stimulus presentations, while FPs depend on elapsed time. A simple two-state stochastic model of multi-item working memory is proposed that guides the account for the main effects of performance and false positives, as well as their interaction. In the model, images enter WM when they are presented, or by spontaneous jump-in. Misses are due to spontaneous jump-out of images previously seen.

Animals↗

PubMed2002

Serial memory strategies in macaque monkeys: behavioral and theoretical aspects.

Serial memory is the ability to encode and retrieve a list of items in their correct temporal order. To study nonverbal strategies involved in serial memory, we trained four macaque monkeys on a novel delayed sequence-recall task and analysed the mechanisms underlying their performance in terms of a neural network model. Thirty fractal images, divided into 10 triplets, were presented repeatedly in fixed temporal order. On each trial the monkeys viewed three sequentially presented sample images, followed by a test stimulus consisting of the same triplet of images and a distractor image (chosen randomly from the remaining 27). The task was to touch the three images in their original order, avoiding the distractor. The monkeys' most common error was touching the distractor when it had the same ordinal position (in its own triplet) as the correct image. This finding suggests that monkeys naturally categorize images by their ordinal number. Additional, secondary strategies were eventually used to avoid distractor images. These include memory of the sample images (working memory) and associations between triplet members. Further direct evidence for ordinal number categorization was provided by a transfer of learning to untrained images of the same ordinal category, following reassignment of image categories within each triplet. We propose a generic three-tier neuronal framework that can explain the components and complex set of characteristics of the observed behavior. This framework, with its intermediate level representing ordinal categories, can also explain the transfer of learning following category reassignment.

Animals↗