Biomedical subjects

Tatiana Pasternak

Publications and source records attributed to Tatiana Pasternak.

6 recordsLinked to original sources

PubMed2006

Directional signals in the prefrontal cortex and in area MT during a working memory for visual motion task.

Neurons in the middle temporal visual area (MT) have been implicated in the perception of visual motion, whereas prefrontal cortex (PFC) neurons have been linked to temporary storage of sensory signals, attentional and executive control of behavior. Using a task that placed demands on both sets of neurons, we investigated their contribution to working memory for visual motion. Monkeys compared the direction of two moving random-dot stimuli, sample and test, separated by a brief memory delay. Neurons in both areas showed robust direction-selective activity during all phases of the task. During the sample, approximately 60% of task-related PFC neurons were direction selective, and this selectivity emerged 40 ms later than in MT. Unlike MT, the PFC responses to sample did not correlate with behavioral choices, but their selectivity was modulated by task demands and diminished on error trials. Reliable directional signals were found in both areas during the memory delay, but these signals were transient rather than sustained by neurons of either area. Responses to the test in both areas were modulated by the remembered sample direction, decreasing when the test direction matched the sample. This decrease arose in the PFC 100 ms later than in MT and was predictive of the forthcoming decision. Our data suggest that neurons in the two regions are functionally connected and make unique contributions to different task components. PFC neurons reflect task-related information about visual motion and represent decisions that may be based, in part, on the comparison in MT between the remembered sample and test.

Action Potentials↗

PubMed2005

Area MT neurons respond to visual motion distant from their receptive fields.

Neurons in cortical area MT have localized receptive fields (RF) representing the contralateral hemifield and play an important role in processing visual motion. We recorded the activity of these neurons during a behavioral task in which two monkeys were required to discriminate and remember visual motion presented in the ipsilateral hemifield. During the task, the monkeys viewed two stimuli, sample and test, separated by a brief delay and reported whether they contained motion in the same or in opposite directions. Fifty to 70% of MT neurons were activated by the motion stimuli presented in the ipsilateral hemifield at locations far removed from their classical receptive fields. These responses were in the form of excitation or suppression and were delayed relative to conventional MT responses. Both excitatory and suppressive responses were direction selective, but the nature and the time course of their directionality differed from the conventional excitatory responses recorded with stimuli in the RF. Direction selectivity of the excitatory remote response was transient and early, whereas the suppressive response developed later and persisted after stimulus offset. The presence or absence of these unusual responses on error trials, as well as their magnitude, was affected by the behavioral significance of stimuli used in the task. We hypothesize that these responses represent top-down signals from brain region(s) accessing information about stimuli in the entire visual field and about the behavioral state of the animal. The recruitment of neurons in the opposite hemisphere during processing of behaviorally relevant visual signals reveals a mechanism by which sensory processing can be affected by cognitive task demands.

Action Potentials↗

PubMed2005

Working memory in primate sensory systems.

Sensory working memory consists of the short-term storage of sensory stimuli to guide behaviour. There is increasing evidence that elemental sensory dimensions - such as object motion in the visual system or the frequency of a sound in the auditory system - are stored by segregated feature-selective systems that include not only the prefrontal and parietal cortex, but also areas of sensory cortex that carry out relatively early stages of processing. These circuits seem to have a dual function: precise sensory encoding and short-term storage of this information. New results provide insights into how activity in these circuits represents the remembered sensory stimuli.

Animals↗

PubMed2004

Training-induced recovery of visual motion perception after extrastriate cortical damage in the adult cat.

Unilateral ibotenic acid lesions of the lateral suprasylvian (LS) cortex severely impair the ability of cats to integrate local motion signals (measured as direction range thresholds) and to extract motion signals from noise (measured as motion signal thresholds) in their contra-lesional visual hemifields. These deficits were found up to several months after the lesions and were limited to thresholds measured with random-dot stimuli, while contrast sensitivity for discriminating the direction of motion of sine-wave gratings remained unaffected. Our goal was to determine whether deficits of complex motion perception could recover and whether the recovery was spontaneous or required retraining. In each cat, a single location in the impaired visual hemifield was selected for visual retraining, which required the animals to discriminate motion direction using random-dot stimuli in which the range of dot directions was varied. Fifteen to 40 days of intensive retraining led to a gradual, complete recovery of motion integration. The recovery was stimulus specific since it did not transfer from direction range to motion signal thresholds, and it was largely restricted to the visual field locations retrained. Delaying the onset of retraining by several days to several months had no significant impact on the extent or rate of recovery. Once recovery was achieved, performance remained stable over a period of several months. These results suggest that recovery of complex visual motion perception after lesions of extrastriate visual cortex is an active process that requires extensive, stimulus- and retinotopically-specific visual retraining.

Animals↗

PubMed2003

Activity of neurons in cortical area MT during a memory for motion task.

We recorded the activity of middle temporal (MT) neurons in 2 monkeys while they compared the directions of motion in 2 sequentially presented random-dot stimuli, sample and test, and reported them as the same or different by pressing one of 2 buttons. We found that MT neurons were active not only in response to the sample and test stimuli but also during the 1,500-ms delay separating them. Most neurons showed a characteristic pattern of activity consisting of a small burst of firing early in the delay, followed by a period of suppression and a subsequent increase in firing rate immediately preceding the presentation of the test stimulus. In a third of the neurons, the activity early in the delay not only reflected the direction of the sample stimulus, but was also related to the range of local directions it contained. During the middle of the delay the majority of neurons were suppressed, consistent with a gating mechanism that could be used to ignore task-irrelevant stimuli. Late in the delay, most neurons showed an increase in response, probably in anticipation of the upcoming test. Throughout most of the delay there was a directional signal in the population of MT neurons, manifested by higher firing rates following the sample moving in the antipreferred direction. Whereas some of these effects may be related to sensory adaptation, others are more likely to represent a more active task-related process. These results support the hypothesis that MT neurons actively participate in the successful execution of all aspects of the task requiring processing and remembering visual motion.

Action Potentials↗

PubMed2003

Stimulus specificity and temporal dynamics of working memory for visual motion.

When asked to compare two moving stimuli separated by a delay, observers must not only identify stimulus direction but also store it in memory. We examined the properties of this storage mechanism in two macaque monkeys by sequentially presenting two random-dot stimuli, sample and test, in opposite hemifields and introducing a random-motion mask during the delay. The mask interfered with performance only at the precise location of the test, 100-200 ms after the start of the delay, and when its size and speed matched those of the remembered sample. This selective interference suggests that the representation of the motion stimulus in memory preserves its direction, speed, and size and is most fragile shortly after the completion of the encoding phase of the task. This precise preservation of sensory attributes of the motion stimulus suggests that the neural mechanisms involved in the processing of visual motion may also be involved in its storage.

Animals↗