Biomedical subjects

Rodrigo F Salazar

Publications and source records attributed to Rodrigo F Salazar.

4 recordsLinked to original sources

PubMed2004

NMDA lesions in the medial prefrontal cortex impair the ability to inhibit responses during reversal of a simple spatial discrimination.

Although lesion studies suggest that the rat medial prefrontal cortex (mPFc) is involved in the process necessary for reversal of a particular set of contingencies, the nature of lesion-induced deficits is unclear. The involvement of rat mPFc in reversal of a simple spatial discrimination was examined in the present study. Our hypothesis was that lesion-induced deficits may reflect a failure to inhibit a learned instrumental response. Lister Hooded rats were trained on a spatial discrimination task (SD), which required a correct barpress matching the cue location, then they were trained on reversal of SD (SDR), which required a correct barpress opposite to the cue location. Rats with mPFc lesions showed a slower learning rate compared to the controls. However, behavior of the lesioned rats during early and later reversal differed. During the initial SDR, the lesioned rats showed a greater number of barpresses during the intertrial interval and a slightly higher percent correct responses than that of the controls. Our data suggest that damage to mPFc may produce a lack of response inhibition, leading to an increase in nondiscriminated bapresses, thereby yielding a 'facilitation' during early reversal. mPFc lesion did not affect either open field activity or prepulse inhibition (PPI), a frequently used measure of sensorimotor gating. Disruption of reversal learning following damage to mPFc is partly due to a failure to inhibit instrumental responses, rather than to disruption of other processes involved in sensorimotor gating or general activity.

Analysis of Variance↗

PubMed2004

Effects of training on neuronal activity and interactions in primary and higher visual cortices in the alert cat.

The effects of behavioral training on early visual representations have been elusive when assessed with firing rates. Learning-induced changes in performance, however, suggest that representations should encompass early cortical stages. Here, we address the question of whether training-induced effects are pertinent to neuronal activity outside the task proper, which is a requirement if subsequent perceptional processes should profit from training. To search for a neuronal signature of training effects beyond firing rates, we measured local field potentials, multiunit and isolated spike activity during passive viewing of previously learned stimulus response associations (S+ and S-) in areas 17/18 and 21a of two alert cats. Evoked potential responses as well as gamma oscillations even during the first 200 msec were found to be stronger for S+ in both areas. Most importantly, the later parts of the response (>200 msec) not only exhibit a highly significant difference in coherent gamma oscillations for S+ and S- both within and across areas, but are also characterized by a pronounced preference in firing rate for S+ in area 21a, whereas primary cortex shows a nonsignificant trend for weaker spike responses. From these results, we conclude that training-induced plasticity occurs in adult visual cortex for behaviorally relevant stimuli by changing primarily the temporal structure of neuronal activity at early stages of cortical processing, whereas later stages of cortical processing express the increased coherence of their input in elevated firing rates.

Action Potentials↗

PubMed2004

Directed interactions between visual areas and their role in processing image structure and expectancy.

During sensory processing, cortical areas continuously exchange information in different directions along the hierarchy. The functional role of such interactions, however, has been the subject of various proposals. Here, we investigate the role of bottom-up and top-down interactions in processing stimulus structure and their relation to expected events. Applying multivariate autoregressive methods to local field potentials recorded in alert cats, we quantify directed interactions between primary (A17/18) and higher (A21) visual areas. A trial-by-trial analysis yields the following findings. To assess the role of interareal interactions in processing stimulus structure, we recorded in naïve animals during stimulation with natural movies and pink noise stimuli. The overall interactions decrease compared with baseline for both stimuli. To investigate whether forthcoming events modulate interactions, we recorded in trained animals viewing two stimuli, one of which had been associated with a reward. Several results support such modulations. First, the interactions increase compared with baseline and this increase is not observed in a context where food was not delivered. Second, these stimuli have a differential effect on top-down and bottom-up components. This difference is emphasized during the stimulus presentation and is maximal shortly before the possible reward. Furthermore, a spectral decomposition of the interactions shows that this asymmetry is most dominant in the gamma frequency range. Concluding, these results support the notion that interareal interactions are more related to an expectancy state rather than to processing of stimulus structure.

Animals↗

PubMed2003

Responses to natural scenes in cat V1.

Studies on processing in primary visual areas often use artificial stimuli such as bars or gratings. As a result, little is known about the properties of activity patterns for the natural stimuli processed by the visual system on a daily basis. Furthermore, in the cat, a well-studied model system for visual processing, most results are obtained from anesthetized subjects and little is known about neuronal activations in the alert animal. Addressing these issues, we measure local field potentials (lfp) and multiunit spikes in the primary visual cortex of awake cats. We compare changes in the lfp power spectra and multiunit firing rates for natural movies, movies with modified spatio-temporal correlations as well as gratings. The activity patterns elicited by drifting gratings are qualitatively and quantitatively different from those elicited by natural stimuli and this difference arises from both spatial as well as temporal properties of the stimuli. Furthermore, both local field potentials and multiunit firing rates are most sensitive to the second-order statistics of the stimuli and not to their higher-order properties. Finally, responses to natural movies show a large variability over time because of activity fluctuations induced by rapid stimulus motion. We show that these fluctuations are not dependent on the detailed spatial properties of the stimuli but depend on their temporal jitter. These fluctuations are important characteristics of visual activity under natural conditions and impose limitations on the readout of possible differences in mean activity levels.

Action Potentials↗