PubMed Health⌕ Search

Biomedical subjects

P Pereyra

Publications and source records attributed to P Pereyra.

6 recordsLinked to original sources
PubMed2000

Closed formulas for tunneling time in superlattices

New formulas for an exact and simple evaluation of the phase time associated with the passage of electrons or photons through a finite superlattice are derived. This time, named here the superlattice-tunneling time tau(n), exhibits a resonant-band structure or a superluminal phase time behavior depending on whether the particle's energy lies in a band or a gap. In the band, tau(n) remains larger than the free motion time tau(f), while in the gap it can be less than tau(f) (with strong substrate effects), but it is larger than the single-cell phase time tau(1). Extremely good agreements with optical-pulse and superluminal delay times measured by Spielmann et al. and Steinberg et al. are found, including the superlattice-tunneling-time limit and the substrate effects. Conditions for measurements of earlier electron arrival are also analyzed.

Journal Article↗
PubMed2000

Long-lasting and context-specific freezing preference is acquired after spaced repeated presentations of a danger stimulus in the crab Chasmagnathus.

A visual danger stimulus elicits an escape response in the crab Chasmagnathus that declines after repeated presentations. Previous results report that such waning may be retained as context-signal memory (CSM) or signal memory (SM): CSM is long lasting, associative, and produced by spaced training, while SM is an intermediate memory, nonassociative, and produced by massed training. The performances of both spaced and massed trained crabs are here examined, using video analysis to determine topographic changes in the behavioral response during and after training. During spaced training, escape vanishes and is mainly replaced by freezing, while during massed training, escape decreases over trials without being replaced by any defensive response. After 24 h, the marked proclivity to freezing persists in spaced trained crabs, while a high level of escaping is shown by massed trained crabs. The long-lasting freezing preference of spaced trained crabs proves to be context-specific and apparent from the very first presentation of the danger stimulus at testing, though freezing is not triggered by the sole exposure to the context. We conclude (a) that freezing preference is the acquired response of the CSM process; (b) that CSM can be properly categorized as an instance of contextual conditioning and SM of classical habituation; (c) that CSM and SM are not two phases of a memory processing but two distinctly types of memory; and (d) that therefore, the temporal distribution of training trials has a drastic effect on crab's memory, more dramatic than that previously described. The possibility that massed and spaced presentations of the same stimulus may represent two different stimulus types is discussed.

Animals↗
PubMed1996

Training-to-testing intervals different from 24 h impair habituation in the crab Chasmagnathus.

A shadow moving overhead elicits an escape response in the crab Chasmagnathus that habituates promptly and for a long period. Experiments were done to test the effects of time of day (light-phase vs. dark-phase) on the acquisition and retention of the habituated response. The short-term habituation produced by the repetitive presentation of the stimulus does not differ between the two phases of the day though their reactivity during training seems to be higher during the dark phase than during the light, in agreement with the peak of circadian locomotor activity. The magnitude of the long-term habituated response, tested 24 or 72 h after training, does not appear to depend either on the time of day of training or on that of testing, but the retention is impaired when testing is conducted at a time of day that differs from that of the original training. Thus, results indicate a) that habituation of a response to a stimulus presented during the dark phase is not generalized to the same stimulus presented during the light phase, and vice versa; and b) that during training not only information regarding the target stimulus is stored but also information about the phase of the day.

Animals↗