Presence/absence-sample matching by pigeons: divergent retention functions may result from the similarity of behavior during the absence sample and the retention interval.

Divergent choose-absence retention functions typically found in pigeons following presence/absence-sample matching have been attributed to the development of a single-code/default coding strategy. However, such effects may result from adventitious differential responding to the samples. In Experiment 1, retention functions were divergent only when differential sample responding could serve as the basis for comparison choice. In Experiment 2, when pecking did not occur during the retention interval, a choose-absence bias was found, but when pecking occurred during the retention interval, a choose-presence bias resulted. In Experiment 3, positive transfer was found when a stimulus associated with the absence of pecking replaced the absence sample but not when a stimulus associated with pecking replaced the presence sample. Thus, presence/absence-sample matching may not encourage the development of a single-code/default coding strategy in pigeons.

Episodic-like memory in pigeons.

It has been proposed that memory for personal experiences (episodic memory, rather than semantic memory) relies on the conscious review of past experience and thus is unique to humans. In an attempt to demonstrate episodic-like memory in animals, we first trained pigeons to respond to the (nonverbal) question "Did you just peck or did you just refrain from pecking?" by training them on a symbolic matching task with differential responding required to the two line-orientation samples and reinforcing the choice of a red comparison if they had pecked and the choice of a green comparison if they had not pecked. Then, in Experiment 1, after providing the conditions for (but not requiring) the pigeons to peck at one new stimulus (a yellow hue) but not at another (a blue hue), we tested them with the new hue stimuli and the red and green comparisons. In Experiment 2, we tested the pigeons with novel stimuli (a circle, which they spontaneously pecked, and a dark response key, which they did not peck) and the red and green comparisons. In both experiments, pigeons chose the comparison appropriate to the response made to the test stimulus. Thus, the pigeons demonstrated that they could remember specific details about their past experiences, a result consistent with the notion that they have the capacity for forming episodic-like memories.

"Work ethic" in pigeons: reward value is directly related to the effort or time required to obtain the reward.

Stimuli associated with less effort or with shorter delays to reinforcement are generally preferred over those associated with greater effort or longer delays to reinforcement. However, the opposite appears to be true of stimuli that follow greater effort or longer delays. In training, a simple simultaneous discrimination followed a single peck to an initial stimulus (S+FR1 S-FR1) and a different simple simultaneous discrimination followed 20 pecks to the initial stimulus (S+FR20 S-FR20). On test trials, pigeons preferred S+FR20 over S+FR1 and S-FR20 over S-FR1. These data support the view that the state of the animal immediately prior to presentation of the discrimination affects the value of the reinforcement that follows it. This contrast effect is analogous to effects that when they occur in humans have been attributed to more complex cognitive and social factors.

Delayed matching in pigeons: can apparent memory loss be attributed to the delay of reinforcement of sample-orienting behavior?

In earlier research using constant-delay matching with pigeons, there is evidence that delay of reinforcement of sample-orienting behavior may contribute to the decline in matching accuracy with increasing delay between sample and comparison stimuli. In the present research using this procedure, we found that a significant decline in matching accuracy between the first and second session can occur when delays are relatively long. This effect cannot be accounted for in terms of either additional memory loss or surprise (generalization decrement) associated with the increase in delay. Furthermore, the decline in matching accuracy occurred regardless of whether the delay was inserted between samples and comparisons (where it would be expected to affect the use of sample memory in making the comparison choice response) or between comparisons and reinforcement (where it would not be expected to affect the use of sample memory in making the comparison choice response). Thus, the decrease in matching accuracy between Session 1 and 2 following an increase in delay appears to be unrelated to sample memory at the time of choice. Instead, the results suggest that delay of reinforcement of sample-orienting behavior may play an important role in the negative slope of the retention functions obtained when constant- or mixed-delay matching procedures are used to assess animal memory.

Development of a single-code/default coding strategy in pigeons.

We tested the hypothesis that pigeons could use a cognitively efficient coding strategy by training them on a conditional discrimination (delayed symbolic matching) in which one alternative was correct following the presentation of one sample (one-to-one), whereas the other alternative was correct following the presentation of any one of four other samples (many-to-one). When retention intervals of different durations were inserted between the offset of the sample and the onset of the choice stimuli, divergent retention functions were found. With increasing retention interval, matching accuracy on trials involving any of the many-to-one samples was increasingly better than matching accuracy on trials involving the one-to-one sample. Furthermore, following this test, pigeons treated a novel sample as if it had been one of the many-to-one samples. The data suggest that rather than learning each of the five sample-comparison associations independently, the pigeons developed a cognitively efficient single-code/default coding strategy.

Single-shot measurement of the amplitude and phase of ultrashort laser pulses in the violet.

Using single-shot, self-diffraction, frequency-resolved optical gating, we measure the complete electric field amplitude and phase of 405-nm second-harmonic pulses from an amplified Ti:sapphire system. The single-shot frequency-resolved optical gating device gives both qualitative and quantitative information, which is useful for analyzing and optimizing the grating compressor in the chirped-pulse amplification system.

Determination of n2 by direct measurement of the optical phase.

We determine the nonlinear refractive index, n(2), in an optical material through the direct measurement of the phase of an ultrashort optical pulse, using the technique of frequency-resolved optical gating. This method results in the accurate measurement of n(2), with the error dominated by the uncertainty in the fluence measurement. We measure n(2) in fused silica and in potassium dihydrogen phosphate at 804 and 402 nm. These results are consistent with those from previous measurements of n(2) in these materials, and the measured dispersion of n(2) in fused silica agrees with theoretical predictions.

Simultaneous discrimination learning in pigeons: value of S- affects the relative value of its associated S+.

In a simple simultaneous discrimination involving a positive stimulus (S+) and a negative stimulus (S-), it has been hypothesized that positive value can transfer from the S+ to the S- (thus increasing the relative value of the S-) and also that negative value can transfer from the S- to the S+ (thus diminishing the relative value of the S+; Fersen, Wynne, Delius, & Staddon, 1991). Evidence for positive value transfer has been reported in pigeons (e.g. Zentall & Sherburne, 1994). The purpose of the present experiments was to determine, in a simultaneous discrimination, whether the S- diminishes the value of the S+ or the S- is contrasted with the S+ (thus enhancing the value of the S+). In two experiments, we found evidence for contrast, rather than value transfer, attributable to simultaneous discrimination training. Thus, not only does the S+ appear to enhance the value of the S-, but the S- appears to enhance rather than reduce the value of the S+.

Amplitude and phase measurements of femtosecond pulse splitting in nonlinear dispersive media.

Frequency-resolved optical gating is used to characterize the propagation of intense femtosecond pulses in a nonlinear, dispersive medium. The combined effects of diffraction, normal dispersion, and cubic nonlinearity lead to pulse splitting. The role of the phase of the input pulse is studied. The results are compared with the predictions of a three-dimensional nonlinear Schrödinger equation.

Dispersion measurements of water with white-light interferometry.

We measure the second- and third-order dispersion coefficients, d(2)k/domega(2) and d(3)k/domega(3), of water for wavelengths from 0.45 to 1.3 mum using a Michelson white-light interferometer. In this interval, the second-order dispersion ranges from 0.068 to -0.1 fs(2)/mum, and the third-order dispersion ranges from 0.048 to 1.18 fs(3)/mum. We observe an oscillation in d(2)k/domega(2) near 1.1 mum that is due to water absorption features near that wavelength. From the dispersion coefficients, derivatives of the index of refraction of water are calculated and compared with available equations. These measured values of d(2)n/dlambda(2) and d(3)n/dlambda(3) should be useful in the evaluation and improvement of existing equations for n(lambda) in water.