Convergence of temporal and spectral information into acoustic images of complex sonar targets perceived by the echolocating bat,Eptesicus fuscus

1. FM echolocating bats (Eptesicus fuscus) were trained to discriminate between a two-component complex target and a one-component simple target simulated by electronically-returned echoes in a series of experiments that explore the composition of the image of the two-component target. In Experiment I, echoes for each target were presented sequentially, and the bats had to compare a stored image of one target with that of the other. The bats made errors when the range of the simple target corresponded to the range of either glint in the complex target, indicating that some trace of the parts of one image interfered with perception of the other image. In Experiment II, echoes were presented simultaneously as well as sequentially, permitting direct masking of echoes from one target to the other. Changes in echo amplitude produced shifts in apparent range whose pattern depended upon the mode of echo presentation. 2. Eptesicus perceives images of complex sonar targets that explicitly represent the location and spacing of discrete glints located at different ranges. The bat perceives the target's structure in terms of its range profile along a psychological range axis using a combination of echo delay and echo spectral representations that together resemble a spectrogram of the FM echoes. The image itself is expressed entirely along a range scale that is defined with reference to echo delay. Spectral information contributes to the image by providing estimates of the range separation of glints, but it is transformed into these estimates. 3. Perceived absolute range is encoded by the timing of neural discharges and is vulnerable to shifts caused by neural amplitude-latency trading, which was estimated at 13 to 18 microseconds per dB from N1 and N4 auditory evoked potentials in Eptesicus. Spectral cues representing the separation of glints within the target are transformed into estimates of delay separations before being incorporated into the image. However, because they are encoded by neural frequency tuning rather than the time-of-occurrence of neural discharges, the perceived range separation of glints in images is not vulnerable to amplitude-latency shifts. 4. The bat perceives an image that is displayed in the domain of time or range. The image receives no evident spectral contribution beyond what is transformed into delay estimates. Although the initial auditory representation of FM echoes is spectrogram-like, the time, frequency, and amplitude dimensions of the spectrogram appear to be compressed into an image that has only time and amplitude dimensions.(ABSTRACT TRUNCATED AT 400 WORDS)     

Evidence for perception of fine echo delay and phase by the FM bat,Eptesicus fuscus

The big brown bat, Eptesicus fuscus, can perceive small changes in the delay of FM sonar echoes and shifts in echo phase, which interact with delay. Using spectral cues caused by interference, Eptesicus also can perceive the individual delays of two overlapping FM echoes at small delay separations. These results have been criticized as due to spectral artifacts caused by overlap between stimulus echoes and extraneous sounds (Pollak 1993). However, no amplitude or spectral variations larger than 0.05 dB accompany delay or phase changes produced by the electronic apparatus. No reverberation falls in the narrow span of delays required to produce the bat's performance curve from echo interference cues. Consistent differences in the durations of sonar sounds for 6 bats that perform the same in the experiments demonstrate that overlap between stimulus echoes and extraneous echoes is not necessary, and changes in the amount of echo overlap have no effect on performance. Noise-induced random variations in echo spectra outweigh putative spectral artifacts, and deliberately-introduced spectral artifacts do not improve performance overall but instead yield new time-frequency images. Amplitude-latency trading of perceived delay, proposed as a demonstration that the latency of neural discharges encodes delay (Pollak et al. 1977), confirms that the bat's fine delay and phase perception depends on a temporal neural code. The perceived delays depend on stimulus delays, not the delays of extraneous sounds. The rejected criticisms are based on physiological results with random-phase FM stimuli which are irrelevant to neural coding of fine echo delay and phase.The contents of this paper first appeared in October 1990 in a letter to G.D. Pollak in response to his unpublished criticisms of echo-jitter experiments. These responses also have been presented at the 1991 and 1992 Association for Research in Otolaryngology midwinter meetings and at the 1992 3rd International Congress of Neuroethology. Several of the control experiments also appeared in Simmons et al. (1990b). The now-published criticisms (Pollak 1993, the preceding paper) have not addressed these responses, including the prior published data demonstrating that the stimulus conditions asserted by these criticisms do not in fact occur.     

Echo delay versus spectral cues for temporal hyperacuity in the big brown bat,Eptesicus fuscus

Big brown bats can discriminate between echoes that alternate in delay (jitter) by as little as 10–15 ns and echoes that are stationary in delay. This delay hyperacuity seems so extreme that it has been rejected in favor of an explanation in terms of artifacts in echoes, most likely spectral in nature, that presumably are correlated with delay. Using different combinations of digital, analog, and cable delays, we dissociated the overall delay of jittering echoes from the size of the analog component of delay, which alone is presumed to determine the strength of the apparatus artifact. The bats' performance remains invariant with respect to the overall delay of the jittering echoes, not with respect to the amount of analog delay. This result is not consistent with the possible use of delay-related artifacts produced by the analog delay devices. Moreover, both electronic and acoustic measurements disclose no spectral cues or impedance-mismatch reflections in delayed signals, just time-delays. The absence of artifacts from the apparatus and the failure of overlap and interference from reverberation to account for the 10-ns result means that closing the gap between the level of temporal accuracy plausibly explained from physiology and the level observed in behavior may require a better understanding of the physiology.Abbreviations FM frequency-modulated - XCR cross-correlation function     

Phase sensitivity in bat sonar revisited

An echolocating bat produces echoes consisting of the convolution of echolocation call and the impulse response (IR) of the ensonified object. A crucial question in animal sonar is whether bats are able to extract this IR from the echo. The bat inner ear generates a frequency representation of call and echo and IR extraction in the frequency domain requires accurate analysis of both magnitude and phase information. Previous studies investigating the phase sensitivity of bats using a jitter paradigm reported a temporal acuity down to 10 ns, suggesting perfect sonar phase representation. In a phantom-target playback experiment, we investigate the perceptual phase sensitivity of the bat Phyllostomus discolor using a novel approach: instead of manipulating IR phase by changing IR delay (jitter paradigm), we randomized IR phase and thus lengthened the IR over time, leaving the magnitude spectrum unchanged. Our results show that phase sensitivity, as reflected in the analysis of signal duration, appears to be much lower than phase sensitivity, as reflected in the analysis of signal onset. The current data indicate that different temporal aspects of sonar processing are encoded with very different temporal resolution and thus an overall claim of “phase sensitivity” as such cannot be maintained.     

Role of broadcast harmonics in echo delay perception by big brown bats

Big brown bats (Eptesicus fuscus) emit frequency-modulated (FM) echolocation sounds containing two principal down-sweeping harmonics (FM₁ ~ 55–25 kHz, FM₂ ~ 105–50 kHz). To determine whether each harmonic contributes to perception of echo delay, bats were trained to discriminate between “split-harmonic” echoes that differed in delay. The bat’s broadcasts were picked up with microphones, and FM₁ and FM₂ were separated with highpass and lowpass filters at about 55 kHz, where they overlap in frequency. Both harmonics then were delivered from loudspeakers as positive stimuli in a 2-choice delay discrimination procedure with FM₁ delayed 3.16 ms and FM₂ delayed 3.46 ms (300 μs delay split). Negative stimuli contained FM₁ and FM₂ with the same filtering but no delay separation. These were presented at different overall delays from 11 down to 3 ms to measure the bat’s delay discrimination acuity for each harmonic in the split harmonic echoes. The bats determined the delays of both FM₁ and FM₂, but performance was overlaid by a broad pedestal of poor performance that extended for 800 μs. Splitting the harmonics by 300 μs appears to defocus the bat’s representation of delay, revealing the existence of a process for recognizing the normally simultaneous occurrence of the harmonics.     

Phase evaluation in hypothetical receivers simulating ranging in bats

Echo delay discrimination by the bat Eptesicus fuscus had been investigated in an experiment with simulated targets jittering in range (Simmons 1979). The dip in the resulting psychometric curve was used by Simmons to suggest the neuronal implementation of a coherent cross-correlation receiver in the auditory system of bats. By computer simulation it is shown here that the dip may be even more pronounced and less susceptible to noise with alternative receiver configurations which not necessarily evaluate signal phase information coherently, e.g., a bank of neuronal filters with envelope-processing. New behavioral experiments are suggested to critically test such model hypotheses.     

Echo SPL, training experience, and experimental procedure influence the ranging performance in the big brown bat, Eptesicus fuscus

Four Eptesicus fuscus were trained in a range discrimination experiment to choose the closer of two phantom targets. Echo attenuation was roving between trials returning echoes ranging from −10 dB to −50 dB SPL (sound pressure level) relative to emission SPL. Discrimination thresholds were determined. After sufficient training, ranging performance was stable and about the same in the range between −20 dB and −50 dB with range difference thresholds around 300 μs. At −10 dB, performance was poor even after long training. After additional training at a constant relative echo SPL of −30 dB and a delay difference of 300 μs the performance measured with roving echo SPL improved at all relative echo SPL between −20 dB and −50 dB but not at −10 dB. The new experimental procedure improved the performance by additional learning, and the bats generalized over a wide range of relative echo SPL. Threshold improved to 100 μs when measured at a constant relative echo SPL of −30 dB, again indicating the influence of the experimental procedure. In correspondence to neurophysiological data the ranging performance deteriorates if the echo SPL is close to the emission SPL. Signal duration and emission SPL were variable during range discrimination. Accepted: 7 March 1998     

Echo SPL influences the ranging performance of the big brown bat,Eptesicus fuscus

Four bats of the species Eptesicus fuscus were trained in a two-alternative forced-choice procedure to discriminate between two phantom targets that differed in range. The rewarded stimulus was located at a distance of 52.7 cm, while the other unrewarded stimulus was further away. Only one target was presented at a time.In the first experiment we measured the range discrimination performance at an echo SPL of –28 dB relative to the bat's sonar transmission. A 75% correct performance level was arbitrarily defined as threshold and was obtained at a delay difference of 80 s, corresponding to a range difference of 13.8 mm.In the second experiment the delay difference was fixed at 150 s and the echo SPL varied between –8 and –48 dB relative to sonar emissions. The performance of the bats depended on the relative echo SPL. At –28 dB the bats showed the best performance. It deteriorated at an increase of the relative echo SPL to –18 dB and –8 dB. The performance also deteriorated when the relative echo SPL was reduced to –38 dB and –48 dB. Only at low relative echo SPLs did the bats partially compensate for the reduction in echo SPL and increased the SPL of their emitted signals by a few dB.Our results support the hypothesis that neurons exhibiting paradoxical latency shift may be involved in encoding target range. This hypothesis predicts a decrease in performance at high echo SPLs as we found it in our experiments. The observed reduction in performance at very low echo SPLs may be due to a decrease in S/N ratio.     

Target detection and range resolution by the big brown bat (Eptesicus fuscus) using normal and time-reversed model echoes

Summary Big brown bats (Eptesicus fuscus) were tested for their ability to detect an electronically simulated target, and to discriminate differences in range to two simulated targets, when receiving either a model of their own sonar emissions or the model reversed in time as the echo. The theory of matched detection predicts a large decrease in performance if bats use matched filtering, unless they are somehow able to adjust their filter to match the novel, time-reversed signal. The detection thresholds we obtained were much lower than Møhl's (1986), but like him we found no difference in threshold for reversed models (Table 2). This suggests either that bats do not use matched filtering for target detection, or, possibly, that they are able to adapt their filter to a highly unnatural signal in some way as yet unknown.Unlike detection, range discrimination was much poorer with reversed echoes (Table 3). Threshold increased from about 1 cm range difference with normal model echoes to 18 cm or more with reversed model echoes. This suggests that range determination, which is based on measuring the time of arrival of echoes, does involve matched filtering. Whether such filtering is ideal (i.e., equivalent to cross-correlation detection) cannot be decided by our results, but there are some indications that the match between an echo and the presumed internal template (the match of matched filtering) must be fairly precise. Also, since performance with phantom targets generated using model echoes was as good as has been found with real targets, the internal template is probably fixed (or only slowly modifiable) rather than re-programmed with each sonar emission. Finally, because synchronization of emission and model echo was not perfect, the apparent distance to targets probably varied by 2 to 4 cm from emission to emission, although both targets would appear to move together thus keeping the range difference constant. This suggests that bats determined range to the targets simultaneously rather than sequentially, as is usually assumed.Abbreviations BP bat-produced echo - FM frequency modulated - M _d detection model echo - M _d reversed detection model echo - M _rd range discrimination model echo - M _rd reversed range discrimination model echo - rms root mean square - SCR signal-to-clutter ratio - SNR signal-to-noise ratio - SPL sound pressure level - XCF crosscorrelation function     

Preference of a revolving target to a stationary one by the big brown bat, Eptesicus fuscus

Utilizing a three-ramp platform, we studied the detection of a revolving and a stationary target in the presence of background clutter by trained Eptesicus fuscus. During the test, the mean amplitude of echo from either target was always larger than that of the background echoes at the bat-to-target distance of 30, 70 and 100 cm. The amplitude of the echo reflected back from a revolving target was modulated between a maximum and a minimum value. An electric motor was used to revolve a target. The frequency contents of the motor noise were mostly below 1 kHz. While the total percent response of approaching either target is always more than 90% at every bat-to-target distance tested, the bats approach a revolving target more frequently than a stationary one. Echolocation pulses emitted by the bats during the test were recorded and analyzed. The bats shortened their pulse durations and interpulse intervals and lowered the frequency contents as they entered into the crawling phase from the searching phase. Potential interference of background echoes and ambient noise with the performance of the bats is discussed. The preference of a revolving target to a stationary one by the bats is perhaps due to the fact that a revolving target has a higher releasing value than a stationary one does.     

Neurons in the inferior colliculus of the big brown bat show maximal amplitude sensitivity at the best duration

The big brown bats, Eptesicus fuscus, emit ultrasonic signals and analyze the returning echoes in multi-parametric domains to extract target features. The variation of different pulse parameters during hunting predicts that analysis of an echo parameter by bats is inevitably affected by other co-varying echo parameters. In this study, we presented data to show that the bat inferior collicular (IC) neurons have maximal amplitude sensitivity at the best duration (BD). A family of rate-amplitude function (RAF) of each IC neuron is plotted with the BD and non-BD sound pulses. The RAF plotted with BD pulses has sharper slope (SL) and smaller dynamic range (DR) than the RAF plotted with non-BD pulses has. All RAFs can be described as monotonic, saturated or non-monotonic. IC neurons with monotonic RAF are mostly recorded at deeper IC and they have the largest average BD, best amplitude (BA) and DR. Conversely, IC neurons with non-monotonic RAF are mostly recorded at upper IC and they have the smallest average BD, BA and DR. Low best frequency (BF) neurons at upper IC have shorter BD, smaller BA and DR than high BF neurons at deeper IC have. These data suggest that IC neurons that tune to an echo duration also have the greatest sensitivity to echo amplitude. These data also suggest that sensitivity in frequency, duration and amplitude appears to be orderly represented along the dorso-ventral axis of the IC.     

A comprehensive computational model of animal biosonar signal processing

Computational models of animal biosonar seek to identify critical aspects of echo processing responsible for the superior, real-time performance of echolocating bats and dolphins in target tracking and clutter rejection. The Spectrogram Correlation and Transformation (SCAT) model replicates aspects of biosonar imaging in both species by processing wideband biosonar sounds and echoes with auditory mechanisms identified from experiments with bats. The model acquires broadband biosonar broadcasts and echoes, represents them as time-frequency spectrograms using parallel bandpass filters, translates the filtered signals into ten parallel amplitude threshold levels, and then operates on the resulting time-of-occurrence values at each frequency to estimate overall echo range delay. It uses the structure of the echo spectrum by depicting it as a series of local frequency nulls arranged regularly along the frequency axis of the spectrograms after dechirping them relative to the broadcast. Computations take place entirely on the timing of threshold-crossing events for each echo relative to threshold-events for the broadcast. Threshold-crossing times take into account amplitude-latency trading, a physiological feature absent from conventional digital signal processing. Amplitude-latency trading transposes the profile of amplitudes across frequencies into a profile of time-registrations across frequencies. Target shape is extracted from the spacing of the object’s individual acoustic reflecting points, or glints, using the mutual interference pattern of peaks and nulls in the echo spectrum. These are merged with the overall range-delay estimate to produce a delay-based reconstruction of the object’s distance as well as its glints. Clutter echoes indiscriminately activate multiple parts in the null-detecting system, which then produces the equivalent glint-delay spacings in images, thus blurring the overall echo-delay estimates by adding spurious glint delays to the image. Blurring acts as an anticorrelation process that rejects clutter intrusion into perceptions.     

Complex sound analysis in the FM bat Eptesicus fuscus,correlated with structural parameters of frequency modulated signals

Big brown bats, Eptesicus fuscus, were presented with artificial frequency modulated (FM) echoes that simulated an object becoming progressively closer to the bat. A stereotyped approach phase behavioral response of the bat to the virtual approaching target was used to determine the ability of the bat to analyze FM signals for target distance information. The degree to which the bats responded with approach phase behavior to a virtual approaching target was similar when they were presented with either a naturally structured artificial FM echo or an artificial FM echo constructed from a series of brief pure tone steps. The ability of the bats to respond to an FM signal structured from a sequence of pure tone elements depended on the number of pure tone steps in the series; the bats required the presentation of tone-step FM signals containing about 83 or greater pure tone elements. Moreover, the duration of the individual tone steps of the tone-step FM signals could not exceed a specific upper limit of about 0.05 ms. Finally, it appears that the bats were able to independently resolve individual tone steps within the tone-step FM signals that were separated by about 450 Hz or more.Abbreviations CF constant frequency - FM frequency modulation     

Neural coding of echo-envelope disparities in echolocating bats

The effective use of echolocation requires not only measuring the delay between the emitted call and returning echo to estimate the distance of an ensonified object. To locate an object in azimuth and elevation, the bat’s auditory system must analyze the returning echoes in terms of their binaural properties, i.e., the echoes’ interaural intensity and time differences (IIDs and ITDs). The effectiveness of IIDs for echolocation is undisputed, but when bats ensonify complex objects, the temporal structure of echoes may facilitate the analysis of the echo envelope in terms of envelope ITDs. Using extracellular recordings from the auditory midbrain of the bat, Phyllostomus discolor, we found a population of neurons that are sensitive to envelope ITDs of echoes of their sonar calls. Moreover, the envelope-ITD sensitivity improved with increasing temporal fluctuations in the echo envelopes, a sonar parameter related to the spatial statistics of complex natural reflectors like vegetation. The data show that in bats envelope ITDs may be used not only to locate external, prey-generated rustling sounds but also in the context of echolocation. Specifically, the temporal fluctuations in the echo envelope, which are created when the sonar emission is reflected from a complex natural target, support ITD-mediated echolocation.     

Some comments on the proposed perception of phase and nanosecond time disparities by echolocating bats

In a series of recent reports, Simmons and his colleagues propose that bats are able to accurately encode the spectral, temporal and phase information of their emitted calls and echoes. The information so encoded is then extracted by the networks of the auditory system with specialized processing. They propose that bats use this information to determine the distance to their target by crosscorrelating the entire structure of the emitted call with the structure of the echo. The idea is that slight deviations in the correlation function can be detected by the bat and the degree of mismatch provides an accurate measure of temporal disparity and hence range. The data in the reports purport to show that bats perceive the phase of ultrasonic signals and that they can resolve temporal disparities of about 10 ns, and thus can distinguish range differences as small as 2 m.The hypothesis also attempts to explain how a variety of acoustic cues are processed and represented in the auditory system and how they are combined to form a unitary percept of space and fine structure. The theory incorporates some time honored processes of extracting information, such as crosscorrelations. The implications of. the hypothesis, however, go far beyond a theory of neural processing and representation of information by ensembles of cells. The hypothesis requires some remarkable abilities, such as the phase coding of ultrasonic signals and a temporal acuity on the order of 10 ns. These features have never been seen in any neurophysiological study of any animal nor has its existence been implied in behavioral studies of other animals. If bats, in fact, detect and process those signals in the manner proposed by Simmons and his colleagues, it would suggest that bats are supermammals whose auditory systems have evolved new and extraordinary mechanisms not possessed by other animals.In view of the extraordinary implications of the hypothesis, it seems prudent to critically evaluate the data upon which the hypothesis is based. The purpose of this review is to point out a number of technical problems and deficiencies in those experiments which undermine the veracity of the purported demonstration of phase perception and nanosecond time resolution by bats.     

Frequency tuning, latencies, and responses to frequency-modulated sweeps in the inferior colliculus of the echolocating bat, Eptesicus fuscus

Neurons in the inferior colliculus (IC) of the awake big brown bat, Eptesicus fuscus, were examined for joint frequency and latency response properties which could register the timing of the bat's frequency-modulated (FM) biosonar echoes. Best frequencies (BFs) range from 10 kHz to 100 kHz with 50% tuning widths mostly from 1 kHz to 8 kHz. Neurons respond with one discharge per 2-ms tone burst or FM stimulus at a characteristic latency in the range of 3–45 ms, with latency variability (SD) of 50 μs to 4–6 ms or more. BF distribution is related to biosonar signal structure. As observed previously, on a linear frequency scale BFs appear biased to lower frequencies, with 20–40 kHz overrepresented. However, on a hyperbolic frequency (linear period) scale BFs appear more uniformly distributed, with little overrepresentation. The cumulative proportion of BFs in FM₁ and FM₂ bands reconstructs a scaled version of the spectrogram of FM broadcasts. Correcting FM latencies for absolute BF latencies and BF time-in-sweep reveals a subset of IC cells which respond dynamically to the timing of their BFs in FM sweeps. Behaviorally, Eptesicus perceives echo delay and phase with microsecond or even submicrosecond accuracy and resolution, but even with use of phase-locked FM and tone-burst stimuli the cell-by-cell precision of IC time-frequency registration seems inadequate by itself to account for the temporal acuity exhibited by the bat. Accepted: 21 June 1997     

Electron spin resonance study of phospholipid membranes employing a comprehensive line-shape model

The electron spin resonance spectra of the 1-myristoyl-2-[6-(4,4-dimethyloxazolidine-N-oxyl)myristoyl]-sn-glycero- 3-phosphocholine spin-label in highly oriented, fully hydrated bilayers of 1,2-dimyristoyl-sn-glycero-3-phosphocholine have been studied as a function of temperature and magnetic field orientation. The oriented spectra show clear indications of slow motional components (rotational correlation times greater than 3 ns) even in the fluid phase (T greater than 23 degrees C), indicating that motional narrowing theory is not applicable to the spectral analysis. The spectra have been simulated by a comprehensive line-shape model that incorporates trans-gauche isomerization in addition to restricted anisotropic motion of the lipid long molecular axis and that is valid in all motional regimes. In the gel (L beta') phase the spin-label chains are found to be tilted at 28 degrees with respect to the normal of the orienting plane. In the intermediate (P beta') phase there is a continuous distribution of tilt angles between 0 degrees and 25 degrees. In fluid (L alpha) phase there is no net tilt of the lipid chains. The chains rotate at an intermediate rate about their long axis in the fluid phase (tau R,parallel = 1.4-6.6 ns for T = 50-25 degrees C), but the reorientation of the chain axis is much slower (tau R, perpendicular= 13-61 ns for T = 50-25 degrees C), whereas trans-gauche isomerization (at the C-6 position) is rapid (tau J less than or equal to 0.2 ns). Below the chain melting transition both chain reorientation and chain rotation are at the ESR rigid limit (tau R greater than or equal to 100 ns), and trans-gauche isomerization is in the slow-motion regime (tau J = 3.7-9.5 ns for T = 22-2 degrees C). The chain order parameter increases continuously with decreasing temperature in the fluid phase (SZZ = 0.47-0.61 for T = 50-25 degrees C), increases abruptly on going below the chain melting transition, and then increases continuously in the intermediate phase (SZZ = 0.79-0.85 for T = 22-14 degrees C) to an approximately constant value in the gel phase (SZZ congruent to 0.86 for T = 10-2 degrees C).(ABSTRACT TRUNCATED AT 400 WORDS)     

The effects of changes in consequences on hens' performance in delayed-matching-to-sample tasks

A delayed-matching-to-sample (DMTS) task was used to investigate remembering with domestic hens. In Conditions 1 and 3 of Experiment 1, six hens responded under a mixed-delay procedure with delays of 0.25, 2, and 8 s. In Condition 2, the reinforcer for correct responding was delayed for 6 s after each correct matching response on 2-s delay trials. In Condition 1, discrimination performance decreased monotonically over the three delays. With the delay to the reinforcer, the decreases were non-monotonic as a result of the considerable drop in the accuracy of discrimination on the 2-s delay trials. Performance at the 2-s delay did not recover completely in Condition 3. In Conditions 1 and 3 of Experiment 2, five hens responded under a mixed-delay procedure with delays of 0, 4, and 16 s. In Condition 2 no reinforcers were provided for correct responding on 0- and 16-s delay trials. When reinforcers were available on all trials discrimination performance decreased monotonically with delay. There were non-monotonic changes in discrimination with delay when there was extinction at two delays resulting mainly from a large drop in discrimination performance at 0 s. In addition, response latencies increased markedly at the two delays associated with extinction. Performance recovered completely in Condition 3. The data support the ideas that remembering involves a temporal discrimination that the effects of delaying reinforcement and removing reinforcement may differ, and that the measurement of response latencies may be a useful tool in DMTS procedures.