People can time travel cognitively because they can remember events having occurred at particular times in the past (episodic memory) and because they can anticipate new events occurring at particular times in the future. The ability to assign points in time to events arises from human development of a sense of time and its accompanying time-keeping technology. The hypothesis is advanced that animals are cognitively stuck in time: that is, they have no sense of time and thus have no episodic memory or ability to anticipate long-range future events. Research on animals' abilities to detect time of day, track short time intervals, remember the order of a sequence of events, and anticipate future events are considered, and it is concluded that the stuck-in-time hypothesis is largely supported by the current evidence.

1. To examine the dependence of y-aminobutyric acid (GABAA) receptor gating on the a-subunit isoform, we studied the kinetics of GABA-gated currents (IGABA) of receptors that differed in the a-subunit subtype, al,82y2S and a3/i2y2S. cDNAs encoding rat brain subunits were co-expressed heterologously in HEK-293 cells and the resultant receptors studied with the whole-cell patch clamp technique and rapidly applied GABA pulses (5-10 s).2. IGABA of both receptors showed a loosely similar dependence on GABA concentration over a wide range (1-5000 ,UM). Generally, IGABA manifested activation reaching an early current peak, subsequent slower spontaneous desensitization, and deactivation of open channels at pulse termination. Lowering GABA concentrations reduced peak currents and slowed activation and desensitization kinetics.3.

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The effects of frequency of presentation and degree of spacing on short-term retention were studied in the pigeon by using a delayed matching-tosample procedure. In four experiments, it was found that (a) retention increased as a direct function of the frequency with which a sample stimulus was presented, (b) temporal spacing of repeated presentations of the sample stimulus lowered retention in comparison to nonspaced presentations, and degree of retention loss was directly related to degree of spacing, (c) degree of spacing and repetition interacted so that the deleterious effect of spaced repetition increased as the number of repetitions increased, and (d) spacing intervals were not commutative in determining retention, a spaced-massed order of presenting the sample stimulus leading to higher retention than a massed-spaced order. It was concluded that a simple trace strength and decay theory of short-term memory can account for all of these findings.

Pigeons' ability to time light and tone stimuli was examined in four experiments. In Experiment 1, two groups of pigeons were trained to discriminate between 2- and 8-s durations of lights or tones and then were transferred to reversal or nonreversal discriminations in the alternate modality. Pigeons learned the light discrimination faster than the tone discrimination and showed immediate positive intermodal transfer from tone to light but not from light to tone. In Experiments 2-4, the peak procedure was used to study birds' timing of 15- and 30-s fixed-interval light and tone signals. Peak times on empty trials under baseline conditions closely approximated the length of fixed-interval signals. When pigeons were tested with time-outs and intermodal switches introduced midway through an empty trial, they tended to reset the timing mechanism and begin timing again from 0 s. With both estimation and production procedures, pigeons were less accurate when timing the tone stimuli than when timing the light stimuli. A comparison of these data with data from timing experiments with rats suggests several possible differences in timing processes between pigeons and rats.