Jonathon D. Crystal

SYSTEMATIC NONLINEARITIES IN THE PERCEPTION OF TEMPORAL INTERVALS

Rats judged time intervals in a choice procedure in which accuracy was maintained at approximately 75% correct. Sensitivity to time (d) was approximately constant for short durations 2.0-32.0 s with 1.0- or 2.0-s spacing between intervals (n = 5 in each group, Experiment 1), 2.0-50.0 s with 2.0-s spacing (n = 2, Experiment 1), and 0.1-2.0 s with 0.1- or 0.2-s spacing (n = 6 in each group, Experiment 2). However, systematic departures from average sensitivity were observed, with local maxima in sensitivity at approximately 0.3, 1.2, 10.0, 24.0, and 36.0 s. Such systematic departures from an approximately constant d are predicted by a connectionist theory of time with multiple oscillators and may require a modification of the linear timing hypothesis of scalar timing theory.

Nonlinear time perception

Sensitivity to time was investigated to test the linear-timing hypothesis. A long duration was adjusted until accuracy was 75% correct for a short duration in a two-choice procedure. Short durations (2, 4, 6, 8, 10, 12, 14, 16 and 18 s) were selected from previous research that suggests that sensitivity to time is nonlinear in this range. Rats were tested with a single short interval (Experiment 1, n=13) or a random order (Experiment 2, n=7). A local maximum in sensitivity (d from signal detection theory) was observed at approximately 8-12 s. Sensitivity to time was reliably correlated (rs=0.759-0.941) with previous data. Weber fractions exhibited a U-shape and were negatively correlated with sensitivity to time (r=-0.800). These results provide additional evidence that sensitivity to time is nonlinearly related to physical time.

Representation of time in time-place learning.

Ordinal, interval, and circadian mechanisms of solving a time-place task were tested. Rats searched for food twice in the morning and once in the afternoon (Group AB-C,n=5) or once in the morning and twice in the afternoon (Group A-BC,n=5) in a box with four food troughs. The location of the food depended on the time of day in a 12:12-h light:dark cycle. Acquisition was documented by foodsite inspections at the correct locations prior to food availability. On nonrewarded probes, the time of the middle search (B) was shifted late (for Group AB-C) or early (for Group A-BC). The rats visited Location B at chance, contrary to an ordinal mechanism. When the posttesting meal and light-dark transitions were omitted, the rats visited correct locations with impaired performance but at abovechance levels on nonrewarded probes. The results are consistent with interval and circadian representations of time.

Circadian time perception.

The variability of anticipating a meal was investigated. Sprague-Dawley rats earned food by inspecting a food source during a 3-hr interval. Food was not available at other times. In Experiment 1, the meal started 3 or 7 hr after light offset in a 12-hr light-dark cycle. Experiment 2 was conducted in constant darkness with 14-, 22-, 22.5-, 24-, 25.5-, 26-, or 34-hr intermeal intervals. Inspections increased before the meal. Rats timed intervals in the circadian range (22-26 hr) with lower variability than that for intervals outside this range (3-14 and 34 hr). Higher precision in timing selected intervals violates the scalar property. Proximity to a circadian oscillator improves timing precision. Variability may be used to identify oscillators with noncircadian periods.

Systematic Nonlinearities in the Memory Representation of Time

The representation of time was investigated by testing rats with intervals that changed by 2 s across trials. In Experiment 1, 2 ranges (20-150 s, 30-160 s; n = 10 rats per group) were examined. The times at which response bursts occurred (start time) were approximately proportional to interval durations. However, systematic departures from linearity were observed. Nonlinearities were related to the absolute duration of intervals, rather than to durations relative to the range. In Experiment 2, 660-s trials were inserted into the sequence of intervals (10-140 s, n = 20). Start and end times of response bursts were approximately proportional to intervals, but nonlinearities in start and end times were correlated, indicating that the source of nonlinearity was in the memory representation of time rather than in a decision process. These results indicate that the representation of time is nonlinearly related to physical time.

TEMPORAL SEARCH AS A FUNCTION OF THE VARIABILITY OF INTERFOOD INTERVALS

We attempted to determine whether timing theories developed primarily to explain performance in fixed-interval reinforcement schedules are also applicable to variable intervals. Groups of rats were trained in lever boxes on peak procedures with a 30-, 45-, or 60-s interval, or a 30- to 60-s uniform distribution (Experiment 1); a 60-s fixed and 1- to 121-s uniform distribution between and within animals (Experiment 2); and a procedure in which the interval between food and next available food gradually changed from a fixed 60 s to a uniform distribution between 0 and 120 s (Experiment 3). In uniform interval schedules rats made lever responses at particular times since food, as measured by the distribution of food-food intervals, the distribution of postreinforcement pauses, and the mean response rate as a function of time since food. Qualitative features of this performance are described by a multiple-oscillator connectionist theory of timing.

Time-place learning in the eight-arm radial maze

Rats (n=4) searched for food on an eight-arm radial maze. Daily 56-min sessions were divided into eight 7-min time zones, during each of which a different location provided food; locations were randomized across subjects before training. The rats obtained multiple pellets within each time zone by leaving and returning to the correct location. Evidence that the rats had knowledge about the temporal and spatial features of the task includes the following. The rats anticipated locations before they became active and anticipated the end of the currently active locations. The rats discriminated currently active locations from earlier and forthcoming active locations in the absence of food transition cues. After the rats had left the previously active location, they visited the next correct location more often than would be expected by chance in the absence of food transition cues. The rats used handling or opening doors as a cue to visit the first location and timed successive 7-min intervals to get to subsequent locations.

Evidence for an alternation strategy in time–place learning

Many different conclusions concerning what type of mechanism rats use to solve a daily time-place task have emerged in the literature. The purpose of this study was to test three competing explanations of time-place discrimination. Rats (n = 10) were tested twice daily in a T-maze, separated by approximately 7 h. Food was available at one location in the morning and another location in the afternoon. After the rats learned to visit each location at the appropriate time, tests were omitted to evaluate whether the rats were utilizing time-of-day (i.e., a circadian oscillator) or an alternation strategy (i.e., visiting a correct location is a cue to visit the next location). Performance on this test was significantly lower than chance, ruling out the use of time-of-day. A phase advance of the light cycle was conducted to test the alternation strategy and timing with respect to the light cycle (i.e., an interval timer). There was no difference between probe and baseline performance. These results suggest that the rats used an alternation strategy to meet the temporal and spatial contingencies in the time-place task.

Simultaneous temporal and spatial processing

Rats searched for food that was contingent on time and place in an open field. One location was active at a time, the active location moved in a clockwise direction after each reward, and each location was repeated several times on each daily session. When a location was active, the first response after a fixed interval produced food. The intervals associated with each of the four locations were consistently 60, 30, 30, and 60 sec. For independent groups, inspecting an inactive location had no consequence (n = 7) or reduced the amount of food delivered at the active location (n = 6). The rates of inspecting active and inactive locations increased before the associated intervals elapsed, with preferential responding at the active locations. Rates of anticipation at active locations failed to superimpose when plotted as a function of proportional time. Simultaneous temporal and spatial processing contributed to the failure of proportional timing.

Timing inter-reward intervals

Abstract Rats judged inter-reward intervals (IRIs) in a two-alternative, forced-choice task. The IRI was either short (S) or long (L). At the end of each IRI, two response levers were inserted into the box. A press to the lever designated as correct or incorrect produced a large (3–5 food pellets) or small (1 pellet) reward, respectively. Psychophysical functions were obtained by testing intermediate IRIs, which were followed by the small reward, independent of the response location. S and L intervals were manipulated across groups, as specified (in seconds) by the group name, S–L: 3–12 ( n =12), 25–100 ( n =12), and 50–200 ( n =7). The psychophysical functions ( p [L] vs. IRI) were ogival in shape and had bisection points ( p [S]= p [L]=0.5) near the geometric mean of S and L intervals. The psychophysical functions did not superimpose in relative time (IRI/L). Instead, 3–12 was timed with greater relative sensitivity than were 25–100 and 50–200.