Speed and accuracy of visual motion discrimination by rats

Abstract

Animals must continuously evaluate sensory information to select the preferable among possible actions in a given context, including the option to wait for more information before committing to another course of action. In experimental sensory decision tasks that replicate these features, reaction time distributions can be informative about the implicit rules by which animals determine when to commit and what to do. We measured reaction times of Long-Evans rats discriminating the direction of motion in a coherent random dot motion stimulus, using a self-paced two-alternative forced-choice (2-AFC) reaction time task. Our main findings are: (1) When motion strength was constant across trials, the error trials had shorter reaction times than correct trials; in other words, accuracy increased with response latency. (2) When motion strength was varied in randomly interleaved trials, accuracy increased with motion strength, whereas reaction time decreased. (3) Accuracy increased with reaction time for each motion strength considered separately, and in the interleaved motion strength experiment overall. (4) When stimulus duration was limited, accuracy improved with stimulus duration, whereas reaction time decreased. (5) Accuracy decreased with response latency after stimulus offset. This was the case for each stimulus duration considered separately, and in the interleaved duration experiment overall. We conclude that rats integrate visual evidence over time, but in this task the time of their response is governed more by elapsed time than by a criterion for sufficient evidence.

Figure 1. Correct trials have longer reaction times than error trials.

a. Distribution of reaction times for one rat in a block with fixed (85%) coherence, for correct responses (green) and error trials (red). Arrows indicate the mean reaction times. b. Cumulative probability distributions of same data shown in panel a. Arrows indicate mean reaction times. c. Mean reaction time of error trials vs. correct trials across 11 experiments from 6 rats. Stimulus parameters were fixed across trials within each experiment, but differed between experiments. Motion coherence was 85% (circles, triangles) or 95% (squares). Response ports were either 90 mm (circles, squares) or 10 mm (triangles) from the central trial-initiation port. Shaded symbol indicates the example experiment of panels a-b. d. Mean estimated decision time of error trials vs. correct trials, for the same data analyzed in c. Decision time is defined as the reaction time in each trial minus each rat’s minimum reaction time over all trials.

Figure 2. Accuracy increases with reaction time.

a. Accuracy (% correct) as a function of reaction time, from same data as Figure 1 a–b. Error bars indicate binomial confidence intervals. b. Accuracy (% correct) is higher in slow trials than fast trials across the population (11 experiments from 6 rats; symbols defined in Figure 1c). Fast and slow trials are defined as the highest and lowest quartile of each rat’s overall RT distribution within the block respectively. Crosses indicate binomial confidence intervals.

Figure 3. Dependence of accuracy and reaction time on motion strength.

a. Psychometric function – accuracy (% correct) as a function of motion strength (coherence) when coherence was varied from 0–1 in randomly interleaved trials. Symbols indicate data; curve is a 2nd order polynomial fit. b. Psychometric function from another experiment in which coherence was drawn uniformly from the interval 0.2–0.9. c. Chronometric function – mean reaction time as a function of coherence, for the experiment of panel a. d. Chronometric function for the experiment of panel b. e. Cumulative probability of reaction times for different coherence ranges, for experiment of panel a. Color indicates motion coherence from low (cool) to high (hot). Cumulative probability was computed over the range of 0–3 s; axis is expanded to focus on the time range of interest. f. Cumulative probability of reaction times for experiment of panel b. Color scale same as in panel e.

Figure 4. Accuracy improves with response time for each coherence, and overall.

a. Accuracy (% correct) as a function of reaction time for different coherence ranges, from the experiment of Figure 3a. b. Accuracy as a function of reaction time from the experiment of Figure 3b. c. Aggregate accuracy as a function of reaction time combining all coherences, for the experiment analyzed in panel a. d. Aggregate accuracy as a function of reaction time for the experiment analyzed in panel b.

Figure 5. Dependence of accuracy and reaction time on stimulus duration.

a. Accuracy (% correct) as a function of stimulus duration, from an experiment in which coherence was fixed at 85% and duration varied from 25 ms –225 ms in randomly interleaved trials. b. Accuracy as a function of motion strength for a second rat tested under same conditions as in a. c. Mean reaction time as a function of stimulus duration, for the experiment of panel a. d. Mean reaction time as a function of stimulus duration, for the experiment of pane b. e. Cumulative probability of reaction times for different stimulus durations from same data as panel a, with duration color coded from warm (long) to cool (brief). Cumulative probability was computed over the range of 0–3 s; axis is expanded to focus on the time range of interest. f. Cumulative probability of reaction times for different stimulus durations from data shown in panel b.

Figure 6. Accuracy decreases with response latency after stimulus offset.

a. Accuracy (% correct) as a function of reaction time for one subject in a block of trials in which stimulus duration varied from 25–225 ms (same experiment as Figure 5a). Bars (lower left) show the range of stimulus durations that contributed to the curves of same color. b. Accuracy as a function of reaction time for a second rat (same experiment as Figure 5b). c. Aggregate accuracy as a function of reaction time, pooling together data from all stimulus durations, for the experiment analyzed in a. d. Aggregate accuracy as a function of reaction time for the experiment analyzed in b.