Discrimination in the sense of flutter: new psychophysical measurements in monkeys

Abstract

Humans and monkeys have similar capacities to discriminate the frequencies of mechanical sinusoids delivered to their hands in the range that corresponds to the sense of flutter (10-50 Hz). Previous studies showed that monkeys can discriminate whether comparison stimuli are higher or lower in frequency than a base stimulus that does not vary from trial to trial during an experiment. We verified this result in two monkeys trained in this manner. To confirm that these animals were able to discriminate, we tested them in a variant of the task in which the frequency of the base stimulus changed randomly from trial to trial. The monkeys failed to discriminate in this new testing mode; instead they seemed to categorize the comparison stimuli, ignoring the base stimulus. After further training in the randomized base condition, the two monkeys learned to discriminate accurately. We then explored how the stimulation parameters affected performance. We found that animals could discriminate accurately with stimulus durations as short as 250 msec, with interstimulus intervals as long as 10 sec, with 50% differences between base and comparison stimulus amplitudes or when stimulated on a different finger. Performance did not degrade in these conditions, even though the monkeys had never been trained or tested under them. The results show that monkeys may try to categorize rather than discriminate when the task allows either strategy, although they are capable of performing true discriminations very robustly. These findings have important implications for investigating the neuronal processes underlying sensory discrimination.

Fig. 1.

Capacity of monkey 1 to discriminate the differences in frequency between two tactile stimuli. A, Psychophysical performance when the base stimulus frequency is held constant at 20, 30, or 40 Hz during a run (100 trials). Data points show the percent of trials in which the frequency of the comparison stimulus was judged as higher than that of the base stimulus, as a function of the frequency of comparison. The curves are logistic functions fitted to the data points. Data were collected during 10 consecutive days and are based on 100 trials per point. Error bars indicate ±1 SD of the 10 daily means and thus indicate the day-to-day variability in performance. B, Failure to discriminate the frequency difference between the two stimuli when the base frequency changes from trial to trial. In each case the comparison frequency was 5 Hz higher or lower than the base frequency. The base frequencies correspond to the midpoints of the line segments.Filled and open symbols correspond to comparison frequencies below and above the base, respectively.C, Capacity of the same monkey to categorize frequencies. Without further training, single stimuli were delivered, and the monkey had to indicate whether they were higher or lower than 30 Hz; the same set of frequencies as in the middle curve of A were used, but without the base stimulus. Monkeys had to discover the limits of the low and high categories through trial and error. The data are shown as the percentage judged high; the first 50 trials in this test were excluded. The animal made accurate categorizations. Each data point inB and C represents 30 trials. In all cases, stimuli were delivered at seven times the detection threshold at 30 Hz, adjusted for equal subjective magnitude. Stimulus duration was 1 sec, with 1 sec of interstimulus interval.

Fig. 2.

Frequency discrimination between two tactile stimuli when the base stimulus frequency changes from trial to trial. Sets of frequency pairs were used in which the difference between base and comparison frequencies was kept constant at 8, 6, 4, and 2 Hz. The monkey had been retrained with similar stimulus sets but with 10 Hz differences. Base–comparison frequency pairs are joined bylines. Each data point in a pair acts as both base and comparison frequency. Filled symbols indicate trials in which the base frequency was higher than the comparison; open symbols indicate trials in which the comparison frequency was higher than the base. Results are indicated as the percentage of trials in which the comparison frequency was judged as higher than the base frequency. The plots show that the difficulty of the task increased with smaller frequency differences. Data points are based on 100 trials. Other stimulation parameters are as in Figure 1.

Fig. 3.

Discrimination capacity of monkey 1 when the base frequency and the frequency difference between the two stimuli are varied simultaneously on every trial. Stimulus sets were constructed in which a reference frequency was paired with eight other frequencies so that it could occupy either the base or the comparison position. These stimulus sets were not used to train the monkeys, only to test them. The base–comparison frequency pairs were chosen pseudorandomly at each trial. For the left panel, filled symbolscorrespond to trials in which 20 Hz was the base frequency; open symbols correspond to trials in which 20 Hz was the comparison frequency. For the right panel the data were sorted similarly but with respect to a reference frequency of 30 Hz. Data points are based on 100 trials, performed during 10 consecutive days. Error bars represent ±1 SD of the 10 daily means. Performance in all cases is comparable to that shown in Figure 1A. Other stimulation parameters are as in Figure 1.

Fig. 4.

Discrimination capacity as a function of stimulus duration. Filled symbols correspond to 1000 msec, andopen symbols correspond to 250 msec duration. Performance is similar in the two conditions. Stimulus sets consisted of frequency pairs separated by 8 Hz in which both frequencies could occupy the base and the comparison positions. These pairs were presented in pseudorandom order. Pairs are joined bylines. Data points are based on 100 trials.

Fig. 5.

Frequency discrimination as a function of interstimulus interval. The same stimulus set, frequency pairs separated by 8 Hz with both frequencies occupying the base and the comparison positions, was used in the four plots. Base and comparison frequency pairs are joined by lines. Results are shown for interstimulus intervals (IS) of 1, 5, 10, and 15 sec. The animal’s performance deteriorated for interstimulus intervals of >10 sec. Data points are based on 20 trials.

Fig. 8.

Frequency discrimination as a function of the locus of stimulation. Sets of frequencies like those in Figure3A were used. Stimuli were delivered to the same digit used throughout the experiments (digit 3) and to two others. The plot on the left shows the discrimination accuracy as a function of frequency for stimulation of fingers 2 (continuous lines) and 3 (dashed lines). The plot on theright shows the results for stimulation of fingers 3 (dashed lines) and 4 (continuous lines).Open symbols indicate trials in which 20 Hz was the comparison frequency; filled symbols indicate trials in which 20 Hz was the base frequency. The dashed curves in the two panels are the same; for clarity, their corresponding data points are not shown. Each point comprises 10 trials; all data were collected during a single day. Performance was the same irrespective of the finger stimulated.

Fig. 6.

Discrimination between frequencies when the first and second stimuli differ in amplitude by 50%. Base and comparison frequencies (20 or 26 Hz) are indicated below each graph, in that order. The numbers in parenthesesindicate the stimulus amplitudes relative to the standard amplitude used in previous tests (equal to 7 times the detection threshold at 30 Hz). Results are plotted as the percentage of trials in which the animal discriminated correctly. The 10 conditions were delivered randomly and were measured in a single run. All data points are from 20 trials per class.

Fig. 7.

Results of a second test in which the first and second stimuli differ in amplitude by 50%. Pairs of stimuli with constant frequency differences of 6 Hz are presented. As in Figure 2, both frequencies in a pair occupy the base and comparison positions and are joined by lines. In the left panelcomparison stimuli were 1.5 times stronger in amplitude than the base stimuli. In the right panel base stimuli were 1.5 times stronger than the comparison stimuli. Data for the two plots were measured in a single run and represent 20 trials per class. Performance was largely insensitive to the amplitude differences.