The easy-to-hard effect in human (Homo sapiens) and rat (Rattus norvegicus) auditory identification

Abstract

The authors examined whether progressively training humans and rats to perform a difficult auditory identification task led to larger improvements than extensive training with highly similar sounds (the easy-to-hard effect). Practice improved humans' ability to distinguish sounds regardless of the training regimen. However, progressively trained subjects were more accurate and showed more generalization, despite significantly less training with the stimuli that were the most difficult to distinguish. Rats showed less capacity to improve with practice but still benefited from progressive training. These findings indicate that transitioning from an easier to a more difficult task during training can facilitate, and in some cases may be essential for, auditory perceptual learning. The results are not predicted by an explanation that assumes interaction of generalized excitation and inhibition but are consistent with a hierarchical account of perceptual learning in which the representational precision required to distinguish stimuli determines the mechanisms engaged during learning.

Figure 1

Spectrograms of selected sample stimuli used in Experiment 1 and 2.

Figure 2

Mean percent correct for pre- and post-tests across deviant sweep rates from 10 to 11.9 sweeps/s in the Easy (n = 9), Hard (n = 9), and Control (n = 6) conditions in Experiment 2. Error bars represent the 95% confidence intervals (CIs).

Figure 3

Adjusted mean percent correct in post-test as a function of deviant sweep rate (sweeps/s) in the Easy and Hard conditions (n = 9) in Experiment 2. The effect of pre-test performance was controlled by ANCOVA (percent correct at corresponding deviant rates in pre-test entered as covariates). Covariates appearing in the model for 10, 11, 11.1, 11.2, 11.3, 11.4, 11.5, 11.6, 11.7, 11.8, 11.9 sweeps/s were evaluated at 84.44, 56.25, 50.28, 49.72, 45.56, 42.08, 40.14, 38.06, 36.67, 33.19, and 37.50, respectively. Error bars represent the 95% confidence intervals (CIs).

Figure 4

The discriminability index (d′) as a function of training session by condition in Experiment 2. Participants in the Hard condition (n = 9) were trained with the critical contrast (11.5 vs. 12 sweeps/s) throughout the eight sessions, whereas those in the Easy condition (n = 9) were trained with progressively more similar contrasts as training sessions proceeded from 8 vs.12 sweeps/s (Session 1), to 10 vs. 12 sweeps/s (Sessions 2 and 3), 11 vs. 12 sweeps/s (Sessions 4 and 5), 11.3 vs. 12 sweeps/s (Sessions 6 and 7), and finally to the critical 11.5 vs. 12 sweeps/s (Session 8). Sessions with the same sound contrast are connected by a line in the figure. The solid curve labeled as ‘Log. (Hard)’ represents the logarithmic trend fitted to the learning curve of the Hard condition.

Figure 5

Performance in Session 8 as a function of Block by Condition in Experiment 2. (A) The discrimination index (d′); error bars represents the 95% confidence intervals (CIs) in the by-participant analysis. (B) Percent correct; error bars represents the 95% confidence intervals (CIs) in the by-trial analysis.

Figure 6

Identification accuracy as a function of session for rats in the Easy and Hard conditions in Experiment 3A. Rats in the Hard condition were trained with the same two FM sounds (8 vs. 12 sweeps/s) throughout the eight sessions, whereas those in the Easy condition were trained with progressively more similar contrasts as training sessions proceeded from tones vs. clicks (Sessions 1 – 4), to 2 vs. 12 sweeps/s FM sounds (Sessions 5 and 6), 4 vs. 12 sweeps/s (Sessions and 7), and finally to 8 vs. 12 sweeps/s sounds (Session 8). Sessions with the same sound contrast are connected by a line in the figure. Error bars represent the 95% confidence intervals (CIs).

Figure 7

Identification accuracy as a function of tone pairs for rats in the Easy and Hard conditions in Experiment 3B. Rats in the Hard condition identified the same two tones (Pairs 1-3: 7.5 and 2.5 kHz) throughout training, whereas those in the Easy condition were trained with progressively more similar tones as training proceeded (Pair 1: 9 and 1 kHz; Pair 2: 8 and 2 kHz; Pair 3: 7.5 and 2.5 kHz). Sessions with the same sound contrast are connected by a line in the figure. Error bars represent the 95% confidence intervals (CIs).