Stimulus Roving and Flankers Affect Perceptual Learning of Contrast Discrimination in <i>Macaca mulatta</i>

<div><p>‘Stimulus roving’ refers to a paradigm in which the properties of the stimuli to be discriminated vary from trial to trial, rather than being kept constant throughout a block of trials. Rhesus monkeys have previously been shown to improve their contrast discrimination performance on a non-roving task, in which they had to report the contrast of a test stimulus relative to that of a fixed-contrast sample stimulus. Human psychophysics studies indicate that roving stimuli yield little or no perceptual learning. Here, we investigate how stimulus roving influences perceptual learning in macaque monkeys and how the addition of flankers alters performance under roving conditions. Animals were initially trained on a contrast discrimination task under non-roving conditions until their performance levels stabilized. The introduction of roving contrast conditions resulted in a pronounced drop in performance, which suggested that subjects initially failed to heed the sample contrast and performed the task using an internal memory reference. With training, significant improvements occurred, demonstrating that learning is possible under roving conditions. To investigate the notion of flanker-induced perceptual learning, flanker stimuli (30% fixed-contrast iso-oriented collinear gratings) were presented jointly with central (roving) stimuli. Presentation of flanker stimuli yielded substantial performance improvements in one subject, but deteriorations in the other. Finally, after the removal of flankers, performance levels returned to their pre-flanker state in both subjects, indicating that the flanker-induced changes were contingent upon the continued presentation of flankers.</p></div

Illustration of changes in the psychometric function over the course of training.

<p>Plots show the psychometric functions obtained from three sessions of roving training, namely from the first (dotted line), middle (dashed line), and last sessions (solid line), as well as during the final session of non-roving training (yellow line). <i>P_reporthigher</i> was plotted against the test contrast, and data were fitted using a Weibull function. Lines represent the fitted curve. Left column: monkey 1; right column: monkey 2. A & B: 20% sample (red); C & D: 30% sample (green); E & F: 40% sample (blue). Improvement in the psychometric function is represented by a shift in the PSE (vertical line) towards the respective sample contrasts; such a shift is clearly visible for the 40% sample condition in monkey 1, and the 20% sample condition in monkey 2.</p

Monkey 2's performance, when stimulus eccentricity was identical to that used for monkey 1 (4.6°).

<p>The drop in performance upon addition of flankers, the gradual improvement during flanker training, and the subsequent return to pre-flanker levels, was similar to that previously seen with a stimulus eccentricity of 1.5°. A: <i>P_correct</i>; B: slope of the psychometric function; C: PSE. Red data points: pre-flankers; green data points (grey background): flankers; blue data points: post-flankers. Unfilled markers: 20% sample contrast conditions; medium-coloured filled markers: 30%; dark-coloured filled markers: 40%.</p

Comparisons of performance levels between early and late sessions during training with roving stimuli, using Mann-Whitney <i>U</i> test.

<p>Results are presented separately for pre-flanker training (left half of the table) and for flanker training (right half). An FDR correction for α-levels was performed for multiple comparisons due to the measures of performance under assessment (pre-flanker training, monkey 1: 20%: α = .05×1/3 = .0167; 30%: α = .05×1/3 = .0167; 40%: α = .05×3/3 = .05; pre-flanker training, monkey 2: 20%: α = .05×3/3 = .05; 30%: α = .05×1/3 = .0167; 40%: α = .05×1/3 = .0167; flanker training, monkey 1: 20%: α = .05×3/3 = .05; 30%: α = .05×3/3 = .05; 40%: α = .05×2/3 = .0333; flanker training, monkey 2: 20%: α = .05×3/3 = .05; 30%: α = .05×2/3 = .0333; 40%: α = .05×2/3 = .0333). For changes in reaction time, an FDR correction was carried out to take the two measures of RT, <i>RT_correct</i> and <i>RT_error</i>, into account (pre-flanker training, monkey 1: 20%, 30% & 40%: α = .05×2/2 = .05; pre-flanker training, monkey 2: 20%%, 30% & 40%: α = .05×1/2 = .025; flanker training, monkey 1: 20%%, 30% & 40%: α = .05×1/2 = .025; flanker training, monkey 2: 20%%, 30% & 40%: α = .05×1/2 = .025). Significant changes are indicated by an asterisk.</p><p>Comparisons of performance levels between early and late sessions during training with roving stimuli, using Mann-Whitney <i>U</i> test.</p

Characteristics of tasks involving non-roving and roving stimuli.

<p>Left panel: in tasks with non-roving stimuli, as in Chen et al. <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0109604#pone.0109604-Chen1" target="_blank">[1]</a>, the sample stimulus was always displayed at a contrast of 30%. Right panel: for the task in the current study, involving roving stimuli, the contrast of the sample stimulus varied randomly from trial to trial and took on a value of 20%, 30% or 40%. Unlike in the non-roving task, subjects had to take note of the contrast of the sample stimulus in order to perform the roving task correctly. For example, for a test stimulus of 25% contrast, they were required to make a saccade to the white target if it had been preceded by a sample of 20% contrast. On the other hand, they were required to make a saccade to the black target if the sample contrast had been 30% or 40%. Note that the contrasts of stimuli in the diagram are exaggerated for illustrative purposes.</p

Proportion of ‘report higher’ trials (<i>P_reporthigher</i>) against session number, for the condition where the test contrast was 35%.

<p>This condition demanded different responses, depending on the sample contrast (termed a ‘conflict condition’). Within each subplot, the leftmost data points indicate subjects' performance during the non-roving task, while those to the right indicate performance during the roving task. The sample contrast was 30% (black markers) or 40% (grey markers). A divergence in data points between response conflict conditions indicated that learning occurred under roving conditions.</p

<i>P_correct</i> for each pairwise comparison between sample contrasts.

<p>A: monkey 1; B: monkey 2. 20% versus 30%: black; 30% versus 40%: cyan; 20% versus 40%: magenta.</p

Comparison of subjects' performance in the absence of flankers, during post-flanker sessions, and during the end of pre-flanker sessions.

<p><i>X</i>_min – <i>X</i>_max: Ranges of performance seen during late pre-flanker sessions, which took place before flankers were introduced. <i>X</i>_a: Performance recorded during the last session of post-flanker training.</p><p>Comparison of subjects' performance in the absence of flankers, during post-flanker sessions, and during the end of pre-flanker sessions.</p

Slopes of the best-fit line to the roving data, shown in Figure 9 and Figure S1, for each response conflict condition.

<p>In the majority of cases, responses diverged with training on the roving task, according to the contrast of the sample.</p><p>Slopes of the best-fit line to the roving data, shown in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0109604#pone-0109604-g009" target="_blank">Figure 9</a> and <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0109604#pone.0109604.s001" target="_blank">Figure S1</a>, for each response conflict condition.</p

Measures of performance of the two subjects during the roving task.

<p>Left column: monkey 1; right column: monkey 2. A & B: <i>P_correct</i>; C & D: slope of the psychometric function; E & F: PSE of the psychometric function. Red markers: pre-flanker training; green markers (grey background): flanker training; blue markers: post-flanker training. Unfilled markers: 20% sample contrast conditions; medium-coloured filled markers: 30%; dark-coloured filled markers: 40%.</p