Fig 1.
Diagrammatic representation of the diffusion model for a simple two-choice response time task.
The vertical axis shows boundary separation (a): the distance between the two decision thresholds. The upper boundary corresponds to a decision to press the high reward lever, while the lower boundary corresponds to a decision to press the low reward lever. The decision is made through information accumulation, which begins from a starting point (zr) located somewhere between the two boundaries. a+ is the distance between the decision starting point and the upper boundary, while a- is the distance between the starting point and lower boundary. a+ = a- would mean the decision starting point is exactly halfway between the two boundaries (signifying no bias). The horizontal axis is the total response time (RT), made up of the decision time plus the non-decision RT (t0). Representative RT distributions with behavioural data (bars) and RT curves used by the model to calculate parameters (black lines) for the midpoint tone are shown above and below the model for decisions cumulating in high and low reward lever presses respectively. The RT distributions are from a single rat on a single control session, which was chosen by selecting the data point with median goodness of fit p-value (p = 0.862) from all control data. The jagged line shows an example decision making process which is noisy, and has an average rate of information accumulation called the drift rate (v). The relative positions of boundaries, starting point and gradient of the drift rate are drawn scaled to the average parameter values for all control data for the midpoint tone.
Fig 2.
The effect of acute negative affective state manipulations on judgement bias.
Acute restraint stress does not alter behaviour on the task, but an acute pharmacological negative affective state manipulation induces a negative bias. For the acute stress manipulation rats were placed in a restraint tube for 15 min immediately prior to testing on the task. For the acute pharmacological manipulation two doses of the anxiogenic drug, FG7142 (a GABAA receptor partial inverse agonist), were administered acutely 30 min prior to testing on the task. (A) Acute restraint stress had no effect on the response latency, did not alter (B) the percentage positive responding, and there was no change in (C) change from baseline in cognitive bias index (CBI). (D) Both doses of FG7142 (3.0 and 5.0 mg/kg) increased response latency across all three tones. (E) FG7142 caused a significant reduction in percentage positive responding for the midpoint tone only for both doses. (F) This is supported by the negative change from baseline in CBI for both doses of FG7142. Data represent mean ± SEM; Acute restraint stress: n = 11; FG7142: n = 10. **p < 0.01, *p < 0.05, #p < 0.06. Veh–vehicle; HT—high reward tone; MT—midpoint tone; LT—low reward tone.
Fig 3.
The effect of a chronic negative affective state manipulation on judgement bias.
Rats assigned to the stress group experienced repeated restraint stress and social isolation (RS&SI) for three weeks. Twice weekly test sessions were conducted one week prior to stress (pre-stress; sessions 1–2), for the three weeks during RS&SI (stress; sessions 3–8) and for three weeks following return to control conditions (post-stress; sessions 9–14). Data are shown for the midpoint tone only. (A) RS&SI seemed to make rats slower to respond during stress and post-stress sessions. (B) There was no clear effect of RS&SI on the percentage positive responding. (C) Rats in the RS&SI group showed a significantly negative change from baseline in cognitive bias index (CBI) compared to controls. (D) Summarising the change from baseline in CBI over the six stress and six post-stress session by calculating the area under the curve for this measure for individual rats indicated that RS&SI caused a significant negative bias, during both the stress and post-stress periods. Data represent mean ± SEM; n = 5 per group. *p < 0.05, #p < 0.06.
Fig 4.
Diffusion model fit validation comparing empirical and predicted data.
Graphical analysis of model fit comparing behavioural (empirical) and model (predicted) data. Data points lie close to the main diagonal (dotted line) for both (A) the percentage of positive responses and (B/C/D) the values for response times (RTs) from the three quartiles of the cumulative distribution functions for the midpoint tone, indicating good model fit. For RTs, squares represent the distribution for high reward responses, while circles represent the distribution for low reward responses. Each symbol corresponds to the data value for one rat on one probe test session. Filled symbols represent rats that were then consistently excluded from all behavioural experiments, and were therefore not included in any further analysis. Importantly, these points are randomly distributed amongst the data.
Fig 5.
Diffusion model parameters for data from the acute restraint stress manipulation.
Three parameters—distance to the upper boundary (a+; A/B), distance to the lower boundary (a-; C/D) and drift rate (v) were analysed for each tone. Change from baseline (control session) for a+ and a- was also calculated for the midpoint tone only to take into account individual differences in underlying bias. Acute restraint stress did not alter (A/B) a+; (C/D) a- or (E) v. (F) Diagrammatic representation of diffusion model parameters for the midpoint tone only for this manipulation. Illustrations are to scale representing mean values for each measure, and are aligned to decision starting point (dotted line). Drift rate is scaled 5:1. Data represent mean ± SEM; n = 11. HT—high reward tone; MT—midpoint tone; LT—low reward tone.
Fig 6.
Diffusion model parameters for data from acute FG7142 treatment.
Three parameters—distance to the upper boundary (a+; A/B), distance to the lower boundary (a-; C/D) and drift rate (v) were analysed for each tone. Change from baseline (control session) for a+ and a- was also calculated for the midpoint tone only to take into account individual differences in underlying bias. (A/B) The higher dose of FG7142 (5.0 mg/kg) only caused a significant increase and positive change from baseline in a+ for the midpoint and low reward tones. (C/D) The lower dose of FG7142 (3.0 mg/kg) only caused a significant increase and positive change from baseline in a- for the midpoint and low reward tones. (E) FG7142 differentially altered v dependent on both tone and dose. (F) Diagrammatic representation of the diffusion model parameters for the midpoint tone only for this manipulation. Illustrations are to scale representing mean values for each measure, and are aligned to decision starting point (dotted line). Drift rate is scaled 10:1 to more clearly illustrate changes. Graph data represent means ± SEM; n = 10. **p < 0.01, *p < 0.05. Veh–vehicle; HT—high reward tone; MT—midpoint tone; LT—low reward tone.
Fig 7.
Diffusion model parameters for data from Experiment 2.
Three parameters—distance to the upper boundary (a+; A/B), distance to the lower boundary (a-; C/D) and drift rate (v) were analysed for each tone. Change from baseline (control session) for a+ and a- was also calculated for the midpoint tone only to take into account individual differences in underlying bias. Data shown are averages for all probe sessions during that experimental period. (A) Changes in a+ in the restraint stress and social isolation (RS&SI) group compared to control group were unclear from the overall analysis of this measure. (B) However, during stress only, change from baseline in a+ was positive in the RS&SI group, and significantly larger compared to controls. (C) The effect of RS&SI on the a- was again unclear from the overall analysis. (D) During the post-stress period only, a positive change from baseline in a- was seen in the RS&SI group, and was also significantly larger compared to controls. (E) There was no difference in v between the RS&SI and control group during any stage of the manipulation. (F) Diagrammatic representation of diffusion model parameters for the control (dashed lines) and RS&SI group (solid lines) for the midpoint tone only for this manipulation. Illustrations are to scale representing mean values for each parameter, and are aligned to decision starting point (dotted line). Drift rate is scaled 5:1. Graph data represent mean ± SEM; n = 5 per group. Independent samples t-tests to compare groups: **p < 0.01, *p < 0.05; one-sample t-tests to compare to baseline: ##p < 0.01, #p < 0.05.
Fig 8.
Independent Component Analysis scores calculated from diffusion model parameter data.
Independent Component Analysis (ICA) was conducted on distance to the upper boundary (a+), distance to the lower boundary (a-) and drift rate (v) data for control data for the midpoint tone only. Negative manipulation data was then projected using the independent components found. (A/C/E) Independent component scores showing the distribution of parameter data for each manipulation. Scores for all three independent components are significantly different for both doses of FG7142 and the stress and post-stress periods of Experiment 2. Control data from all negative affective state manipulations are shown together. Each symbol represents the data from a single session for a single rat for that manipulation, mean ± SEM is shown by the black line and error bar for each manipulation. (B/D/F) Diagrammatic representations of the direction of change of diffusion model parameter values for control data (yellow) and negative affective state manipulation data (blue) described by the independent component. Black lines represent mean values for parameters from all data (control and negative manipulations). (B) The first independent component describes increased expectation for the low reward and more negative interpretation of the ambiguous cue for negative affective state manipulations compared to control. (D) The second independent component reflects reduced anticipation for the high reward for negative manipulation data. (F) The third independent component comprises reduced expectation for the high reward and more negative interpretation of the ambiguous cue for the negative affective state manipulations. ***p < 0.001, **p < 0.01, *p < 0.05. RS—restraint stress; RS&SI—restraint stress and social isolation.