Figure 1.
Schematic representation of the centre-surround contrast discrimination task.
Consecutive stimuli were presented to participants who indicated which of the two intervals had the centre stimulus of higher contrast. The target stimulus (front panel) was presented for 500 ms followed by a 500 ms inter-stimulus interval. A second stimulus was then displayed for 500 ms and was comprised of either the target stimulus alone (no-surround condition) or surrounded by an annulus of 95% contrast with a radius of 4 degrees (surround condition – back panel). The smaller centre stimulus had a radius of 0.67 degrees and seven different contrast levels were randomly presented.
Figure 2.
Example psychometric function for a single participant.
The filled and open symbols show the raw data collected for the no surround and surround conditions respectively. Fitted curves are the best fitting cumulative Gaussian distributions to the data. The curves have similar spreads (slope of the psychometric function), however, the presence of the annular surround results in a leftwards shift of the curve (unfilled symbols) because the target patch appears to be lower contrast than the veridical contrast of 40% (dashed vertical line). Bias was determined as the shift in the point-of-subjective equality (PSE, mean of the best fitting cumulative Gaussian) caused by the annular surround.
Figure 3.
Centre-surround suppression for parallel static gratings.
Panel A shows the PSE for the isolated centre patch (right hand side of panel: LHS) and when presented within the surround (left hand side of panel: RHS). Group means (±95% confidence intervals of the mean) are shown for control participants (C), migraine with aura (MA) and migraine without aura (MO) groups. Panel B shows the precision (spread of psychometric functions for the same groups, also for the no surround (LHS) and surround (RHS) conditions. Individual performance for each participant is shown in Panel C which plots the bias (shift in PSE) against their precision for the surround condition. Panel D shows the group mean (±95% confidence intervals of the mean) suppression ratio for the controls and all pooled migraine participants. The suppression ratio was determined as the PSE for the surround condition divided by that for the no surround condition. A ratio of 1 indicates that the surround has no effect. A reduction in the apparent contrast of the central patch due to the surround results in a suppression index less than 1.
Figure 4.
Centre surround suppression for parallel drifting gratings.
Panel A shows the PSE for the isolated centre patch (right hand side of panel: LHS) and when presented within the surround (left hand side of panel: RHS). Group means (±95% confidence intervals of the mean) are shown for control participants (C), migraine with aura (MA) and migraine without aura (MO) groups. Panel B shows the precision (spread of psychometric functions for the same groups, also for the no surround (LHS) and surround (RHS) conditions. Individual performance for each participant is shown in Panel C which plots the bias (shift in PSE) against their precision for the surround condition. Panel D shows the group mean (±95% confidence intervals of the mean) suppression ratio for the controls and all pooled migraine participants. The suppression ratio was determined as the PSE for the surround condition divided by that for the no surround condition. A ratio of 1 indicates that the surround has no effect. A reduction in the apparent contrast of the central patch due to the surround results in a suppression index less than 1.