Figure 2. Depiction of the masked image presentation protocol.

During presentations a to-be-masked image of a snake or wallaby alternated with a colored mask between being presented to either eye at a rate of 2 Hz. The brightness of the to-be-masked image was ramped on, from black to full brightness, across the first 3 seconds of the presentation, meaning that CS were presented at full intensity for the final 3 seconds of each 6 second test presentation, culminating in a shock for CS+ presentations.