Final answer:
In LeDoux's emotional processing pathways, 'fast' refers to the direct route from the thalamus to the amygdala, while 'slow' refers to the indirect route via the cortex.
Step-by-step explanation:
With respect to LeDoux's emotional processing pathways, fast is to slow as amygdala is to cortex. This reflects the two different pathways that a fear stimulus can take within the brain. The fast pathway goes directly from the thalamus to the amygdala, allowing a quicker but less detailed response to emotional stimuli. The slow pathway, on the other hand, involves the stimulus being relayed from the thalamus through the cortex and then to the amygdala, which allows more processing and contextual interpretation of the stimulus before an emotional response is generated.