I'm not sure if such a figure of speech exists in English, and "unidirectional gaga" is certainly not correct.
But which wording expresses that a person becomes dumber from having exercised/performed something too frequently or in only one particular fashion.
- It's an adverb or phrase that is usually applied to a person. But can also describe activities.
- It primarily expresses "to make stupid" caused by a particular behaviour/performance.
- The persistence or repetetion of past activites is implied to turn the person more dull / less open to alternative approaches.
- Does usually but not necessarily have a derogatory meaning.
Possible alternative ways to express this:
to become stultified
- from ingrained habits
- through monotony
somebodys behaviour became dull
- from tedious repetetion
- ever the same approach
But of course I'm searching for a more catchy phrase or expression which coveres that sentiment.