Following satnav instructions "switches off" activity in parts of the brain used for navigation, a study has shown.
Researchers carried out brain scans of 24 volunteers as they explored a simulation of the streets of Soho, in London's West End.
When they had to find their own way around, spikes of activity were seen in the hippocampus and pre-frontal cortex, the regions of the brain involved in memory and planning.
Activity went up when the number of potential routes was increased. But no activity spike was detected when people followed satnav instructions.
Psychologist Dr Hugo Spiers, from University College London (UCL), said: "Entering a junction such as Seven Dials in London, where seven streets meet, would enhance activity in the hippocampus, whereas a dead-end would drive down its activity.
"If you are having a hard time navigating the mass of streets in a city, you are likely putting high demands on your hippocampus and pre-frontal cortex.
"Our results fit with models in which the hippocampus simulates journeys on future possible paths while the pre-frontal cortex helps us to plan which ones will get us to our destination.
"When we have technology telling us which way to go, however, these parts of the brain simply don't respond to the street network.
"In that sense our brain has switched off its interest in the streets around us."
Previous research at UCL has shown that the hippocampus brain regions of London taxi drivers expand as they learn "the knowledge" - memorising the streets and landmarks of the capital.
The new results, published in the journal Nature Communications, suggest that drivers who rely on satnav instructions do not engage their hippocampus and find it harder to commit city streets to memory.
With its complex network of small streets, London seems to be especially challenging to the hippocampus.
Much less mental effort might be needed to navigate Manhattan's grid layout in New York, said the scientists.
At most junctions in the American city the only options are to go straight, left or right.
Dr Beatrix Emo, who led the city street analysis at UCL and now works in Switzerland, said: "Linking the structure of cities to behaviour has been around since the 1980s but this is the first study to reveal the impact of that structure on the brain."
Commenting on the findings Dr Christopher Connolly, from the University of Dundee, said: "We still need to navigate ourselves around other maps not covered by satnav, such as at work/school or when shopping.
"Furthermore, the hippocampus plays a critical role in the consolidation of short-term memory to long-term memory, a function that is lost during Alzheimer's disease.
"Although previous studies have reported that brain training exercises improve the skills in that task, they do not necessarily improve our performance in other learning tasks."