I want to quote at length because it's fascinating in the context of the left/right hemisphere concepts Iain McGilchrist has been best known for (especially the idea that left-hemisphere dominance is a relatively recent human adaptation):

https://www.youtube.com/watch?v=dFs9WO2B8uI

https://www.ted.com/talks/iain_mcgilchrist_the_divided_brain

https://www.thersa.org/globalassets/pdfs/blogs/rsa-divided-b...

https://en.wikipedia.org/wiki/The_Master_and_His_Emissary

Linguists also administered several brain exams specifically geared towards measuring Genie's language comprehension. On one such test she had no difficulty giving the correct meaning of sentences containing familiar homophones, demonstrating that her receptive comprehension was significantly better than her expressive language. Genie also did very well at identifying rhymes, both tasks that adult split-brain and left hemispherectomy patients had previously been recorded performing well on. During these tests an EEG consistently picked up more activity from the two electrodes over the right hemisphere of her brain than from those over the normal locations of the Broca's area and Wernicke's area, and found especially high involvement from her right anterior cerebral cortex, lending further support to the researchers' conclusion that Genie was using her right hemisphere to acquire language.

As early as 1972 Genie scored between the level an 8-year-old and an adult on all right-hemisphere tasks the scientists tested her on, and showed extraordinarily rapid improvement on them. Her ability to piece together objects solely from tactile information was exceptionally good, and on spatial awareness tests her scores were reportedly the highest ever recorded. Similarly, on a Mooney Face Test in May 1975 had the highest score in medical literature at that time, and on a separate gestalt perception test her extrapolated score was in the 95th percentile for adults. On several other tests involving right-hemisphere tasks, her results were markedly better than other people in equivalent phases of mental development; in 1977 the scientists measured her capacity for stereognosis at approximately the level of a typical 10-year-old, significantly higher than her estimated mental age. The scientists also noted in 1974 that Genie seemed to be able to recognize the location she was in and was good at getting from one place to another, an ability which primarily involves the right hemisphere.

Genie's performance on these tests led the scientists to believe that her brain had lateralized, and that her right hemisphere had undergone specialization. Because Genie's performance was so high on such a wide variety of tasks predominantly utilizing the right hemisphere of her brain, they concluded her exceptional abilities extended to typical right-hemisphere functions in general and were not specific to any individual task. They attributed her extreme right hemisphere dominance to the fact that what very little cognitive stimulation she did receive was almost entirely visual and tactile. While even this had been extremely minimal it had been enough to commence lateralization in her right hemisphere, and the severe imbalance in stimulation caused her right hemisphere to become extraordinarily developed.

By contrast, Genie performed significantly below average and showed much slower progress on all tests measuring predominantly left-hemisphere tasks. Stephen Krashen wrote that by 2 years after the first examinations on her mental age Genie's scores on left-hemisphere tasks consistently fell into the 2½- to 3-year-old range, only showing an improvement of 1½ years. On sequential order tests she consistently scored well below average for someone with a fully intact brain, although she did somewhat better on visual than on auditory tests. The scientists especially noted that she did not start to count until late 1972, and then only in an extremely deliberate and laborious manner. In January 1972 the scientists measured her in the 50th percentile for an 8½- to 9-year-old on Raven's Progressive Matrices, although they noted she was outside of the age range of the test's design. Similarly, when the scientists administered Knox Cubes tests in 1973 and 1975 Genie's score improved from the level of a 6-year-old to a 7½-year-old, more rapid than her progress with language but significantly slower than that of right hemisphere tasks.