  Instruction-tuning is a widely adopted method of finetuning that enables
large language models (LLMs) to generate output that more closely resembles
human responses to natural language queries, in many cases leading to
human-level performance on diverse testbeds. However, it remains unclear
whether instruction-tuning truly makes LLMs more similar to how humans process
language. We investigate the effect of instruction-tuning on LLM-human
similarity in two ways: (1) brain alignment, the similarity of LLM internal
representations to neural activity in the human language system, and (2)
behavioral alignment, the similarity of LLM and human behavior on a reading
task. We assess 25 vanilla and instruction-tuned LLMs across three datasets
involving humans reading naturalistic stories and sentences. We discover that
instruction-tuning generally enhances brain alignment by an average of 6%, but
does not have a similar effect on behavioral alignment. To identify the factors
underlying LLM-brain alignment, we compute correlations between the brain
alignment of LLMs and various model properties, such as model size, various
problem-solving abilities, and performance on tasks requiring world knowledge
spanning various domains. Notably, we find a strong positive correlation
between brain alignment and model size (r = 0.95), as well as performance on
tasks requiring world knowledge (r = 0.81). Our results demonstrate that
instruction-tuning LLMs improves both world knowledge representations and brain
alignment, suggesting that mechanisms that encode world knowledge in LLMs also
improve representational alignment to the human brain.
