Neural evidence for progressive decline of visual face recognition ability in Alzheimer's disease

Alzheimer's disease (AD) is associated with deficits in recognizing familiar faces already at prodromal stages due to significant episodic and semantic memory impairment. However, whether the ability to visually discriminate and match unfamiliar faces for their identity also declines in AD remains unclear. We addressed this issue with a robust implicit electroencephalographic (EEG) measure of face identity recognition (FIR) relying on fast periodic visual stimulation (FPVS). Thirty cognitively unimpaired (CU) individuals and thirty AD patients, including seventeen patients with mild cognitive impairment (MCI) due to AD and thirteen patients with dementia due to AD viewed stimulation sequences of the same unfamiliar face identity repeated at 6 Hz across size changes, with other identities appearing every 5 faces, eliciting a neural 1.2 Hz response indexing FIR. Stimuli were also presented upside down to control for low-level visual effects. While CU individuals exhibited robust FIR EEG responses at 1.2 Hz (and harmonics) over the occipito-temporal cortex, larger for upright than inverted faces, this response and the inversion effect were significantly reduced in patients with dementia due to AD but preserved in patients with MCI due to AD. These results indicate that unfamiliar FIR is preserved in patients at the prodromal stage, suggesting that familiar FIR difficulties previously observed during the prodromal phase of AD may stem from other cognitive factors, such as long-term memory impairments. In contrast, deficits in discriminating unfamiliar faces seem to emerge as the disease progresses. Overall, this study provides novel evidence for the processes underlying face identity recognition deficits in AD and highlights the diagnosticity of FPVS-EEG for assessing specific cognitive processes in neurodegenerative populations.