Selective visual attention is one of the most fundamental cognitive functions in humans. Although there is a plethora of literature that uses neuroimaging to probe the neural mechanisms of visual attention, very few studies have investigated visual attention at the single-neuron level in humans. Our lab conducts one of the very first studies to investigate visual attention at the single-unit level in humans. Importantly, we are investigating a network of brain regions critical for selective visual attention, especially attention to social stimuli (i.e., social attention), and we are investigating both goal-driven and stimulus-driven attention. The primary objectives of our research are two-fold: (1) to characterize social attention signals in the medial temporal lobe (amygdala and hippocampus) and prefrontal cortex (in particular the orbitofrontal cortex—a relatively unexplored brain region for social attention); and (2) to analyze attention signals from different brain regions using functional connectivity analysis. Our research will provide the most comprehensive analysis of the neural circuits underlying social attention in humans. The outcomes of our research will shed light on the neural mechanisms of impaired visual attention in patients with psychiatric and neurological disorders, such as autism spectrum disorder (ASD) and Attention-Deficit / Hyperactivity Disorder (ADHD).

In our recent publications (Wang et al., Curr Biol, 2018; Wang et al., Brain, 2019), we have shown a distinct population of target-selective neurons in both the medial temporal lobe (MTL) and medial frontal cortex (MFC) whose response signals whether the currently fixated stimulus is a target or distractor. This target-selective response is invariant to visual category and predicts whether a target is detected or missed behaviorally during a given fixation. The response latencies, relative to fixation onset, of MFC target-selective neurons precede those in the MTL by 200 ms, suggesting a frontal origin for the target signal. The human MTL thus represents not only fixed stimulus identity, but also task-specified stimulus relevance due to top-down goal relevance.

Visual search task and single-neuron responses from the medial temporal lobe (MTL). (A) Task structure. (B) An example visual search array with fixations indicated. Each circle represents a fixation. Green circle: first fixation. Magenta circle: last fixation. Yellow line: saccades. Red box: target. (C) An example face-selective neuron. Each raster (upper) and peristimulus time histogram (PSTH) (lower) is shown with color coding as indicated. Trials are aligned to cue presentation (gray lines). Trials within each category are sorted according to reaction time (black line). Waveforms for each unit are shown in the raster plot. In the PSTH plot, asterisk (*) indicates a significant difference between the response to face-target trials and non-face-target trials in that bin (P<0.05, two-tailed t test, Bonferroni-corrected; bin size = 250ms). Shaded area denotes ± SEM across trials. (D) An example target-selective neuron increased firing rate when fixating on targets vs. distractors. Fixations are sorted by fixation duration (black line shows start of the next saccade). t = 0 is fixation onset. Asterisk (*) indicates a significant difference between fixations on targets and distractors in that bin (P<0.05, two-tailed t-test, Bonferroni-corrected; bin size = 50ms). (E) Cue-aligned average peristimulus time histogram (PSTH) of all face-preferred neurons in face (tan) and non-face (green) trials. (F) Cumulative distribution of the single-fixation response of 50 target neurons.

Frontal target neurons respond before MTL target neurons. (A, B) Example target neurons from the pre-SMA. (C, D) Two target neurons simultaneously recorded in the pre-SMA and MTL. (E) Cumulative firing rate for target neurons from the pre-SMA (dotted lines; n=31 neurons) and MTL (solid lines; n=27 neurons). Shaded area denotes ± SEM across neurons. Red: fixations on targets. Blue: fixations on distractors. Top bars show clusters of time points with a significant difference. Arrows indicate the first time point of the significant cluster. Magenta: MTL neurons. Black: pre-SMA neurons. (F) Difference in cumulative firing rate (calculated from [E]).