The face-selective response in the human occipito-temporal cortex has been extensively explored at the level of large neural populations (e.g., using functional MRI [fMRI], electroencephalography / magnetoencephalography, or intracranial local field potential), but not at the level of single or multi-units. We have recently reported a rare case of two face-selective units located in the vicinity of the Fusiform Face Area, recorded from a patient with epilepsy (Axelrod et al., 2019, Neurology). These units exhibited a robust (300% and more) modulation for a variety of facial stimuli. In addition to the stimulus-evoked response, it is well established that neurons also fire spontaneously, without any task. Notably, the degree of similarity between the magnitudes of stimulus-evoked activity and spontaneous activity is still unclear. In the present study we capitalized on a rare face-selective multi-unit recording from a human subject, to compare stimulus-evoked activity elicited by static images of faces and spontaneous activity recorded during a 6-minute continuous resting-state session. We found that generally, the magnitude of the face-selective stimulus-evoked response was much greater than the magnitude of spontaneous activity. However, this difference also depended on the duration of the time-window (i.e. a period of interest) utilized to examine the response. In particular, for time-windows of 150 ms and more, there were few spontaneous responses with comparable firing rates to those found in the face-selective evoked response. However, for shorter periods of interest (e.g. 50 ms), when comparing an equal number of windows, about 10-20% of the firing rates recorded during spontaneous activity were comparable to those recorded during the face-selective stimulus-evoked response. Overall, the present results provide a unique perspective on the relationship between stimulus-evoked and spontaneous neural activity.
Abstract The stimulus-evoked neural response is a widely explored phenomenon. Conscious awareness is associated in many cases with the corresponding selective stimulus-evoked response. For example, conscious awareness of a face stimulus is associated with or accompanied by stimulus-evoked activity in the fusiform face area (FFA). In addition to the stimulus-evoked response, spontaneous (i.e. task-unrelated) activity in the brain is also abundant. Notably, spontaneous activity is considered unconscious. For example, spontaneous activity in the FFA is not associated with conscious awareness of a face. The question is: what is the difference at the neural level between stimulus-evoked activity in a case that this activity is associated with conscious awareness of some content (e.g. activity in the FFA in response to fully visible face stimuli) and spontaneous activity in that same region of the brain? To answer this question, in the present study, we had a rare opportunity to record two face-selective multi-units in the vicinity of the FFA in a human patient. We compared multi-unit face-selective task-evoked activity with spontaneous prestimulus and a resting-state activity. We found that when activity was examined over relatively long temporal windows (e.g. 100–200 ms), face-selective stimulus-evoked firing in the recorded multi-units was much higher than the spontaneous activity. In contrast, when activity was examined over relatively short windows, we found many cases of high firing rates within the spontaneous activity that were comparable to stimulus-evoked activity. Our results thus indicate that the sustained activity is what might differentiate between stimulus-evoked activity that is associated with conscious awareness and spontaneous activity.
