Unsuppressible Repetition Suppression and exemplar-specific Expectation Suppression in the Fusiform Face Area

Abstract

Recent work casts Repetition Suppression (RS), i.e. the reduced neural response to repeated stimuli, as the consequence of reduced surprise for repeated inputs. This research, along with other studies documenting Expectation Suppression, i.e. reduced responses to expected stimuli, emphasizes the role of expectations and predictive codes in perception. Here, we use fMRI to further characterize the nature of predictive signals in the human brain. Prior to scanning, participants were implicitly exposed to associations within face pairs. Critically, we found that this resulted in exemplar-specific Expectation Suppression in the fusiform face-sensitive area (FFA): individual faces that could be predicted from the associations elicited reduced FFA responses, as compared to unpredictable faces. Thus, predictive signals in the FFA are specific to face exemplars, and not only generic to the category of face stimuli. In addition, we show that under such circumstances, the occurrence of surprising repetitions did not trigger enhanced brain responses, as had been recently hypothesized, but still suppressed responses, suggesting that repetition suppression might be partly ‘unsuppressible’. Repetition effects cannot be fully modulated by expectations, which supports the recent view that expectation and repetition effects rest on partially independent mechanisms. Altogether, our study sheds light on the nature of expectation signals along the perceptual system.

Immediate stimulus repetition abolishes stimulus expectation and surprise effects in fast periodic visual oddball designs

“…Our results instead align with recent work reporting distinct mechanisms underlying repetition suppression and perceptual expectations (e.g. Grotheer & Kovacs, 2015;Pajani et al, 2017;Feuerriegel et al, 2018).…”

“…In contrast, the Size Change oddball indexes a combination of low-level stimulus differences relative to base rate faces (due to increased image size) and surprise signals resulting from a violation of expectations for the common oddball face identity. This oddball (as well as the Identity Change oddball) were included to test the hypothesis that face identity repetition suppression (i.e., smaller signals for identity repetitions compared to identity changes from base rate faces) would not be found when identity repetitions are unlikely, indicating that repetition suppression effects simply reflect perceptual expectations (Summerfield et al, 2008; but see Pajani et al, 2017). We could test this hypothesis by comparing oddball responses between each size-matched identity repetition and identity change oddball stimulus type.…”

Evaluating the neurophysiological evidence for predictive processing as a model of perception

“…Numerous subsequent functional magnetic resonance imaging (fMRI), magnetoencephalography (MEG), and electroencephalography (EEG) studies have reported modulations of RS by the probability of a repetition 45,46,58–67 (i.e., expectation) (but see Refs. 68–70). Also consistent with PP, and seemingly at odds with neural adaptation accounts, is evidence that unexpected repetitions evoke greater neural responses than frequent alternations 37 .…”

“…A key observation from this work has been that the observed neural modulations are not generic to broad categories of stimuli, but specific to individually predicted exemplars. For instance, Pajani et al 70 . observed ES in FFA BOLD activity in response to specifically predicted facial identities relative to nonpredicted faces.…”

“…A key observation from this work has been that the observed neural modulations are not generic to broad categories of stimuli, but specific to individually predicted exemplars. For instance, Pajani et al 70 observed ES in FFA BOLD activity in response to specifically predicted facial identities relative to nonpredicted faces. Although many of the above studies could not segregate the distinct contributions of error and expectation units to the observed global neural response modulations, some studies have applied alternative experimental designs and methodologies that were tailored to do so.…”