Rat superior colliculus encodes the transition between static and dynamic vision modes

Abstract

The continuity illusion occurs when visual stimuli are presented at a sufficiently high frequency, thereby triggering a shift from the static to the dynamic vision mode. This facilitates perception of continuous and moving objects, which is key for interactions with the surrounding environment. However, how the continuity illusion is encoded along the entire visual pathway remains poorly understood, with disparate Flicker Fusion Frequency (FFF) measured at the retinal, cortical, and behavioural levels. Here, we combine a behavioural paradigm, functional–MRI (fMRI), and electrophysiological validation for studying the mechanisms underlying the encoding of the continuity illusion effect in the rat. Our behavioural measurements reported a Flicker Fusion Frequency (FFF) of 18±2 Hz. Remarkably, whole–pathway fMRI revealed marked zero–crossings from positive to negative fMRI signal regimes at the FF in the superior colliculus (SC) – an important visual saliency detector – but not in higher cortical or thalamic visual areas. Our electrophysiological recordings in SC explained the sources of these observations as arising from strong neuronal suppression when the continuity illusion is achieved. Combined, our data suggests that activation and suppression balances in SC play a critical role in encoding the continuity illusion effect.

Dissecting push/pull interactions in the rat subcortical auditory pathway

“…These findings highlight the importance of subcortical interactions in auditory processing and lays the foundation for deeper investigations of push-pull effects, as e.g. recently also observed in superior colliculus (Gil et al, 2024a), using fMRI.…”

“…No ipsilateral responses (barring CN), or evidence of an auditory push-pull mechanism have been previously reported using BOLD fMRI. Recently, we showed that high-field rodent systems and cryogenic probes can sufficiently enhance sensitivity towards detecting negative BOLD signals upon population-level silencing (Gil et al, 2024b, 2024a). Here, we hypothesized that a push-pull mechanism due to silencing should produce negative BOLD responses (NBRs) in the iIC.…”

Extensive topographic remapping and functional sharpening in the adult rat visual pathway upon first visual experience

“…For all time points, immediately after the initial overshoot the VD SC exhibited a negative BOLD response to the retinotopic stimulus, contrasting with the positive BOLD response measured in HC, as it is evident in the BOLD activation maps and in the BOLD time series. Note that BOLD signal in SC (key player in saliency detection) has 3 distinct phases, an initial positive peak (likely representing the detection of stimulation onset), a positive plateau phase during the repetitive stimulus duration, and another post-stimulus peak (likely representing a signaling of cessation of activity) [ 46 ]. In Fig 4D, 4G, 4J and 4M , we averaged the BOLD signal across the entire stimulation period, merging the initial phases of the SC signal (initial overshoot and plateau).…”

“…Here, we used far more complex stimuli: the duration of stimulation, stimulus size and content of the stimulus, the contrast, the spatial frequency, and the movement direction very likely influenced the dynamics of the measured BOLD-fMRI responses and are likely to originate the differences between the studies. Gil and colleagues (2022) varied the temporal flickering frequency of the LED visual stimulation, this resulted in dramatic changes in the BOLD response profile across the entire visual pathway (including the changes in the positiveness of the BOLD responses) [ 46 ]. The fact that the change of a single stimulus parameter can result in such dramatic changes in the BOLD responses suggests that the different findings between this study and the one of Gil and colleagues (2021) can be attributed to the different types of stimulus used.…”

“…They found that when continuity illusion is achieved, the SC fMRI signal changes from positive to negative regimes. Importantly, electrophysiological recordings in SC point to neuronal suppression as a source of the negative BOLD signal [ 46 ]. This study also points to the idea that chronic visual deprivation disrupts the activation–suppression balance in SC, leading to negative BOLD responses in the SC in VD animals throughout the 4 scanning sessions.…”