Dissection of tectal feedback connections in the rat visual system using MRI

1334 

Abstract Submission 

Rita Gil¹, Mafalda Valente¹, Noam Shemesh¹

¹Champalimaud Research, Champalimaud Foundation, Lisbon, Lisbon

Rita Gil  
Champalimaud Research, Champalimaud Foundation
Lisbon, Lisbon

Mafalda Valente  
Champalimaud Research, Champalimaud Foundation
Lisbon, Lisbon

Noam Shemesh  
Champalimaud Research, Champalimaud Foundation
Lisbon, Lisbon

Previous work from the lab has shown positive to negative BOLD transitions in the rat superior colliculus (SC) when moving from low to high frequency visual stimulation¹. Such modulations have been associated with high visual frequency stimulation-induced neuronal suppression and the entrance in the continuity illusion regime: individual light flashes discriminated in the low frequency static vision mode become fused in the high frequency dynamic vision mode and animals report seeing an illusory continuous light. Since retinal evoked potentials can still track individual light flashes at very high frequencies^2,3, SC's response modulation could result from feedback arriving to the region, namely from the primary visual cortex (V1) - corticotectal connections^4,5- and between SCs –tectotectal connections^5-7. In this work we investigate the effect of such feedback connections on the SC response modulations using fMRI.

Study in Long-Evans rats under medetomidine sedation in accordance with European Directive 2010/63.

Stimulation: Binocular/monocular stimulation at 1Hz and 25Hz (flash duration=10 ms) was performed with an optic fibre connected to a blue LED (λ  = 470nm).

Ibotenic Acid Lesions: Injections of 1% ibotenic solution diluted in a 0.1M NaOH solution were performed along the left SC and/or both V1s. Animals were imaged one week after surgery to avoid inflammation.

fMRI: Spin-echo echo planar imaging (SE-EPI) sequence in a 9.4T scanner: TR/TE=1500/40ms; resolution=268x268μm², slice=1.5mm. Pre-processing: Outlier, motion and slice-timing correction, isotropic smoothing. Runs were detrended with a polynomial fit to rest periods.

The visual pathway is shown in Fig.1A. BOLD t-maps for the controls (Fig.1B) and V1 lesion group (Fig.1C), with reduced cortical feedback, show SC response modulation as a function of frequency: monocular stimulation at high frequency reveals positive/negative responses in the ipsilateral/contralateral SC (iSC/cSC), respectively. V1 lesioning reduces response amplitudes in both stimulation frequencies and modalities suggesting a general cortical gain effect (Fig.1D).
To investigate the tectotectal connections, monocular stimulation (Fig.2A) is used, along with iSC and/or V1 lesions (Fig.2B).
While for the 1Hz regime iSC lesions have little effect on the cSC responses, for the 25Hz regime the cSC responses are driven towards stronger negative values (Fig.2C). High frequency regimes lead to iSC positive responses that appear to counterbalance the negative cSC responses. Once such positive responses are abolished (through iSC lesions) the cSC responses are amplified. In Fig.2D we propose a mechanism for the tectotectal interactions in the different frequency stimulation regimes.

While cortical feedback serves as general gain control of the SC responses, the interaction between the SCs appears to depend on the operating vision mode. During the static vision mode, tectotectal feedback does not play a big role; however, at high frequency stimulation regime (when animals enter the dynamic vision mode) tectotectal feedback appears to exert a push-pull effect where the iSC counterbalances the neuronal suppression in the visually stimulated SC (cSC). Future studies are needed to better understand the behavioural relevance of these measured opposite responses in SCs at high frequency regimes.

Activation (eg. BOLD task-fMRI) ¹

fMRI Connectivity and Network Modeling

Subcortical Structures ²

BOLD fMRI

Perception and Attention Other

ANIMAL STUDIES

Cortex

FUNCTIONAL MRI

MRI

Sub-Cortical

Vision

Other - Brain Lesions