| 1:17a |
A new oculomotor model demystifies "Remarkable Saccades"
Hering's Law of binocular eye movement control guides most oculomotor research and supports diagnosis and treatment of clinical eye misalignment (strabismus). The law states that all eye movements are controlled by a unitary conjugate signal and a unitary vergence signal that sum. Recent evidence of temporally asynchronous inter-eye rotations during vergence (Chandna et al., 2021) challenges the viability of a unitary vergence signal. An alternative theory that might explain these anomalous results posits that the eyes are controlled independently. Yet independent control fails to explain a phenomenon known as "Remarkable Saccades" where an inappropriate saccade occurs from an eye aligned on a target during asymmetric vergence (Enright, 1992). We introduce a new model formulated to describe the Chandna et al. (2021) midline vergence result that generates remarkable saccades as an emergent property. The Hybrid Binocular Control model incorporates independent controllers for each eye with a cortical origin that interact with a unitary conjugate controller residing in brainstem. The model also accounts for behavioral variations in remarkable saccades when observers attend to an eye. Furthermore, it suggests more generally how the eyes are controlled during vergence and other voluntary eye movements, thus challenging documented oculomotor neural circuitry and suggesting that refinements are needed for clinical oculomotor interventions. |
| 12:20p |
Structural connectivity matures along a sensorimotor-association connectional axis in youth
Childhood and adolescence are associated with protracted developmental remodeling of cortico-cortical structural connectivity. However, how heterochronous development in white matter structural connectivity spatially and temporally unfolds across the macroscale human connectome remains unknown. Leveraging non-invasive diffusion MRI data from both cross-sectional (N = 590) and longitudinal (baseline: N = 3,949; two-year follow-up: N = 3,155) developmental datasets, we found that structural connectivity development diverges along a pre-defined sensorimotor-association (S-A) connectional axis from ages 8.1 to 21.9 years. Specifically, we observed a continuum of developmental profiles that spans from an early childhood increase in connectivity strength in sensorimotor-sensorimotor connections to a late adolescent increase in association-association connectional strength. The S-A connectional axis also captured spatial variations in associations between structural connectivity and both higher-order cognition and general psychopathology. Together, our findings reveal a hierarchical axis in the development of structural connectivity across the human connectome. |
| 1:31p |
Interval timing clock property in the rat granular retrosplenial cortex
The rodent granular retrosplenial cortex (gRSC), densely interconnected with the hippocampal formation and the anterior thalamic nuclei, plays an important role in learning and memory. We had revealed that small pyramidal neurons in the superficial layers of the rat gRSC exhibit late-spiking (LS) firing properties. It has been suggested that neural circuits containing LS neurons can encode time intervals on the order of seconds, known as "interval timing". To test the possibility that the rat gRSC is involved in the processing of interval timing, we employed a trace fear conditioning paradigm in which the conditioned stimulus (CS) and the unconditioned stimulus (US) were temporally separated. First, we examined the effect of cytotoxic lesions made in the RSC prior to trace fear conditioning. We found that intact rats exhibited freezing behavior after CS tone presentation, whereas lesioned rats did not exhibit such freezing behavior. Next, we conducted in vivo chronic or acute recordings of neural activity from the rat gRSC in a test session conducted one week after the conditioning. In both recordings, we observed a distinct spike activity in which there was a transient increase in the firing rate around the presentation of the CS tone, followed by a rapid suppression and then ramping activity (a gradual elevation of the firing rate) until the next CS presentation. This "ramping activity" is thought to be one way in which interval timing is represented in the brain. Post stimulus histogram analysis revealed the existence of ramping activity in the gRSC, which reached its peak at various time intervals after the onset of the CS tone. Interestingly, this activity was specifically observed in response to the CS tone but not to the non-CS tone. Moreover, in naive rat gRSC (no trace fear conditioning), no such ramping activity was observed. These results indicate that gRSC neurons can encode time information on the order of tens to hundreds of seconds, integrating incoming sensory input with past memory traces.The rodent granular retrosplenial cortex (gRSC), densely interconnected with the hippocampal formation and the anterior thalamic nuclei, plays an important role in learning and memory. We had revealed that small pyramidal neurons in the superficial layers of the rat gRSC exhibit late-spiking (LS) firing properties. It has been suggested that neural circuits containing LS neurons can encode time intervals on the order of seconds, known as "interval timing". To test the possibility that the rat gRSC is involved in the processing of interval timing, we employed a trace fear conditioning paradigm in which the conditioned stimulus (CS) and the unconditioned stimulus (US) were temporally separated. First, we examined the effect of cytotoxic lesions made in the RSC prior to trace fear conditioning. We found that intact rats exhibited freezing behavior after CS tone presentation, whereas lesioned rats did not exhibit such freezing behavior. Next, we conducted in vivo chronic or acute recordings of neural activity from the rat gRSC in a test session conducted one week after the conditioning. In both recordings, we observed a distinct spike activity in which there was a transient increase in the firing rate around the presentation of the CS tone, followed by a rapid suppression and then ramping activity (a gradual elevation of the firing rate) until the next CS presentation. This "ramping activity" is thought to be one way in which interval timing is represented in the brain. Post stimulus histogram analysis revealed the existence of ramping activity in the gRSC, which reached its peak at various time intervals after the onset of the CS tone. Interestingly, this activity was specifically observed in response to the CS tone but not to the non-CS tone. Moreover, in naive rat gRSC (no trace fear conditioning), no such ramping activity was observed. These results indicate that gRSC neurons can encode time information on the order of tens to hundreds of seconds, integrating incoming sensory input with past memory traces. |