Biology subjects

Ahmed, O. J.

Publications and source records attributed to Ahmed, O. J..

4 recordsLinked to original sources

bioRxiv2021

Sea of Electrodes Array (SEA): Extremely Dense and High-Count Silicon-Based Electrode Array Technology for High-Resolution High-Bandwidth Interfacing with 3D Neural Structures

In this work, we propose a new silicon-based micro-fabrication technology to fabricate 3D high-density high-electrode-count neural micro-probe arrays scalable to thousands and even millions of individual electrodes with user-defined length, width, shape, and tip profile. This unique technology utilizes DRIE of ultra-high aspect-ratio holes in silicon and refilling them with multiple films to form thousands of individual needles with metal tips making up the "sea-of-electrodes" array (SEA). World-record density of 400 electrodes/mm2 in a 5184-needle array is achieved. The needles are ~0.5-1.2mm long, <20m wide at the base, and <1m at the tip. The silicon-based structure of these 3D array probes with sharp tips, makes them stiff enough and easily implantable in the brain to reach a targeted region without failing. Moreover, the high aspect ratio of these extremely fine needles reduces the tissue damage and improves the chronic stability. Functionality of the electrodes is investigated using acute in vivo recording in a rat barrel field cortex under isoflurane anesthesia.

neuroscience↗

bioRxiv2020

Thalamus and claustrum control parallel layer 1 circuits in retrosplenial cortex

The granular retrosplenial cortex (RSG) is critical for both spatial and non-spatial behaviors, but the underlying neural codes remain poorly understood. Here, we use optogenetic circuit mapping in mice to reveal a double dissociation that allows parallel circuits in superficial RSG to process disparate inputs. The anterior thalamus and dorsal subiculum, sources of spatial information, strongly and selectively recruit small low-rheobase (LR) pyramidal cells in RSG. In contrast, neighboring regular-spiking (RS) cells are preferentially controlled by claustral and anterior cingulate inputs, sources of mostly non-spatial information. Precise sublaminar axonal and dendritic arborization within RSG layer 1, in particular, permits this parallel processing. Observed thalamocortical synaptic dynamics enable computational models of LR neurons to compute the speed of head rotation, despite receiving head direction inputs that do not explicitly encode speed. Thus, parallel input streams identify a distinct principal neuronal subtype ideally positioned to support spatial orientation computations in the RSG.

neuroscience↗

bioRxiv2020

Instantaneous amplitude and shape of postrhinal theta oscillations differentially encode running speed

Hippocampal theta oscillations have a temporally asymmetric waveform shape, but it is not known if this theta asymmetry extends to all other cortical regions involved in spatial navigation and memory. Here, using both established and improved cycle-by-cycle analysis methods, we show that theta waveforms in the postrhinal cortex are also temporally asymmetric. On average, the falling phase of postrhinal theta cycles lasts longer than the subsequent rising phase. There are, however, rapid changes in both the instantaneous amplitude and instantaneous temporal asymmetry of postrhinal theta cycles. These rapid changes in amplitude and asymmetry are very poorly correlated, indicative of a mechanistic disconnect between these theta cycle features. We show that the instantaneous amplitude and asymmetry of postrhinal theta cycles differentially encode running speed. Although theta amplitude continues to increase at the fastest running speeds, temporal asymmetry of the theta waveform shape plateaus after medium speeds. Our results suggest that the amplitude and waveform shape of individual postrhinal theta cycles may be governed by partially independent mechanisms and emphasize the importance of employing a single cycle approach to understanding the genesis and behavioral correlates of cortical theta rhythms.

neuroscience↗

bioRxiv2019

Uniquely excitable neurons enable precise and persistent information transmission through the retrosplenial cortex

The retrosplenial cortex (RSC) is essential for both memory and navigation, but the neural codes underlying these functions remain largely unknown. Here, we show that the most prominent cell type in layers 2/3 (L2/3) of the granular RSC is a uniquely excitable, small pyramidal cell. These cells have a low rheobase (LR), high input resistance, lack of spike-frequency adaptation, and spike widths intermediate to those of neighboring fast-spiking (FS) inhibitory neurons and regular-spiking (RS) excitatory neurons. LR cells are excitatory but rarely synapse onto neighboring neurons. Instead, L2/3 of RSC is an inhibition-dominated network with dense connectivity between FS cells and from FS to LR neurons. Biophysical models of LR but not RS cells precisely and continuously encode sustained input from afferent postsubicular head-direction cells. Thus, the unique intrinsic properties of LR neurons can support both the precision and persistence necessary to encode information over multiple timescales in the RSC.

neuroscience↗